How-To Guide

P31
• U ltrasoundexposuretim e.Exposure tim eisanespecially usefulvariable,becauseitis
controlledby theusers.
Acoustic Output Limits
• IS P T A ≤ 720 mW/cm spta.3
• M I ≤ 1.9
• T I ≤ 6.0
Applying ALARA
T hesystem im agingm odeoftheoperatorselectedthatisdependsontheuserinform ation
needed.U nderstandingthe natureoftheim agingm odeused,thetransducerfrequency,
system setupvalues,scanningtechniques,and operatorexperienceallow thesonographerto
m eetthe definition oftheAL AR A principle.T heam ountofacousticoutputisuptothesystem
operator.T hisdecisionm ustbebased onthefollow ing factors:typeofpatient,typeofexam ,
patienthistory,easeordifficulty ofobtainingdiagnostically usefulinform ation,thepotential
localized heating ofthepatientduetotransducersurfacetem peratures.P rudentuse ofthe
system occursw hen patientexposure islim itedto thelow estindex readingfortheshortest
am ountoftim enecessary toachieveacceptablediagnosticresults.A highindex readingdoes
notnecessarily indicate the occurrenceofabioeffect,how ever,itm ustbetakenseriously.Itis
the operatorresponsibility tom akeevery effortto reducethepossibleeffectsofahighindex
reading by lim itingexposuretim e.
L im itingexposuretim eisaneffectivew ay toaccom plishthisgoal.T hereareseveralsystem
controlsthattheoperatorcanusetoadjusttheim age quality andlim ittheacousticintensity.
T hesecontrolsarerelatedtothetechniquesthatanoperatorm ightusetoim plem entAL AR A.
T hesecontrolscanbedivided intothreecategories:direct,indirect,and receivercontrols.
Using System Controls to Implement ALARA
DirectControls
T hesystem L U 700 hasno directcontrolforoutput,thereforethesonographerm ust
controlexposuretim eandscanningtechniquetoim plem enttheAL AR A principle.
T o ensurethatacousticand therm allim itsarenotexceededforallim agingm odes,
the system L U 700 isdesignedtoautom atically adjustoutput.T hesystem doesnot
exceed aspatialpeaktem poralaverageintensity (IS P T A)of720 m W /cm forall
im agingm odes.T heequipm ent'sm echanicalindex (M I)doesnotexceedvalues
greaterthan1.9 and therm alindex(T I)doesnotexceedvaluesgreaterthan6.0.
IndirectControls
T heindirectcontrolsarethosethathaveanindirecteffectonacousticintensity.
T hesecontrolsaffectim agingm ode,pulserepetition frequency(P R F),pulselength.
T hechoiceofim agingm odedeterm inesthe nature ofthe ultrasoundbeam .2D isa
scanningm ode;Dopplerisastationary orun-scanned m ode.A stationary
ultrasoundbeam concentratesenergy inasinglelocation.A m ovingorscanned
ultrasoundbeam dispersestheenergy overan areaandthe beam isconcentrated
onthe sam eareaforafraction ofthetim easthatofan un-scanned m ode.