resident physics lectures ultrasound basics principles george david, m.s. associate professor of...

Resident Physics Lectures

Ultrasound Ultrasound Basics Basics PrinciplesPrinciples

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

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

US Transducer Operation

alternating voltage (AC) applied to piezoelectric element

Causesalternating dimensional changesalternating pressure changes

pressure propagates as sound wave

Ultrasound Basics

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

AcmeUltra-Sound

Co.

I’m a scanner, Jim,

not a magician.

What does your scanner know about echoed sound?

How loud is the echo?

inferred from intensity of electrical pulse from transducer

What does your scanner know about echoed sound?

What was the time delay between sound broadcast and

the echo?

What else does your scanner know about echoed sound?

The sound’s pitch or frequency

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)

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)

Dot Placement on ImageDot position ideally

indicates source of echoscanner has no way of

knowing exact locationInfers location from echo

?

Dot Placement on Image

Scanner aims sound when transmitting

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

ain’t necessarily so

?

Positioning DotDot positioned along assumed linePosition on assumed line calculated based

uponspeed of soundtime delay between sound transmission & echo

?

Distance of Echo from TransducerTime delay accurately measured by scanner

distance = time delay X speed of sound

distance

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

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

Gray Shade of Echo

Ultrasound is gray shade modality

Gray shade should indicate echogeneity of object

? ?

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

Based upon intensity (volume, loudness) of echo

? ?

Gray Shade

Loud echo = bright dotSoft echo = dim dot

Complication

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

Gray Shade of Echo

Correction needed to compensate for sound attenuation with distance

Otherwise dots close to transducer would be brighter

Echo’s Gray Shade

Gray Shade determined byMeasured echo strength

accurateCalculated attenuation

Charles LaneWho am I?

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)

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

How Do We Move the Beam?

ElectronicallyPhased Arrays

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.”

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”

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!

IntensityIntensity of Sound Beam

intensity = power / cross sectional area

Sound Wave VariationFreeze timeMeasure some acoustic variable as a

function of position

Position

AcousticVariableValue

PressureDensityTemperature

MOREMake multiple measurements of an

acoustic variable an instant apartResults would look the same but appear

to move in space

1

2

MORETrack acoustic

variable at one position over time

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

Transverse WavesParticle moves perpendicular to wave

travelWater ripple

surface height varies with timepeak height moves outward

water does not move outward

Compression (Longitudinal) Waves

Particle motion parallel to direction of wave travel

1

2

1

2

Wave Travel

Motion ofIndividual Coil

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.

Sound WavesInformation may be encoded in wave energy

radioTVultrasoundaudible sound

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

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

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

Period

as frequency increases, period decreases

if frequency in Hz, period in seconds/cycle

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

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

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

Sound Period & Frequency are

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

Who am I?

Burt Mustin

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”

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

Wavelength Unitslength per cycle

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

millimeter for clinical ultrasound

Wavelength Equation

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

As frequency increases, wavelength decreasesbecause speed is constant

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

Wavelength is a function of both the

sound source and the medium!

Who am I?

John Fiedler

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)

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

Pulsed Sound Example

ringing telephoneringing tone

switched on & offPhone rings with a

particular pitch sound frequency

sound silence sound

Parameters

frequencyperiodwavelengthpropagation

speed

• pulse repetition frequency

• pulse repetition period

• pulse duration• duty factor• spatial pulse

length• cycles per pulse

Sound Pulse

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

Pulse Repetition Periodtime from beginning of one pulse until

beginning of nexttime between corresponding points of

adjacent pulses

Pulse Repetition Period

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

Higher FrequencySame PulseRepetition Frequency

Pulsed SoundPulse repetition frequency & period

independent sound frequency & period

Same FrequencyHigher PulseRepetition Frequency

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

Pulse Durationunits

time per pulse (time/pulse)equation

pulse duration = Period X # cycles per pulse

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

Pulse Duration Period

Pulse Duration

Longer Pulse Duration

Shorter Pulse Duration

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

Pulse Duration

Pulse duration is a controlled by

the sound source, whatever

that means.

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

Spatial Pulse Lengthdistance in space traveled by

ultrasound during one pulse

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

Spatial Pulse Length

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)

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

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

Why is Spatial Pulse Length Important

Spat. Pulse Length = # cycles per pulse X wavelength

Wavelength = Speed / Frequency

Spatial pulse length determines axial resolution

Acoustic ImpedanceDefinitionAcoustic Impedance = Density X Prop.

Speed

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

increases with higherDensityStiffnesspropagation speed

independent of frequency

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

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