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81 Cards in this Set
- Front
- Back
What is sequencing?
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Exciting groups of elements in a specific patern to scan a region in a linear fashion.
Generally done with large linear or curved linear array |
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What are the limitation of pencil probes?
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no image
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Frame rate
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- the ability to create multiple frames per second
- HZ (images per second) |
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temporal resolution
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ability to position moving structures from one instant to another
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relation between Frame rate and temporal resolution
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direct proportional
frame rate increase = temporal resolution increase |
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What influence the frame rate?
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- speed of sound in medium
- depth of imaging |
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frame time
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the time it takes to generate one frame
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relation between frame rate and frame time
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frame rate is reciprocal of frame time
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Calculate the frame time for an image at the depth of 10cm and 200 lines.
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13microsec (cm/Hz) * 200 lines = 26 ms/frame
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If the frame rate is 1/26 msec, what is the frame rate in Hz?
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0.038 KHz = 38 Hz
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Relation between
depth frame time frame rate |
debth increase
frame time incrrease frame rate decrease |
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Calculate the frame time for an image at 10 cm debth and 400 lines in msec and in Hz.
What is PRF ? PRP ? |
13microsec * 400 lines = 52 ms/frame
frame rate is 1/52=19Hz PRF = debth * 13microsec/cm = .13ms PRP = 1/PRF =1/0.1ms=10Hz |
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What is line density?
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ability to alter the spacing between sound beams or scan lines
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Lateral resolution
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- lateral resolution is defined by the beam width
- best lateral resolution results from a narrow beam |
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Electronic focusing
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- can only be done if there is more than one crystal in the tx
- also called array (multiple crystals) |
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How can we change the focusing?
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1. lenses
2. curved elements 3. electronic focusing 4. mirrors We can only affect the beamwidth in the near field. If we need a deeper focus, use a larger diameter crystal |
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Lenses used to change the focus
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advantage:
-sound can be converged more rapidly -produce a shallower focus disadvantage: -creates a supl. acoustic impedance mismatch -heating of tx |
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Curved surface focusing
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disadvantage:
- PZT very brittle, difficult to construct advantage: -newer materials are more flexible -eliminates the acoust. imped.mismatch and absorbtion from lens |
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Focus and intensity
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By focusing the beam:
- intensity is increased - power is increased - area diminish intensity = power / area |
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A mode
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amplitude mode
- can only look straight ahead, creating a one dimensional scaning technique |
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B mode
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- Brightness mode
- 2 dimensions, can see more than one region simultaneously |
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1D arrays tx
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- can not focus in elevational plane
- can not acquire data in the 3rd dimension |
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1.5 D arrays tx
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- developed in 90'
- 32 elements in elevational plane 64 elem. in lateral dimension - allow focusing at two different focal depths in elevation |
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Relation between line density, lateral resolution, color and frame rate
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line density increased
= better lateral resolution line density decreased = better color = better frame rate |
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If we change the sector from 90 degrees to 45 degrees, what will happen to the scan lines? what about the frame rate ?
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=scan lines * 2 (will double)
= frame rate * 2 (will double) |
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Non imaging modalities (CW Doppler, Non imaging PW, blind M mode) are governed by PRF
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PRF = round trip travel time
pulser drive voltage Since there is no image formation, temporal resolution is high |
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Imaging modalities are governed by the frame rate or frame frequency
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that makes the temporal resolution less than for nonimaging modalities
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What influence beam divergency ?
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- tx diameter
- frequency of sound OBS: they also affect natural focus |
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Diameter of a crystal and the beam divergence
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Crystal Diameter and beam divergence are inversely related
Larger crystal diverge less, therefore have a better resolution in Fraunhofer zone |
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Frequency, Attenuation, Divergence
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low frequency:
- attenuates less - diverge more - lower lateral resolution in far field higher frequency - diverge less in fraunhofer zone - create a better resolution in the far field |
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Larger diameter crystals and higher frequency will create less divergence
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Smaller diameter crystals and lower frequency will create more divergence
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Huygens wavelets
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Spherical waves
diffraction patterns |
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Huygens' Principle
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A large element can be thought as a milion of tiny sound sources. Each of those sources is than a huygen source and creates a huygens wavelet (v shaped)
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Linear Switched Array Tx
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- has a group of elements that can be switched on off electronically
- no steering possibilities - rectangular image (wider than thicker) advantage: - create a wider image in near field (in their time) disadvantage: - we can not change focus - no focus - expensive in comparison with single element. |
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Mechanical steered tx
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- make a sector image
- single crystal, wobbled by a motor rotating the crystal - symetrical beam in elevation and lateral planes - broad depth of field for a deep focus - cardiac use |
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Mechanical steered tx
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- advantage over phased array: less expensive (only two crystals)
- disadvantage; - fixed focus for transmit and receive - parts wear out - motion artifact - air pockets in gel creates artefact |
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Mechanical annular array
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- sector image with curved top
- multiple concentric elements |
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lateral plane
synonims |
azimuthal
side by side transverse angular |
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axial plane
synonims |
radial
depth longitudinal range |
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fo (pulsed mode)
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fo = c/ 2*thickness
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fo(continuous mode)
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determined by the frequency of the drive (trasmit) voltage
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Blind (Doppler only) tx have a broad depth of field
True / False |
True
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A phased array tx must have multiple elements.
True / False |
True
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A mechanical system can steer and focus the beam electronically
True / False |
False
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An annular array can be focused electronically but must be steered mechanically.
True / False |
True
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An annular tx has the distinct benefit of variable focus in both elevation and lateral planes. A disadvantege with respect to phased tx is that annular tx:
a. cannot image deeper than 10 cm b. is inherently less sensitive c. must be steered mechanicaly d. can not perform Doppler or color flow |
c. must be steered mechanicaly
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Which of the following has a rectangular image?
a. annular b. sector c. linear switched array d. more than one of the above |
c. linear switched array
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A fundam. difference between a linear swithced array and a linear phased array is that a linear switched array:
a. has a fixed focus and cannot steer b. has a variable focus and can steer c. has a lower maximum PRF |
a. has a fixed focus and cannot steer
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Sector phased array principally create a sector image
a. for rib acces b.to allow for more lines per scan c. for cost effectiveness d. to minimize the time it takes to shoot a frame |
a. for rib acces
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The foot print of a tx has 2 dimensions. The beamwidth is associated with the......... dimension and the slice thickness is determined by the ..... dimension.
a. axial, elevation b. elevation, lateral c. elevation, axial d. lateral, elevation |
d. lateral, elevation
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Which of the following does not belong?
a. axial b.longitudinal c. depth d. radial e. azimuthal |
e. azimuthal
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Which of the following is not the same as lateral resolution?
a. radial b. angular c. Azimuthal d. Transverse |
a. radial
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Which focusing technique best improves the focus in the far field?
a. lens b.mirrors c.electroning focusong d.curved elements e. none, one can only improve focus in the near field |
e. none, one can only improve focus in the near field
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radial resolution equals
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SPL/2
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Lateral resolution equals the ...... and varies with depth since the .... varies with depth
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beamwidth, beamwidth
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When a tx is refered to as mechanical, it means that the ... is performed mechanically
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steering
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A..... is used to help focus a beam in elevation plane
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lens
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The ..... plane coresponds to the beam thickness
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elevation
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the ..... ...... is used to minimize the acoustic impedence mismatch between the PZT and tissue
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matching layer
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The ........ ........is used to shorten SPL , improuving longitudinal resolution
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backing material
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the focus is where beam reaches its minimum..... and maximum........
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diameter, intensity
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the focal region is also called ..... of field and refers to area of the beam which is most tightly......
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depth, focused
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...... is the ability to distinguish between 2 objects
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resolution
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....... resolution is also called axial, radial, and depth resolution
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longitudinal
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..... resolution is the ability to distinguish between objects along the beam direction
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longitudinal
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...... resolution is also called angular, azimuthal, or transverse resolution
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lateral
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...... resolutin is the ability to distinguish between structures that are side by side
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lateral
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In .... the operating freq. is determined by the speed of sound in the crystal and the crystal thickness
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pulsed mode
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In pulse mode a thicker crystal will have a .... operating freq
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lower
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In .......... a thicker crystal will not change the operating freq.
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continuous mode
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the .... appears at a distance of one NZL
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focus
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A phased array tx uses ...... focusing in the lateral dimension and a ...... for focusing in the elevation dimension
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electronic, lens
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Beyond Fresnel zone, the beam is said to be ...... limited
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diffraction
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Bmode is an example of ..... wave mode
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pulsed
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color flow is an example of ....wave mode
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pulsed
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CW Doppler is an example of ...... wave mode
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continuos
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The use of the name mechanical for a tx implies the means by which it is:
a. focused b. steered c. assembled d. all of abave |
b. steered
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Which tx style has a variable focus in both lateral and elevation plane?
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annular array (almost extinct)
2-D arrays |
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Which of following tx can be electronically steered?
a. sector phased array b. annular array c. phased linear array d. linear switched array |
a. sector phased array
c. phased linear array |
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What factors can influence the lateral resolution ?
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Any factor that can affect the beamwidth; frequency, aperture, focus, multiple transmit foci, parallel procesing
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What factors can influence the longitudinal resolution ?
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Any factor that can influence SPL such as: frequency, wavelenght, propagation speed, backing or dumping material, excitation pulse driving, crystal...etc
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