lange review

what is ultrasound?

a longitudinal mechanical wave with a frequency over 20KHz (20,000 cycles Hz)

how are ultrasound waves produced?

oscillatory motion of particles in a medium, creating regions of rarefaction and compression, collision allows the movement of particles to propagate

longitudinal wave

the particles are in a direction parallel to the propagation

transverse wave

aka shear wave, particles are perpendicular to direction of propagation

mechanical wave

a wave that requires a medium to pass through

acoustic variable

pressure, temperature, density, particle motion (distance) - these may change as an acoustic wave passes through the medium

parameters of a wave

cycle, frequency, period, wavelength, propagation speed (common to all waves)

cycle

one compression and one rarefaction, or a complete positive and negative change in an acoustic variable

frequency

number of cycles per second (how many times the acoustic variable changes in one second)

period

the time it takes for one cycle to occur, inverse of frequency

wavelength

the distance a wave travels in one cycle

propagation speed

the maximum speed with which an acoustic wave can travel through a medium - proportional to stiffness, inversely proportional to densitt

opposite of stiffness

compressibility

propagation speeds in mediums

highest in solids > liquids ? gases

PRF

number of pulses per second - decreases as depth increases

PRP

the amount of time it takes for a pulse to repeat/time from one pulse to the next - increases as depth increases

pulse duration

the length of a pulse/time it takes for a pulse to occur

duty factor

the fraction of time the transducer is generating a pulse (range from 0 - 1)

SPL

the length of a pulse in space/distance over which a pulse occurs

amplitude

maximum variation that occurs in an acoustic variable, indicates the strength of the sound wave

power

rate of energy transferred, proportional to wave amplitude squared (rate at which work is done, work divided by time needed to do it)

intensity

power in a wave divided by the area of the beam

SP

intensity at the centre of the beam (beam uniformity ratio x spatial average)

SA

intensity averaged over the beam (intensity / beam uniformity ratio)

TP

maximum intensity in the pulse

TA

averaged intensity over one on-off beam cycle

PA

intensity averaged over the duration of the single pulse

the six intensities

SPTP > SATP > SPTA (heating) > SATA

* SAPA and SPPA

TP and TA are the same when

using continuous wave

DF in terms of TP

DF = TA/TP

factors contributing to attenuation

absorption, scattering, reflection

absorption

conversion of sound energy into heat

scattering

diffuse scattering is the redirection of the sound beam after it strikes rough of small boundaries where the wavelength is larger than the reflecting surface (liver parenchyma, RBCs)

reflection

return of a portion of the US beam back towards the transducer - specular (wavelength of pulse smaller than boundary, smooth surface, diaphragm, liver capsule, GB walls)

what does US reflection depend on?

acoustic impedance mismatch at the boundary

most important factor for echo strength

angle of incidence

acoustic impedance

product of the density of a medium and the velocity of sound in that medium

IRC

reflected intensity over incident intensity

ITC

intensity reflection transmission, transmitted intensity over incident intensity or 1 - IRC

types of incidnece

perpendicular and oblique

how to calculate angle of transmission

angle of incidence x speed1/spped2

snell’s law

sin(theta2) = sin(theta1) x c2/c1

range equation

relationship between the round trip travel time of the pulse and the distance to a reflector (distance = 1/2vt

harmonic imaging is the result of

nonlinear propagation of the sound beam as it passes through tissue

advantages of the harmonic beam

less dispersion (narrower) and smaller side lobes - narrower beam = better lateral resolution, less clutter

transducer

device that converts one form of energy to another

piezoelectric principle

converts pressure into electrical energy

reverse piezoelectric principle

converts electrical energy into pressure

what determines frequency of an acoustic wave by a pulsed wave system?

thickness of the PZT and the propagation speed of the crystal

what does the diameter of the crystal affect?

lateral resolution - does NOT affect frequency

what is the frequency of a CW ultrasound wave equal to?

frequency of the electric voltage that drives the crystal

damping material/backing material

absorbs vibrations and reduces number of cycles to: reduce PD and SPL, increasing AR; increase bandwidth, decreasing Q, decrease duty factor

matching layer

in front of the element to decrease reflection at the transducer-tissue interface

thickness of the matching layer

1/4 of the wavelength

CW bandwidth

very narrow

a low Q factor indicates?

broad BW, low operating frequency, short pulse length, uniform near field

mechanical transducer

scan head with a single disk shaped active element e.g. oscillatory or rotary. mechanically steered and mechanically focused. sector shaped image, focussing at a specific depth

annular array

type of mechanical transducer with 5-11 rings of elements. mechanically steered, electronically focused. more elements than single element counterpart, allowing for the electronic focussing and better depth resolution. also sector.

electronic transducer

assembly of multiple elements called an array - 3 types: linear, curved, sector phased

linear array

produces a rectangular image

curved array

no beam steering, internal and electronic focusing

sector phased array

has time delays for electronic steering and electronic focusing. sector

mechanical focusing

uses a curved crystal or acoustic lens. improves lateral reso by decreasing beam width

types of electronic focusing

transmit and receive

transmit focusing

firing a group of elements with a time delay between elements. increases lat resolution and can do multizone focus

receive focus

electronic focusing of received echoes by delaying the return of signals to the processing system. enhances clarity

huygen’s principle

the resultant beam is a combination of all sound arising from different sources on the transducer

focal length

distance from transducer to focal point

AR improves with

smaller wavelength or number of cycles, increased frequency

pulser

produces an electric voltage that activates the PZT, causing it to produce a sound beam. also signals the receiver and scan converter that the transducer has been activated.

pulser role in array transducers

delay and variations in pulse amplitude needed for electronic control of beam scanning, steering and shaping, apodization

scan converter

transforms incoming echo data into a suitable format for the display

analog scan converter

in older machines, semiconducters in square matrices. current in the electronic beam is proportional to intensity of returning echoes. brightness of display varied by stored charge in each element

digital scan converter

store image brightness as numbers instead of electrical charges. 3 components: ADC, digital memory and circuit (DAC)

largest value of n bits

2^n - 1

how many bits in most US machines?

4-8, 16 - 256 shades of gray. CDI needs more

formula for how many bits an US image has

number of bits/image = number of image rows x number of image columns x number of bits/pixels OR number of pixels x number of bits/pixels

coded excitation

a digital technology allowing good penetration and high resolution, improves AR, contrast, SNR and penetration depth - includes write zoom and read zoom

FR, PRF and LPF are directly related to

propagation speed

A mode

1d. y-axis is amplitude or strength, x-axis is distance in time

what axis is missing on B mode?

y-axis

section thickness

finite width of the beam producing debris or exhoes in normal anechoic or echo free structures

propagation artifacts

reverb, refraction, multipath, mirror image

attenuation artifacts

shadowing, enhancement, edge shadowing

comet tail

produced by a strong reflector, similar to reverb but thin lines of closely spaced discrete echoes - gas bubbles, surgical clips, biopsy needles, bullet

ring down

resonance phenomenon assoc w gas bubbles - similar to reverb, numerous parallel echoes

propagation speed error

assumption of c = 1540 not always true - calculated distance may differ based on if speed is greater or lower

range ambiguity

echoes from a transmitted pulse that return after a time equal to the PRP will be erroneously recorded at a depth closer to the transducer

QA programs test

1. imaging performance
2. equipment performance and safety
3. beam measurements
4. acoustic output


1. preventative maintenance

beam profiler

records 3d reflection amplitude information

hydrophone

measures acoustic output, small transducer element mounted on a narrow tube - can be used to calculate period, PRP, PD, can be used as a beam profiler

what applies to universal precautions?

vaginal secretion, semen, amniotic fluid, CSP, pleural and peritoneal fluid - NOT feces, sweat or urine

required methods for EV probe disinfection/prevention

cold chemical disinfectants, disposable probe covers

angiodynography

another term for CDI

multigate analysis

a method of collecting doppler data from several adjacent spatial locations

synchronous signal processing

system uses the same signal to make the gray-scale and doppler image. earliest was M/Q system. more in echo. uses phased array. grating lobes and side lobs do not detract from the images

asynchronous signal processing

different signals to form the grey scale and doppler images - more common

FFT provides

doppler signal frequencies and doppler signal amplitudes

power doppler

amplitude signal processing - determines power spectrum of the signal amplitudes

CDI artifacts

range ambiguity, aliasing, soft tissue vibrations (bruit etc)