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72 Cards in this Set
- Front
- Back
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crystals that exhibit the piezoelectric effect, do so because the molecules are ____
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polar
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alternately reversing the polarity of voltage causes piezoelectric crystals to
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alternately expand and contract
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higher amplitude of electrical signal produces higher/lower acoustical signal
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higher
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the measure of the efficiency of conversion between electrical and mechanical signals
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coupling coefficient
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material most commonly used for piezoelectric crystals in US
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zironate titanate (PZT)
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PZT has the positive characteristics below. But, what it's drawback
high coupling coefficient high natural resonance f repeatable and stable |
has a 30 to 40 MRayls Z
this causes significant impedance when its interfaced with tissue |
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the process of heating a material and applying electrical field to polarize molecules
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poling
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the critical temperature where a material looses its poling
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Curie point
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"the frequency of operation depends on what (2)
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thickness of the piezoelectric material
propagation velocity of sound through the piezoelectric material |
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PW operating frequency (MHz) equation
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propagation speed (mm/µsec)
--------------------------------------- 2 * thickness (mm) |
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CW operating frequency
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same as the frequency of transmit voltage (drive voltage frequency)
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the response of a crystal to a single, short duration pulse
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impulse response
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important physical dimensions of a crystal (2)
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diameter, thickness
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crystal diameter determines
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beam width
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crystal thickness has a primary role in determining
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operating frequency
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____ is the path the wave travels
____ is the energy at which it is traveling |
beam
wave |
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Difference between beam shape for PW and CW
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CW beam is continuous and shape is present all the time
PW beam is not continuous and shape is not present all the time |
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two beam parameters
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depth, beam width
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synonyms for depth (a beam parameter)
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Memory aid: LADRR
Longitudinal Axial Depth Radial Range |
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synonyms for beam width (a beam parameter)
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ATLAS
Azimuthal Transverse Lateral Angular Side-by-Side |
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the depth at which the beam reaches its narrowest beam width
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natural focus
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you say a transducer is _______ when nothing has been added to the it to affect natural focus
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unfocused
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beam width is D/2 at …
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natural focus
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the beam width equals the ______ at (2 * focal length)
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diameter
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two parameters that principally affect the depth of the natural focus
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diameter, operating frequency
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limit of simple crystal (4)
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PZT has high Z
impulse response has many cycles fixed natural focus only image straight ahead |
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purpose of matching layer
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reduce the difference in impedance between PZT and tissue
many transducers use multilayered matching layers |
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how are reverberation artifacts eliminated in the matching layers
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making matching layer thickness 1/4 λ of the operating frequency
causes destructive adding of unwanted wavelengths |
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how is the ring time of a crystal reduced
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backing layer
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what is the formula for axial resolution
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SPL / 2
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lateral resolution equals
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lateral beam width
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techniques to change focus (4)
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lenses
curved elements mirrors electronic focusing |
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least used focusing technique
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mirrors
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lenses introduce another layer of ____ into the transducer
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impedance
lenses are also very absorptive |
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curved elements are not used as much because
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crystals are brittle
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beyond the near field, the beam cannot be focus because it is _____ ______
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diffraction limited
there is no way to get waves to constructively or destructively interfere all focusing techniques affect the focus only within the near field |
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material that converts electropotential (voltage) into acoustic waves and reverse
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piezoelectric crystals
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shortens spatial pulse length improving longitudinal resolution
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backing material
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minimize acoustic impedance mismatch from crystal to tissue
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matching layer
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helps focus beam in elevation plane
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lens
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NZL (near zone length) equation
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NZL = (D^2)/4λ, or
NZL = (radius^2)/λ |
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NZL (near zone length) units
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NZL = mm * MHz / (m/sec)
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when frequency goes up the NZL gets longer/shorter
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longer
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when radius goes up the NZL get longer/shorter by a factor of _____
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longer, 4
because the radius gets squared |
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name given to the region of the beam that is above and below the focus where the beam is approximately the same width as the diameter of the aperture
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depth of field, or focal region
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a small range of lateral resolution and intensity happens with a depth of field that is shallow/broad
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shallow
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a large range of lateral resolution and intensity happens with a depth of field that is shallow/broad
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broad
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theoretically, beam intensity should always be higher at the focus. This is not true, in reality, because (2)
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attenuation is ignored
simplistic beam models used for instruction |
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what two situations related to intensity work in opposite directions
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area perspective: intensity increases as beam converges
power perspective: intensity decreases as depth increases |
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the reason why sonographers should care about the opposing changes to intensity in terms of power and area
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putting the focus a little deeper than region of interest can improve signal strength
system is allowed to increase transmit power highest intensity is a little shallower than the focus beam intensity drops dramatically in far field |
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what are the imaging dimensions (3)
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axial, lateral, elevation
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exiting specific groups of elements in a specific pattern
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sequencing
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the major limitation of linear switched array transducers
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no steering capability
keyword is 'switched' as opposed to 'phased' |
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what kind of transducer
round crystal sliced into concentric rings focus from shallow to deep |
annular array
prone to bad grating lobe artifacts until advent of 2-D arrays, only transducer that could control elevation |
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group of transducer elements
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array
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small time or phase delays of the elements within an array
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electronic steering
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principle that predicts how many small wavelets will construct a beam and which direction it will travel
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Huygens's principle
wavefront is determined by the tangential surface of the wavelets in array the wavefront of individual wavelets experience constructive interference to build wavefront of beam in array |
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a parabolic profile in firing elements is the primary way to electronically _____
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focus
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a more severe parabolic profile results in nearer/farther
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nearer
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a less severe parabolic profile results in nearer/farther
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farther
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transducer that produced lateral dimension by "sliding" it across the patient
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B-scan
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transducers that automated sweeping across the body with a motor
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mechanical
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multiple elements in one dimension
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1D
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transducer that has 3 elements in the elevation direction
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1.5D
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multiple elements in both the lateral and elevation directions
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2D
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this transducer requires both sequencing and phasing separately to create a true trapezoidal image
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linear phased array
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can be focus and steered in both the lateral and elevation direction
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2D
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what is the separation of the elements in a transducer
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pitch
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image produced by sequencing only, but phasing also used for other things
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curved linear phased array
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elevation resolution depends on _____ ____ ____
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elevation beam width
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the ability to distinguish between two objects in any of the 3 dimensions
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detail resolution
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the center frequency of the transmit bandwidth
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operating frequency
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