Ultrasound PhysicsUltrasound PhysicsHave no fearHave no fear

Presentation by Alexis Palley MDPresentation by Alexis Palley MDDepartment of Emergency MedicineDepartment of Emergency Medicine

Cooper University Hospital Cooper University Hospital

“ “How does it do that?” How does it do that?” Lecture Objectives:Lecture Objectives:

Review basic physics vocabularyReview basic physics vocabulary Explain the principles of sound wavesExplain the principles of sound waves Use ultrasound physics to explain how Use ultrasound physics to explain how

images are producedimages are produced Teach how to use these principles to help Teach how to use these principles to help

your diagnostic abilities your diagnostic abilities

BasicsBasics

Sound is energy traveling though matter Sound is energy traveling though matter as a wave as a wave

The wave travels by compressing and The wave travels by compressing and rarefacting matterrarefacting matter

Depending on the matter- the wave will Depending on the matter- the wave will travel at different velocities or directionstravel at different velocities or directions

U/S probes emit and receive the energy as U/S probes emit and receive the energy as waves to form pictures waves to form pictures

Physical PrinciplesPhysical Principles

CycleCycle 1 Cycle = 1 repetitive periodic oscillation1 Cycle = 1 repetitive periodic oscillation

Cycle

FrequencyFrequency

# of cycles per second# of cycles per second Measured in Hertz (Hz)Measured in Hertz (Hz)

-Human Hearing 20 - 20,000 Hz-Human Hearing 20 - 20,000 Hz-Ultrasound > 20,000 Hz-Ultrasound > 20,000 Hz-Diagnostic Ultrasound 2.5 to 10 MHz-Diagnostic Ultrasound 2.5 to 10 MHz

(this is what we use!)(this is what we use!)

frequencyfrequency1 cycle in 1 second = 1Hz1 cycle in 1 second = 1Hz

1 second

= 1 Hertz

High FrequencyHigh Frequency

High frequency (5-10 MHz)High frequency (5-10 MHz)greater resolutiongreater resolutionless penetrationless penetration

Shallow structuresShallow structuresvascular, abscess, t/v gyn, vascular, abscess, t/v gyn, testiculartesticular

Low FrequencyLow Frequency

Low frequency (2-3.5 MHz)Low frequency (2-3.5 MHz)greater penetration greater penetration less resolutionless resolution

Deep structuresDeep structuresAorta, t/a gyn, card, gb, renalAorta, t/a gyn, card, gb, renal

Wavelength Wavelength

The length of one complete cycleThe length of one complete cycle A measurable distanceA measurable distance

WavelengthWavelength

Wavelength

Amplitude Amplitude

The degree of variance from the normThe degree of variance from the norm

Amplitude

Producing an imageProducing an image Probe emits a sound wave pulse-Probe emits a sound wave pulse-

measures the time from emission to return measures the time from emission to return of the echoof the echo

Wave travels by displacing matter, Wave travels by displacing matter, expanding and compressing adjacent expanding and compressing adjacent tissuestissues

It generates an ultrasonic wave that is It generates an ultrasonic wave that is propagated, impeded, reflected, refracted, propagated, impeded, reflected, refracted, or attenuated by the tissues it encounters or attenuated by the tissues it encounters

Producing an imageProducing an image

Important concepts in production of an U/S Important concepts in production of an U/S image:image:

• Propagation velocityPropagation velocity• Acoustic impedanceAcoustic impedance• ReflectionReflection• RefractionRefraction• AttenuationAttenuation

Propagation Velocity Propagation Velocity Sound is energy transmitted through a medium-Sound is energy transmitted through a medium- Each medium has a constant Each medium has a constant

velocity of soundvelocity of sound (c)(c) Tissue’s resistance to compression Tissue’s resistance to compression

density or stiffnessdensity or stiffness Product of frequency Product of frequency (f)(f) and wavelength and wavelength (λ)(λ)

c=fλc=fλ Frequency and Wavelength therefore are Frequency and Wavelength therefore are

directly proportional- if the frequency increases directly proportional- if the frequency increases the wavelength must decrease.the wavelength must decrease.

Propagation VelocityPropagation Velocity

Propagation velocity Propagation velocity Increased by increasing Increased by increasing

stiffnessstiffnessReduced by increasing Reduced by increasing

densitydensity Bone: 4,080 m/secBone: 4,080 m/sec Air: 330 m/secAir: 330 m/sec Soft Tissue Soft Tissue AverageAverage: 1,540 m/sec: 1,540 m/sec

ImpedanceImpedance

Acoustic impedance Acoustic impedance (z) (z) of a material is the of a material is the product of its density and propagation product of its density and propagation velocityvelocity

Z= pcZ= pc Differences in acoustic impedance create Differences in acoustic impedance create

reflective interfaces that echo the u/s reflective interfaces that echo the u/s waves back at the probewaves back at the probe

Impedance mismatch = Impedance mismatch = ΔZΔZ

Acoustic ImpedanceAcoustic Impedance

Homogeneous mediums reflect no soundHomogeneous mediums reflect no sound acoustic interfaces create visual acoustic interfaces create visual

boundaries between different tissues. boundaries between different tissues. Bone/tissue or air/tissue interfaces with Bone/tissue or air/tissue interfaces with

large large ΔzΔz values reflect almost all the values reflect almost all the soundsound

Muscle/fat interfaces with smaller Muscle/fat interfaces with smaller Δz Δz values reflect only part of the energyvalues reflect only part of the energy

RefractionRefraction A change in direction of the sound wave as it A change in direction of the sound wave as it

passes from one tissue to a tissue of higher or passes from one tissue to a tissue of higher or lower sound velocitylower sound velocity

U/S scanners assume that an echo returns U/S scanners assume that an echo returns along a straight pathalong a straight path

Distorts depth reading by the probeDistorts depth reading by the probe Minimize refraction by scanning perpendicular to Minimize refraction by scanning perpendicular to

the interface that is causing the refractionthe interface that is causing the refraction

ReflectionReflection

The production of echoes at reflecting The production of echoes at reflecting interfaces between tissues of differing interfaces between tissues of differing physical properties. physical properties.

Specular - large smooth surfaces Specular - large smooth surfaces Diffuse – small interfaces or nooks and Diffuse – small interfaces or nooks and

cranniescrannies

Specular ReflectionSpecular Reflection

Large smooth interfaces (e.g. diaphragm, Large smooth interfaces (e.g. diaphragm, bladder wall) reflect sound like a mirrorbladder wall) reflect sound like a mirror

Only the echoes returning to the machine Only the echoes returning to the machine are displayedare displayed

Specular reflectors will return echoes to Specular reflectors will return echoes to the machine only if the sound beam is the machine only if the sound beam is perpendicular to the interfaceperpendicular to the interface

Diffuse ReflectorDiffuse Reflector Most echoes that are imaged arise from Most echoes that are imaged arise from

small interfaces within solid organssmall interfaces within solid organs These interfaces may be smaller than the These interfaces may be smaller than the

wavelength of the soundwavelength of the sound The echoes produced scatter in all The echoes produced scatter in all

directionsdirections These echoes form the characteristic These echoes form the characteristic

pattern of solid organs and other tissuespattern of solid organs and other tissues

ReflectorsReflectors

Specular Specular Diffuse Diffuse

AttenuationAttenuation The intensity of sound waves diminish as The intensity of sound waves diminish as

they travel through a mediumthey travel through a medium In ideal systems sound pressure In ideal systems sound pressure

(amplitude) is only reduced by the (amplitude) is only reduced by the spreading of wavesspreading of waves

In real systems some waves are scattered In real systems some waves are scattered and others are absorbed, or reflectedand others are absorbed, or reflected

This decrease in intensity (loss of This decrease in intensity (loss of amplitude) is called attenuation. amplitude) is called attenuation.

The MachineThe Machine

Ultrasound scannersUltrasound scanners Anatomy of a scanner:Anatomy of a scanner:

TransmitterTransmitter TransducerTransducer ReceiverReceiver ProcessorProcessor DisplayDisplay StorageStorage

TransmitterTransmitter a crystal makes energy into sound waves a crystal makes energy into sound waves

and then receives sound waves and and then receives sound waves and converts to energyconverts to energy

This is the Piezoelectric effectThis is the Piezoelectric effect u/s machines use time elapsed with a u/s machines use time elapsed with a

presumed velocity (1,540 m/s) to calculate presumed velocity (1,540 m/s) to calculate depth of tissue interfacedepth of tissue interface

Image accuracy is therefore dependent on Image accuracy is therefore dependent on accuracy of the presumed velocity. accuracy of the presumed velocity.

TransducersTransducers

Continuous mode Continuous mode continuous alternating current continuous alternating current doppler or theraputic u/sdoppler or theraputic u/s 2 crystals –1 talks, 1 listens2 crystals –1 talks, 1 listens

Pulsed modePulsed mode Diagnostic u/s Diagnostic u/s Crystal talks and then listensCrystal talks and then listens

ReceiverReceiver

Sound waves hit and make voltage across Sound waves hit and make voltage across the crystal- the crystal-

The receiver detects and amplifies these The receiver detects and amplifies these voltagesvoltages

Compensates for attenuationCompensates for attenuation

Signal AmplificationSignal Amplification

TGC (time gain TGC (time gain compensation)compensation)

Manual controlManual control Selective enhancement or Selective enhancement or

suppression of sectors of suppression of sectors of the imagethe image

enhance deep and enhance deep and suppress superficialsuppress superficial

*blinders*blinders

GainGain

Manual controlManual control Affects all parts of the Affects all parts of the

image equallyimage equally Seen as a change in Seen as a change in

“brightness” of the images “brightness” of the images on the entire screenon the entire screen

*glasses*glasses

Changing the TGC

Changing the Gain

DisplaysDisplays

B-modeB-mode Real time gray scale, 2DReal time gray scale, 2D Flip book- 15-60 images per secondFlip book- 15-60 images per second

M-modeM-mode Echo amplitude and position of moving targetsEcho amplitude and position of moving targets Valves, vessels, chambersValves, vessels, chambers

““B” ModeB” Mode

““M” ModeM” Mode

Image propertiesImage properties Echogenicity- amount of energy reflected Echogenicity- amount of energy reflected

back from tissue interfaceback from tissue interface

Hyperechoic - greatest intensity - white Hyperechoic - greatest intensity - white Anechoic - no signal - black Anechoic - no signal - black Hypoechoic – Intermediate - shades of grayHypoechoic – Intermediate - shades of gray

Hyperechoic

Hypoechoic

Anechoic

Image ResolutionImage Resolution

Image quality is dependent onImage quality is dependent on Axial ResolutionAxial Resolution Lateral ResolutionLateral Resolution Focal ZoneFocal Zone Probe SelectionProbe Selection Frequency SelectionFrequency Selection Recognition of ArtifactsRecognition of Artifacts

Axial ResolutionAxial Resolution

Ability to differentiate two objects along the Ability to differentiate two objects along the long axis of the ultrasound beamlong axis of the ultrasound beam

Determined by the pulse lengthDetermined by the pulse length• Product of wavelength Product of wavelength λ and # of cycles in pulseλ and # of cycles in pulse• Decreases as frequency Decreases as frequency ff increases increases

Higher frequencies produce better Higher frequencies produce better resolutionresolution

Axial ResolutionAxial Resolution 5 MHz transducer5 MHz transducer

Wavelength 0.308mmWavelength 0.308mm Pulse of 3 cyclesPulse of 3 cycles Pulse length Pulse length

approximately 1mmapproximately 1mm Maximum resolution Maximum resolution

distance of two objects distance of two objects = 1 mm = 1 mm

10 MHz transducer10 MHz transducer Wavelength 0.15mmWavelength 0.15mm Pulse of 3 cyclesPulse of 3 cycles Pulse length Pulse length

approximately 0.5mmapproximately 0.5mm Maximum resolution Maximum resolution

distance of two objects distance of two objects = 0.5mm = 0.5mm

Axial ResolutionAxial Resolution

body

screen

Lateral ResolutionLateral Resolution

The ultrasound beam is made up of The ultrasound beam is made up of multiple individual beams multiple individual beams

The individual beams are fused to appear The individual beams are fused to appear as one beamas one beam

The distances between the single beams The distances between the single beams determines the lateral resolutiondetermines the lateral resolution

Lateral resolutionLateral resolution

Ability to differentiate objects along an axis Ability to differentiate objects along an axis perpendicular to the ultrasound beamperpendicular to the ultrasound beam

Dependent on the width of the ultrasound Dependent on the width of the ultrasound beam, which can be controlled by focusing beam, which can be controlled by focusing the beamthe beam

Dependent on the distance between the Dependent on the distance between the objectsobjects

Lateral ResolutionLateral Resolution

body

screen

Focal ZoneFocal Zone Objects within the focal zoneObjects within the focal zone Objects outside of focal zoneObjects outside of focal zone

Focal zone Focal zone

Probe optionsProbe options Linear ArrayLinear Array Curved ArrayCurved Array

Ultrasound ArtifactsUltrasound Artifacts

Can be falsely interpreted as real Can be falsely interpreted as real pathologypathology

May obscure pathologyMay obscure pathology Important to understand and appreciateImportant to understand and appreciate

Ultrasound ArtifactsUltrasound Artifacts Acoustic enhancementAcoustic enhancement Acoustic shadowingAcoustic shadowing Lateral cystic shadowing (edge artifact)Lateral cystic shadowing (edge artifact) Wide beam artifactWide beam artifact Side lobe artifactSide lobe artifact Reverberation artifactReverberation artifact Gain artifactGain artifact Contact artifactContact artifact

Acoustic EnhancementAcoustic Enhancement

OppositeOpposite of acoustic shadowing of acoustic shadowing Better ultrasound transmission allows Better ultrasound transmission allows

enhancement of the ultrasound signal enhancement of the ultrasound signal distal to that regiondistal to that region

Acoustic EnhancementAcoustic Enhancement

Acoustic ShadowingAcoustic Shadowing

Occurs distal to any highly reflective or Occurs distal to any highly reflective or highly attenuating surfacehighly attenuating surface

Important diagnostic clue seen in a large Important diagnostic clue seen in a large number of medical conditionsnumber of medical conditions Biliary stonesBiliary stones Renal stonesRenal stones Tissue calcificationsTissue calcifications

Acoustic ShadowingAcoustic Shadowing

Shadow may be more prominent than the Shadow may be more prominent than the object causing itobject causing it

Failure to visualize the source of a shadow Failure to visualize the source of a shadow is usually caused by the object being is usually caused by the object being outside the plane of the ultrasound beamoutside the plane of the ultrasound beam

Acoustic ShadowingAcoustic Shadowing

Acoustic ShadowingAcoustic Shadowing

Lateral Cystic ShadowingLateral Cystic Shadowing A type of refraction artifactA type of refraction artifact Can be falsely interpreted as an acoustic Can be falsely interpreted as an acoustic

shadow (similar to gallstone)shadow (similar to gallstone)

X

Lateral Cystic ShadowingLateral Cystic Shadowing

Beam-Width ArtifactBeam-Width Artifact

Gas bubbles in the duodenum can Gas bubbles in the duodenum can simulate a gall stonesimulate a gall stone

Does notDoes not assume a dependent posture assume a dependent posture Do notDo not conform precisely to the walls of conform precisely to the walls of

the gallbladderthe gallbladder

Beam-Width ArtifactBeam-Width Artifact

Beam-width artifact Gas in the duodenum simulating stones

Side Lobe ArtifactSide Lobe Artifact

More than one ultrasound beam is More than one ultrasound beam is generated at the transducer headgenerated at the transducer head

The beams other than the central axis The beams other than the central axis beam are referred to as beam are referred to as side lobesside lobes

Side lobes are of low intensitySide lobes are of low intensity

Side Lobe ArtifactSide Lobe Artifact Occasionally cause Occasionally cause

artifactsartifacts The artifact by be The artifact by be

obviated by obviated by alternating the angle alternating the angle of the transducer of the transducer headhead

Side Lobe ArtifactSide Lobe Artifact

Reverberation ArtifactsReverberation Artifacts

Several typesSeveral types Caused by the echo bouncing back and Caused by the echo bouncing back and

forth between two or more highly reflective forth between two or more highly reflective surfacessurfaces

Reverberation ArtifactsReverberation Artifacts

On the monitor parallel bands of On the monitor parallel bands of reverberation echoesreverberation echoes are seen are seen

This causes a This causes a “comet-tail” “comet-tail” patternpattern Common reflective layersCommon reflective layers

Abdominal wallAbdominal wall Foreign bodiesForeign bodies GasGas

Reverberation ArtifactsReverberation Artifacts

Reverberation ArtifactsReverberation Artifacts

Gain ArtifactGain Artifact

Contact artifactContact artifact

Caused by poor probe-Caused by poor probe-patient interfacepatient interface

Take homes!Take homes!

reviewreview

u/s uses waves echoed off reflective u/s uses waves echoed off reflective interfaces to display the structures of the interfaces to display the structures of the body-body-

works by “talking and listening” works by “talking and listening” Must be careful when interpreting imagesMust be careful when interpreting images We have more control than we thought! We have more control than we thought!

Think before you ultrasound!Think before you ultrasound!

Think before you ultrasound!Think before you ultrasound! Choose the right frequency probe – Choose the right frequency probe –

frequency = shallow & detailedfrequency = shallow & detailed Hold the probe perpendicular to the organ wall Hold the probe perpendicular to the organ wall

being studiedbeing studied Adjust the depth, tgc and gain to help your Adjust the depth, tgc and gain to help your

imageimage Artifact may be affecting your image Artifact may be affecting your image Use knowledge of physical properties of tissues Use knowledge of physical properties of tissues

to help with positioning- ie. use bladder for to help with positioning- ie. use bladder for acoustic window to ta u/s.acoustic window to ta u/s.

Thank youThank you

Any questions?Any questions?