Post on 16-May-2023
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
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!)
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
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
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.
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
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
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
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
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
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
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 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
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)
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
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
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
Contact artifactContact artifact
Caused by poor probe-Caused by poor probe-patient interfacepatient interface
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! 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.