resident physics lectures ultrasound basics principles george david, m.s. associate professor of...
TRANSCRIPT
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Resident Physics Lectures
Ultrasound Ultrasound Basics Basics PrinciplesPrinciples
George David, M.S.Associate Professor of Radiology
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Ultrasound TransducerActs as both speaker & microphone
Emits very short sound pulse Listens a very long time for returning echoes
Can only do one at a time
Speakertransmits sound pulses
Microphonereceives echoes
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Piezoelectric PrincipleVoltage generated when certain
materials are deformed by pressureReverse also true!
Some materials change dimensions when voltage applied dimensional change causes pressure
changewhen voltage polarity reversed, so is
dimensional change V
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US Transducer Operation
alternating voltage (AC) applied to piezoelectric element
Causesalternating dimensional changesalternating pressure changes
pressure propagates as sound wave
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Ultrasound Basics
What does your scanner know about the sound echoes it hears?
AcmeUltra-Sound
Co.
I’m a scanner, Jim,
not a magician.
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What does your scanner know about echoed sound?
How loud is the echo?
inferred from intensity of electrical pulse from transducer
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What does your scanner know about echoed sound?
What was the time delay between sound broadcast and
the echo?
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What else does your scanner know about echoed sound?
The sound’s pitch or frequency
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What Does Your Scanner Assume about Echoes(or how the scanner can lie to you)
Sound travels at 1540 m/s everywhere in bodyaverage speed of sound in soft tissue
Sound travels in straight lines in direction transmitted
Sound attenuated equally by everything in body (0.5 dB/cm/MHz, soft tissue average)
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Luckily These Are Close Enough to Truth To Give Us Images
Sound travels at 1540 m/s everywhere in bodyaverage speed of sound in soft tissue
Sound travels in straight lines in direction transmitted
Sound attenuated equally by everything in body (0.5 dB/cm/MHz, soft tissue average)
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Dot Placement on ImageDot position ideally
indicates source of echoscanner has no way of
knowing exact locationInfers location from echo
?
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Dot Placement on Image
Scanner aims sound when transmitting
echo assumed to originate from direction of scanner’s sound transmission
ain’t necessarily so
?
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Positioning DotDot positioned along assumed linePosition on assumed line calculated based
uponspeed of soundtime delay between sound transmission & echo
?
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Distance of Echo from TransducerTime delay accurately measured by scanner
distance = time delay X speed of sound
distance
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What is the Speed of Sound?scanner assumes speed of sound is that of soft
tissue1.54 mm/sec1540 m/sec13 usec required for echo object 1 cm from
transducer (2 cm round trip)
distance = time delay X speed of sound
1 cm13 sec
Handy rule
of thumb
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So the scanner assumes the wrong speed?
Sometimes
?
soft tissue ==> 1.54 mm / sec
fat ==> 1.44 mm / sec
brain ==> 1.51 mm / sec
liver, kidney ==> 1.56 mm / sec
muscle ==> 1.57 mm / sec
•Luckily, the speed of sound is almost the same for most body parts
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Gray Shade of Echo
Ultrasound is gray shade modality
Gray shade should indicate echogeneity of object
? ?
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How does scanner know what gray shade to assign an echo?
Based upon intensity (volume, loudness) of echo
? ?
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Gray Shade
Loud echo = bright dotSoft echo = dim dot
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Complication
Deep echoes are softer (lower volume) than surface echoes.
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Gray Shade of Echo
Correction needed to compensate for sound attenuation with distance
Otherwise dots close to transducer would be brighter
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Echo’s Gray Shade
Gray Shade determined byMeasured echo strength
accurateCalculated attenuation
Charles LaneWho am I?
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Attenuation Correctionscanner assumes
entire body has attenuation of soft tissueactual attenuation
varies widely in body
• Fat 0.6
• Brain 0.6
• Liver 0.5
• Kidney 0.9
• Muscle 1.0
• Heart 1.1
Tissue Attenuation Coefficient (dB / cm / MHz)
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Ultrasound DisplayOne sound pulse
producesone image scan line
one series of gray shade dots in a line
Multiple pulsestwo dimensional image
obtained by moving direction in which sound transmitted
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How Do We Move the Beam?
ElectronicallyPhased Arrays
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Sound Wave Definition?Sound is a WaveWaveWaveWave is a propagating
(traveling) variation in a “wave wave variablevariable”
“An elephant is big, gray, and looks like an elephant.”
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Sound Wave Variable
Examples pressure (force / area) density (mass / volume) temperature
Also called acoustic variableacoustic variable
Sound is a propagating (moving) variation in a “wave variablewave variable”
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Energy & PowerPower
rate of energy useUnits: watts or milliwatts
Energy = Power X TimeUnits: kilowatt-hours
ElectricBill
300 KW-hr.
Electricity billed in energy!
Light Bulbs rated in power!
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IntensityIntensity of Sound Beam
intensity = power / cross sectional area
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Sound Wave VariationFreeze timeMeasure some acoustic variable as a
function of position
Position
AcousticVariableValue
PressureDensityTemperature
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MOREMake multiple measurements of an
acoustic variable an instant apartResults would look the same but appear
to move in space
1
2
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MORETrack acoustic
variable at one position over time
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Sound WavesWaves transmit energyWaves do not transmit matter“Crowd wave” at sports event
people’s elevation varies with timevariation in elevation moves around stadium
people do not move around stadium
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Transverse WavesParticle moves perpendicular to wave
travelWater ripple
surface height varies with timepeak height moves outward
water does not move outward
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Compression (Longitudinal) Waves
Particle motion parallel to direction of wave travel
1
2
1
2
Wave Travel
Motion ofIndividual Coil
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MediumMaterial through which wave movesMedium not required for all wave types
no medium required for electromagnetic waves radio x-rays infrared ultraviolet
medium is required for sound sound does not travel through vacuum
Talk louder! I can’t hear
you.
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Sound WavesInformation may be encoded in wave energy
radioTVultrasoundaudible sound
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Sound Frequency# of complete variations (cycles) of an
acoustic variable per unit time
Unitscycles per second1 HzHz = 1 cycle per second1 kHzkHz = 1000 cycles per second1 MHzMHz = 1,000,000 cycles per second
Human hearing range 20 - 20,000 Hz
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Sound Frequency
Ultrasound definition> 20,000 Hznot audible to humans
dog whistles are in this range
Clinical ultrasound frequency range1 - 10 MHz
1,000,000 - 10,000,000 Hz
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Periodtime between a point in one
cycle & the same point in the next cycletime of single cycle
Unitstime per cycle (sometimes
expressed only as time; cycle implied)
period
Magnitude of acoustic
variable
time
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Period
as frequency increases, period decreases
if frequency in Hz, period in seconds/cycle
1Period = ------------------- Frequency
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Period
if frequency in kHz, period in msec/cycleif frequency in MHz, period in sec/cycle
1 kHz frequency ==> 1 msec period1 MHz frequency ==> 1 sec period
Period = 1 / Frequency
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Reciprocal Units
Frequency Units
Period Units
Hz (cycles/sec) seconds/cycle
kHz (thousands of cycles/sec)
msec/cycle
MHz (millions of cycles/sec)
sec/cycle
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Sound Period & Frequency are
determined only by the sound source. They are independent of medium.
Who am I?
Burt Mustin
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Propagation SpeedSpeed only a function of mediumSpeed virtually constant with respect to
frequency over clinical rangeSpeed depends on medium’s
Density (mass per unit volume) more dense ==> lower speed
Stiffness (or bulk modulus; opposite of elasticity or compressibility) more stiffness ==> higher speed
“same letter, same effect”
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Wavelengthdistance in space over which single
cycle occurs OR
distance between a given point in a cycle & corresponding point in next cycle
imagine freezing time, measuring between corresponding points in space between adjacent cycles
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Wavelength Unitslength per cycle
sometimes just length; cycle impliedusually in millimeters or fractions of a
millimeter for clinical ultrasound
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Wavelength Equation
Speed = Wavelength X Frequency [ c = X (dist./time) (dist./cycle) (cycles/time)
As frequency increases, wavelength decreasesbecause speed is constant
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WavelengthSpeed = Wavelength X Frequency
c = X (dist./time) (dist./cycle) (cycles/time)
mm/sec mm/cycle MHz
Calculate Wavelength for 5 MHz sound in soft tissue
Wavelength = 1.54 mm/sec / 5 MHz
Wavelength = 1.54 / 5 = 0.31 mm / cycle
5 MHz = 5,000,000 cycles / sec = 5 cycles / sec
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Wavelength is a function of both the
sound source and the medium!
Who am I?
John Fiedler
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Pulsed SoundFor imaging ultrasound, sound is
Not continuousPulsed on & off
OnOn Cycle (speak)Transducer produces short duration
soundOffOff Cycle (listen)
Transducer receives echoesVery long duration
ON OFF ON OFF
(not to scale)
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Pulse CycleConsists of
short sound transmissionlong silence period or dead time
echoes received during silence same transducer used for
transmitting soundreceiving echoes
sound silence sound
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Pulsed Sound Example
ringing telephoneringing tone
switched on & offPhone rings with a
particular pitch sound frequency
sound silence sound
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Parameters
frequencyperiodwavelengthpropagation
speed
• pulse repetition frequency
• pulse repetition period
• pulse duration• duty factor• spatial pulse
length• cycles per pulse
Sound Pulse
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Pulse Repetition Frequency
# of sound pulses per unit time# of times ultrasound beam turned on
& off per unit timeindependent of sound frequency
determined by sourceclinical range (typical values)
1 - 10 KHz
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Pulse Repetition Periodtime from beginning of one pulse until
beginning of nexttime between corresponding points of
adjacent pulses
Pulse Repetition Period
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Pulse Repetition PeriodPulse repetition period is reciprocal
of pulse repetition frequency
as pulse repetition frequency increases, pulse repetition period decreases
units time per pulse cycle (sometimes simplified to just time)
pulse repetition period & frequency determined by source
PRF = 1 / PRP
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Higher FrequencySame PulseRepetition Frequency
Pulsed SoundPulse repetition frequency & period
independent sound frequency & period
Same FrequencyHigher PulseRepetition Frequency
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Pulse DurationLength of time for each sound
pulseone pulse cyclepulse cycle =
one sound pulse and one period of silence
Pulse duration independent of duration of silence
Pulse Duration
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Pulse Durationunits
time per pulse (time/pulse)equation
pulse duration = Period X # cycles per pulse
(time/pulse) (cycles/pulse) (time/cycle)
Pulse Duration Period
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Pulse Duration
Longer Pulse Duration
Shorter Pulse Duration
Same frequency; pulse repetition frequency,period, & pulse repetition period
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Pulse Duration
Pulse duration is a controlled by
the sound source, whatever
that means.
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Duty FactorFraction of time sound generatedDetermined by sourceUnits
none (unitless)Equations
Duty Factor = Pulse Duration / Pulse Repetition Period
Duty Factor = Pulse Duration X Pulse Repetition Freq.
Pulse Duration
Pulse Repetition Period
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Spatial Pulse Lengthdistance in space traveled by
ultrasound during one pulse
HEYH.......E.......Y
Spatial Pulse Length
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Spatial Pulse Length
depends on source & mediumas wavelength increases, spatial pulse
length increases
Spat. Pulse Length = # cycles per pulse X wavelength
(dist. / pulse) (cycles / pulse) (dist. / cycle)
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WavelengthCalculate SPL for 5 MHz sound in soft tissue, 5 cycles per pulse
(Wavelength=0.31 mm/cycle)
SPL = 0.31 mm / cycle X 5 cycles / pulse = 1.55 mm / pulse
Spat. Pulse Length = # cycles per pulse X wavelength
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Spatial Pulse Length
as # cycles per pulse increases, spatial pulse length increases
as frequency increases, wavelength decreases & spatial pulse length decreasesspeed stays constant
Spat. Pulse Length = # cycles per pulse X wavelength
Wavelength = Speed / Frequency
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Why is Spatial Pulse Length Important
Spat. Pulse Length = # cycles per pulse X wavelength
Wavelength = Speed / Frequency
Spatial pulse length determines axial resolution
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Acoustic ImpedanceDefinitionAcoustic Impedance = Density X Prop.
Speed
(rayls) (kg/m3) (m/sec)
increases with higherDensityStiffnesspropagation speed
independent of frequency
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Acoustic Impedance of Soft Tissue
Density: 1000 kg/m3
Propagation speed:1540 m/sec
Acoustic Impedance = Density X Prop.
Speed
(rayls) (kg/m3) (m/sec)
1000 kg/m3 X 1540 m/sec = 1,540,000 rayls
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Why is Acoustic Impedance Important?
DefinitionAcoustic Impedance = Density X Prop.
Speed
(rayls) (kg/m3) (m/sec)
Differences in acoustic impedance determine fraction of intensity echoed at an interface