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You are here: Home / 08. Ultrasound / Physics and Knobology

Physics and Knobology

July 14, 2011 by CrashMaster

 

 

 

http://www.upei.ca/~vca341/usphysics/img0.html

 

http://courses.washington.edu/radxphys/Lectures05-06/Ultrasound%20-%20Chapter%2016%20-%20Lecture%202%20-%20060302_files/frame.htm

 

http://courses.washington.edu/radxphys/PhysicsCourse.html

 

Physics

T=1/f

Hertz=cycles per second

1 million hertz=1MHz

Medical UTS=1-30 MHz

Audible sound = 20-20000 Hz

Impedance=C/density

 

Wave Length

(insert equation)

 

wave length is the distance between two peaks or valleys in the wave

it is determined by speed divided by frequency

For soft tissue,

use speed of sound as 1540 m/s

change it to mm

1.54 mm/f (MHz) gives the answer in mm

 

air and lung has the lowest impedance, bone has the highest

 

Reflection

Perpendicular Incidence

if materials with two different impedances are next to each other, some of the beams energy will be reflected and some will be transmitted.

Non-Perpendicular Sound Beam Incidence  Can cause refraction, i.e. bending of the sound beam

Rayleigh Scatterers

structures smaller than the wavelength

they increase as frequency increases

 

 

Attenuation in tissue

caused by reflection and scattering  and direct absorbtion

the higher the frequency, the higher the attenuation. almost proportional

 

beam characterisitcs

near field=rough waves

far field=smooth waves

with increasing frequencies, the near field length increases

beam divergence is less at higher frequencies

the diameter of the probe also increases the NFL

 

 

Ring-Down Artifact

wherever small bubbles or partial liquids exist

look like comet tails

 

Mirror Images

echo bounces off the bottom of the image hits an object and bounces back to the bottom

 

Doppler Spectral Mirroring

when gain is set too high or when angle is perpendicular to flow

 

Masses can show enhancement or shadowing

low attenuating masses generate large enhancement i.e. a cyst or fluid filled structure causes enhancement behind it

 

Refraction

at the interface of tissues with different speeds of sound

bone has major refraction problems

fat which is slower than tissue

Edge shadowing of structures like GB is another example

 

if a structure with a slower speed of sound is imaged everything distal to it will look further away

 

wavelength = c/f (just divide 1.54/freq)

 

Impedance=C/density

 

Intensity=mW/cm2

 

Incident Beam= 10 log (I2/I1)

 

Spatial resolution

axial and lateral resolution

Axial Resolution

Longitudinal discrimination the minimum separation of two targets in tissue in a direction parallel to the beam which results in their being imaged as two distinct structures

Relates to pulse length (about 3 wavelengths)

Made better with higher frequency (b/c wavelengths are shorter)

Does not vary with depth

Axial resolution improves with increased damping, increased frequency, increased bandwidth, decreased pulse length

Transducers have a tendency to “ring” after being excited by an electrical impulse, creating an acoustic pulse which has an extended length in

Moreover, shortening the length of an ultrasound pulse while keeping the total energy of the pulse constant, results in a higher peak acoustic intensity. Thus a compromise is reached between the peak pressure to which tissue is exposed and the effective axial resolution of the ultrasound image.

Lateral Resolution

relates to beam width

number of scan lines

wider transducer

higher frequency has a longer near zone and therefore narrower beam as long as area in focal zone

depth dependent

Shadowing of stones is lateral resolution

Near zone length is increased by increased frequency and wider transducer

 

Lateral resolution better with increased frequency and focusing (also by curving transducer)

 

focal zone=where beam is narrowest

fresNel=near

fraunhFer=far

 

focusing decreases bandwidth, improves lateral resolution not axial

focusing increases pulse length which hurts axial resolution

 

increased transducer diameter=increased near zone length=better LATERAL resolution

 

power does not affect it

receiver gain does not affect it

 

convex=wider image in near field and increased resolution at depth

Elevational Resolution

works just like lateral, relates to depth. has a focal zone as well

elevation=across the width of transducer=z thickness=slice thickness=mechanical focus by manufacturer

small cystic structures=elevational resolution

can cause pseudo-sludge b/c back of gb may wider beam than front

 

Temporal Resolution

frame rate

no phantom for this one

to improve temporal resolution decrease scan line density

frame rate=images per second 10-50 is the norm

scan depth is operator control which affects frame rate

decreasing focal zones increases temporal resolution as does increased prf

 

Contrast resolution

resolution of objects with similar reflective properties

contrast resolution-change of gray scale map

 

 

Frequency

all frequencies have identical transit times and sound propagation speeds

Propagation  speed=speed of sound through substance, not user adjustable

Increased frequency=better spatial resolution and poorer penetration

 

increase frequency to see shadows because the stone must be wider than sound beam to see shadows

As you increase the frequency and focus the beam, the beam width narrows; therefore better axial resolution

shortest wavelength=highest frequency

 

As frequency increases, scattering, absorption, and attenuation all increase

 

2.5 Mhz to 5 Mhz probe, wavelength halves

 

Scattering intensity=frequency 4th power

 

If frequency is doubled, absorption is doubled

 

Resonance frequency=voltage frequency and thickness of an element

 

thin elements=high frequency

voltage of pulser determines final frequency

rate determines PRF

Power

increased power=increased penetration, acoustic power, brightness, and voltage

Power=Energy/Time

 

Decreasing Db

half value layer=decrease 3DB, shallower with higher frequency, point at which beam intensity is reduced by half

 

10 Db decrease is 10% of original

3 Db decrease is ½ of original

Propagation Speed

Tissue=1540 m/s or 1.54

Bone=4080

increased density causes decreased propagation speed

molecules oscillate (compressions and rarefactions) to propagate sound

rarefactions=low pressure/density formed during sound propagation

compression=elevated pressure during sound propagation

 

the stiffer the material, the quicker the sound

ONLY MEDIA determines the SPEED of SOUND

 

fat causes axial misrepresentation things look FURTHER AWAY, because fat is slower

Impedance

lung has highest rate of attenuation

fat has slowest propagation  speed of tissue

 

impedance increases if density increases or speed increases and affected by stiffness, unaffected by frequency

 

air reflects all sound, 99.9% reflection coefficient

 

Gel reduces impedance difference between transducer and skin

 

unit of impedance=Rayl

 

Impedance=density(propagation speed)

Z=pc

 

 

Pulsed ultrasound-# of pulses to element per second=Pulse Repetition Frequency (PRF)

thickness of piezoelectric determines frequency

 

 

 

If number of cycles increase but wavelength stays the same, pulse duration is increased???

PRF=# of impulses to transducer/second

 

 

 

Period=time for one cycle

frequency=cycles per second

Period=1/frequency

 

 

milli=10 -3

micro=10 -6

 

 

 

absorption=sound converted to heat

 

 

transmission + reflection coefficient=100%

 

Reflection & Refraction

RBC=rayleigh scatterer

 

refraction=edge shadowing=different propagation  speeds

 

reflection requires a difference in acoustic impedance

 

Specular reflections=renal capsule or diaphragm. LARGE SMOOTH INTERFACE

Specular reflection=crap image from oblique angles

 

Scattering=non-specular reflection-THIS IS WHAT ALLOWS IMAGING IN THE FIRST PLACE

 

interference=summation of waves

 

diffuse reflection=rough surface

 

diffraction=passage through aperture

 

Refraction described by Snell’s law=angle of sound c oblique interface and different speeds. Refraction

 

Attenuation

attenuation in soft tissue=0.5 dB/cm/MHz so as frequency goes up, attenuation goes up

have to double the depth b/c it is roundtrip

 

High attenuation (gallstone)=shadow

Low attenuation (bladder)=enhancement

 

½ power distance (1/2 value thickness) in cm water 380, blood 15, tissue 5, muscle 1, lung 0.05

 

Increased pressure = increased intensity

 

Absorption=sound to heat

 

Normal incidence=perpendicular incidence

 

Huygen’s instructive to deconstructive interference from each sound source

 

air is best reflector

 

 

 

Crystals & Elements

Curie=temperature point at which ceramics go piezo

crystal material=lead zirconate

aperture focusing=# of elements changed

 

Mechanical Sector

Mechanically steered

Mechanically focused

Linear

only one mechanical focus on width of beam

Phased array

Sector image, pointed top

Electronically Steered and focused

Annular array

mechanically steered, electrically focused

Beam is symmetrical about beam axis

Annular arrays are transducer assemblies with circular or ringlike elements, used to focus the beam. Annular arrays must be steered mechanically since they can only be fired in an outward-inward progression due to the rings. Annular arrays reduce section thickness artifacts

 

Side Lobes/Grating Lobes

Dynamic apodization=reduces side lobes makes all energy come from center of elements

subdicing=reduces grating lobes, breaking elements into sub-elements

Matching layer

reduces acoustic mismatch

matching layer should be 1/4 of wavelength

Focusing

dynamic receive focusing holds sound waves with others at same depth return

 

 

 

 

backing material

dampens ringing

dynamic damping-stop crystal from ringing after it rings

Gain

gain-volume knob of stereo

TGC-works on received echoes at depth

rejection-lowers electric noise, rejects low level echoes

the whole pulsed thing is to allow depth calculations

elements only fire about 1% of the time

 

reduce gain if background noise (i.e. not black background)

Processing

if it can be performed on a frozen image, it is POST PROCESSING

frame averaging-compares betweens frames and reduces random noise

tissue harmonics-improved contrast resolution, 2x transmitted frequency

Beam former apodization, beam steering, focusing aperture control?

interpolation-fills in skipped lines with decreased scan line density

 

pulser to beam former to receiver to memory to display

 

typical frame rate=10-50 Hz=10-50 frames per second

 

improved signal to noise=frame averaging

 

if only PRF is increased, frame rate will increase because it will take less time to fire all the pulses to make one frame. If too high, you get range ambiguity. New pulse fired before the first one returns

 

Signal to noise-system sensitivity; greater this ratio, the smaller the signal that can be differentiated

 

Rectification converts negative portion of signal to positive

 

Increasing dynamic range decreases image contrast because more levels to assign colors

 

Gain is at the receiver

 

range equation d=1/2 ct

 

3 DB decreased by 1/2

 

radio frequency to video=demodulater

AKA amplitude or envelope detection

 

transducer to electricity to acoustic pulses

duty factor is only time

increased PRF = increased duty factor

 

threshold is another name for rejection

 

read zoom-uses stored data

write zoom gets new data

 

scan converter-makes 2d image

 

Artifacts

Reverb-closely spaced reflections, like metal fragment

 

Pulse echo imaging

M-Mode

motion

depth of reflections with respect to time

m-mode=time, motion pattern

 

A-mode

amplitude

width of spike=strength of the echo

amplitude/distance (time) used in opthal

 

B-Mode

Brightness

 

Pixels

brightness level limited by bit depth or bits per pixel

numbers of shades of gray=contrast resolution, bits per pixel

 

television is 30 frames per second, 525 lines

 

digital scan converter = image memory storage area  # of pixels in matrix=better spatial resolution

 

8 bit=1 byte=256 shades of gray

 

color needs 24 bis/pixel

 

if image is washed out, check processing equipment

 

 

memory to proportional voltages to brightness on monitor to digital to analog

 

Doppler

beat frequency=reference wave + reflected wave

 

continuous wave doppler-no max velocity

pulsed wave-max velocity

 

with pulse wave doppler, frequency shift > 1/2 PRF you get aliasing

 

to improve sensitivity to slow ???; decrease PRF as slow frequency=low freq shifts can also decrease wall filter and increase the doppler frequency

 

ensemble length=pulses per scan line

 

spectral broadening-fill in of spectral window associated with turbulent flow

 

indeterminate doppler angle-velocity estimate inaccurate

 

max frequency shift at 0° to flow; if angle is 90, no shift detected

 

determine direction, phase quadrature detection

 

 

Continuous wave Doppler

no range

 

flow towards=red=POSITIVE SHIFT

 

increased wall filter=reduced display of low frequency

 

Increased packet size=decreased frame rate, improved signal to noise

 

Fourier Analysis-used to perform spectral analysis for pulsed doppler

 

angle near 90°-you get spectral mirroring

 

aliasing top clipped off seen at bottom–fix by increased PRF, i.e. velocity scale, range, flow rate

 

 

 

doppler shift=difference between transmitted and received frequency

 

change F=2 V cos

smaller the angle, the larger the shift

 

Nyquist limit=aliasing frequency

 

reduce aliasing by increasing angle, lower zero baseline, increased PRF, decrease doppler frequency

 

aliasing=shift >1/2 PRF

 

color doppler uses autocorrelation

 

power doppler

encodes amplitude

does not use phase

angle does not matter

only strength of frequency

 

 

spectral broadening= turbulent flow

color gate=axial length of the sampling volume

 

spectral analysis-determines distribution and magnitude of frequency

 

to better image deep vessels, decrease doppler frequency

 

Image Features and Artifacts

 

partial volume artifact-from slice thickness that is too wide

ringdown=gas bubble

 

Quality Assurance

string phantom-doppler velocity

 

doppler flow phantom-velocity estimation, accuracy of flow directions

 

hydrophone-acoustic output level

 

sensitivity-ability to detect weak echoes

 

Closely spaced targets at various distances from the probe=AXIAL resolution

 

clear anechoic tubes oriented perpendicular =elevational resolution

 

width of point target=lateral resolution

 

adjust to maximum output and gain when testing

 

dead zone=distance from transducer to 1st echo

 

focal point=best lateral resolution

 

SMPTE-evaluates the display

 

Safety

 

1° C max increase in temperature

 

SATA=lowest for pulsed-wave field

 

increased focusing = increased heat

 

SPTA=AIUM statement on mammalian in vivo  100 mW/cm=safe

 

mech index <1=safe = likelihood of cavitation

 

effects from

cavitation

heating

mechanical interactions

acoustic streaming

NOT IONIZATION

 

ALARA-as low as reasonably ACHIEVABLE

 

therm index-max rise in tissue

 

SPPA and SARA-nor applicable to continuous wave

 

hydrophone can measure amplitude

 

duty factor= time actually transmitting

=pulse length*pulses per second

 

mc gapasial units of peak negative pressure

 

bone takes on most heat

 

high frequency/high intensity=increased thermal index

 

acoustic streaming=circular motion of fluids in tissues

 

 

TIB > temp increase in bone

 

SPTA for the eye is the lowest

 

TIC for brain

 

cavitation occurs with high pressure and low frequency

 

 

ULTRASOUND EXPOSURE

(AIUM) Statement on clinical safety: “Diagnostic ultrasound has been in use for more than 40 years. Given its known benefits and recognized efficacy for medical diagnosis, including use during human pregnancy, the American Institute of Ultrasound in Medicine herein addresses the clinical safety of such use: No confirmed biological effects on patients or instrument operators caused by exposures at intensities typical of present diagnostic instruments have ever been reported.

 

First, the acoustic intensity averaged over time (the Spatial Peak Temporal Average intensity, SPTA) is considerably higher in pulsed Doppler mode with many duplex scanners than in most imaging instruments. One survey reports values up to 750 mW/cm2 ISPTA, but some pulsed Doppler systems are known to deliver SPTA intensities as high as 1,000 to 2,000 mW/cm2.

 

Second, the beam must be stationary during a Doppler examination will ‘dwell’ on a target area for a longer period than for imaging, sometimes for a period of minutes. Finally, it is widely felt that of all tissues, those of the fetus are likely to be among the most sensitive to biological effects of ultrasound, and Doppler has begun to play a part in the ultrasound examination of the fetus. Only recently has the U.S.Food and Drug Administration approved the marketing of a single-gate pulsed Doppler duplex system for fetal use, bringing questions to many users’ minds as to whether this modality is indeed safe for clinical use. There are two classes of interaction of ultrasound with tissue that it is relevant to consider.

 

Heating  1°C) are of no consequence. Local temperature rise will increase with the SPTA intensity but will also be affected by physiological factors such as local blood flow.

 

more dangerous phenomenon of transient cavitation is certainly capable of destroying tissue but can only occur at high instantaneous (that is, spatial peak temporal peak, SPTP) intensities.

 

SPTA intensities below 100 mW/cm2

 

Pulsed>Color>Mmode>B Mode

 

 

 

 

Lectures and Stuff

 

 

 

 

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