User's Manual

Table Of Contents
98
penetration,resolution,andfieldofview.Thedefaultsystempresetsareresetatthestartof
eachnewpatient.Itisthescanningtechniqueofthequalifiedultrasounduseralongwith
patientvariabilitythatdeterminesthesystemsettingsthroughouttheexam.
Thevariableswhichaffectthewaythequalifiedultrasounduser
implementstheALARA
principleinclude:patientbodysize,locationofthebonerelativetothefocalpoint,attenuati o n
inthebody,andultrasoundexposuretime.Exposuretimeisanespeciallyusefulvariable,
becausethequalifiedultrasoundusercancontrolit.Theabilitytolimittheexposureovertime
supportstheALARA
principle.
Applying ALARA
Thesystemimagingmodeselectedbythequalifiedultrasounduserisdeterminedby the
diagnosticinformationrequired.2Dimagingprovidesanatomicalinformation;CPDimaging
providesinformationabouttheenergyoramplitudestrengthoftheDopplersignalovertime
atagivenanatomicallocationandisusedfordetectingthepresenceof
bloodflow;Color
imagingprovidesinformationabouttheenergyoramplitudestrengthoftheDopplersignal
overtimeatagivenanatomicallocationandisusedfordetectingthepresence,velocity,and
directionofbloodflow;TissueHarmonicImaginguseshigherreceivedfrequenciestoreduce
clutter,artifact,andimproveresolution
onthe2Dimage.Understandingthenatureofthe
imagingmodeusedallowsthequalifiedultrasoundusertoapplytheALARAprinciple.
Prudentuseofultrasoundrequiresthatpatientexposuretoultrasoundbelimitedtothelowest
ultrasoundoutputfortheshortesttimenecessarytoachieveacceptablediagnosticresults.
Decisionsthat
supportprudentusearebasedonthetypeofpatient,examtype,patienthistory,
easeordifficultyofobtainingdiagnosticallyusefulinformation,andpotentiallocalized
heatingofthepatientduetotransducersurfacetemperature.
Thesystemhasbeendesignedtoensurethattemperatureatthefaceofthetransducerwillnot
exceedthelimitsestablishedinSection42ofEN60601237:Particularrequirementforthe
safetyofultrasoundmedicaldiagnosticandmonitoringequipment.See“Transducersurface
temperaturerise”onpage 104.Intheeventofadevicemalfunction,thereareredundant
controlsthatlimittransducerpower.Thisisaccomplishedby an
electricaldesignthatlimits
bothpowersupplycurrentandvoltagetothetransducer.
Thesonographerusesthesystemcontrolstoadjustimagequalityandlimitultrasoundoutput.
Thesystemcontrolsaredividedintothreecategoriesrelativetooutput:controlsthatdirectly
affectoutput,controlsthatindirectlyaffectoutput,andreceivercontrols.
Direct controls
Thesystemdoesnotexceedaspatialpeaktemporalaverageintensity(ISPTA)of720 mW/cm
2
forallimagingmodes.(ForeithertheOphthalmicorOrbitalexam,theacousticoutputis
limitedtothefo llowingvalues:ISPTAdoesnotexceed50 mW/cm
2
;TIdoesnotexceed1.0, and
MIdoesnotexceed0.23.)Themechanicalindex(MI)andthermalindex(TI)mayexceedvalues
greaterthan1.0onsometransducersinsomeimagingmodes.OnemaymonitortheMIandTI
valuesandadjustthecontrolstoreducethesevalues.See“Guidelines
forreducingMIandTI”