Ultrasound Output Display Standard Copyx

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Ultrasound+Output+Display+–+What+do+the+numbers+mean?+

G.!Wayne!Moore,!B.Sc.,!MA,!FASE!

July!13,!2015!

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The!ultrasound!Output!Display!Standard*+(ODS)!has!a!specific!technical!definition!

but!that!definition!is!generally!found!standing!in!contextual!isolation.!Specifically,!

how!do!these!dimensionless!MI!and!TI!numbers,!displayed!on!the!ultrasound!system!

monitor!(see!photo!below),!relate!to:!(1)!the!laws!of!ultrasound!physics,!(2)!FDA!

prescribed!regulatory!constraints!placed!on!the!limits!of!the!transmitted!acoustic!

energy,!(3)!the!other!user!controls!on!the!ultrasound!system,!and!(4)!the!ALARA!

principle.!Related!to!the!potential!for!bioQeffects!occurring!at!certain!levels!of!

ultrasound!acoustic!intensity,!the!MI!and!TI!indices!displayed!serve!as!a!guide,!and!a!

reminder!to!the!clinical!endQuser!that!although!ultrasound!is!generally!regarded!as!a!

safe!imaging!modality,!one!must!always!use!the!minimal!amount!of!energy!

necessary!to!obtain!a!diagnostic!quality!image!(including!Doppler).!MI,!or!

mechanical!index,!relates!to!the!potential!of!nonQthermal!effects!from!ultrasound!

occurring!within!tissue!(e.g.,!cavitation).!TI,!or!thermal!index,!relates!to!the!potential!

for!tissue!temperature!increases!within!the!acoustic!beam.!There!are!three!types!of!

TI!that!may!be!displayed!on!the!monitor,!depending!on!the!clinical!application!

selected:!(1)!TIS,!for!soft!tissues,!(2)!TIB,!for!bone!tissue!(shown!in!the!photo!below),!

and!(3)!TIC,!for!transcranial.!!!!!!!

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When!designing!an!ultrasound!system!there!is!a!classical!macroQlevel!engineering!

tradeQoff!that!must!be!dealt!with!and!resolved;!balancing!image!resolution!and!

depth!of!penetration.!Both!of!these!parameters!are!related!to!the!ultrasonic!

frequency!being!used!and!are!in!opposition.!Specifically,!the!higher!the!frequency!

being!used!the!better!the!resolution,!but!at!the!expense!of!the!depth!of!penetration.!

That’s!the!physics!of!it.!Although!a!tad!more!complicated!an!easy!math!example!will!

explain;!ultrasound!energy!is!lost!in!soft!tissue!at!the!rate!of!approximately!

0.7dB/cm/MHz,!where!MHz!is!the!ultrasonic!frequency.!So!at!3MHz!the!attenuation!

would!be!2.1dB/cm,!while!at!8MHz!the!attenuation!would!be!at!5.6dB/cm.!This!

relationship!means!that!!

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*!The!official!title!of!the!ODS!is:!“Standard'for'Real-Time'Display'of'Thermal'and'

Mechanical'Output'Indices'on'Diagnostic'Ultrasound'Equipment”'

as!the!frequency!(let’s!relate!this!to!the!probe!frequency)!is!increased!and!image!

resolution!improved,!the!depth!of!penetration!will!decrease,!unless!we!increase!the!

ultrasound!system’s!output!power,!which!the!FDA!says!is!a!noQno!beyond!an!

estimated!in-situ!intensity!of!720mW/cm2.!So!ultrasound!systems!and!their!

attendant!probes!are!designed!with!the!intended!clinical!application!clearly!in!mind;!

striking!the!optimal!balance!between!resolution!and!required!depth!of!penetration!

within!the!FDA!acoustic!output!power!constraints.!So!within!each!clinical!application!

there!is!a!range!of!MI!and!TI!numbers.!The!MI!and!TI!“values”!change!as!the!clinical!

user!changes!various!parameters!of!the!image!or!modes!of!operation!(e.g.,!BQmode,!

MQmode,!spectral!Doppler,!or!color!flow!Doppler),!but!especially!the!“power”!knob.!

Enter!the!ALARA!principle;!As!Low!as!Reasonably!Achievable.!Even!though!through!

regulation!there!is!a!maximum!upper!limit!on!acoustic!output,!user!groups!have!

developed!the!ALARA!principle!with!the!guiding!premise!that!the!clinical!user!should!

only!use!(that!is,!only!expose!the!patient!to)!as!much!acoustic!power!required!to!

obtain!a!diagnostically!acceptable!image;!hence!the!range!of!MI!and!TI!values!

adjustable!by!the!acoustic!output!knob.!Original!Equipment!Manufacturers!(OEMs)!

are!required!by!the!FDA!to!submit!testing!results!in!the!form!of!acoustic!output!

power!tables!for!each!probe!and!mode!of!operation!for!which!they!seek!510(k)!

market!clearance.!This!testing!is!an!onQgoing!requirement!throughout!the!product!

lifeQcycle!of!any!given!probe!on!the!cleared!system!to!ensure!that!no!significant!

changes!have!occurred!in!the!manufacturing!process,!or!if!new!acoustic!materials!

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are!being!substituted**.!This!ensures!that!the!output!power!of!any!given!probe!is!still!

within!the!statistical!range!of!the!reported!MI!and!TI!values!displayed!on!the!

monitor,!and,!therefore,!still!a!valid!indicator!to!the!clinical!user!relative!to!how!

much!power!they!are!inputting!into!the!patient.!!

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Safe!and!efficacious!are!the!byQwords!for!an!FDA!cleared!ultrasound!system!and!

probe!sold!in!the!United!States.!Unfortunately!there!are!some!probe!“repair”!

companies!in!the!United!States!that!replace!the!acoustic!stack!of!a!probe!with!an!

array!manufactured!by!someone!other!than!the!OEM!without!performing!acoustic!

power!testing.!This!practice!has!the!potential!for!rendering!the!MI!and!TI!values!

displayed!as!unreliable!indicators!for!the!clinical!user,!and!may!lead!to!higher!than!

necessary!acoustic!exposure!to!the!patient,!and/or!producing!subQdiagnostic!images.!

Replacing!OEM!arrays!with!unQtested!and!unQvalidated!arrays!manufactured!by!nonQ

OEM!companies!also!renders!the!probe!as!an!“adulterated”!product!as!defined!by!

the!FDA**.!Before!giving!permission!to!any!probe!repair!company!to!replace!an!

acoustic!array,!the!user!should!first!demand!to!see!the!acoustic!power!testing!

results!for!that!array,!the!name!of!the!manufacturer!of!that!array,!and!the!

comparison!chart!of!the!OEM!array!test!results!with!those!from!the!nonQOEM!array.!

To!do!less!would!be!a!disservice!to!the!patient.!

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**Guidance!for!Industry!and!FDA!Staff!“Information'for'Manufacturers'Seeking'

Marketing'Clearance'of'Diagnostic'Ultrasound'Systems'and'Transducers”!Document!

Issued!on!Q!09/09/2008!!!

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