How-To Guide
P33
Output Display
T herearetw otypesofindicesm ightbedisplayed:oneism echanicalindex(M I)and the
otheristherm alindex(T I).T hem echanicalindex(M I)providesanindicationoftherisk
duetom echanicalornontherm alm echanism s.T hetherm alindex(T I)providesan
indicationoftheriskofharm duetotherm alm echanism s.T hem echanicalindex(M I)is
continuously displayedovertherangeof0.0 to1.9,inincrem entsof0.1.
T hetherm alindex furtherconsistsofthefollow ingindices:softtissue(T IS ),bone(T IB),
and cranialbone(T IC).O nly oneoftheseisdisplayed atany tim e.Eachtransducer
applicationhasadefaultselectionthatisappropriateforthatcom bination.T heT IB,T IS ,
orT IC iscontinuously displayed overtherangeof0.0 tom axim um output,basedonthe
transducerandapplication.T heapplication-specificnatureofthedefaultsettingisalso
anim portantfactorofindex behavior.A defaultsetting isasystem controlstate thatis
presetby them anufacturerortheoperator.T hesystem hasdefaultindex settingsforthe
transducerapplication.T he defaultsettingsareinvoked autom atically bytheultrasound
system w henpow eristurned on,w hennew patientdataisenteredintothesystem
database,orw henanapplicationchangeoccurs.T hedecisionastow hichofthethree
therm alindicestodisplay should bebasedonthefollow ingcriteria:
• Appropriateindex fortheapplication:T IS isused forim agingsofttissue,
T IB forafocusatornearbone,andT IC forim agingthroughbone near
the surface,asinacranialexam .
• M itigatingfactorsthatm ightcreate artificially highorlow therm alindex
readings:locationoffluid orbone,orblood flow.Forexam ple,istherea
highly attenuatingtissuepathsothattheactualpotentialforlocalzone
heatingislessthanthetherm alindex displays?
• S canned m odesversusun-scanned m odesofoperationaffectthe
therm alindex.Forscannedm odes,heatingtendsto benearthesurface;
forun-scannedm odes,thepotentialforheatingtendsto bedeeperin
thefocalzone.
• Alw ayslim itultrasound exposuretim e.Do notrushtheexam .Ensure
thattheindicesarekepttoam inim um andthatexposuretim eislim ited
w ithoutcom prom isingdiagnosticsensitivity.
Mechanical Index (MI) Display
T hescientificevidencesuggeststhem echanicalbioeffectsarethreshold
phenom enathatdoesoccurw henacertainlevelofoutputisexceeded.T he
thresholdlevelvariesdependingonthetissue.T hepotentialform echanical
bioeffectsvariesw ith peakrarefactionalpressureand ultrasound frequency.T he
higherM Ivaluereading,thegreaterthepotential.T hereisnospecificM Ivalue,
w hichm eansthatam echanicaleffectisoccurringinfact.T he M Ishouldbeused as
aguideforim plem entingtheAL AR A principle.