User's Manual

70
Sincetheultrasonicpathduringtheexamislikelytopassthroughvaryinglengthsandtypes
oftissue,itisdifficulttoestimatethetrueIn Situintensity.Anattenuationfactorof0.3isused
forgeneralreportingpurposes;therefore,theIn Situvaluecommonlyreportedusesthe
formula:
In Situ(derated)=Water
[e
‐(0.069lf)
]
SincethisvalueisnotthetrueIn Situintensity,theterm“derated”isusedtoqualify it.
Themaximumderatedandthemaximumwatervaluesdonotalwaysoccuratthesame
operatingconditions;therefore,thereportedmaximumwaterandderatedvaluesmaynotbe
relatedbytheIn Situ(derated)formula.For
example:amulti‐zonearraytransducerthathas
maximumwatervalueintensitiesinitsdeepestzone,butalsohas thesmallestderatingfactor
inthatzone.Thesametransducermayhaveitslargestderatedintensity inoneofitsshallowest
focalzones.
Tissue models and equipment survey
TissuemodelsarenecessarytoestimateattenuationandacousticexposurelevelsIn Situfrom
measurementsofacousticoutputmadeinwater .Currently,availablemodelsmay belimitedin
theiraccuracybecauseofvaryingtissuepathsduringdiagnosticultrasoundexposuresand
uncertaintiesintheacousticpropertiesofsofttissues.Nosingletissuemodel
isadequatefor
predictingexposuresinallsituationsfrommeasurementsmadeinwater,andcontinued
improvementandverificationofthesemodelsisnecessaryformakingexposureassessments
forspecificexamtypes.
Ahomogeneoustissuemodelwithattenuationcoefficientof0.3 dB/cm MHzthroughoutthe
beampathiscommonlyusedwhenestimatingexposurelevels.
Themodelisconservativein
thatitoverestimatestheIn Situacousticexposurewhenthepathbetweenthetransducerand
siteofinterestiscomposedentirelyofsofttissue.Whenthepathcontainssignificantamounts
offluid,asinmanyfirstandsecond‐trimesterpregnanciesscannedtransabdominally,this
modelmayunderestimate
theIn Situacousticexposure.Theamountofunderestimation
dependsuponeachspecificsituation.
Fixed‐pathtissuemodels,inwhichsofttissuethicknessisheldconstant,sometimesareusedto
estimateIn Situacousticexposureswhenthebeampathislongerthan3cmandconsistslargely
offluid.Whenthismodelisused
toestimatemaximumexposuretothefetusduring
transabdominalscans,avalueof1dB/cmMHzmaybeusedduringalltrimesters.
Existingtissuemodelsthatarebasedonlinearpropagationmayunderestimateacoustic
exposureswhensignificantsaturationduetonon‐li neardistortionofbeamsinwaterispresent
duringtheoutputmeasurement.
The
maximumacousticoutputlevelsofdiagnosticultrasound devicesextendoverabroad
rangeofvalues:
•Asurveyof1990‐equipmentmodelsyieldedMIvaluesbetween0.1 and1.0attheirhighest
outputsettings.MaximumMIvaluesofapproxim ately2.0areknowntooccurforcurrently
availableequipment.MaximumMIvaluesaresimilar
forreal‐time2DandMModeimaging.