Physics Final Study Guide

What kind of waves are ultrasound waves?

mechanical and longitudinal

Rarefactions are regions of ________ and ________.

low pressure and density

Compressions are regions of _______ and ________.

high pressure and density

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

the medium only; stiffness and density

Stiffness is also called what?

bulk modulus

If you increase stiffness what happens to propagation speed?

it increases

Sound cannot travel through a ________. It needs a medium

vacuum

What is the average speed of sound in soft tissue?

1,540 m/s or 1.54 mm/us

What is frequency determined by?

the sound source

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)

Audible frequency:

Ultrasound:

Infrasound:

20Hz-20,000Hz

above 20,000Hz (20kHz)

below 20Hz

Constructive interference

waves are in phase; stronger; reinforcement

Deconstructive interference

waves are out of phase; weaker

What is period?

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

What is wavelength?

the length of one cycle (c/F)

What is pulse duration?

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

What is duty factor?

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

Attenuation

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

Attenuation is highest in ___ and lowest in _____.

air; fluid

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

reduce penetration and increase attenuation

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

3\.5 MHz for an adult; 5 MHz for a child

Half value layer thickness (HVLT)

level at which 3dB attenuation has occurred

3dB =

1/2

What is absorption?

when energy is converted into heat

Reflection

specular reflectors are large and smooth like the kidney capsule

Rayleigh scatterers

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

Attenuation coefficient in soft tissue

0\.5 dB per MHz

What compensates for attenuation?

TGC

3 operator controls

intensity, power, and frequency

What does increasing power result in?

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

What would decrease intensity?

increasing beam area

Power increases =

intensity increases

What is amplitude?

the maximum cyclic change in a quantity

What are the acoustic variables?

pressure, density, temperature, and motion (vibration)

What is the intensity associated with bioeffects?

SPTA

Intensities highest to lowest

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

Impedance

density x propagation speed (rayls)

What must there be in order for reflection to occur?

a change in impedances

Intensity transmission coefficient (ITC)

the percentage of sound transmitted at a boundary

Intensity reflection coefficient (IRC)

the percentage of sound that is reflected

Diffraction

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

Snells law

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

Refraction:

shows lateral misrepresentation

Acoustic enhancement

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

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

How will multipath artifacts displace structures?

axially

What do grating lobes do?

put an extra echo on the image

What do slice thickness artifacts do?

fill in anechoic spaces

What produces comet tail artifact?

metal (surgical clips)

What produces ring down artifact?

gas

Shadowing

decrease in amplitude behind a highly attenuating structure

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

decreases spatial resolution

Curving the element, or lens, will do what?

improve lateral resolution? (focusing)

Temporal resolution decreases with what 3 things?

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

Which is superior, axial or lateral resolution?

axial

Where is the most accurate measurement taken?

along the beam path

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

divide by 13

What increases NZL?

increasing the frequency or the transducer diameter

FZ

the narrowest part of the beam; max intensity here

The far field is also called__________.

fraunhofer

The near zone is also called_________.

fresnel

Wide bandwidth transducers have what?

shorter pulses and better axial resolution

Array transducers have what?

multiple elements

Phasing

electronic time delays to steer and/or focus the beam

Annular arrays

electronically focused but mechanically steered

Linear phased array

best for cardiac and intercostal scanning

What is axial resolution improved by?

damping, increasing frequency, wider bandwidth, shorter pulse length

huygen’s principle

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

Linear sequenced

rectangle shape

Linear phase

sector shape

Dynamic apodization (subdicing)

reduces grating lobes

What is lateral resolution improved by?

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

What does the backing layer do?

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

Formula for axial resolution

SPL/2

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

Dynamic receive focus

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

Q factor

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

What does multifocusing achieve? (frame rate)

reduces frame rate

Contrast resolution

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

What will increasing line density do?

improve spatial resolution and decrease frame rate

Piezoelectric effect

acoustic to electric

F number

refers to the focusing of the sound beam

PZT (lead zirconate titanate)

material used for TRX

Shades of gray (bits per pixel) describes what?

contrast resolution

How many lines are in a TV frame?

525

Gain control

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

Output power control affects what?

the pulse component

TGC

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

Read magnification

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

write magnification

preprocessing control; does not lose resolution

PRF

number of pulses per second

Compression

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

Beam former

responsible for apodization, beam steering, and focus aperture

Order of the instrument:

pulser, beam former, receiver, memory, display

Dynamic range

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

Rectification

converts negative voltages to positive

Demodulation

converts from radio frequency to video form

D to A converter

numbers to proportional values of brightness

A to D

analog to digital form

Scan converter

where images are stored