SonoSite S-Cath - Tissue Models and Equipment Survey

Chapter 6: Safety 87
Safety
where:
In Situ=In Situintensityvalue
Water=Waterintensityvalue
e=2.7183
a=attenuationfactor(dB/cm MHz)
Attenuationfactor(a)forvarioustissue types:
brain=0.53
heart=0.66
kidney=0.79
liver=0.43
muscle=0.55
l=skinlinetomeasurementdepthincm
f=centerfrequencyofthetransducer/system/mode
combinationinMHz
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