User's Manual

Chapter 6: Safety 63
Safety
penetration,resolution,andfieldofview.Thedefaultsystempresetsareresetatthestartof
eachnewpatient.Itisthescanningtechniqueofthequalifiedultrasounduseralongwith
patientvariabilitythatdeterminesthesystemsettingsthroughouttheexam.
Thevariableswhichaffectthewaythequalifiedultrasounduser
implementstheALARA
principleinclude:patientbodysize,locationofthebonerelativetothefocalpoint,attenuation
inthebody,andultrasoundexposuretime.Exposuretimeisanespeciallyusefulvariable,
becausethequalifiedultrasoundusercan controlit.Theabilitytolimittheexposureovertime
supportstheALARA
principle.
Applying ALARA
Thesystemimagingmodeselectedbythequalifiedultrasounduserisdeterminedbythe
diagnosticinformationrequired.2Dimagingprovidesanatomicalinformation;CPDimaging
providesinformationabouttheenergyoramplitudestrengthoftheDopplersignalovertime
atagivenanatomicallocationandisusedfordetectingthepresenceof
bloodflow;Color
imagingprovidesinformationabouttheenergyoramplitudestrengthoftheDopplersignal
overtimeatagivenanatomicallocationandisusedfordetectingthepresence,velocity,and
directionofbloodflow;TissueHarmonicImaginguseshigherreceivedfrequenciestoreduce
clutter,artifact,andimproveresolution
onthe2Dimage.Understandingthenatureofthe
imagingmodeusedallowsthequalifiedultrasoundusertoapplytheALARAprinciple.
Prudentuseofultrasoundrequiresthatpatient exposure toultrasoundbelimitedtothelowest
ultrasoundoutputfortheshortesttimenecessarytoachieveacceptablediagnosticresults.
Decisionsthat
supportprudentusearebasedonthe typeofpatient,examtype,patienthistory,
easeordifficultyofobtainingdiagnosticallyusefulinformation,andpotentiallocalized
heatingofthepatientduetotransducersurfacetemperature.
Thesystemhasbeendesignedtoensurethattemperatureatthefaceofthetransducerwillnot
exceedthelimitsestablishedinSection42ofEN60601‐2‐37:Particularrequirementforthe
safetyofultrasoundmedicaldiagnosticandmonitoringequipment.See“Transducersu rface
temperaturerise”onpage 68.Intheeventofadevicemalfunction, thereareredundantcontrols 
thatlimittransducerpower.Thisisaccomplishedbyan
electricaldesignthatlimitsbothpower
supplycurrentandvoltagetothetransducer.
Thesonographerusesthesystemcontrolstoadjustimagequalityandlimitultrasoundoutput.
Thesystemcontrolsaredividedintothreecategoriesrelative tooutput:controlsthatdirectly
affectoutput,controlsthatindirectlyaffectoutput,andreceivercontrols.
Direct controls
Thesystemdoesnotexceedaspatialpeak temporala verageintensity(ISPTA)of720 mW/cm
2

forallimagingmodes.Themechanicalindex(MI)andthermalindex(TI)mayexceedvalues
greaterthan1.0onsometransducersinsomeimaging modes.OnemaymonitortheMIandTI
valuesandadjustthecontrolstoreducethesevalues.See“GuidelinesforreducingMIandTI”
onpage 64.
Additionally,onemeansformeetingtheALARAprincipleistosettheMIorTI
valuestoalowindexvalueandthenmodifyingthisleveluntilasatisfactoryimageorDoppler
modeisobtained.FormoreinformationonMIandTI,seeBSEN60601‐2‐37:2001:AnnexHH.