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540 Cards in this Set
- Front
- Back
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is changing the crystal size to affect beam diameter practical?
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no
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What terms are used to describe performance and specifications
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transmit power, dynamic range, signal, noise, noise floor, signal to noise ratio (SNR), Compression, Pre-processing, Post processing
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name the four methods used for focusing
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lenses, curved elements, electronic focusing, and mirrors
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What is focal depth proportional to?
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the diameter of the crystal squared
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why are acoustic mirrors not used
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because they aren't practical or popular
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What is transmit power also called
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acoustic power, output power, transmit gain, power gain, acoustic gain, output intensity, transmit voltage, output voltage
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when can electronic focusing be done?
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when there is more than one crystal
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What is the equation for focus depth?
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Diameter squared over four times the wavelength
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multiple crystals are referred to as an
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array
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What does transmit power control
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the amplitude of the excitation voltage that drives the crystal(s)
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can a technologist alter the focusing characteristic using electronic focusing?
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yes
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Where is the beam diameter smallest at?
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the end of the near zone, fresnel zone, focal zone
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can focusing be used with multi element transducers?
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yes, it doesn't work with single element transducers
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Higher voltage= ___ amplitude mechanical oscillation of the crystal = ____ amplitude sound wave
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higher, higher
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what is the the difference between single element and multi element transducers
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they are more versatile
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What is beam divergence?
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the gradual spreading of the beam past the focus, in the fraunhofer zone
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what do lenses do
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they enable the sound waves to converge more rapidly that would naturally occur
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increasing transmit power and resulting intensity increase can cause
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bioeffects and tissue damage
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lenses produce a ____ focus
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shallower
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When the cross sectional ares is ______ there is less intensity.
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greater
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shallower focus means
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the beam diverges at a shallower depth
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What is the dynamic range
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the ratio of maximum to minimums of any quantity
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are lenses still in use, if not how long were they used?
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no, extensively for many years
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what are the two factors governing beam divergence?
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Transducer diameter and frequency of sound.
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what are the setbacks of lens focusing?
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the lenses are very absorptive and heat up with use, also they create another acoustic impedance mismatch
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name the different types of dynamic range
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input, output, display, and gain
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what shape of surface is used to help beam convergence?
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concave shape
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The smaller the crystal diameter the _____ the divergence in the fraunhofer zone.
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greater
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how is curved surface focusing achieved
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by mounting pzt to a curved surface
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what is input dynamic range
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ratio of the maximum input signal to the minimum possible input signal
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what are the setbacks to curved surface focusing?
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pzt material is brittle, not willing to bend, mounting to a curved surface is challenging
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the larger the crystal diameter the _____ the divergence in the fraunhofer zone.
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smaller
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how often are curved surface focuses used
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less than the lens
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what is output dynamic range
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ratio of the maximum to the minimum output signal
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what are the pluses for curved surface focusing?
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newer materials are more flexible and it eliminates acoustic impedance mismatch and heating setbacks of lens
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How are crystal diameter and beam divergence related?
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inversely
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can focusing techniques affect the beamwidth in the far field relative to natural focus?
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no only the near field
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what is the default dynamic range
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input dynamic range
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what does it mean that beyond the near field the beam is diffraction limited?
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there is no real way to get the waves to interfere either constructively or destructively to creat a narrower beam
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Because larger crystals diverge less in the far field they will have.....
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better lateral resolution
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what if you need to focus beyond the transducers natural focus?
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use a transducer with a larger diameter crystal
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What is the input dynamic range formal definition
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range of the signal amplitudes a system can receive and process without causing harmonic distortion
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How do we define intensity?
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power of cross sectional area
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frequency and divergence have a(n) ______ relationship.
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inverse
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what happens by our model of intensity as we focus the beam
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it goes up at the focal point
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what is signal
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any phenomenon desired to be measured
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what does the relationship of focus and intensity mean to us technologists?
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we want to place focus at area of interest on the screen
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lower frequency sounds more or less in the fraunhofer zone
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more
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what were simple single disc transducers used for?
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A-mode or amplitude mode
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what is noise
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unwanted signals
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characterize a simple single disc transducers abilities
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can only look straight ahead creating a one dimensional scanning technique
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higher frequency sound will create better .......
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lateral resolution in the far field.
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What is B-scan and what was it created for
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b-scan is brightness scan and it was created from the desire to see two dimensions and more than one region simultaneously
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what is noise floor
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amplitude level below which no signals are visible because of the presence of noise
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how was b scanning performed
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with depth preset by crystal abilities, the technologist physically slid the transducer over the patient to create the lateral dimension
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larger diameter crystals and higher frequency will create less or more divergence?
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less
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Why was a automated system created
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so that you could see two dimensions without having to physically mover scan head
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what is signal to noise ratio (SNR)
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amplitude of the signal divided by the amplitude of the noise
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explain the set-up of a mechanical transducer
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it was a simple crystal attached to the head of a motor which wobbled at various angles to generate the image
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smaller diameter crystals and lower frequency will create more or less divergence?
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more
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what were the setbacks of mechanical transducers?
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they had mechanical issues such as parts wearing out and pockets of air in gel inside scan head
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Signal to noise ratio specifies...
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the quality of the signal and how much faith we should put in the data
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which kind of transducers introduced phasing?
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array transducers
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Which is the most divergent:
a) 6cm and 4mHz b) 2cm and 3mHz c) 12 cm and 8mHz |
b
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explain 1-D arrays setup
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they had multiple crystals but only in one dimension
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What does a Higher SNR imply
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better imaging situation excluding artifact and more trustworthy data
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explain 1.5 D arrays setup
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developed in the 90's they had three elements in the elevation plane with 64+ in the lateral dimension
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Which is the least divergent:
a) 6cm and 4mHz b) 2cm and 3mHz c) 12 cm and 8mHz |
c
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what were the setbacks of 1-D array
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they could not focus in the elevation plane or acquire third dimension data automatically
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A strong signal by itself does/does not guarantee a good SNR
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does not
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how did 1.5 D arrays work
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you could switch on or off, individually, the elevation elements (affecting the diameter of the transducer) allowing the elevation focus to be changed
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What shape are sound waves that are produced by tiny PZT diverge in?
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V shape
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what did 1.5 D array transducers use as a guideline equation?
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focus depth is proportional to diameter squared
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a weak signal by itself does/ does not guarantee bad SNR
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does not
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explain the setup of 2-D arrays
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they have multiple crystals in both the lateral and elevation direction
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When do spherical waves occur?
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when the source is about the size of a wavelength
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how are 1.5D and 2D arrays different
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2D has a completely variable focus in elevation and dominate today's technology
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Changing the receiver gain does/does not improve the signal to noise ratio
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does not, it changes both unless some component of the signal is saturated
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What is the simplest probe still in use today
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pedoff, pencil probe
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What are V shaped waves known as?
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Spherical waves, diffraction patterns, or Huygens wavelets
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explain the pedoff probe
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it has a narrow bandwidth, symmetrical beam in elevation and lateral dimensions, it doesn't create an image, is manually steered, and it has fixed focus for transmit and receive
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So even if the true SNR does not change the
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apparent SNR might
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what is sequencing
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it is exciting groups of elements in a specific pattern to scan a region in a linear fashion
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The tiny source that produces wavelets are known as?
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huygens sources
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what is sequencing performed with
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with a large linear or curved linear array transducer
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A certain amount of what is necessary to make the signal bright enough so that it can be visualized on the display within the dynamic range of the monitor and the human eye
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gain
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According to Huygens' principle a large active element may be thought of as .....
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millions of tiny distinct sound sources
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What happens if the gain doesn't map the signal into the proper dynamic range
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the signal will appear weak at best because of the eyes or the displays lack of sensitivity
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A huygens source creates....
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a huygens wavelet with a v shape
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When you increase the gain the signal and noise...
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are amplified by the same amount, giving the appearance of improved SNR "apparent SNR"
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Why is Huygens' Principle important?
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It helps us understand why a circular active element is going to form and hour glass shape.
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What is electronic noise
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randoms signals caused by electric amplification of small returning echoes
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The hour glass shaped main beam is formed by
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wavelets overlapping and interfering constructively
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What creates electronic noise?
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random excitations of electrons within the electronics
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In the area where the sound beam is absent ____ interference occurs
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destructive
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What does electronic noise looks like in doppler spectrum, color doppler, or image
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random white speckle with high receiver gain, or random color pixels where there is no flow
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When stated simply what does Huygens' principle explain?
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Why the sound doesn't just go out straight and come back in straight.
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When does clutter happen?
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when larger returning echoes obliterate weaker signals
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What is sequencing?
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The excitation of elements in a specific pattern to linearly scan a region.
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How is clutter classified?
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as noise but the signal from clutter and other noise sources are distinctly different
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How are groups of elements switched on or off in a linear transducer?
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electronically
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where does haze come from
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artifact
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About how many elements are in a linear switched array transducer
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200+
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give an example of when haze is produced
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sidelobe returning echoes, poor skin contact, beam distortion (aberration) from tissue characteristic.
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Describe a linear switched array transducer
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the beam is asymmetric in the lateral and elevation planes (its wider than thick), the large lateral dimension is created by sequencing
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What is electrical interference
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when the transducer receives energy from other electrical devices or electromagnetic waves such as radio transmission
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What is a linear switched array transducer typically used for?
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Used for vascular 2D, color and Doppler
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How does electrical interference occur
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it can be carried through the air or from the power supplying the system
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What shape is the image produced by a linear switched array transducer?
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rectangular
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what does electrical interference look like
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a bright flashlight down the middle of an image or a barber pole flashing
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What are the disadvantages of a linear switched array transducer?
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Fixed focus, no steering, and expensive in comparison to a single element mechanical transducer
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what does electrical interference appear as on spectral doppler?
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bright white horizontal or zigzag lines in the spectrum called doppler tones
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What was the advantage of a linear switched array transducer?
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it allowed for a wide linear image in the near field, but it couldn't steer
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what is preprocessing
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signal conditioning that occurs in real time and cannot be removed from an image once acquired
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Why was a mechanically steered transducer developed?
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For cardiac imaging to create an automated image and get in between rib spaces.
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what is postprocessing
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any processing which can be changed after the data is acquired
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Describe how a mechanically steered transducer worked
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an element was mounted on a motor head which swept from one point to another acquiring scan lines from multiple positions over time
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where is postprocessing usually done?
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in the scan converter
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What is a mechanically steered transducer also known as and why
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a mechanical sector transducer, because the beam is sector shaped
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postprocessing can be performed on...
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frozen data as well as real time
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What was a mechanically steered transducer used for?
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2D imaging, M mode, Doppler, color Doppler
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what are examples of preprocessing?
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receiver gain, receive focusing, received compression
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Other than being sector shaped, what other qualities did a mechanical steered transducer beam have?
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it was symmetrical in elevation and lateral planes with a broad depth of field and deep focus
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what are examples of postprocessing?
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data compression colorization, and rejection
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What are the disadvantages of a mechanical steered transducer?
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fixed focus, parts wearing out, motion artifact, little imaging flexibility, air pockets in the gel inside the transducer head
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what is the pulser also known as
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the transmit beamformer
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What are the advantages mechanical steered transducers had over phased array transducers?
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they were less expensive to make, less expensive to run, and ran on a single channel
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what does the pulser do
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it creates electrical signals that excite the transducer crystal thus forming sound beams
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What is the difference between a mechanical annular array and a mechanical steered transducer
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annular array crystals are cut into concentric circular rings
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What must the pulser be able to do
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produce infinite numbers of electrical waveforms to drive the variety of transducers used in ultrasound, and produce continuous and pulsed waves
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How did cutting the crystal into circles solve the fixed focus problem?
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the varying diameter of the circles, and adjusting which ones were on at any given time.
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what does the pulser also do
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regulates the amplitude of the signals produced, therefore changing the acoustic power output
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What was a mechanical annular array used for
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2D, M-mode, Doppler, and Color
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what is the beamformer
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a part of the transmitter that functions with array transducers during transmission and reception
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What did a mechanical annular array image look like
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A sector image with a curved top
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what are two important functions of the beam former
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creates the appropriate phase delays and pulse sequencing to create the transmit beam and creates the appropriate phase delays and pulse sequencing to create the receive beams
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How was a mechanical annular array steered?
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by the elements being mounted on a wobbling motor
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what does the beamformer do to the signals that return from each element of the transducer?
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applies the appropriate processing summing together all the signals
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What were the advantages of a mechanical annular array transducer?
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it had variable focus in the lateral and elevation dimension, and variable depth of field
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what is apodization
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it is the limiting or restricting of which elements are active
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What were the disadvantages of a mechanical annular array?
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Limited temporal resolution, excessive grating lobe artifact, more expensive to manufacture due to cut elements, and more expensive to run electronically because of multiple channels
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what are active elements collectively known as
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the aperture
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Phasing is a term used to determine a ___ ___
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time reference
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each active element is connected to an ___ and ____ _____
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amplifier and processing chain
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For a wave, a phase difference is?
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the amount of time shift necessary to make two waves align
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what are the amplifier and processing chain collectively known as
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the receive channel
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in order to create constructive and destructive interference we need....
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multiple waves
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how many active receive channels are found in high end systems
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256-512
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Each transducer element acts as an individual _____ _____, so many elements collected together can create _____ _____ simultaneously.
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wave source, multiple waves
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the benefit of more receive channels is
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greater processing flexibility
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What is a collection of elements known as?
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an array
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the benefits of greater processing flexibility can only really be recognized when transducers exist that have...
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as many elements ad the system has channels (512 channels and 500 elements vs 512 channels and 6 elements) Ferrari in a school zone
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What is an array
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a group of small transducers which can be used together to form a larger more flexible transducer
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what operations does the receiver perform?
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amplification, compensation, compression, demodulation, rejection, and alalog to digital conversion.
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an array is really a group of small transducers which can be used together to form....
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a larger more flexible transducer
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what is amplification also known as
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receiver gain
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what is electronic steering
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steering achieved by using a small phase delay between excitation pulses to each of the elements in an array
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why is amplification necessary
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the returning signals from the body are too small to be processed within the electronics or visualized on a monitor
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What are the advantages of electronic steering?
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varying focus, phasing, parallel processing
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what is amplification controlled by?
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the user and partly by the machine
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What are the biggest disadvantages of electronic steering?
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cost of production and complexity of the design
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If the gain reads 0, does this mean there is no gain applied to the image?
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no, the machine always applies gain, it is always necessary
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What is electronic steering for receive?
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listening with the same phasing we transmitted
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what does the gain nob do
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amplifies or deamplifies the gain already on the image based on the unpredictability of the size of the returning signals from the patient.
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What are phased delays also used for?
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focus transmit and receive beams
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what is preamplification
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a process to improve the quality of the signal before it gets amplified
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True of False. You can ONLY focus shallower or at the natural focus of the transducer
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true
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where is preamplification done?
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as close to the transducer as possible (in the chain)
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For deeper focusing you use ____ extreme phasing
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less
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what are preamplifiers designed to do?
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prevent electronic noise from contaminating the tiny signals created by the crystal transducers
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for shallower focusing you use ____ extreme phasing
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more
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what is receiver gain system control also known as
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receiver gain, gain, amplification
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The receive phase delay must match the....
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transmit phase delay
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what does receiver gain do
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it affects the amount of amplification of the received signal and does not affect eh intensity on the patient
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True or false there will be many instances when we will want to focus and steer at the same time
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true
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what is TGC or DGC
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time or depth gain compensation
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How do we focus and steer together?
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by adding the phase profile for steering to the focus profile
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what do TGCs do
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its the application of extra amplification to compensate for increasing attenuation with depth
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What was a phased array sector transducer designed for?
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scanning between ribs
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what are the TGC pods normally ranged at
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0-60dB to avoid over sensitivity of the pods
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How large is the footpring of a phased array sector transducer?
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small, 64-128 elements
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are TGCs modifiable from a tech stand point?
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no, there are internally applied ones that cannot be adjusted
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What is the frequency ranges of a phased array sector transducer?
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2-4Mhz for adults and 6-10Mhz in children
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We want to use the ___ as our coarse adjustment and the ___ as our fine adjustment of the image
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gain, TGCs
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What is the phased array sector transducer's doppler frequency?
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1.8-4 Mhz
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what is dynamic range?
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maximum to minimum range in anything
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What does a phased array sector do?
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2D imaging, M-mode, Doppler, Color
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___ dynamic range is generally much greater than the ___ dynamic range and usually the ___ dynamic range will exceed our ___ dynamic range
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signal, display, display, visual
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What shaped image does a phased array sector transducer produce?
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a sector shaped image
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what ranges are usually outside our visible dynamic range
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range of returning signals and monitor or display ranges
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Describe the element(s) of a phased array sector transducer
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multiple square or rectangular elements
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what is compression
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a general term used for any technique that maps a larger dynamic range into a smaller dynamic range
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a phased array sector transducer uses phasing in the lateral dimension to achieve
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electronic steering and variable transmit and receive focus
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what does compression have to do with the range of returning signals
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it maps the range of signals into a smaller dynamic range that our eyes can distinguish
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Does a phased array sector use a lens, if so what for
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Yes, to create fixed elevation focus
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about how many shades of gray can the eye distinguish
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20
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Is the beam from a phased array sector transducer symmetrical?
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no its not symmetrical in elevation and lateral plane
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what does compression allow
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for us to visualize different tissues within the 20 visible shades
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What are the disadvantages of a phased array sector transducer
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more expensive than a single element transducer, it has more expensive electronics, and it has a fixed elevation focus
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is compression performed with or without altering the rank between signals
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without
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What are the advantages of a phased array sector transducer
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variable focus in the lateral dimension, no motion artifact, and flexibility to perform parallel processing and other advanced techniques
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can the user control compression?
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yes, but there is internal compression that is not modifiable
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A sound beam has only one ___ or "___"
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focus, waist
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is the receiver function of compression user controlled?
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no its set by manufacturer
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how does a machine create multiple foci?
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by transmitting multiple sound beams down each scan line, each impulse having a different phase profile creating a focus and more than one depth
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receiver function of compression is
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a preprocessing function of compression
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As the returning sound waves arrive at the transducer....
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elements on the front of the probe are excited
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compression process that happens later in the signal processing can be though of as
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video compression, it compresses the displayed gray scale appearance
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In dynamic receive focusing when the elements are excited it
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creates an electrical impulse that returns through multiple channels to the machines receiver
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what does compression warn us of
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the limitation of ultrasound
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Why does the delay pattern change continuously?
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because the transducer is listening for reflections from different depths
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compressing information gives potential to
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compress important signals out of visibility relative to the surrounding tissue
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The deeper the depth of return signal the ____ the time delay is applied to that signal
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greater
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what is bistable
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an image that looks black and white
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Can the dynamic receive focusing be changed by the technologist?
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no, it is internally regulated
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what is demodulation
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a two part process that changes the electrical signals within the receiver into a form more suitable for display
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Why was the linear phased array transducer created?
|
for scanning vascular and small parts, with wide field of view in the fresnel zone
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as a wave propagates through a medium the interactions cause changes or ___ in the wave
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modulations
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What does the linear phased array transducer replace?
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the linear switched array transducer
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demodulation is a process by which
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modulations are removed or detected
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What is the difference between the linear phased and the linear switched in terms of switching and sequencing?
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the linear phased can be steered and focused when desired
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what is demodulation also known as
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signal detection
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About how many elements are in a linear phased array transducer head
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200-300+ elements
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what are the two parts of demodulation
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smoothing and rectification
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What are the advantages of the linear phased array transducer?
|
in has variable focus in the lateral dimension, in allows for creation of wider linear image in the near field, and it has flexibility for advanced techniques and parallel processing
|
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what is smoothing also known as
|
envelope detection
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What are the disadvantages of the linear phased array transducer?
|
its more expensive than a single element mechanical, it has more expensive electronics, it has a fixed elevation focus, and it has no elevation steering.
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what does rectification do
|
it converts the negative components of a signal to positive
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Are the lateral and elevation planes of linear phased array transducer symmetrical?
|
no
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what does smoothing do
|
it traces the signal peaks and valleys, applying some averaging or smoothing
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Does a linear phased array use a lens? if so why
|
yes, for elevation focus
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when is envelope detection done?
|
after rectification
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What kind of control does a linear phased array have over the lateral dimension?
|
It can electronically steer, it has variable received electronic focus, and it has dynamic receive focus
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what is demodulation like?
|
the early modality of a mode
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What shaped image does a linear phased array create?
|
rectangular imaged through sequencing and parallelogram image by phasing each group of sequenced elements
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what is rejection
|
it sets threshold below which signals will not be visible on the display
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What can a linear phased array transducer do?
|
2D, Doppler, Color
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what does rejection do
|
it suppresses low level noise signals caused by signal through the body, transducer, cable, or system electronics.
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Describe the characteristics of a Curved Linear Phased Array transducer
|
Basically the same as linear with Curved, convex face creating a wider image in the near and far field, it needs steering as the scan head created the desired image geometry
|
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|
rejection affects all
|
low level signals on the image regardless of location, birhgt or strong reflectors are unchanged.
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A lack of elevational focus is a result of only 1D array (one row of crystals). Describe what 1.5D and 2D transducers are
|
1.5D adds a small row of crystals to either side of the main element allowing for limited control over elevational focus, 2D has multiple elements in lateral and elevation planes so that both can be focused
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the reject threshold is set/not set in the receiver
|
not actually actively set in the receiver but rather is just the limit of the sensitivity of the system
|
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1.5D added a small row of crystals to either side of the main element which allowed...
|
limited control over elevation focusing
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is there a preset reject level?
|
no, users adjust it
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2D has multiple elements in both lateral and elevation so beams can...
|
focus and steer in both lateral and elevation directions
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a level is reached below which the signals are not detected, this level is referred to as the
|
noise floor
|
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what creates greater system sensitivity
|
the lower the noise floor the smaller the signals that can be detected
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what is the balance for reject
|
pushing the noise floor as low as possible while preserving the required signal input dynamic range and amplification
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why is a-mode called thus
|
the demodulated signal detection was shown along a horizontal line, displaying the amplitude of signals
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in a-mode the variation of amplitudes corresponded with
|
impedance difference between the propagating medium
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is a-mode still used
|
rarely, although b-mode uses that same principles
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how is a mode translated to b mode
|
a mode line is converted into a brightness mapped line, the horizontal line is now represented as depth instead of time, amplitude is reflected as brightness of reflector
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where is a to b mode conversion done
|
the scan converter
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what are the two main functions of the scan converter
|
conversion of a mode lines to b mode lines and organization of the successive lines of data into a formatted image
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is the conversion of a to b mode easier on a linear or a sector
|
linear
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what is the scan converter responsible for
|
keeping track of which line of data should be presented at what location on the screen
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once the frame is complete the scan converter is sent a
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flag, that tells it the next line received is the first line of the next frame
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pre and post processing functions vary
|
vendor to vendor
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what is one of the pluses of more processing power and memory
|
the machine can store more raw RF data, which can make it easier to pre and post process at any time
|
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|
post processing system controls which are user controlled:
|
compression, dynamic range, grayscale, post processing curves or maps, contrast (display)
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adding colorization to an image
|
extends the dynamic range of the eye (theoretically)
|
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colorization is intended to improve visualization of
|
low level signals preserving a greater dynamic range
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the overall effect of colorization depends on the ___ ___ uses and the colorization __ chosen.
|
compression maps, hue
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does colorization really cause enhancement of the image?
|
no; compression, gain, etc are much more important
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where does the analysis package reside?
|
the back end software
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what allows for scrolling back in time, freezing data, and placing calipers?
|
data storage in digital format in the memory in the back end of the system
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with area measurements you want to be
|
perfectly orthogonal to the structure with the transducer
|
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what plane are lateral and AP measurements best?
|
transverse
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name three different kinds of video displays and monitors
|
CRT, LCD, and DLP
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CRT is
|
cathode ray tube which up until 5 yrs ago was the most common monitor used in ultrasound systems
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LCD stands for
|
liquid crystal display
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DLP stands for
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Digital light processing chips
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which display types are used in flat panel displays?
|
LCD and DLP
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What is the great thing about flat panel displays?
|
the matte finish reduces ambient glare
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CRT vs LCD
|
LCD is much lighter, more flexible as far as swiveling, and the monitor is larger
|
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what is NTSC
|
the National Television Standards Committee, they set US standards
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What is the NTSC standard for black and white
|
525 horizontal lines and 30fps (frames per second), interlaced monitor
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Color was added to the NTSC standard which took
|
longer for frame creating dropping the frame rate to 29.97 Hz
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If the machine has a higher FR then the monitor, we.... so...
|
lose data, so FR of monitor must be more than machines
|
|
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Interlaced monitors can display approx
|
30fps
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Non-interlaced monitors were developed to
|
compensate for HD broadcasts, frame rates are double of interlaced to 60fps
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What is a pixel
|
the smallest division of the monitors display
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each pixel can be representes as ___ ___ which can stay light or dark
|
multiple layers
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each layer cna be lit or not chanign the level of
|
brightness of the pixel, regulating the shade of gray
|
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|
ranme each layer of a pixel with the
|
bit
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if there is only one layer of a pixel it is either _ or _
|
lit up (1) or dark (0)
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|
one bit displays are
|
either black or white (bistable)
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|
|
two layers or a two bit would give us _ shades of gray
|
4
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What is the equation for gray levels
|
gray levels = 2 raised to the power of bits
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|
|
most current monitors are
|
8 and 10 bit monitors (which are well beyond our visual discrimination of shades of gray)
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Whare are most current monitors outside our visual discrimination?
|
this has to do with room ambient light
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what are linear array traducers best for
|
superficial imaging eg small parts and vascular
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what do linear array transducers image
|
a rectangle
|
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sector array transducers image
|
like a piece of pie, comes to a point,
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|
what are sector array transducers best for
|
cardiac, lower frequency, small scan head that is square in shape
|
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what is frame rate
|
the ability of the system to create multiple frames per second
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what is temporal resolution
|
the ability to position moving structures from one instant to the next accurately
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how are temporal resolution and frame rate related?
|
directly
|
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what are the two factors that determine frame rate
|
speed of sound in medium and depth of imaging
|
|
|
in clinical ultrasound the frame rate is determined by
|
maximum imaging depth since we generally assume the speed of sound in soft tissue is constant (1540)
|
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what is the unit of frame rate
|
hertz (images per second)
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|
frame rate is the reciprocal of the
|
frame time
|
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|
frame time is
|
time it takes to generate one frame
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what is the calculation for frame time
|
13 micro seconds times imaging depth
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what is the calculation for frame rate
|
1/frame time
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what can you think of frame rate as
|
frame frequency, how quickly can we produce a new frame
|
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deeper depth will increase our ___ therefore reduce our ____
|
frame time, frame rate
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what happens if we double our number of scan lines
|
our frame time doubles, our frame rate halves
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Is the PRF greater or less than the frame rate
|
greater, always greater
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what is line density
|
the ability of the machine to alter the spacing between sound beams or scan liens
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|
why would we double the amount of scan lines to halve our frame rate
|
to improve image quality
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is line density adjustable?
|
most are factory preset, though mindray, GE, etc are adjustable
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what if we change the sector from 90 degrees to 45 degrees
|
frame rate doubles, frame time halves
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What happens to the area in between scan lines
|
the pixels are averaged and interpolated, something could be averaged out
|
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Non imaging modalities such as CW doppler, non-imaging PW, blind M mode are governed by what
|
PRF (drive voltage)
|
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|
for non imaging modalities there is no image formation thus
|
the temporal resolution is high
|
|
|
when frame rate increases temporal resolution
|
increases
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|
CW speed is dictated by
|
sound travel in medium
|
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PW speed is dictated by
|
round trip travel time
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When it comes to temporal resolution we must ask
|
are we trying to make a perfect picture or a perfect movie
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What would we do to create a perfect picture
|
multi foci focusing, wide field of view, and high line density (low frame rate, but good picture)
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What is the basic process of real time imaging
|
transmit beams, receive beams, process returned data, perform measurements on processed data, display processed data, store the processes data
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What are terms used to describe performance and specifications
|
transmit power, dynamic range, signal, noise and noise floor, signal to noise ratio, compression, preprocessing, and post processing
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transmit power is also called
|
acoustic power, output power transmit gain, power gain, acoustic gain, output intensity, transmit voltage, and output voltage
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what is transmit power
|
it controls the amplitude of the excitation voltage that drives the crystals
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___ voltage = ___ amplitude mechanical oscillation of the crystal = ___ amplitude sound wave
|
higher, higher, higher
|
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|
increasing transmit power and resulting intensity increase can cause
|
bioeffects to the patient, to much intensity can cause tissue damage
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what is input dynamic range
|
the ratio f the maximum input signal to the minimum possible input signal
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what is dynamic range
|
the ratio of maximum to minimums of any quantity
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what is output dynamic range
|
the ratio of maximum to minimum output signal
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what kinds of dynamic ranges are there
|
input, output, display, and gain
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what is the default dynamic range in ultrasound
|
input dynamic range
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what is the formal definition for the US system input dynamic range
|
the range of the signal amplitudes a system can receive and process without causing harmonic distortion
|
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What is signal
|
any phenomenon desired to be measured
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what is noise
|
unwanted signals
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what is noise floor
|
amplitude level below which no signals are visible because of the presence of noise
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what is signal to noise ration (SNR)
|
amplitude of the signal divided by the amplitude of the noise
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what does SNR specify
|
the quality of the signal and how much faith we should put in the data
|
|
|
higher SNR implies
|
better imaging situation excluding artifact and more trustworthy data
|
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|
Strong signal by itself does or does not guarantee a good SNR
|
does not (same for weak signal and poor SNR)
|
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Does changing the receiver gain improve the signal to noise ration?
|
no
|
|
|
changing the amplification usually changes both the ___ and the ___ (unless some component of the signal is saturated)
|
signal and noise
|
|
|
so even if the true SNR does not change...
|
the apparent SNR might
|
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|
certain amount of ___ is necessary to make the signal bright enough so that it ca be visualized on the display within the dynamic rang eof the monitor and the human eye
|
gain
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|
|
if the gain doesn't map the signal into the visible range the signal will appear
|
weak at best, because of a lack of sensitivity on our display
|
|
|
Increase the gain and the signal and noise are...
|
amplified by the same amount (apparent SNR increases, but true SNR stays the same)
|
|
|
Even with a matching layer the impedance mismatch
|
is still relatively large between both interfaces
|
|
|
What does a large impedance mismatch mean
|
there will be a significant amount of reflection, reverb artifact
|
|
|
The reverb can be reduced significantly by regulating the thickness of the
|
matching layer
|
|
|
how thick is the ideal conventional matching layer?
|
quarter wavelength thick at the operating frequency
|
|
|
How many degrees is in a full wavelength
|
360 degrees
|
|
|
how many degrees is a quarter wavelength
|
90 degrees
|
|
|
The idea behind the quarter wavelength is based on the principles of
|
constructive and destructive interference
|
|
|
So essentially what does the matching layer accomplish
|
creating a 180 degree out of phase, destructive interference with reverberating waves, cleaning up the picture
|
|
|
What are the problems with materials that are not PZT
|
their sensitivity and efficiency are poorer than PZT
|
|
|
What is Axial Resolution
|
the ability to distinguish between two structures in the axial dimension, or the ability to distinguish structures @ depth
|
|
|
What is the important difference between PW and CW
|
PW sees depth
|
|
|
What is best spatial pulse length in relation to axial resolution
|
shorter SPL means better axial resolution
|
|
|
To distinguish two separate objects that are along the same beam path the reflected echoes from the first and the second object have to be
|
distinct in time and not connect
|
|
|
What is happening to the wave if two objects appear as one
|
the echo from the second object would have to return while the pulse is still insonifying the first object
|
|
|
How long is the wavelength if two objects appear as one
|
twice the distance between the two objects
|
|
|
So to define two objects separately the axial resolution is
|
1/2 the spatial pulse length
|
|
|
The shorter the spatial pulse length the better/worse the axial resolution
|
shorter
|
|
|
How do we determine the SPL
|
the # of cycles in pulse times the wavelength
|
|
|
What is the relationship of wavelength and frequency
|
inversely related
|
|
|
So an increase in frequency improves/worsens axial resolution
|
improves
|
|
|
Backing material is used to
|
shorten the ring time of a transducer by absorbing the energy, which decreases the number of cycles in a pulse
|
|
|
If there is less cycles in a pulse, and less SPL, then does bandwidth increase or decrease?
|
increase
|
|
|
Changing the composition and thickness of the backing material will create various degrees of
|
damping
|
|
|
higher damping = longer/shorter pulse = better/worse axial resolution
|
shorter, better
|
|
|
What are the negatives of backing material
|
in decreases the efficiency of the transducer and decreases the quality factor
|
|
|
What is lateral resolution
|
the ability to resolve two structures in the lateral dimension (side by side)
|
|
|
What is happening if two structures that are side by side appear as one structure
|
they are being insonified simultaneously
|
|
|
Best lateral resolution results from a narrower/wider beam
|
narrower
|
|
|
Lateral resolution can be defined by
|
beamwidth
|
|
|
What is impulse response
|
the response of a crystal to a single short duration pulse
|
|
|
short impulse response = more/fewer cycles in the pulse, which improves/degrades axial resolution
|
fewer, improves
|
|
|
longer impulse response = many/few cycles in the pulse, which improves/degrades axial resolution
|
many, degrades
|
|
|
In simple round crystals the diameter effects... and the thickness effects...
|
beam width, operating frequency
|
|
|
beam and wave are
|
the path the wave travels, and the energy that is traveling. the words are interchangeable
|
|
|
the beam shape or dimensions of the beam is essentially
|
the region of the body through which the sound waves propagate
|
|
|
what is elevation also known as
|
thickness
|
|
|
in CW the beam appears
|
continuous; the width the same size as the transducer, which narrows and then expands beyond original width
|
|
|
in PW the beam appears
|
waves of beams that follow the same type of beam path as CW, but in snapshots
|
|
|
What is Depth also known as
|
axial, longitudinal, and radial
|
|
|
Depth is
|
the direction of a beam away from the transducer
|
|
|
what is beamwidth also known as
|
lateral, azimuthal, side by side, transverse, and angular
|
|
|
beamwidth is: for round crystals... for non-symmetric crystals
|
symmetrical in all planes, different in two different planes which creates another dimension of elevation
|
|
|
The depth that the beam reaches its narrowest beamwidth is called
|
the natural focus
|
|
|
what is the fresnel zone also called
|
the near field, near zone
|
|
|
what is the fresnel zone
|
the region before (shallower) than the natural focus depth
|
|
|
what is the franhoefer zone also called
|
the far field or far zone
|
|
|
what is the fraunhoefer zone
|
the region deeper than the natural focus
|
|
|
what does unfocused mean
|
that nothing has been done to alter the natural focus of the crystal
|
|
|
can unfocused beams be modified?
|
no
|
|
|
what makes a focused transducer different from a unfocused transducer
|
focused transducer have had a technique employed to move or allow the focus to be moved to a depth other than the natural focus
|
|
|
for an unfocused transducer the beamwidth is
|
1/2 the diameter of the crystal a the focus
|
|
|
At twice the focal depth, the beam....
As depth increases past that point the beam will.... |
returns to the size of the crystal diameter, continue to diverge
|
|
|
What is the Near Zone Length (NZL)
|
the distance from the surface of the transducer to the natural focus
|
|
|
what is the equation for NZL
|
Diameter squared divided by four times the wavelength
|
|
|
What is the modified (assuming soft tissue) NZL equation
|
Diameter squared times operating frequency divided by six
|
|
|
Where is the natural focus for a 3MHz transducer with a diameter of 1cm
|
5cm
|
|
|
NZL is proportional to
|
the diameter of the crystal squared
|
|
|
so if we increase the diameter of the crystal by a factor of 2 the depth of the natural focus will increase by
|
4
|
|
|
A small/large diameter crystal is better for superficial imaging
|
small
|
|
|
NZL and operating frequency are ___ related
|
directly
|
|
|
If we double the operating frequency of a crystal the corresponding natural focal depth will
|
also double
|
|
|
higher frequency produces shallower/deeper focus
|
deeper
|
|
|
by design the focus is usually controlled by the ____ since higher frequency attenuates much faster
|
crystal diameter
|
|
|
beamwidth effects resolution in such that the wider the beam width the better/worse the resolution
|
worse
|
|
|
beam intensity is ___ ___ to the diameter
|
inversely proportional
|
|
|
larger beam - less beam intensity =
|
less reflected signal power
|
|
|
what are some of the limitations of simple transducers
|
can only image straight ahead, processing is minimal, steering isn't possible
|
|
|
How do we minimize the acoustic impedance from the crystal to the patient
|
adding a matching layer
|
|
|
Sound propagates through the matching layer then into the body because of the
|
closer impedance match
|
|
|
Matching layer also helps reduce
|
reflection back into the body with returning echo
|
|
|
Most current transducer designs are using multiple matching layers with each layer having
|
a lightly lower impedance to facilitate greater transmission
|
|
|
Transducers can be defined as any device that
|
converts one form of energy to another
|
|
|
During transmission ___ energy is converted to ___ energy, then during the receive time the ___ energy is converted back to _____ energy
|
electrical, sound, sound, electrical
|
|
|
what is resoprosity
|
the transducer converts bi-directionally
|
|
|
the Piezoelectric effect is the phenomenon by which a
|
mechanical deformation occurs when an electric field (voltage) is applied to certain material or a carrying electrical signal is produced when the crystal structure mechanically deformed
|
|
|
Piezoelectric materials are those which convert
|
sound into electricity and visa versa
|
|
|
What is PZT material made out of
|
a ceramic that is not naturally piezoelectric
|
|
|
why is PAT the manufactured material of choice for ultrasound?
|
because of its high coupling coefficient, high frequency of natural resonance, and very good reproducible characteristics for a stable design
|
|
|
Explain the poling process
|
Put a crystal in an extremely hot oven (allowing molcules to move more freely), apply large electrical field to crystal, molecules align to poles of electric field, crystal is then removed form heat and electrical field, and molecules stay aligned
|
|
|
poling allows for
|
greater physical distortion of crystal when varying electrical fields are applied
|
|
|
what is the curie temperature
|
the temperature at which the poling of PZT becomes undone, 300 degrees C or 572 degrees F
|
|
|
for pulse wave the operating frequency is primarily determined by the
|
thickness of the crystal and the propagation velocity within the crystal
|
|
|
a thicker crystal results in longer/shorter time for the crystal to expand and contract
|
longer
|
|
|
slower compression rate - longer/shorter period= higher/lower frequency
|
longer, lower
|
|
|
a thinner crystal will expand and contract at a faster/slower rate so the compression of the neighboring medium will occur faster
|
faster
|
|
|
faster compression = longer/shorter period = higher/lower frequency
|
shorter, higher
|
|
|
operating frequency is proportional to
|
one over thickness (propagation speed of the crystal)
|
|
|
what is the equation for operating frequency
|
propagation speed (mm/micro-s)over two times the crystal thickness (mm)
|
|
|
for CW the frequency is determined by
|
the transmit signal or the drive voltage frequency of the pulser
|
|
|
in CW the crystals natural resonant frequency is
|
overridden by the pulser or drive voltage (resulting in the frequency of the CW transducer to equal the frequency of the transmit voltage)
|
|
|
what is bandwidth
|
the useful range of frequencies over which anything can operate
|
|
|
What kinds of bandwidth are there
|
transmit, receive, system receiver, and display
|
|
|
how is bandwidth displayed on a graph
|
bell curve with frequency range on the horizontal axis and sensitivity (dB) on the vertical axis
|
|
|
what is the equation to find the frequency center for bandwidth
|
maximum frequency minus minimum frequency (so -6dB=4MHz)
|
|
|
What is fractional bandwidth determined by
|
dividing the bandwidth center by the operating frequency
|
|
|
when are transducers considered broadband
|
when they have a fractional bandwidth of greater than 80%
|
|
|
what is quality factor
|
a unit less number that is related to bandwidth and is the reciprocal of the fractional bandwidth
|
|
|
what is the equation for QF (quality factor)
|
operating frequency / bandwidth
|
|
|
What is the general rule of bandwidth wideness
|
wider is better, however there are instances where there is no real advantage
|
|
|
What is another name for transducers with wide bandwidth
|
multi-Hertz or frequency agility
|
|
|
Wide bandwidth transducers add ___ since it can be operated at different frequencies
|
flexibility
|
|
|
What is the advantage of multi hertz transducers
|
they are flexible and allow B-mode imaging at higher frequency, while color and doppler are performed at lower frequencies
|
|
|
What is Rayleigh scattering
|
the high attenuation of blood or other small particles due to their being to small to be good reflectors
|
|
|
What is Dynamic Frequency Tuning also called
|
sliding receive filters
|
|
|
What do sliding receive filters do
|
optimizes both penetration and resolution by having higher resolution in the more shallow scan area, and more penetration in the deeper scan area
|
|
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What is one type of harmonic imaging called
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second harmonic imaging
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what is second harmonic imaging
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for a transducer that has enough bandwidth, the transmit is done at the fundamental frequency and the receive is done at twice the fundamental frequency
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what is frequency compounding or frequency fusion
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a type of parallel processing technique
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how does frequency compounding work
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transmission over a broad range of frequencies and then receiving and processing two or more different narrower frequency bands, each band is then fused together into one image
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When is more bandwidth bad
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doppler and CW doppler do not need much bandwidth. More bandwidth means less sensitivity
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What does it mean when we say we are using a 4MHz transducer?
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our center bandwidth is 4MHz
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The more pulse response the ___ frequency response
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less, and visa versa
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when will interference occur
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if there are more than wave traveling through a medium at the same time
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What is constructive interference
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when two or more wave's maximum and minimum values occur at the same time, this will join and form a wave with greater amplitude
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what is destructive interference
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when two or more wave's maxs and mins do not occur at the same time, their joining will result in a wave with an amplitude which is less
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what is the attenuation coefficient used to describe
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it simplifies the report of attenuation
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what are the units of measure for the attenuation coefficient
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dB/cm
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What is the attenuation rate in soft tissue
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.5dB/cm
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what is the equation for the attenuation coefficient
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1/2F times d
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What is the total attenuation for an imaging depth of 10cm using an 8MHz transducer
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40dB (one way)
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what are decibels
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logarithmic power ratios
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Why doesn't an 8MHz transducer work at 10cm
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80dB = 10 log{ If / Ii }
-80/10 = log{ If / Ii } 10^-8 = { If / Ii } 1/100000000 = { If / Ii } of the original frequency, not visible |
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What is an incident angel
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the angle which the propagating sound wave strikes the tissue interfaces
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Acute angle is
Right angle is obtuse angle is oblique angle is |
less than 90 degrees
exactly 90 degrees greater than 90 degrees anything other than 90 degrees |
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What is normal incidence, and what is it also known as
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exactly 90 degrees; perpendicular, orthogonal, and right angle
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what is incident intensity
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the intensity of a sound wave immediately before it strikes an interface
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what is reflected intensity
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intensity of the portion of wave that is reflected after striking an interface and returning back in the direction it came
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what is transmitted intensity
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the portion of the incident that continues forward after striking an interface
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what is the conservation of energy at interface
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incident intensity = reflected + transmitted ; without loss due to heat
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What is the Intensity Reflection Coefficient (IRC)
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the percentage of the intensity that bounces back when sound strikes a tissue interface
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For ultrasound in soft tissue interfaces the majority/minority of the intensity is transmitted?
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majority (99%)
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for soft tissue and bone or air the greater/lesser percentage (IRC) is reflected
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greater
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What is the Intensity transmission coefficient (ITC)
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the percentage of the intensity that passes forward at the interface
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Soft issues interfaces will transmit/propagate most of the intensity
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propagate (99%)
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soft tissue and bone or air will have a high/low ITC
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low, meaning a high reflection coefficient
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IRC + ITC =
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100%, energy is conserved at interfaces
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With normal angle of incidence at an interface, reflection will only occur if there is
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an impedance between the media
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the % of the incident beam that is reflected is related to the
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impedance difference of the tissues
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If impedance is equal there is
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no reflection
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if there is a slight impedance difference then there is a
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small reflection
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if there is large impedance the difference will yield
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a greater reflection
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what is the acoustic impedance equation
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Z = p x c
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What is the units for acoustic impedance
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rayls
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impedance =
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resistance to
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what is acoustic impedance a measure of
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amplitude
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for acoustic impedance Z2-Z1 is proportional to
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reflected intensity
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for acoustic impedance Z2+Z1 is equal to
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incident intensity
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what is the equation for the reflection percentage
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reflection % = Ir / IT = (Z2-Z1 / Z2+Z1) ^ 2
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What if Z2=Z1 in acoustic impedance?
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0% there is no reflection
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What happens when Z2 is significantly higher than Z1
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then its pretty much 100% reflection
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Calculate the IRC assuming the Z1 = 40rayls and Z2 = 60 rayles (assuming a normal angle of incidence)
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(60-40/60+40) ^ 2
(2/10) ^ 2 ~4% |
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What is the ITC of an IRC that is 4%
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96%
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1-IRC =
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ITC
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What are the two physical principles that must apply to the reflection at oblique incidence
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conservation of energy and reflection angle = incidence angle
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what is refraction
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a transmission with a change in direction
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in order for refraction to occur what are the two conditions that must be met
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oblique incidence angle, and propagation speeds of the two media are different
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What is Snells Law
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if the sound speeds of two mediums are equal then no refraction will occur
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According to Snells Law if the propagation velocity in medium 2 is greater than medium 1 it will
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bend the wave to the right
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When will refraction occur
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when the propagation speed is different between mediums
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As propagation velocity changes between interface increase or the incident angles increases from normal incidence the refracted angle will
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increase
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What is the critical angle
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the point at which no energy is transferred into medium 2
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What is absorption
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the conversion of wave energy to heat, the dominant factor creating attenuation
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More energy is lost to heating the tissue than
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is redirected through reflection or refraction
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What is the problem with absorption
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it adds to attenuation, no propagation, and potentially damages tissue
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what increases exponentially with increase in frequency
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absorption, thus we use the lowest frequency possible to avoid access absorption
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what is the range
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the distance form the transducer to an echo generating structure
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for proper echo positioning on the display we need what two items of information
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direction from which the echo came (the source), and the distance to the reflector or scatterer where the echo was produced (which we don't know)
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time of flight is also known as
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round trip travel time
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what is time of flight
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the elapsed time from pulse generation to pulse reception
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what is the equation for range
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depth = 1/2 {c (mm/micro-s) x t (micro-s)}
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for every 13 micro seconds of round trip travel time the reflector is ___ deeper in soft tissue
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1cm
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what is the time of flight for a reflector 6cm deep in soft tissue?
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78 micro seconds
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What is the actual distance traveled you are looking at a 6cm deep reflector?
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12 cm
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An echo returns 104 micro seconds after a pulse was emitted. locate the depth of the structure that produced that echo. use the range equation
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1.54 micro second/mm x 104 micro seconds = 160 mm
1/2 160 = 80mm 8cm |
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An echo returns 104 micro seconds after a pulse was emitted. locate the depth of the structure that produced that echo. use the 13 micro second rule
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104 / 13 = 8cm
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What is the pulse repetition period (PRP)
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the pulser being on and off for 1 cycle
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What is the PRP equation
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depth (cm) x 13 micro seconds/ cm
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We can calculate the PRP with the 13 micro second rule because
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they are directly related
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According to the PRF sound can travel to ___ and back in 1 sec
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7.7 cm
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PRF at a shallow depth is high/low
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high
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what is the equation for PRF
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77,000cm/s
__________ depth (cm) |
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what is PRF's unit
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Hz
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What is the wavelength equation
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wavelength = c/f
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wavelength has a big impact on
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axial resolution and the type of reflection occurring
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how can we control wavelength
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by changing the transducer or the frequency of the transducer
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calculate wavelength for a 5MHz in a mediium with a sound speed of 1550m/sec
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1550/5 = 310 micro m
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what is amplitude
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maximum reflection, voltage, loudness and brightness of a sound wave
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Can we control the amplitude with the machine?
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yes!
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the greater the power/voltage the more/less the crystal is deformed sending a bigger/smaller wave
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more, bigger
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increasing the voltage applied to the transducer creates a greater
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mechanical distortion of the crystals, making a stronger acoustic wave
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Greater amplitude means greater/lesser degree of compression
greater amplitude means greater/lesser degree of rarefaction |
greater, greater
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why is amplitude and power important
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because they could cause bioeffects
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what is the relationship between power and amplitude
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P = A^2
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if we increase the power on a machine the acoustic power produced will
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also be higher
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