PHYSICS ARRT SONOGRAPHY

piezoelectric effect?

property of certain materials to create a voltage when pressure is applied

-indirect piezoelectric effect

reverse piezoelectric effect?

property of certain materials to deform when voltage is applied to them and create pressure (sound)

-direct piezoelectric effect

what piezoelectric material is used in ultrasound transducers?

lead zirconate titanate (PZT)

PZT crystal thickness?

1/2 wavelength thick

5 parts of ultrasound transducer?

1. pzt

2. case

3. wire

4. matching layer

5. damping element

matching layer of transudcer?

impedance of matching layer is between those of the skin and active element which increases percentage of sound transmitted

*1/4 wavelength thick*

impedances: PZT > matching layer > gel > skin

damping element of transducer?

epoxy resin impregnanted with tungsten bonded to the back of active element to limit it's vibrations

damping increases range of frequencies present in any pulse and shorter the pulse greater the range of frequencies present - wide bandwith better image qual

bandwith?

range of frequencies between highest and lowest transducer

bandwidth = max frequency - min frequency

other names for main frequency?

center, resonant, primary or natural frequency

quality factor?

Q = resonant frequency (MHz)/bandwith(MHz)

low Q better quality images

frequency equation?

f = propagation speed/ wavelength

focus or focal point?

location where beam reaches its minimum diameter

beam width?

as sound travels, width of the beam changes

starts out same size of aperature of transducer

gets narrower until reaches focal point

then diverges

focal depth, focal length, or near zone length of transducer?

distance from transducer face to focal point

fresnel zone?

aka near zone - area of beam that converges between transducer and focus

fraunhofer zone?

aka far zone - area where beam diverges beyond focal point

huygen's principle?

explains the hourglass shape of an imaging transducer's sound beam

each sound source on transducer face creates many sound wavelets emitted that constructively and destructively interfere with each other to produce hourglass shape

axial resolution? other names for axial resolution?

ability to distinguish two structures that are parallel to beam

shorter pulses create better axial resolution

LARD:

longitudinal, axial, radial, range, depth

equation of axial resolution?

AX (mm) = SPL(mm)/2

typical range 0.05-0.5 mm

lateral resolution? other names for lateral resolution?

minimum distance that two structures are separated perpendicular to the sound beam

LATA: lateral, angular, transverse, azimuthal

equation of lateral resolution?

lateral resolution = beam diameter

methods of focusing?

1. lens (external focusing)

2. curved piezoelectric crystal (internal focusing)

3. electronic focusing (dynamic, variable, or multi-focusing)

display modes?

1. A-mode

2. B-mode

3. M-mode

4. Doppler (color, spectral, power)

A-mode?

amplitude mode

X axis: reflector depth

Y axis: amplitude of reflection

b-mode?

brightness mode

X axis: reflector depth

Z axis: brightness of dot measured by reflection amplitude

no Y axis

m-mode?

motion mode: only mode that displays the changing position of the reflectors with respect to time

x axis: time

y axis: reflector depth

types of transducer arrays?

"array" collection of separate PZT elements

1. linear array- elements arranged in a line

i. linear switched or sequential array

ii. linear phased array

2 annular array - elements arranged in a ring

3. convex, curved, or curvilinear array - elements arranged in an arc

i. convex switched or sequential array

ii. convex phased

"swtiched or sequential" in transducer name indicates?

no beam steering and fixed focusing

"phased array" in transducer name indicates?

focus and steering are electronic

electronic slope?

creates beam steering

electronic curvature?

creates beam transmit focusing

beam former?

system in transducer that creates electronic patterns of applying 10 nanosecond delays to the separate elements of transducer - creates electronic focusing and steering

dynamic receive focusing?

aka dynamic aperature

time delays during reception can be applied to electrical signals from transudcer to us system allows focusing at many depths

annular phased array?

ring of elements - provides focusing in all planes at all depths - beam steering mechanical

multidimensional arrays?

2-D arrays creat 3-D or 4-D images

1 1/2 dimensional arrays improve elevational resolution

vector arrays?

combine linear sequential and linear phased technologies - trapezoidal, electronic focusing and steering

mechanical transducer?

1 disc of element, mechanical steering, fixed focus, sector shape image

types of resolution?

1. axial

2. lateral

3. temporal

4. elevational

5. contrast

contrast resolution vs spatial resolution

types of resolution that affect detail

more shades of gray, more bits, better contrast

more pixels, more detail, better spatial resolution

temporal resolution?

ability to accurately locate moving structures - determined by frame rate

frame rate?

20 Hz - 100 Hz

determined by: imaging depth and number of pulses per frame

how can sonographer affect temporal resolution?

1. max imaging depth - shallow

2. # of pulses per scan line (multi-focus systems) - single focus

3. sector size - narrow

4. line density (lines per angle of sector) - low

6 system components of US machine?

1. master synchronizer

2. transducer

3. pulser - beam former

4. receiver - processes electronic signal

5. display

6. storage

high vs low signal to noise ratio?

high signal to noise: high quality image

low signal to noise: low quality image

receiver functions?

1. amplification - gain

2. compensation - TGC

3. compression - dynamic range

4. demodulation

i. rectification - corrects for or eliminates negative voltages

ii. smoothing - evens out signal

5. rejection

output power?

affects brightness by adjusting the strength of sound pulse sent to the body by the transducer

reciever gain?

affects the brightness by changing the amplication of the electronic signals after returning to the receiver?

ALARA?

as low as reasonably achievable

harmonics?

harmonic imaging creates ultrasound scans from sound reflections at twice the transmitted frequency - harmonics created in the tissues, not in the transducer

how does us system convert signals to display image?

electronic signal (analog) from transducer > A to D converter > scan converter (interpret digital information) > D to A converter > display on screen

read magnification vs write magnification?

read: post processing zoom

write: pre processing zoom - improved spatial resolution

fill in interpolation?

preprocessing

improves image detail (spatial resolution) by filling in the missing data especially for deeper parts of the image

spatial compounding?

scan lines are steered b the transducer in different directions, so structures are interrogated by more than one pulse

-phased array transducers only

-frames are averaged, improving signal-to-noise ratio

persistence?

temporal averaging - consolidation of images displays a smoother image and reduces noise

-useful for stationary or slow-moving structures

edge enhancement?

most useful to emphasize the boundary between different tissues - distinguish interfaces between structures with different gray scale characteristics

coded excitation?

a sophisticated method of creating sound pulses carrying individual identity codes - controlled by pulser - US system knows what reflection is associated with each transmitted pulse - improves penetration and contrast resolution

dynamic range?

ratio of the largest to the smallest signal strength that each component processes - dynamic range of the display indicates the number of gray shades

PACS?

picture archiving and communication system

DICOM?

digital imaging and communications in medicine - provides standards or guidelines for medical imaging networks

preprocessing?

manipulating data before storage in scan converter

TGC

log compression

write magnification

fill-in interpolation

persistence (frame averaging)

spatial compounding

post processing?

manipulating the data after it has been stored in the scan converter memory but prior to display

done on frozen images

log compression?

log compression lowers the high level echoes and boosts the low level echoes - image data's dynamic range is reduced - controlled preprocessing by the sonographer

beam divergence?

wide in far field

pulsatile vs phasic?

blood moves with variable velocity

pulsatile : high pressure, high rate

phasic: low pressure, low rate

types of laminar flow?

plug: uniform flow

parabolic flow: flow in center travel faster

types of turbulent flow?

vortex and eddy currents

turbulence may be identified as spectral broadening

reynold's number?

unitless number indicating laminar or turbulent flow

less than 1500 laminar

greater than 200 turbulent

kinetic vs pressure energy?

kinetic: moving object

pressure: potential or stored energy

forms of energy loss as blood flows?

1. friction: conversion of other forms of energy into heat

2. viscosity: thickness of a fluid (pose)

3. inertia: energy is lost when the velocity of a fluid changes

factors that determine resistance to blood flow?

1. radius of lumen (most important)

2. length

3. viscosity of fluid

stenosis and flow through stenosis?

narrowing or irregularity of a lumen

-highest velocity at point of max narrowing (lowest pressure)

-turbulence at exit

hydrostatic pressure?

weight of blood pressing on the vessel from heart level to the point of measurement

laying down patient: hydrostatic pressure is zero when patient is laying down

standing patient: hydrostatic pressure is present especially below heart

explain how venous flow is affected by breathing

inspiration - diaphragm presses into the abdomen -increasing pressure in abdomen forcing *venous return* from arms and abdomen

expiration - diaphragm presses into thorax - increasing pressure in thorax - venous flow in legs increases

doppler shift?

Difference between received and transmitted frequencies

positive: reflected frequency higher than transmitted

negative: reflected frequency lower than transmitted

typically 20 Hz - 20 kHz

what is the doppler equation?

[2 x reflector speed x incident frequency x cos(angle)]/ propagation speed

doppler shift directly related to: reflector speed, frequency of transducer, cosine of angle between flow and the sound beam

doppler shift in inversely related to propagation speed

degree of angle to measure true velocity through doppler?

0 degrees or 180 degrees

< 60 degrees is normally accepted

90 degree angle of incidence will show no flow because cos (90) = 0

continuous wave doppler?

type of spectral doppler where

two crystals used - 1 sending 1 recieving

-advantage: able to measure high velocity accurately (high sensitivity)

-disadvantages: range ambiguity (no damping, narrow bandwith and high Q)

pulsed wave doppler

a type of spectral doppler where

one crystal alternates between sending and receiving

-advantage: sample volume or gate and range resolution/ specificity

-disadvantage: aliasing, errors in measuring velocity

aliasing?

high velocities appear negative

high velocity measurements are inaccurate if the PRF sampling rate is too low in comparison to measured blood velocity

nyquist frequency?

doppler frequency at which aliasing occurs

nyquist limit (kHz) = PRF/2

5 ways to eliminate aliasing?

1. adjust the scale (increase PRF and nyquist limit)

2. select a transducer with a lower frequency (reduces doppler shift for a given velocity)

3. select new view with a shallower sample volume (this increases the PRF and nyquist limit)

4. use continuous wave doppler (no aliasing with CW)

5. baseline shift

color doppler?

color doppler provides information regarding direction of flow - shows average/mean velocities

colors tell us flow direction, 90 degree angle of incidence between sound beam and flow will show no flow

variance mode vs velocity mode with color doppler?

velocity mode: flow towards and away from transducer

variance mode: laminar flow and turbulent flow

doppler packet?

multiple ultrasound pulses are needed to accurately determine RBC velocities by doppler - called packet or ensemble length

advantages of more pulses: greater accuracy of velocity measurement, sensitivity to low flow

disadvantages of more pulses: more time required to acquire information, frame rate and temporal resolution reduced

power doppler?

direction and velocity is not calculated - any doppler shift is colorized without consideration of direction or speed

advantages: increased sensitivity to low flow, not affected by doppler angle (unless angle of incidence 90 degrees), no aliasing

limitations: no measurement of velocity or direction, slower frame rate, susceptible to motion (*flash artifact*)

spectral analysis?

methods used to extract or identify the individual frequencies making up the complex signal

methods of spectral analysis?

fast fourier transform: for pulsed wave doppler and CW doppler

autocorrelation: color flow

bidirectional doppler uses "phase quadrature" distinguishes between positive and negative flow

causes of artifacts?

1. violation of assumptions

2. equipment malfunction or poor design

3. the physics of ultrasound

4. interpreter error (anatomic pitfalls)

5. operator error

reverberation artifact?

multiple echoes appearing on display as a result of "ping ponging" between two reflectors"

parallel to sound beam

comet tail or ring down artifact?

appears as a solid line directed downward, merged reverberation

shadowing artifact?

too much attenuation of a structure results in shadowing under structure

edge shadowing or shadowing by refraction?

refraction at the edge of a circular structure

enhancement?

low attenuation rate of a structure that causes hyperechogenicity behind structure

mirror imaging artifact?

sound may bounce off a strong reflector (mirror) and be redirected - mirror artifact will be located deeper than true reflector

propagation speed errors?

if media through which US travels doesn't propagate at 1540 m/s then the assumed relationship between time and distance is invalid

will appear as step-off, split, or cut

refraction artifact?

sound changes direction striking a second boundary - second copy of the true reflector will appear side by side

side lobes and grating lobes artifact?

extra acoustic energy may be transmitted in directions other than the beams main axis - second copy of the true reflector (artifact appears side by side)

side lobes: mechanical or single crystal transducers create side lobes

grating lobes: arrays create grating lobes - reduced by subdicing and apodization

slice thickness artifact?

occurs when beam has a greater width than the reflector

-true reflector located outside of the idealized imaging plane

slice thickness?

elevational resolution

speckle artifact?

grainy appearance not directly from reflections from tissues created by interference effects - usually in shallow areas of image