Introduction, Physics, and Introduction, Physics, and “Knobology”“Knobology”

Keith Boniface, MD, RDMSKeith Boniface, MD, RDMSDepartment of Emergency MedicineDepartment of Emergency Medicine

George Washington University Medical George Washington University Medical CenterCenter

Introduction to Emergency Ultrasound

ObjectivesObjectives

Understand the reasons for ED ultrasoundUnderstand the reasons for ED ultrasound Learn the primary and other ED Learn the primary and other ED

applications for ultrasound (US)applications for ultrasound (US) Discuss the physics behind ultrasound and Discuss the physics behind ultrasound and

the relevance to obtaining optimal imagesthe relevance to obtaining optimal images Understand the concept of image Understand the concept of image

orientation and probe movementsorientation and probe movements

ED U/S vs. RadiologyED U/S vs. Radiology

ED Ultrasound is a limited study that ED Ultrasound is a limited study that seeks to answer highly focused questions seeks to answer highly focused questions with yes/no answers:with yes/no answers:– Is there free fluid in the abdomen, pleural Is there free fluid in the abdomen, pleural

space, or pericardium?space, or pericardium?– Is there an AAA?Is there an AAA?– Is there an IUP?Is there an IUP?– Is there a DVT?Is there a DVT?– Is the heart contracting? Normally or Is the heart contracting? Normally or

abnormally?abnormally?

What are ED docs using U/S What are ED docs using U/S for?for?

ACEP’s “primary” core 7ACEP’s “primary” core 7– FASTFAST– AortaAorta– PelvicPelvic– RUQRUQ– CardiacCardiac– RenalRenal– ProceduralProcedural

What are your needs as a department?What are your needs as a department?

Where are we going to Where are we going to focus?focus?

Exams to rapidly ID critically ill patients:Exams to rapidly ID critically ill patients:– FAST, AAA, Cardiac in PEA or hypotensionFAST, AAA, Cardiac in PEA or hypotension

Exams to improve patient flow in the ED:Exams to improve patient flow in the ED:– DVT, Pelvic sono in early pregnancyDVT, Pelvic sono in early pregnancy

Exams to help us with procedures:Exams to help us with procedures:– Central lines, abscesses, ankle taps, LPsCentral lines, abscesses, ankle taps, LPs

PhysicsPhysics

What is ultrasound?What is ultrasound?

Mechanical longitudinal waveMechanical longitudinal wave Propagates through a medium, Propagates through a medium,

interacting with the substance it interacting with the substance it passes throughpasses through

Human hearing 20 Hz - 20,000 Hz (20 Human hearing 20 Hz - 20,000 Hz (20 KHz or 0.02 MHz)KHz or 0.02 MHz)

Ultrasound >20 KHz (0.02 MHz)Ultrasound >20 KHz (0.02 MHz) Diagnostic ultrasound 2.5 MHz-10 MHzDiagnostic ultrasound 2.5 MHz-10 MHz

Wave CharacteristicsWave CharacteristicsPeriod

Amplitude

Wave CharacteristicsWave Characteristics

Period Period FrequencyFrequency WavelengthWavelength Velocity (propagation speed)Velocity (propagation speed) Amplitude/IntensityAmplitude/Intensity

PeriodPeriod

Seconds per Cycle (units=μsec)Seconds per Cycle (units=μsec) Function of the wave source Function of the wave source

(transducer)(transducer)

FrequencyFrequency

Cycles per second (= Hertz, Hz)Cycles per second (= Hertz, Hz) Function of source (transducer)Function of source (transducer) Inverse of periodInverse of period Major factor in determining depth of Major factor in determining depth of

beam penetrationbeam penetration– increase frequency, decrease penetrationincrease frequency, decrease penetration– decrease frequency, increase penetrationdecrease frequency, increase penetration

WavelengthWavelength

Length for complete cycle (= mm)Length for complete cycle (= mm) Function of source and mediumFunction of source and medium Wavelength = velocity/frequencyWavelength = velocity/frequency As frequency increases, wavelength As frequency increases, wavelength

decreases and vice versadecreases and vice versa Major determinant of image resolutionMajor determinant of image resolution

– increased frequency, increased resolutionincreased frequency, increased resolution– decreased frequency, decreased resolutiondecreased frequency, decreased resolution

Propagation SpeedPropagation Speed

Velocity of wave (= m/s)Velocity of wave (= m/s) Function of mediumFunction of medium

– air = 331 m/sair = 331 m/s– water = 1430 m/swater = 1430 m/s– soft tissue = 1540 m/ssoft tissue = 1540 m/s– bone = 4080 m/sbone = 4080 m/s

Amplitude and IntensityAmplitude and Intensity

Height of waveHeight of wave Function of source (transducer)Function of source (transducer) Major determinant of power outputMajor determinant of power output Intensity is proportional to amplitudeIntensity is proportional to amplitude22

– intensity = power/area (units = mw/cm2)intensity = power/area (units = mw/cm2) Theoretical concern of bioeffects of US Theoretical concern of bioeffects of US

mediated by amplitude/powermediated by amplitude/power

Wave Propagation Wave Propagation PhenomenaPhenomena

Reflection - gives us the image on the Reflection - gives us the image on the screenscreen– a function of differences of a function of differences of acoustic impedanceacoustic impedance

AttenuationAttenuation– Refraction - Refraction - occurs at interfaces with differing propagation occurs at interfaces with differing propagation

speeds - eliminate by perpendicular incidencespeeds - eliminate by perpendicular incidence

– Scatter - Scatter - reflected in multiple directionsreflected in multiple directions

– Absorption - Absorption - converted to heatconverted to heat

– Attenuation = attenuation coefficient x depthAttenuation = attenuation coefficient x depth– Attenuation coefficient proportional to frequencyAttenuation coefficient proportional to frequency

Target CharacteristicsTarget Characteristics

Echogenic (=bright)Echogenic (=bright)– large reflection componentlarge reflection component– waves returning to transducerwaves returning to transducer

Anechoic or Hypoechoic (=dark)Anechoic or Hypoechoic (=dark)– large attenuation componentlarge attenuation component– waves not returning to transducerwaves not returning to transducer

Mixed echogenicityMixed echogenicity

Acoustic WindowAcoustic Window

Allows sound waves to penetrate Allows sound waves to penetrate into the bodyinto the body

Good acoustic windowsGood acoustic windows– Liver, spleenLiver, spleen– Urine-filled bladderUrine-filled bladder

Poor acoustic windowsPoor acoustic windows– Gas (intestines, lung)Gas (intestines, lung)– Strong reflectors (bone)Strong reflectors (bone)

How is the image formed?How is the image formed?

Piezoelectric effect (receiving)Piezoelectric effect (receiving)– wave induced crystal deformation leads to wave induced crystal deformation leads to

electrical charge generationelectrical charge generation Reverse Piezoelectric effect (generating)Reverse Piezoelectric effect (generating)

– electrically induced crystal deformation leads electrically induced crystal deformation leads to wave generationto wave generation

Pulsed Echo Mode - crystal both receives Pulsed Echo Mode - crystal both receives (99%) and generates (1%) impulses(99%) and generates (1%) impulses

Knobology - Tuning the Knobology - Tuning the ImageImage

PowerPower Probe selectProbe select DepthDepth GainGain FrequencyFrequency FocusFocus Freeze and printFreeze and print

PowerPower

Probe SelectProbe Select

FAST, AAA, Pelvic, Cardiac

DVT, Musculoskeletal, Procedures

DepthDepth

GainGain

Occurs in Receiver, not GeneratorOccurs in Receiver, not Generator Total GainTotal Gain

– amplifies returning echo and allows lower amplifies returning echo and allows lower power (patient intensity exposure) use and power (patient intensity exposure) use and brighter imagebrighter image

Time Gain Compensation (TGC)Time Gain Compensation (TGC)– amplifies returning echoes by factor of time amplifies returning echoes by factor of time

required for echo returnrequired for echo return– allows homogenous tissue to be isoechoic allows homogenous tissue to be isoechoic

regardless of depthregardless of depth

GainGain

TGC adjustmentTGC adjustment

Make liver homogenous in appearance

Frequency and Axial Frequency and Axial ResolutionResolution

Axial Resolution = ability of machine Axial Resolution = ability of machine to distinguish between 2 distinct to distinguish between 2 distinct objects lying parallel to beam axisobjects lying parallel to beam axis

Axial resolution improves with Axial resolution improves with increased frequencyincreased frequency

Resolved Unresolved

FrequencyFrequency

Menu buttonMenu button Pulls up on-screen menuPulls up on-screen menu

S=standardS=standard R=resolution (high freq)R=resolution (high freq) P=penetration (low freq)P=penetration (low freq)

Focus and Lateral Focus and Lateral ResolutionResolution

Lateral Resolution = ability of Lateral Resolution = ability of machine to distinguish between two machine to distinguish between two distinct targets lying side by sidedistinct targets lying side by side

Depends on beam width - primary Depends on beam width - primary determinant is determinant is focal zonefocal zone

Resolved Unresolved

Focus and Beam ProfileFocus and Beam Profile

Focal Region

Focus

Near Field Far Field

FocusFocus

Freeze, Cine Loop,Freeze, Cine Loop, and Print/Save and Print/Save

PrinterPrinter

Text annotationText annotation

Image OrientationImage Orientation

Area nearest transducer is at top of screenArea nearest transducer is at top of screen Transverse scan is viewed like CT, with Transverse scan is viewed like CT, with

patient’s right at left side of screenpatient’s right at left side of screen– in abdomen, turn orientation marker to pt’s Rin abdomen, turn orientation marker to pt’s R

Sagittal scan is viewed so head is to L of Sagittal scan is viewed so head is to L of screen and feet are at Rscreen and feet are at R– in abdomen, turn orientation marker to headin abdomen, turn orientation marker to head

Orientation marker on probe corresponds to Orientation marker on probe corresponds to orientation marker on screenorientation marker on screen

Image OrientationImage Orientation

Probe MovementsProbe Movements

FanFan RockRock RotateRotate Slide (in 2 axes)Slide (in 2 axes)

We’ll talk more about this in the We’ll talk more about this in the hands-on sessionshands-on sessions

Common ArtifactsCommon Artifacts

Acoustic shadowAcoustic shadow Acoustic window/enhanced Acoustic window/enhanced

through-transmissionthrough-transmission ReverberationReverberation MirrorMirror

Acoustic ShadowAcoustic Shadow

Decreased echoes distant to Decreased echoes distant to strong attenuator (i.e. gallstones, strong attenuator (i.e. gallstones, kidney stones, ribs)kidney stones, ribs)

Acoustic WindowAcoustic Window

Increased echoes distant to weak Increased echoes distant to weak attenuator (“enhanced thru-attenuator (“enhanced thru-transmission)transmission)

May be used as window to enhance May be used as window to enhance visualizationvisualization

ReverberationReverberation

Tail of echoes or “comet tail” Tail of echoes or “comet tail” formed by sound bouncing formed by sound bouncing between 2 strong reflectors, between 2 strong reflectors, usually in cystic structuresusually in cystic structures

To minimize, decrease gainTo minimize, decrease gain Do not confuse with acoustic Do not confuse with acoustic

shadowing or acoustic windowshadowing or acoustic window

Mirror artifactMirror artifact

Secondary to echoes from strong Secondary to echoes from strong reflector (esp. at diaphragm)reflector (esp. at diaphragm)

Guide to image acquisitionGuide to image acquisition

Rest the side of your hand on the Rest the side of your hand on the patient and use plenty of gelpatient and use plenty of gel

Watch the screen, not the patientWatch the screen, not the patient Small, slow movementsSmall, slow movements Move patient (if possible), use Move patient (if possible), use

respirations to move structures respirations to move structures into viewinto view

Guide to image tuningGuide to image tuning

Choose the correct transducerChoose the correct transducer Adjust depth and focus to center Adjust depth and focus to center

area of interestarea of interest Actively use gain and TGC to keep Actively use gain and TGC to keep

like tissue looking alike (i.e. liver), like tissue looking alike (i.e. liver), and cystic structures anechoicand cystic structures anechoic

Acquire images in at least 2 planesAcquire images in at least 2 planes

Safety of UltrasoundSafety of Ultrasound

AIUM 1993AIUM 1993– ““No confirmed biological effects on patients or No confirmed biological effects on patients or

instrument operators caused by exposure at intensities instrument operators caused by exposure at intensities typical of the present diagnostic ultrasound instruments typical of the present diagnostic ultrasound instruments have ever been reported. Although the possibility exists have ever been reported. Although the possibility exists that such biological effects may be identified in the that such biological effects may be identified in the future, current data indicate that the benefits to patients future, current data indicate that the benefits to patients of the prudent use of diagnostic ultrasound outweigh the of the prudent use of diagnostic ultrasound outweigh the risks, if any, that may be present.”risks, if any, that may be present.”

ALARA principleALARA principle– minimize duration and intensity of exposure as much as minimize duration and intensity of exposure as much as

possiblepossible

Machine CareMachine Care

Machine CareMachine Care

Clean machine with soft clothClean machine with soft cloth NO commercial cleaners on machineNO commercial cleaners on machine Wipe gel off probe with towelWipe gel off probe with towel Disinfect probe with recommended Disinfect probe with recommended

disinfectant - NO alcoholdisinfectant - NO alcohol Do NOT drop probe, run over cords, or Do NOT drop probe, run over cords, or

park machine on ambulance dock...park machine on ambulance dock...

How do you get better at How do you get better at US?US?

Put the ultrasound probe onto your Put the ultrasound probe onto your patients! (especially those getting patients! (especially those getting “confirmatory studies”)“confirmatory studies”)

When you know a patient has an When you know a patient has an abnormality, take a look! (PD patients, abnormality, take a look! (PD patients, known AAA, DVT, pregnant, ovarian known AAA, DVT, pregnant, ovarian cyst, pleural effusion, heart failure, ST cyst, pleural effusion, heart failure, ST elevation MI, gallstones, etc.)elevation MI, gallstones, etc.)