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65 Cards in this Set
- Front
- Back
What is piezoelectric effect?
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The phenomen by which a mehanical deformation occurs when an electric field (voltage) is applied to a certain material or a varying electrical signal is produced when the crystal structure is mechanically deformed
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What is a transducer?
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Any device that converts one form of energy to another
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Why PZT is the material of choice for ultrasound?
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Because of its high coupling coeficient: high frequency of natural resonance and very good reproductible characteristics for a stable design
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What is Curie temperature?
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300 C, 572 F
If a material is overheated it can loose its poling and piezoelectric efficiency |
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If the frequency doubles, the NZL
a) increases by a factor of 2 b) decreases by a factor of 2 c) increases by a factor of 4 d) decreases by a factor of 4 |
a) increases by a factor of 2
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What is the relation between operating frequency and the thickness of a crystal?
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1.A thicker crystal has a slower compression and a lower frecquency
2.A thinner crystal has a higher compression rate and a higher frecquency |
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If he diameter of a tx is increased by a factor of 2, the near zone lenght will:
a) increases by a factor of 2 b) decreases by a factor of 2 c) increases by a factor of 4 d) decreases by a factor of 4 |
c) increases by a factor of 4
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Which of the following txs will give the shallowest focus?
a) D=2mm, f=2MHz b) D=1mm, f=2MHz c) D=2mm, f=10MHz d) D=1mm, f=10MHz |
NZL=(D*D*F)/6
corect answer is b |
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What is bandwidth?
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Bandwidth is described as the useful range of frequencies over which anything can operate
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What is fractional bandwidth?
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Fractional bandwidth is determined by dividing the bandwidth by the operating frequency
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What is a broadband transducer?
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Transducers are considered broadband
when they have a fractional bandwidth > 80% |
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What is the quality factor ?
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Q factor is a unitless number
Q-factor=Operating frequency / bandwidth |
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What techniques can be used to improve performance of a multi-Hertz Transducer?
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1.Dynamic Frequency Tuning
2.Harmonic imaging 3.Frequency Fusion (Compounding) |
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How are the pulse duration and the bandwidth related?
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Shorter PD = Wider Bandwidth
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How can you decrease the impulse response?
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1. Dumping
2. Increasing the frequency |
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What is the relation between axial resolution and the impulse response time?
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1. short impulse response = fewer cycles in the pulse ; improves axial resolution
2. longer impulse response = many cycles in the response; degrades axial resolution |
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What is the beam shape for continous and pulse waves?
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For continuous waves, beamshape is consistent (as long as the signal is ON);
For pulse wave, the beamshape vary depending on the time it had been traveling |
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What are the beam parammeters?
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1. Depth (Axial, Longitudinal, Radial)
2. Beamwidth: Lateral (Azimuthal, side by side, Transverse, Angular) |
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What is elevation?
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For nonsymetrical crystals, the beamwidth will be different in 2 different planes which creates another dimensional definition: elevation
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What is the beamwidth for an unfocused transducer?
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For an unfocused tx. the beamwidth is 1/2 the diameter of the crystal at the focus
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Unfocused tx. do not have a focus point.
TRUE / FALSE |
FALSE
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What is natural focus?
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The depth where the beam reaches it's narrowest beamwidth
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For an unfocused tx, what is the approx. beamwidth at the focal ditance and twice the focal distance, given a crystal diameter of 5mm?
a) 5 mm, 10 mm b) 2.5 mm, 10 mm c) 5 mm, 5 mm d) 2.5 mm, 5 mm |
d)
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How thick would a crystal have to be for 2 MHz pulsed mode operation given a propagation velocity through the crystal of 5,000 m/sec?
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fo = c / 2*thickness
thickness = c / 2 *fo = 5,000m/sec / 2*2*10power6 1/sec thickness = 1.25 mm |
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What is NZL ?
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1. Near Zone Lenght
2. Distance from tx to the natural focus 3.Fresnel Zone |
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How do we calculate NZL?
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1. NZL = D*D/ 4λ
most frequently in its modified form 2. NZL = D*D * Fo / 6 |
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Which crystalis is better for superficial imaging; a smaller or a larger one?
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The crystal with smaller diameter
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If we increase the diameter of a crystal by 2, how much will increase the depth of natural focus (NZL) ?
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Will increase 4 times
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Axial resolution is determined by ?
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Wavelenght
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Lateral resolution is determined by ?
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Beamwidth
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How thick should be a matching layer?
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A quarter wavelenght
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What is axial resolution ?
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Ability to distinguish between 2 structures that are along the same beam path, in axial dimensions
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What is the relation between SPL and axial resolution ?
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1.shorter SPL = better axial resolution
2. axial resolution = 1/2 SPL |
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What is SPL?
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SPL = no. cycles * wavelenght
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What is lateral resolution ?
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Ability to resolve 2 structures in the lateral dimension
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What is backing material ?
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tunsgsten powder and epoxy resins
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How can you change the focus ?
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1. Lenses
2. Curved elements 3. Electronic focusing 4. Mirrors |
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What are the disadvantage of using lenses for changing the focus?
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1. creates another impedance mismatch between the matching layer and patient
2. decrease efficiency of tx because it is very absorptive 3. cause some surface heating on the tx. |
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What are the disadvantage of using curved surface focusing for changing the focus?
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1. PZT is very brittle
2. newer materials are flexible composites |
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Near field (Fresnel)
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the area between the face of tx. and the beam focus
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Far fiels (Fraunhofer)
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the region past the focus
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Focal region
(Depth of field) |
Region over which the beam is most tightly focused
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Detail Resolution
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bility to distinguish between 2 objects in any of 3 dimensions; axial, lateral or elevation
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Operating frequency
fo |
The center frequency of the transmit bandwidth
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fo (pulsed mode)
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Determined by the crystal thickness and speed of sound in crystal material
fo=c / 2*(thickness) |
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fo (continuous mode)
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Determined by the frequency of the drive (transmit) voltage
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The footprint of a tx has 2 dimensions. The beamwidth is associated with .......... dimension and 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 resoluion is the same as lateral resolution
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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 is the same as Range or axial resolution
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In pulse mode what determines the operating frequency fo ?
Give the equation |
fo= c / 2 * thickness
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In continuous mode what determines the operating frequency fo ?
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fo is determined by the frequency of the transmitter (of the pulser)
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If the thickness of a crystal doubles, fo for PW mode will:
a) double b)halves c)quadruples d) quarters |
b) halves
fo= c / 2*thickness |
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A tx is anything which converts .......... from one form to another form
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energy
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An ........... tx converts voltage into sound above human hearing and sound into voltage
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ultrasound
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An ultrasound tx converts electropontential .............. into .............. and mechanical energy into electropotential energy
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energy, mechanical energy
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Ultrasound tx use the...... effect
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piezoelectric
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A commonly used material for ultrasound tx is ....... Zirconate Titanate (PZT)
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Lead
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If a tx is heated above its ............... temperature will lose its piezoelectric properties.
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Curie
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For PZT the Curie temperature is
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300 degree C
572 defree F |
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When a tx is reffered as mechanical, it means that the ....... is performed mechanical
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steering
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A............ is used to help focus a beam in the elevation plane
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lens
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The primary purpose of dumping mateerial is to:
1. decrease the no of cycles in the pulse 2. to improuve the axial resolution 3. create broadband tx |
1. decrease the no of cycles in the pulse
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HIgher frequency tx will have a deeper natural focus.
TRUE / FALSE |
TRUE
because they alternate quicklier |
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Lateral resolution is the best
1. in the far field 2. at the focus point 3. in the near field |
2. at the focus point
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Regarding beam formation, what occurs in the Fraunhoffer zone?
1. better resolution 2. increased difraction 3. increased attenuation |
2. increased difraction
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