SonoSite S-Cath - ALARA Principle; Applying ALARA

Chapter 6: Safety 79
Safety
ALARA principle
ALARAistheguidingprinciplefortheuseofdiagnosticultrasound.Sonographersand other
qualifiedultrasoundusers,usinggoodjudgmentandinsight,determinetheexposurethatis
“aslowasreasonablyachievable.”Therearenosetrulestodeterminethecorrectexposurefor
everysituation.Thequalifiedultrasounduserdeterminesthe
mostappropriatewaytokeep
exposurelowandbioeffectstoaminimum,whileobtainingadiagnosticexamination.
Athoroughknowledgeoftheimagingmodes,transducercapability,systemsetupand
scanningtechniqueisnecessary.Theimagingmodedeterminesthenatureoftheultrasound
beam.Astationarybeamresultsinamoreconcentrated
exposurethanascannedbeam,which
spreadsthatexposureoverthatarea.Thetransducercapabilitydependsuponthefrequency,
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surface
temperaturerise”onpage 85.Intheeventofadevicemalf unction,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.