How-To Guide
P29
Cavitation
T heinteractionofultrasound w ithgasbubblesorcontrastagentscausesrapid and potentially
largechangesinbubblesize.T hisprocess,term ed cavitation,m ay increasetem peratureand
pressurew ithinthebubbleandthereby causem echanicalstresson surroundingtissues,
precipitatefluid m icrojetform ation,andgeneratefreeradicals
[5]
.Gas-containingstructures(e.g.,
lungs,intestines)arem ostsusceptibletotheeffectsofacousticcavitation.U ltrasound w avelength
hasanim portantroleinbubbleform ationand grow th:shortw avelengthultrasound (observed at
higherfrequencies)doesnotprovidesufficienttim eforsignificantbubble grow th;therefore,
cavitationislesslikely underthesecircum stancescom paredw ith longw avelengths.T heshorthalf-
life ofcavitationnucleipreventsm ostcavitation-relatedbiologicaleffects,unlessultrasound
contrastagentsarealsopresent.Contrastagentsm arkedly reducethethreshold intensity for
cavitation.How ever,becauseoftherelatively highviscosity ofblood and softtissue,significant
cavitationisunlikely,and cavitationhasnotbeenshow n to occurw iththeultrasound exposure
com m only used duringadiagnosticexam ination.
N ote:Cavitation depends on:
• Frequency
• Pressure
• Focused/unfocused beams
• Pulsed/continuous ultrasound
• Degree of standing waves
• Nature and state of material
• Boundaries
Other effects
A variety ofotherphysicalforcesm ay also beproduced by ultrasoundenergy.Althougheachof
these effectscan bedem onstrated invitro,thereisnoevidencethatany ofthesephysical
phenom enahasasignificantbiologicaleffectonpatients.
ALARA Principles
T heguidingprinciplefortheuseofdiagnosticultrasound isdefinedby theAL AR A (w hich
m eansthatw e keeptotalultrasound exposureaslow asreasonably achievable w hile
optim izingdiagnosticinform ation).T hedecisionastow hatisreasonablehasbeenlefttothe
judgm entand insightofqualifiedpersonnel.According toAIUM Medical Ultrasound Safety
(Third Edition),therearethefollow ingdescription"W ithnew ultrasoundequipm ent,theon-
screenoutputdisplay (therm alindex [T I]and m echanicalindex [M I])letsusdeterm inethe
exposurelevelinterm softhepotentialforbioeffects.Forequipm entthatdoesnothavean
outputdisplay,w edependonw hateveroutputinform ation,suchasintensity,decibels,orthe
percentageofpow er,thatthesystem provides.Becausethethreshold,ifoneexists,for
diagnosticultrasound bioeffectsisundeterm ined,itbecom esourresponsibility tocontrolthe
totalexposuretothepatient.Controllingthetotalexposuredependsontheoutputleveland
exposuretim e.T he outputlevelrequired foranexam inationdependsonthepatientand the
clinicalneed.N otalldiagnosticexam inationscanbeperform edatvery low levels.Infact,