SonoSite S-Cath - Acoustic Output Tables

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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situa tion.
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‐linear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app roximately2.0areknowntooccurforcurrently
availableequipment.MaximumMIvaluesaresimilarforreal‐time2DandMMode
imaging.
•Computedestimatesofupperlimitstotemperatureelevationsduringtransabdominal
scanswereobtainedinasurveyof1988and1990pulsedDopplerequipment.Thevast
majorityofmodels
yieldedupperlimitslessthan1°and4°C(1.8°and7.2°F)forexposures
offirst‐trimesterfetaltissueand second‐trimesterfetalbone,respectively.Thelargestvalues
obtainedwereapproximately1.5°C(2.7°F)forfirst‐trimesterfetaltissueand7°C(12.6°F)for
second‐trimesterfetalbone.Estimatedmaximumtemperatureelevationsgiven
herearefor
a“fixedpath”tissuemodelandarefordeviceshavingI
SPTA
valuesgreaterthan500 mW/
cm
2
.Thetemperatureelevationsforfetalboneandtissuewerecomputedbasedon
calculationproceduresgiveninSections4.3.2.1‐4.3.2.6in“BioeffectsandSafetyof
DiagnosticUltrasound”(AIUM,1993).
Acoustic output tables
Table 7throughTable 31indicatetheacousticoutputforthe systemandtransducer
combinationswithaTIorMIequaltoorgreaterthanone.Thesetablesareorganizedby
transducermodelandimagingmode.Foradefinitionoftermsusedinthetables,see“Terms
usedintheacousticoutputtables”
onpage 119.