Signal Processing Overview of Ultrasound Systems for ...

1 WhitePaperSPRAB12 November2008 SignalProcessingOverviewofUltrasoundSyst emsforMedicalImagingMurtazaAli, :thefrontend(thatincludestransmission,re ception,anddigitizationofultrasoundsigna l),followedbybeamforming(tofocusonaparti cularscanline),themidend(thatperformsdem odulation,envelopedetection,andcompressi on),andfinally,thebackend(thatperformsva riousimageenhancementandrenderingfunctio ns).TheprocessingrequiredinDopplermodeis describedand,finally,theprocessingandiss uesrelatedto3 ,obstetrics,gynecology,abdominalimaging, , ,( ,inguidinginterventionalprocedures). ,digitalsignalprocessors(DSP) (SOC) [20]. , (DAS) , ,whichiscomposedofimageenhancement,noise reduction, , ,Section8brieflyintroducesthebasicconcep tsusedfor3 November2008 SubmitDocumentationFeedback2 UltrasoundSystem:BasicConceptsRegion of InterestScan LinesFocal ,bynomeans, , ,ultrasoundsystemsoperateinthe2 MHzto20 MHzfrequencyrange,althoughsomesystemsare approaching40 MHzforharmonicimaging[3].

2 Inprinciple, , , , ,asetoftransducerelementsareenergizedwit hasetoftime-delayedpulsestoproduceasetof soundwavesthatpropagatethroughtheregiono finterest, , , , ,inwhichtheultrasoundsystemusuallyoperat es,doesnotmatchthedisplaycoordinatesyste ms,acoordinatetransformation,calledscanc onversion, November2008 SignalProcessingOverviewofUltrasoundSyst emsforMedicalImaging3 SubmitDocumentationFeedbackHVTransmitter HVMUXS ignalChainREFT ransmitBeamformerFront EndBackEndReceiveBeamformerBeamformerCon trol UnitPreprocessingSpectralDopplerProcessi ngB ModeProcessingColor/PowerDopplerProcessi ngScan ConversionPost ProcessingCore & LineAC/DC SupplyWith Green ModeControllerMain Power SupplyBacklightTouchscreenSystemPowerSup plyVoltT/RSwitchesMemoryInterfacesMMC/SD IOIDE/ATAU ltrasound OS/UIMidClockingTransducerTx BufferAmpPassiveLPFCW (analog)BeamformerLow PassFilterDisplayAudio OutputAudioAmpLEGENDP rocessorInterfaceAmplifierLogicPowerADC/ DACO therAmplifier + FilterPGALNAA mplifier StageVCAADCA nalog , 100 Vand (Tx/Rx)switch, , , (VCA)

3 Beforebeingsampledbyanalog-to-digitalcon verters(ADC).TheVCAisconfiguredsothatthe gainprofilebeingappliedtothereceivedsign alisafunctionofthesampletimesincethesign alstrengthdecreaseswithtime( ,ithastraveledthroughmoretissue). , (ASIC),field-programmablegatearray(FPGA) , ,whichdeterminestheinput/output(I/O) :BasicConceptsOncethedataisbeamformed,de pendingontheimagingmodes, , (Brightness)mode, , ,finally, ,anwindowedfastFouriertransform(FFT) ,ASIC, , ,modernSOCsthatincludeDSP,RISC, ,thissectionhasconcentratedondigitalbeam forming,whichiscommonforpulsewave(PW) ,inthesesystems, , (CW) , ,thedynamicrangerequirementbecomesveryhi ghtouseADCtodigitizethereflectedultrasou ndsignalandmaintainenoughsignaltonoise(S NR) , (usuallyinKHzrange) sultrasoundsystem. A-mode(Amplitude) , B-mode(Brightness)imagingisthesameasA-mo de, ,multiplesetsofpulsesaregeneratedtoprodu cesoundwavesforeachscanline,eachsetofpul sesareintendedforauniquefocalpointalongt hescanline.

4 ForCW(ContinuousWave)Doppler, , ,thebloodflowinveinscanbeestimatedusingt heDopplerfrequency(seeSection7fordetails ).However,sincethesensoriscontinuouslyre ceivingdatafromvariousdepths,thevelocity locationcannotbedetermined. ForPW(PulseWave)Doppler, ,thevelocitylocationcanalsobedetermined. ForColorDoppler, InPowerDoppler,insteadofestimatingtheact ualvelocityofthemotion, (upto1000pulsespersecond),thisisusefulin assessingratesandmotionandisstillusedext ensivelyincardiacandfetalcardiacimaging. HarmonicImagingisanewmodalitywheretheB-m odeimagingisperformedonthesecond(orpossi blyother)harmonicsoftheimaging[3].Duetot heusualhighfrequencyoftheharmonic, ,duetohigherloss, Elasticity/StrainImagingisanewmodalitywh eresomemeasuresofelasticity(likeYoung smodulus)ofthetissue(usuallyundercompres sion)isestimatedanddisplayedasanimage[17 ],[18].

5 , :specular,diffusiveanddiffractive[24]. ,thereflectionprocesscanbeapproximatedas anincidentraywiththescatteredwavefrontfo llowingtheshapeoftheobject..Bycentrallim ittheorem, , (specularscattering), ,thoughmodeledwithadistributionovertheim age, , (likeboundarydetection) , ( )()MTF f , zexp ( f )zg=(1)( f )( f )i ( f )gab= --(2)y(f )f0 1aaa=+(3)( )( )( )fkff0 Ebb=+(4)( )( )( )P f , zP f MTF f , z0=(5) (MTF)writtenasafunctionoffrequency,f,and depthofpenetration,z,inthefollowingmanne r[24]:wherek0=2 f/c0,c0beingthevelocityofsoundusuallytak enatthecenterfrequencyofthespectrum, (f)isanexcessdispersiontermresultinginfr equencydependentphasevelocity,and (f)isthefrequencydependentattenuationfol lowingpowerlaw( 0, 1,andyareconstants).IfapressureP0(f)isap pliedatdepthz=0,thepressureatanydepthzis thengivenby:TheinverseFouriertransformof MTF(f,z) ,thefollowingcharacteristicscanbederived : ,theattenuationislow( )inblood.

6 Velocitydispersion,thoughsmall,mayneedto beconsideredforwidebandsignalsasisthecas eofmodernultrasoundtransducers. Intimedomain, ,responseisimpulselikeandwithdepth,itspr eadsovertime. Infrequencydomain,thereisaslightdownward shiftofpeakintheresultingfrequencyspectr umwithdepthofpenetrationwhenashaped( ,Gaussian) , :linear,curved, , , , , ,thesignalflexisusuallysolderedtoagroupo fwiresinacablebundlethatisconnectedtothe systemviaastandardizedconnector( ,ZIF156). , , , , , , , ,alternatetransducerstructuresusingcapac itivemicro-machinedultrasoundtransducers (CMUTs)isanactiveareaofresearch[33].8 SignalProcessingOverviewofUltrasoundSyst emsforMedicalImagingSPRAB12 PointXZidq( )222sinRxx Rfpii fptciq+-=(6)( )22Rx2 xRsinmaxmax fpfptmaxcq++=(7)( )()ptp tiit=-(8) ,theprocessofsteeringandfocusingthesound beaminanultrasoundsystemiscommonlyreferr edtoasphasedarraybeamforming[26].

7 Inthismodeofoperation, , ,p(t).Thepropagationtime,ti,fortheithpie zo-electricelementtothefocalpointcanbewr ittenas:wherexiisthex-axiscoordinateofth eithpiezo-electricelement,Rfp,istheradia ldistancefromtheorigintothefocalpoint, ,istheangleofthefocalpointwithrespecttot hez-axis, , , ,themaximumpropagationtimeforagiven(Rfp, )configuration( ,focalpoint) ,thepulsewaveformfortheithpiezo-electric elementbecomes:where I=tmax-tiSPRAB12 November2008 SignalProcessingOverviewofUltrasoundSyst emsforMedicalImaging9 SubmitDocumentationFeedback( )()NNr tAA p tritjri tji 1 j 1tt=--== (9)( )( )2cos2 Rsinxxfpii2 Rfpticqq-+ (10)( )sin,xciq-( )2cos2, , , ,r(t),fromagivenfocalpointafterithasbeen timelignedcanbewrittenas:whereAriistheap odizationappliedtoeachreceivesignal,Atji stheapodizationappliedtoeachtransmitsign al, riisthetime-delayvalueappliedtoeachrecei vedsignal,and , , ,Equation6cannotbeseparatedintotermsthat justdependonRfpand.

8 [28].Thelinearterm,isoftenreferredtoasth ebeamsteeringcomponentbecauseitonlydepen dsupon , x,whereiisthechannelnumberand , , ,detection,andlogcompression(seeFigure6) . (TFC) (conventionalimaging)orthesecondharmonic (harmonicimaging). , (conventionalversusharmonic),orchangesin thecenterfrequencieswithtime( ,aphysicalphenomenoninthetissuethatresul tsinachangeoffrequencywiththedepthofpene tration).However,thisoperationismorecomp lexcomparedtothealternativebelow. , [24].Theactualdynamicrangeofthereceiveds ignaldependsontheADCbits,theTGCamplifier usedinthefrontend, (usually7or8bits). [16].Thecodedexcitationisusedtoincreaset hedepthofpenetrationandincreasesensitivi tywithoutincreasingthetransmitpulsepower ,orequivalently, Codedexcitation:Severalcodedsequenceslik eBarkerandGolaysequences[10]havebeenused .

9 Chirps:Thisisborrowedfromradarwherethetr ansmitwaveformconsistsofalinearsweepoffr equencymodulatedpulseofagivenduration[11 ].Asisclearfromthechoiceofcodedsequences , ,thenthereceivedsignalneedtobefilteredth roughthecorrespondingmatchedfilter(orseq uence).Toreducetheeffectofchangingcenter frequencywiththedepthofpenetration, ()()1y(i )x(i ) (i )y(i 1) ,1(i ) f x(i ) y(i 1)aaa=+-+=--(11) ,theyareweightedandadded[24]. , ,andwhensummed, (TGC), , , ( ,gallbladder).Insuchcases,itiscommontopr ovideadditionalgaincompensationbothinthe axialandlateraldirections[15],[25].These gaincompensationscanbeunderusercontrol,b utitisalsopossibletoautomaticallydetects uchanechoicregionsandprovideanautomaticg aincontrol(AGC) , , (FIR/IIR) ,asingletapmotionadaptiveIIRfilteringcan beimplementedusingEquation11[22]:Here,y( I)istheoutputpixelvalueattimeI,x(I)thein putpixelvalue, Data in Polar CoordinatesZoomed VersionData Converted to Cartesian CoordinatesExtrapolated Cartesian Point (Outside Region)Interpolated Cartesian PointOriginal Polar (Spatial) , (forlinearprobes)orinpolarcoordinates(fo rcurvilinearorphasedarrayprobes).

10 , (2X2interpolation).Somehigh-endsystemsma yuseinterpolationbasedon16nearestneighbo rs(4X4interpolation).Thisisamulti-imagem ethodusedforspecklereduction[24].Theidea isthatbycombiningviewsofthesameobjectfro mdifferentangles,eachhavinganuncorrelate dspecklerealization, ,compensatingthemfortranslation/rotation (involvesmultiplyingwitha2 Dmatrix) November2008 SignalProcessingOverviewofUltrasoundSyst emsforMedicalImaging13 SubmitDocumentationFeedback0 degree acquisition20 degree acquisition-20 degree acquisitionWeighted averagingCompounded ( x, y )w( s,t )f ( x s, y t )sa tb=++= -= - (12) , (illustratedas+20 ,0 and-20 ). (duetolessreflection);therefore, (-1,-1)w(-1,0)w(-1,1)w(0,-1)w(0,0)w(0,1) w(1,-1)w(1,0)w(1,1) November2008 SubmitDocumentationFeedback221x1yh( x,y ) g ( x,y ) (exp()) (exp()),2D222222g ( x) g (y )1D1Dp sp sss==- -= (13) f22f ( ) ( ), wherexyx yfff( x,y ) f( x 1,y ) andf( x,y ) f( x,y 1)xy =+@+ =--=-- (14) [1].