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175 Cards in this Set
- Front
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Why are natural piezoelectric materials not used in ultrasound?
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because some are better at transmitting sound, while others are good at recieving sound, but none are very good at doing both.
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how did pierre and jacque curie contribute to ultrasound? When?
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they discovered the piezoelectric phemenon in 1880
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Descibe piezoelectricity?
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when you take a piezoelectric substance, and apply force, you will get a voltage. Voltage is directly proportional to ennergy. also, when you apply oltage to a crystal, the crystal will deform. THat will set up a sound wave in the human body if it is placed on the skin.
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How do you create sythetic crytal?
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take a ferroelectric material(material which has magnetic domains which can be rearranged under the proper conditions.)
-heat the material to a certain temperature in the presence of a strong magnetic field. -this will induce piezoelectric qualitites. |
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WHat did Dr's Howry and Bliss do? when?
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They were the first to:
-use pulsed ultrasound -detect gallstones and foreign ojects in soft tissue -use compound scanning(1951) -First cross-section image |
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What did doctor Ian donald do?
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Developed contact scanning technique.
-first to describe full bladder technique This was in 1960 |
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what is A-mode?
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stands for amplitude mode
-amplitude is on the verticle axis, and time is on the horzontal axis. Verticle deflections on a time baseline A-mode is only used for opthalmology -distance is extremely accurate |
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HOw did fresnel and frauenhofer contribute to ultrasound? when?
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They came up with the theory of diffraction(spreading of the sound beam) They called the zone before the focal zone the near zone(fresnel zone) and the zone after the zone the far zone(or frauenhofer zone). This was in the 19th century
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Describe amplitude
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the maximum displaceent f particles from their positions of rest.
-as amplitude increases, intensity and power increase -Can be calculated as the maximum value minus the normal value |
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what is the amplitude intensity formula
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FS
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what is the wavelenth formula?
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FS
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what is the BUR formula?
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spacial peak intensity over spacial average intensity
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what 3 factors affect propegation speed?
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-density(mass per unit volume)
stiffness(hardness, ---resisstance of material to compression) -temperatures |
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what is the formula for decibles?
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FS
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What is the distance formula?
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FS
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state the 4 wave variables
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1. preasure-consentration of force(lb/square inch)
2. density-consentration of mass for particles in a a given volume(Kg/cm cubed) 3. temperature-consentration of heat enery. More preasure=more heat 4. distance-measure of particle motion Increase in preasure= increase in distance |
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what is a decible?
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unit of power/intensity ration
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differentiate btw power and intensity.
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power-energy over the whole cross-section of a beam
intensity-average rate of flow of energy /cm squared |
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PA
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pulse average intensity-the intensity averaged across the 1/2 cycle
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what is the formula for wavelength?
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FS
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name all the intensities from highest to lowest
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SPTP
SPPA SPTA SATP SAPA SATA |
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what is bulk modulus?
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synonym for stiffness
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what are the ultrasound wave characteristics?
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amplitude
propagation velocity wavelength frequency period |
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What is the duty factor formula?
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DF=TA/PA
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amplitude can refer to change in what?
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-postiiton or particle displacement
-preassure -density -particle velocity |
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what is duty factor?
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the percentage of time pulses are occuring.
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In order to calculate to axial resolution, what information besides frequency do you need?
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wavelenth, and number of cycles
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What is FFT used for?
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spectral doppler.
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Why do ultraound beams narrow to a point and then become wider agian(not including focusing)
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This has to do with huygens principle, and the interactions of the waves, and wavelets natrually at a certain point
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what is diffraction?
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what is diffraction?
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dead zone
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in a large source, there are smaller sources within it which send out wavelets. It takes a while for these wavelets to become one cohernt wave front. THe time before this wavefront forms is termed the dead zone.
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what is the principle of superposition?
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adding the values at different phases of the waves which interact with one another to produce a resultant wave.
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What are huygen's 2 principles?
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1st-a single sound source is comprised many sound sorces.
2nd-any phase on a coherent wave is capable of acting as another sound source |
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What is the difference btw beam width, and effective beam with?
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eam width can stay the same while changeing the effective beam width. The effective beam width is the width at a certain point of the beam. ie. increasing the effective beam with will cause there to be a less drastic focal zone, and the focal zone will be wider. Some factors affecting the effective beam with are:
-Gain -Outpu |
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how can we(as sonographers)minimize the effects of attenuation?
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-use a lower frequency transducer
-increase output -increase gain -adjust TGC -alter scanning path -use breathing techniques |
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What attenuation?
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the reduction of amplitude of a soundwave as it passes through a medium/
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How does absorption contribute to attenuation?
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greater angle of incidence =greater absorption=greater attenuation
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what is considered to be a specualar reflector?
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if the interface or object is larger than the wavelength of the beam. ie organ capsules, vessel walls, pericardium, diaphragm, etc.
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How is half value layer calculated?
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6/MHz
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What is another term for half value layer?
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half intensity layer
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what is the half value layer?
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the distance from the transducer where the intensity is half of what it originally was.
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what is absorption determined by?
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-beam frequency
-viscosity of beam -relaxation time |
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tranmit/receive switch
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Gate keeper of the system:
directs large voltages from the pulse decrement counter to the transducer and weaker frelected echoes to the reciever. |
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Explain temperal resolution?
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the ability to accurately display events occuring rapidly over time.
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How else does spectral broadening happen?
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when scatters enter and leave the area of the sample volume. This is aka intrinsic or geometric spectral broadening.
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what is the bernouli effect?
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the decrease in pressure in areas of high speed flow
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what is poiseulles equation used for?
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used to predict flow rates in long straight tubes
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Compare continuous wave doppler vs. Pulsed doppler
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continuous wave:
-cannot determine depth -has a high signal to noise ratio, so it is useful in smaller superficial vessels -efficient transmitters -no limits to velocity Pulse wave: -can determne depth -can use color -niquist limit-has upper levels for velocities, so you can correctly show on screen |
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how does an increase in propegation speed of sound affect the doppler shift?
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it decreases the doppler shift
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what is a continuous wave transducer?
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a transducer that is constantly emitting an ultrasound beam and is constantly receiving. There are 2 crystals. One emits, and one receives.
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What are the components and functions of a continous wave system?
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a)transmitter-voltage generator
b)transducer-constantly emitts an ultrasoundb beam and is constantly receiving. Converts electical energy to mechanical c)receiver-amplifies weak signals so they can make it through a system d)comperator-compares trasitted and recieved frequencies e)doppler anylizer-anylizes frequency spectrum-can be autivle or on a strip char |
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what is module frequency?
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the most common frequency over time.
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what does SNR stand for?
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signal to noise ratio-the amplitude of the signal divided by the amplitude of the noise
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what determines the size of the sample volume?
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SPL or pulse duration
-the width of the pulse is derermined by the diwth of the beam |
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Factors that determine the width of the beam
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-frequency
-aperture(#of crystals) -distance from the crystal(beam is not the same width all the way along) -focus |
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What are the components of a pulsed wave system?
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-master oscilator
-PRF decrement counter -gated transmitter -transmit/recieve switch -transducer -receiver -quadrate detector -doppler anylizer -doppler detector -display |
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how is dwell time calculated?
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pulse repetition period x packet size
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What are the disadvantages of colour doppler?
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-measures average, not peak velocity
-incorrect doppler angles are not compensateed for -frame rate may be limited -receiver must be dynamic in order to handle 2D and Doppler -Color is not standardized -weak signals may appear to be no flow |
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What are 4 factors that determine colour doppler image quality?
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-motion descrimination
-temporal resolution -spatial resolution -uniformity |
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What are 2 formulas for volumetric flow rate?
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volumetric flow rate=
volume of blood past a point/ unit time or Q=pressure differences(dynes/cm)/ resistance(poise) |
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explain gated transmitter
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allows the exact number of cycles to be emitted from the transducer
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What are the 2 effects seen in pulsitile flow?
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windkessel effect and flow reversal
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doppler detector in a pulsed wave system
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determines frequency content
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what are 2 doppler shift equations?
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reflected frequncy-transmitted frequency
D(f)=2fvCos0/C-v |
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What is the formula for resistance?
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resistance=8xLxn/piX r to the 4th.
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what is the reynalds formula?
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FS
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what is poiseilles equation?
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FS
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What is starlings law?
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the more volume of blood during diastole, the more ejected blood there will be during systole.
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what is parabolic flow?
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a type of laminar flow seen in long tubes. The average speed is 1/2 the maximum in the center
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What are 4 factors that determine colour doppler image quality?
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-motion descrimination
-temporal resolution -spatial resolution -uniformity |
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what is poiseulles law? What is it used to predict?
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see FS, used to predict the average flow velocity
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as diameter increases, what happens to flow rate?
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it increases
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doppler anylyzer in a pulsed system
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Puts it in the format that you want to see it
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what is beat frequency?
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same as doppler shift value
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components of a CW system
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a)transmitter-voltage generator
b)transducer-constantly emitts an ultrasoundb beam and is constantly receiving. Converts electical energy to mechanical c)receiver-amplifies weak signals so they can make it through a system d)comperator-compares trasitted and recieved frequencies e)doppler anylizer-anylizes frequency spectrum-can be autivle or on a strip chart |
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explain the master oscillator in a PW system?
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internal clock which acts as a reference point for phasic information.
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PRf decrament counter
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ensures all phases of the pulses are similar
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gated transmitter
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allows the exact number of cycles to be admitted from the transducer
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quadrate detector in a pulsed wave system
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determines forward and reversed flow.
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Describe a rotating crystal transducer?
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-real time
-rotating one or more transducers in a pivot. -sector shaped image produced |
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what are the types of mechanical transducers?Say if the are still effective today?
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-oscellating transducer(obsolete)
-Oscillating mirror(obsolete) -Rotating crystal(obsolete) |
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describe a oscillating mirror transducer?
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THe transducer is stationary, and the bearm is directed toward an ossilating mirror which sweeps the beam through the tissue. THis results in a sector shaped image.
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why is an annular array considered both
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because it has circular rings of crystal material. IT cannot angle crystals or fire them sequentially, so there needs to be an oscilatting mirror to change.
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name the overall sensitivity controls
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name the overall sensitivity controls
-power/intensity(output) -Gain(amplification) -reject -frequency |
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Describe a mechanical transducer?
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-it is not used anymore
-the reflective apperatus is oscellating, so It feels like it's moving. -sector shaped(pie)image |
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Describe an osscilating transducer?
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-known as a "wobbler"
-a single crystal is ossilating on a pivot in the center of the crystal -could be submerged and combined with an ossilating mirror to produce a rectangular image |
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tuning wires
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allow the maunfacture to fine tune the transducer so that the crystal more closely matches the electronics of the scanner. This allows for optimal imaging.
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what determines the crystal thickness?
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crystal thickness is about 1/2 the wavelength of sound emitted by the crystal.
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what material decreases or eliminates the need for matching layers? why?
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PVF(polyvinylidene flouride)-it has an acoustic impedence closer to that of human soft tissue.
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what is the reverse peizoelectric effect?
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when a voltage is applied to peizoelectric material, it is deformed.
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what are ferroelectric materials?
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materials that are able to retain their dipole arrangementa after electrical field is removed.
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what is a composite element? what are it's qualities?
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materials that are able to retain their dipole arrangementa after electrical field is removed.
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Insulators
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crystal has a tendancy to vibrate up and down, so the insulating rings damp this vibration.
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damping(backing)material
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This dampens or reduces the number of cycles of the ultrasound wave.
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electric sheild.
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-lines the inside of the housing
-provides a shield from external electrical interference(ie. radio frequencies,etc) This increases the signal to noise ratio. |
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Matching layer; what is the optimal thickness of a matching layer?
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minimizes the acoustic impedence difference at the transducer patient interface. The optimal thickness of the matching layer is 1/4 of the wavelength. Use of a matching layer causes the beam to have a wider bandwidth which allows better matching of the crystal to electronics of the ultrasound system.
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What are the 2 ways in which ultrasound crystals can be activated?
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-shock excited mode-single burst of non-alternating voltage
-burst excited mode-crystals are activated by one or two cycles of an alternating current. This allows the sonographer to select the frequency operated by the transducer. |
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what is bandwith? What is a synonymn for it?
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the difference btw the highest and lowest frequency emitted. Synonymn is transducer pass band
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what are some advantages of wide bandwidth tranducers?
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-are necessary for multihertz transducers
-impove sensitivity -improve image quality -better axial resolution -crystal more easily matched to electronics -low frequencies in the beam can be controlled to decrease beam divergnece in the far field. |
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what are 1/4 and 1/2-power bandwidths?
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pass bands that eliminate the weaker 1/2 or 1/4 of the frequencies.
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what is fractional bandwidth, what is the formula?
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fractional bandwidth=bandwidth/frequency
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what is Q factor?
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a measure of how efficiently the transducer changes electrical voltage to a sound wave. Also indicates how good of a transmitter a crystal is.
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What are the formulas for Q factor?
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Q=operating frequency/bandidth
Q=energy stored per cycle/energy lost per cycle Q=1/fractional bandwidth. |
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what 3 things happen as the SPL becomes shorter?
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-axial resolution improves
-bandwidth increases -Q factor will decrease |
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electromechanical coupling co-efficient
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how efficiently the transducer converts electrical stimuli into ultrasound energy and recieved ultrasound energy back into electrical energy.
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dielectric constant
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characterises mechanical and electrical properties of a crystal. Describes movement of a crystal when voltage is applied ot it,and to voltage induced when a strain is applied to it.
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why is damping material not inculded in trasducers used specifically for continuous wave applications.
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because damping material causes SPL to be shorter, but since continuous waves don't have SPL, it doesn't matter. Also, using damping material causes a loss in energy, so it is better not to use it.
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a transducer which is .22 mm thick, will produce what frequency?
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element thickness is half the wavelength, so a transducer with .22 with produce a wavelength of .44. This goes into the c=f/wavelength to get 3.5 mHZ
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For a given frequency, what will happen to the fractional bandwidth as the bandwith increases?
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decreases
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what benefit does the presence of the electrical field bestow?
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increased image quality, prevent radio frequencies from reaching the transducer, produce artifacts
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the lack of which transducer part could result in unacceptable side lobes.
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insulator
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what is the advantage of a 2-d transducer over a 1.5D transducer?
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can focus in more dimentions. THere is an increase n the # of rows increasing the possiblity for timing delays.
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what are some free hand scanning methods with sensors?
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-articulated arm-gantry system
-magnetic field-transmitter transmitts several different magnetic fields so the machine can tell the orientation of the transducer -acoustic ranging-have recievers in the ceiling -imaged base corrilation-camera looks for similar traits in am=n imge and it matches them up |
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what are grating lobes?
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beams of energy not in much of the ultrasound beam-angle depends on wavelength and element spaceing
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which part of the transducer is responsible for improving the signal to noise ratio?
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electrical field
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explain how a color monitor works?
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-three phosphors are required(red, blue, green). Each group represents one crystal. A different electron gun is required for each color.
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binary system
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foundation of computerization based on powers of 2.
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what does a beam former do in a digital scan converter?
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beam former is a device located within the housing that generates a large voltage by the pulser when initiated by the central timer.
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bit depth
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the number of colors a monitor is capable of displaying and it is determined by the number of pixels associated with each pixel location.
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explain transparency and laser camera
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A laser replaces CRT as a light source. A laser beam sweeps across the film and light is modified according to the memory of each pixel location. Each image is comprised of 5120 lines of information.
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Explain the wafer of a storage target or the dielectric matrix.
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one side of the wafer is divided into small squares, and each square is smaller than the diameter of the sound beam.
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What is refresh rate?
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the number of images per second in a color monitor
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DLP
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digital light processors-have small chip which has one reflector chip for each pixel. By directing light towards the screen from the chip, an image is produced. To get color, you interpose color wheels.
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what are the additional aspects of a storate CRT?
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-storage mesh-where the beam hits the mesh, electrons are knocked off leavinga positively charged latent image still in the form of electrons
-collector mesh-attracts dislodged electrons and safely grounds them -flood guns-make the invisible latent image visible by spraying a gentle flood of electrons toward the phosphor which are attracted to the positive areas of the storage mesh. At these places, electrons go through the storage mesh to the phosphor causing it to glow which we see as an image. |
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What is an ADC?
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analog scan converter that allowed for a greater range of grayscale, and post-processing. It stores an image as electrical charges on a dielectric matrix, and reads info by displaying dots of light on a TV. (was in the mid seventies)
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Explain how a non-storage Cathode ray tube works?
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-Evacuated glass tube-evacuated, so it is a vacuum where a high voltage(10,000volts) is applied across the tube, and the positive end is the phosphor end.
-cathode/filament/electron gun-cathode is heated to cook off electrons and they are moved towards the phosphore due to a high voltage, and positive phosphore. -control grid-controls the brightness of the dot by controlling the number of electrons that go to the phosphore -focusing grid-focuses electrons into a narrow beam -phosphor-phosphore emitts light if struck by electrons causing an image to appear |
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why are CRT's the most common method of displaying ultrasound?
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-displays both grayscale and color
-high light outpudt -dynamic range -resolution is comperable to transducer -frame rate is sufficient -cost is resonable |
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apodization
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applying different voltages to different crystals
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name the 5 functions of the reciever, and what their pupose is?
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-amplification-converting small voltages into larger ones.
-compensation-TGC-compensates for attenuation -compression-reduces the dynamic range of echos(monitor is a source of compression) -demodulation-converts voltages from one form to another: includes flipping a wave from negative to posative and smoothing. -rejection-reducing low amplitude echos |
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what are tuned amplifyers?
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reduce amplifier noise by changing the frequency range of the reciever to match the reduction in echo bandwidth from increasingly deep structes.
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what were some significant problems with storage CRT's when they were first invented?
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-image deterioration
-very few shades of gray(bistable) |
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what are the aspects of a ADC?
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cathode/filament-metal structure which is heated so electrons are given off.
-field mesh-electrons pass through the field mesh on their way to the targed, and this mesh causes electrons to strike the target at right angles, and collects stray electrons which are knocked off the dielectric matrix. -storage target/dielectric matrix-this is in the place of phosphor. As electrons are hurled toward the target, they are slowed down in the region btw the mesh and the target. When they hit the target, electrons ar dislodged leaveing a positive charge on the matrix which causes a latent image to be formed. |
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explain transparency film and multiformat cameras
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-Film is a layer of emulsion(with silver halide crystals) on a clear polyester base. Areas where light is the most intense is the darkest. `
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plasma
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gas filled cells that when voltage passes through, phosphores are excited
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explain LCD's
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polarized plates that are twisted and the amount of twisting determines the amount of light that gets through
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why was the scan converter very NB in the development of U/S?
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It allowed grayscale demonstration
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what is the f-number?
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the focal length divided by the aperture
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what effects do the pulse width and legths have on the ultrasound beam?
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-pulse length determines axial resolution and Q of the beam
-Pulse width determines width of the beam and varies with distance. Also determines beam width(lateral resolution) at any point |
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how are side lobes controlled?
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apodization
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Describe how f-number corrispond to focusing?, and how the applies to NZL?
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-weak focusing=f-number >6=NZL=10 cm or greater
medium focusing=f-number2-6 NZL approximately 3 cm strong focusing=f-number<2 NZL approximately 0.5 cm or less |
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what are the ways to focus a single crystal?
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nternal focus-crystal is shaped to a concave surface
external focus-lens attached to the front of the crystal |
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What is transmit focus, and mulitzone transmit focus?
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transmit focus-timing of crystal firings are changed to allow electronic focus at a selected depth.
-multizone transmit focus collects data from a number of lines of sight and combinees them producing a besam which is focused oer a relatively long distance. THe problem is that this slows doen the frame rate. |
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what causes side lobes?
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width and length vibrations of the crystal, reverb, and transducer interface and interference phenomenon.
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what determines the diameter of the beam?
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-wavelength(frequency of transducer)
-diameter of the crystal -distance from the crystal that you want to measure |
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envelope detection is another term for what?
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amplification
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what are the advantages of focusing a beam?
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-improvement in lateral resolution
-increased sensitivity in the focal zone |
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How are ultrasound beams focused?
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-electronically-transmit focus, and dynamic recieve focus
-mechanically-curving the crystal and attaching the lens |
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Line voltage fluxuations are least problematic with which system?
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digital scan converter
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how can one control beam width inthe elevational axis?
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controlled by lens, curving the crystal, transmit focusing, dynamic recieve focusing
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what is the formula that determines the angle of divergence in the farfield?
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sin0=1.22wavelength/d`
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explain co-processing
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aka parallel processing:
where multiple scan lines of different focal zones are being generated simultaneously. This improves the frame rate with multiple trasmit focusing. |
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grating lobes
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due to the regular spacing of crystals in a linear array.
-echos returned from grating lobes can be mistaken for echos along the main axis of the beam.(echos can be placed closer or further away from the source.) -effects can be significant with bright reflectors. -part of clutter and can cause reduced contrast in the image. |
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how does a channel work for receiving?
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each channel will have n independant element, path, analog to digital converteer
-normally the number of channels does not exceed the number of elements |
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what is the formula for f-number?
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fnumber=F/2a
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what are some cons to side lobes
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effective beam width widening(clutter), not as much energy down the center.
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what is needed in order to provide dynamic focusing?
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continuously changing aperature
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Electrical Isolation
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accomplished by having separate electrical communication among adjacent crystals and grounding.
-it is impossible to have complete isolation because crystals act as a type of capicatore(store electrical charge), but this effect is quites small. |
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what are grating lobes controlled by?
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subdicing-when the distance btw elements is reduced to less than 1 wavelength, the angle of side love and main beam exceeds 90 degrees, effectively eliminating the grating lobe.
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how are side lobes reduced?
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the use of dynamic apodization and increasing aperature.
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what is the formula for frame rate?
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FR=c/2DNF
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what is a channel? how does this affect imaging? what are some typical numbers of channels?
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each independant crystal and delay(electrial pathway) is called a channel. The greater number of channels, the greater flexibility of imaging
-typical numbers of channels are 32, 64, 128, 256 |
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side lobes
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lobes of energy that is not part of the main beam.(degrades lateral resolution)
-the result of width and length vibrations of crystals |
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dynamic damping
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a voltage is applied to crystales to generate a pulse but inorder to counteract the voltage, a voltage pulse of opposite polarity is applied to the crystal wich limits the ringing of the trasducer, and thus the SPL
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confocal imaging
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-a different central frequency is used to focus the beam at each depth. This allows a very narrow beam, and good SPL, but frame rate tends to be compromised unless co-processing is involved.
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where is section thickness artifact most often seen
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gallbldder, and urinary bladder(side wall appears thicker because lateral resolution is worse than axial res.
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what is comet tail artifact caused by? how can it be controlled?
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-occurs btw 2 close anatomical reflectors(same mechanism as reverb)
-complexity depends on nature, shape and size of reflector, and distance and orientation from transducer. -controlled by sensitivity settings, but there artifacts can be diagnostically useful. |
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how is ringdown artifact produced?
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resonance artifact seen behind air, or metal objects
-looks like enhancement but there is no fluid. -often iatrogenic(doctor caused) |
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twinkle artifact
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doppler artifact where the reverb phenomenon is applied. A slight shift in the pase of the wave going back will cause a color twinkle posterior to a Lg reflector.
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coded excitation
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-a series of pulses and gapes along each line of site which improves:
-penetration contrast resolution -this can be done because of parallel processing or else the frame rate would drop dramatically. |
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acoustic speckle
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-often with, mid to low gray scale echos, the dot on the screen is a result of various constructive and destructive interference, so it cannot be traced to one reflector. The best way to help with this is to make pixels smaller.
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aliasing
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improper sampling of flow velocities; when the doppler shift exceed half the PRF
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range ambiguity
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occurs in cases where the PRF is too high for the depth resulting in deeper structures appearing close because deep echos have arrived after the transducer has fired again. (usually PRF is automatically reduced as depth is increased, but in doppler PRF is adjusted to control aliasing, so this artifact sometimes appears)
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poling
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the process of inducing piezoelectric qualities into a substance
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what assumptions do we make with imaging?
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-echos are coming directly from the trasducer
-propegation speed through tissues is 1540/s -magnitude of reflection is an indication of magnitude of the reflector. |
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slice thickness artifact
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-perpendicular to the long axis of the array
-people assume that the eam is very narrow, but ther is a certain volume of tissue interrogated, and tissue volume doesn't return echos. -through mechanical or electrical focusing, volume is minimized. |
