Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of...

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Resident Physics Lectures Ultrasound Ultrasound Basics Basics Principles Principles George David, M.S. Associate Professor of Radiology

Transcript of Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of...

Page 1: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Resident Physics Lectures

Ultrasound Ultrasound Basics Basics PrinciplesPrinciples

George David, M.S.Associate Professor of Radiology

Page 2: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 3: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 4: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

US Transducer Operation

alternating voltage (AC) applied to piezoelectric element

Causesalternating dimensional changesalternating pressure changes

pressure propagates as sound wave

Page 5: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Ultrasound Basics

What does your scanner know about the sound echoes it hears?

AcmeUltra-Sound

Co.

I’m a scanner, Jim,

not a magician.

Page 6: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

What does your scanner know about echoed sound?

How loud is the echo?

inferred from intensity of electrical pulse from transducer

Page 7: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

What does your scanner know about echoed sound?

What was the time delay between sound broadcast and

the echo?

Page 8: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

What else does your scanner know about echoed sound?

The sound’s pitch or frequency

Page 9: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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)

Page 10: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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)

Page 11: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Dot Placement on ImageDot position ideally

indicates source of echoscanner has no way of

knowing exact locationInfers location from echo

?

Page 12: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Dot Placement on Image

Scanner aims sound when transmitting

echo assumed to originate from direction of scanner’s sound transmission

ain’t necessarily so

?

Page 13: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Positioning DotDot positioned along assumed linePosition on assumed line calculated based

uponspeed of soundtime delay between sound transmission & echo

?

Page 14: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Distance of Echo from TransducerTime delay accurately measured by scanner

distance = time delay X speed of sound

distance

Page 15: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 16: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 17: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Gray Shade of Echo

Ultrasound is gray shade modality

Gray shade should indicate echogeneity of object

? ?

Page 18: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

How does scanner know what gray shade to assign an echo?

Based upon intensity (volume, loudness) of echo

? ?

Page 19: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Gray Shade

Loud echo = bright dotSoft echo = dim dot

Page 20: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Complication

Deep echoes are softer (lower volume) than surface echoes.

Page 21: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Gray Shade of Echo

Correction needed to compensate for sound attenuation with distance

Otherwise dots close to transducer would be brighter

Page 22: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Echo’s Gray Shade

Gray Shade determined byMeasured echo strength

accurateCalculated attenuation

Charles LaneWho am I?

Page 23: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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)

Page 24: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 25: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

How Do We Move the Beam?

ElectronicallyPhased Arrays

Page 26: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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.”

Page 27: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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”

Page 28: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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!

Page 29: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

IntensityIntensity of Sound Beam

intensity = power / cross sectional area

Page 30: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Sound Wave VariationFreeze timeMeasure some acoustic variable as a

function of position

Position

AcousticVariableValue

PressureDensityTemperature

Page 31: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

MOREMake multiple measurements of an

acoustic variable an instant apartResults would look the same but appear

to move in space

1

2

Page 32: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

MORETrack acoustic

variable at one position over time

Page 33: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 34: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Transverse WavesParticle moves perpendicular to wave

travelWater ripple

surface height varies with timepeak height moves outward

water does not move outward

Page 35: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Compression (Longitudinal) Waves

Particle motion parallel to direction of wave travel

1

2

1

2

Wave Travel

Motion ofIndividual Coil

Page 36: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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.

Page 37: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Sound WavesInformation may be encoded in wave energy

radioTVultrasoundaudible sound

Page 38: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 39: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 40: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 41: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Period

as frequency increases, period decreases

if frequency in Hz, period in seconds/cycle

1Period = ------------------- Frequency

Page 42: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 43: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 44: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Sound Period & Frequency are

determined only by the sound source. They are independent of medium.

Who am I?

Burt Mustin

Page 45: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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”

Page 46: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 47: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Wavelength Unitslength per cycle

sometimes just length; cycle impliedusually in millimeters or fractions of a

millimeter for clinical ultrasound

Page 48: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Wavelength Equation

Speed = Wavelength X Frequency [ c = X (dist./time) (dist./cycle) (cycles/time)

As frequency increases, wavelength decreasesbecause speed is constant

Page 49: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 50: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Wavelength is a function of both the

sound source and the medium!

Who am I?

John Fiedler

Page 51: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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)

Page 52: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 53: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Pulsed Sound Example

ringing telephoneringing tone

switched on & offPhone rings with a

particular pitch sound frequency

sound silence sound

Page 54: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Parameters

frequencyperiodwavelengthpropagation

speed

• pulse repetition frequency

• pulse repetition period

• pulse duration• duty factor• spatial pulse

length• cycles per pulse

Sound Pulse

Page 55: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 56: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Pulse Repetition Periodtime from beginning of one pulse until

beginning of nexttime between corresponding points of

adjacent pulses

Pulse Repetition Period

Page 57: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 58: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Higher FrequencySame PulseRepetition Frequency

Pulsed SoundPulse repetition frequency & period

independent sound frequency & period

Same FrequencyHigher PulseRepetition Frequency

Page 59: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 60: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Pulse Durationunits

time per pulse (time/pulse)equation

pulse duration = Period X # cycles per pulse

(time/pulse) (cycles/pulse) (time/cycle)

Pulse Duration Period

Page 61: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Pulse Duration

Longer Pulse Duration

Shorter Pulse Duration

Same frequency; pulse repetition frequency,period, & pulse repetition period

Page 62: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Pulse Duration

Pulse duration is a controlled by

the sound source, whatever

that means.

Page 63: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 64: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Spatial Pulse Lengthdistance in space traveled by

ultrasound during one pulse

HEYH.......E.......Y

Spatial Pulse Length

Page 65: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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)

Page 66: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 67: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 68: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Why is Spatial Pulse Length Important

Spat. Pulse Length = # cycles per pulse X wavelength

Wavelength = Speed / Frequency

Spatial pulse length determines axial resolution

Page 69: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

Acoustic ImpedanceDefinitionAcoustic Impedance = Density X Prop.

Speed

(rayls) (kg/m3) (m/sec)

increases with higherDensityStiffnesspropagation speed

independent of frequency

Page 70: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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

Page 71: Resident Physics Lectures Ultrasound Basics Principles George David, M.S. Associate Professor of Radiology.

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