Physics Final Study Guide

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What kind of waves are ultrasound waves?

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What kind of waves are ultrasound waves?

mechanical and longitudinal

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Rarefactions are regions of ________ and ________.

low pressure and density

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Compressions are regions of _______ and ________.

high pressure and density

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Speed of sound (propagation speed) is determined by what?

the medium only; stiffness and density

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Stiffness is also called what?

bulk modulus

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If you increase stiffness what happens to propagation speed?

it increases

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Sound cannot travel through a ________. It needs a medium

vacuum

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What is the average speed of sound in soft tissue?

1,540 m/s or 1.54 mm/us

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What is frequency determined by?

the sound source

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What is frequency and what two things determine it?

the number of cycles per second; propagation speed of the crystal and thickness of the element (c/wavelength)

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Audible frequency:

Ultrasound:

Infrasound:

20Hz-20,000Hz

above 20,000Hz (20kHz)

below 20Hz

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Constructive interference

waves are in phase; stronger; reinforcement

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Deconstructive interference

waves are out of phase; weaker

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What is period?

the time for one cycle to occur (1/F)

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What is wavelength?

the length of one cycle (c/F)

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What is pulse duration?

the time that the pulse is on (# of cycles in a pulse x period)

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What is duty factor?

the fraction or percentage of time that the pulse is on (pulse duration/PRP x 100)

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Attenuation

the weakening of sound as it travels through the body through reflection, scattering and absorption

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Attenuation is highest in ___ and lowest in _____.

air; fluid

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What will increasing the frequency of the TRX do? (attenuation)

reduce penetration and increase attenuation

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What is the average frequency TRX for an adult and a child?

3.5 MHz for an adult; 5 MHz for a child

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Half value layer thickness (HVLT)

level at which 3dB attenuation has occurred

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3dB =

1/2

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What is absorption?

when energy is converted into heat

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Reflection

specular reflectors are large and smooth like the kidney capsule

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Rayleigh scatterers

smaller than the wavelength (RBCs) - frequency to the 4th

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Attenuation coefficient in soft tissue

0.5 dB per MHz

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What compensates for attenuation?

TGC

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3 operator controls

intensity, power, and frequency

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What does increasing power result in?

it increases exposure to patient, increases penetration, image brightness, and voltages applied to the crystal

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What would decrease intensity?

increasing beam area

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Power increases =

intensity increases

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What is amplitude?

the maximum cyclic change in a quantity

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What are the acoustic variables?

pressure, density, temperature, and motion (vibration)

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What is the intensity associated with bioeffects?

SPTA

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Intensities highest to lowest

SPTP, SATP, SPTA, SATA (TP is always highest, TA is always lowest)

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Impedance

density x propagation speed (rayls)

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What must there be in order for reflection to occur?

a change in impedances

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Intensity transmission coefficient (ITC)

the percentage of sound transmitted at a boundary

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Intensity reflection coefficient (IRC)

the percentage of sound that is reflected

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Diffraction

the interaction associated with divergence of a sound beam after passing through a small aperture

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Snells law

refers to the angle of sound transmission at an interface with oblique incidence and different propagation speeds

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Refraction:

shows lateral misrepresentation

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Acoustic enhancement

the hyperechoic region posterior to a low attenuating structure (fluid)

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If the speed of sound is slower than the average in soft tissue, what will happen?

the instrument with place the structure further away than it actually is

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How will multipath artifacts displace structures?

axially

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What do grating lobes do?

put an extra echo on the image

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What do slice thickness artifacts do?

fill in anechoic spaces

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What produces comet tail artifact?

metal (surgical clips)

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What produces ring down artifact?

gas

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Shadowing

decrease in amplitude behind a highly attenuating structure

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What does lowering the frequency of the transducer do? (spatial resolution)

decreases spatial resolution

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Curving the element, or lens, will do what?

improve lateral resolution? (focusing)

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Temporal resolution decreases with what 3 things?

an increase in scan lines, multifocus zones, and an increase in sector width

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Which is superior, axial or lateral resolution?

axial

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Where is the most accurate measurement taken?

along the beam path

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How do you find the reflector depth if GRT is given?

divide by 13

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What increases NZL?

increasing the frequency or the transducer diameter

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FZ

the narrowest part of the beam; max intensity here

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The far field is also called__________.

fraunhofer

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The near zone is also called_________.

fresnel

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Wide bandwidth transducers have what?

shorter pulses and better axial resolution

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Array transducers have what?

multiple elements

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Phasing

electronic time delays to steer and/or focus the beam

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Annular arrays

electronically focused but mechanically steered

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Linear phased array

best for cardiac and intercostal scanning

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What is axial resolution improved by?

damping, increasing frequency, wider bandwidth, shorter pulse length

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huygen’s principle

constructive and destructive interference; v shaped wavelets; hourglass shaped beam

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Linear sequenced

rectangle shape

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Linear phase

sector shape

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Dynamic apodization (subdicing)

reduces grating lobes

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What is lateral resolution improved by?

focusing; beam diameter for an unfocused beam is 1/2 the transducer diameter

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What does the backing layer do?

improves axial resolution by reducing ringing and shortening the SPL; also reduces sensitivity

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Formula for axial resolution

SPL/2

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What does the matching layer do?

reduces acoustic impedance mismatch between element and tissue; gel acts as a matching layer; 1/4 a wavelength thick

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Dynamic receive focus

uses delay lines after the echoes are detected (focusing during reception)

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Q factor

characterizes the frequency bandwidth of the transmitted ultrasound wave; F/BW

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What does multifocusing achieve? (frame rate)

reduces frame rate

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Contrast resolution

the ability to distinguish between adjacent structures that produce echoes of similar amplitude

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What will increasing line density do?

improve spatial resolution and decrease frame rate

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Piezoelectric effect

acoustic to electric

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F number

refers to the focusing of the sound beam

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PZT (lead zirconate titanate)

material used for TRX

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Shades of gray (bits per pixel) describes what?

contrast resolution

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How many lines are in a TV frame?

525

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Gain control

determines the amount of amplification that occurs in the receiver (increasing gain is not associated with increasing risk of bioeffects)

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Output power control affects what?

the pulse component

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TGC

equalizes the differences in echo amplitudes received at different depths; compensates for attenuation

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Read magnification

post processing control; increases pixel size so some resolution is lost in the image

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write magnification

preprocessing control; does not lose resolution

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PRF

number of pulses per second

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Compression

receiver function that decreases the difference between the smallest and largest receiver signal amplitude

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Beam former

responsible for apodization, beam steering, and focus aperture

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Order of the instrument:

pulser, beam former, receiver, memory, display

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Dynamic range

the ratio of the largest to smallest signal a system can handle

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Rectification

converts negative voltages to positive

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Demodulation

converts from radio frequency to video form

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D to A converter

numbers to proportional values of brightness

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A to D

analog to digital form

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Scan converter

where images are stored

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