Engineering4(2018)702–713Contents lists available at ScienceDirectEngineeringResearch
Robotics—Article
AHigh-PrecisionUS-GuidedRobot-AssistedHIFUTreatmentSystemforBreastCancer
TianhanTanga,TakashiAzumab,?,ToshihideIwahashia,HidekiTakeuchia,EtsukoKobayashia,IchiroSakumaaabGraduateSchoolofEngineering,TheUniversityofTokyo,Tokyo113-8656,JapanGraduateSchoolofMedicine,TheUniversityofTokyo,Tokyo113-8654,Japanarticleinfoabstract
Breast cancer is the most commonly diagnosed cancer in women. A strong treatment candidate is high-intensity focused ultrasound (HIFU), a non-invasive therapeutic method that has already demonstrated its promise. To improve the precision and lower the cost of HIFU treatment, our group has developed an ultrasound (US)-guided, ?ve-degree-of-freedom (DOF), robot-assisted HIFU system. We constructed a fully functional prototype enabling easy three-dimensional (3D) US image reconstruction, target seg-mentation, treatment path generation, and automatic HIFU irradiation. The position was calibrated using a wire phantom and the coagulated area was assessed on heterogeneous tissue phantoms. Under the US guidance, the centroids of the HIFU-ablated area deviated by less than 2 mm from the planned treatment region. The overshoot around the planned region was well below the tolerance of clinical usage. Our system is considered to be suf?ciently accurate for breast cancer treatment.
ó 2018 THE AUTHORS. Published by Elsevier LTD on behalf of Chinese Academy of Engineering and Higher Education Press Limited Company. This is an open access article under the CC BY-NC-ND license
Articlehistory:Received31March2017Revised21December2017Accepted10July2018Availableonline17July2018Keywords:HIFUBreastcancerUS-guidednavigationMedicalroboticsAccuracyevaluation1.IntroductionBreastcanceristhemostcommoncancerinwomenworldwide[1],andthemostfrequentlydiagnosedcancerinwomenthrough-outJapan[2].Therefore,itposesamajorthreattoqualityoflife.High-intensityfocusedultrasound(HIFU)isanon-invasive,therapeuticultrasound(US)technologythatdeliversapreciselyfocusedacousticbeamtoasmallvolumeinthebody.ThehighacousticenergyoftheHIFUbeamcausesthermalnecrosisofthetargettissue.Mostbene?cially,thistechnologycanselectivelydestroydeepmalignanttissues,withlittledamagetotheoverlyingandsurroundingtissuesalongthebeampath.HIFUisapromisingmethodfortreatingbreastandothertypesofcancer[3–5].TheconceptviewoftheHIFUtreatmentforbreastcancerisshowninFig.1.Tofullyexploitthenon-invasivenessofHIFU,theHIFUtechnologyshouldbeintegratedintothemodernimage-guidedrobot-assistedsurgicalsystemparadigm.Animage-guidedrobot-assistedsystemtypicallycomprisesthreemainparts:amedical?Correspondingauthor.E-mailaddress:azuma@fel.t.u-tokyo.ac.jp(T.Azuma).imagingsystem,amedicalrobotsystem,andamedicalnavigationsystem.Theimagingandrobotsystemsextendthefunctionalitiesofthesurgeon’seyesandhands,respectively,andthenavigationsystemcoordinatesandassociateswiththehand–eyesystem.Therefore,thenavigationsystemisvitallyimportantinaprecisesurgicaltreatment.Atpresent,HIFUnavigationtasksareguidedbymagneticreso-nanceimaging(MRI).MRIhashighspatialresolutionandcanmon-itortissuetemperaturethroughthetemperature-dependenteffectsontheprotonresonantfrequency[6,7].However,MRI-guidedHIFUhasseveraldrawbacks.First,thelowframe-refreshrateofMRIimpedesreal-timenavigation.Second,thebulksizeofMRIisproblematicforsystemintegration.Third,thecon?nedspaceoftheMRIgantrylimitsthepossibleanglesalongwhichtherobotsystemcanapproachthetargetregion,therebycompro-misingthetreatmentdeliveryandincreasingtheriskofskinburn.Fourth,thestrongmagnetic?eldgeneratedbyMRIimposesadditionalconstraintsontherobotsystem.Mostofthecommonrobotcomponentscannotbeusedincloseproximitytomagnets,sooperationnearMRIrequiresspecialstrategiesthatincreasethesystemcomplexity[8].Fifth,thehighcosthaspreventedthepopularizationofHIFUtreatment.T.Tangetal./Engineering4(2018)702–713703Fig.1.ConceptviewoftheHIFUtreatmentforbreastcancer.Incontrast,USimagingisacompact,cheaptechnologywithafastframe-refreshrate.Owingtothesefeatures,inadditiontoitsversatilityandeasyaccessibility,USimaginghasbecomeanincreasinglypopularmodalityinimage-guidedsurgery.However,theconventionalUSdiagnosticsystemprovidesonlyatwo-dimensional(2D)imageofthetargetregion.Tocreateanaccuratethree-dimensional(3D)atlasfortheHIFUtreatmentplan,itispreferabletoutilizearobotsystemtocollectastackof2DUSimagesandreconstructa3DUSimagefromthestack.Tothisend,ourgrouphasproposedaUS-guidedrobot-assistedHIFUsystemasaprecise,low-costbreastcancertreatmentsolution.TheultimategoalistopromoteHIFUtreatmentandimprovethequalityoflifeofpatients.Here,wefocusonthenavigationsystemoftheproposedtreat-mentsystem.ToachieveaneffectiveandaccurateHIFUtreatment,thenavigationsystemshouldtaketheuserthroughthetreatmentwork?ow,fromimageacquisitiontotreatmentexecution.Theinternalfunctionalblocksshouldinclude3DreconstructionoftheUSimages,targetde?nition,andtreatmentpathgeneration.ToobtainthecorrectspatialrelationbetweentheUSimagespaceandtherobotworkingspace,hand–eyecalibrationisalsoneces-sary.Thesefunctionsareimplementedassubtaskstoachievethemainobjective.Finally,theoverallaccuracyofthetreatmentsystemshouldbeevaluated.Theremainderofthispaperisorganizedasfollows.AfteroverviewingtheconstitutionofoursysteminSection2,wedetailthedevelopmentoftheUS-guidednavigationsystemforHIFUtreatmentanditsevaluationmethodinSection3.TheexperimentresultsarepresentedinSection4.WediscusstheresultsinSection5,andconcludewithSection6.2.Systemoverview2.1.HardwarecomponentsoftheHIFUsystemThesystemconsistsofthreemainparts:aHIFUtransducerwithanappropriatecontrollerunitthatprovidestherequiredenergyfortreatment,aUSimagingsystem(ProsoundòF75,Hitachi,Ltd.,Tokyo,Japan)thatprovidesthenecessarydiagnosticinformationfornavigation,andacustom-made?ve-degree-of-freedom(DOF)parallel-linkrobotsystemthatmanipulatestheHIFUtransducerandthemodi?edUSimagingtransducer.Table1
Speci?cationsoftheHIFUandimagingtransducers.ParameterHIFUtransducerImagingtransducerTypeConcavehemisphericalLinearNumberofelements256128Diameterofaperture(mm)120—Geometricfocallength(mm)100—Elevationfocallength(mm)—60Pitchofelement(mm)—0.2Lengthofelement(mm)—8Centralfrequency(MHz)25Fractionalbandwidth(%)$5065Probelength(mm)—30Probewidth(mm)—16Thetwotransducersarecon?guredconfocallyand?xedontheend-effectoroftherobotsystem.Thespeci?cationsofbothtrans-ducersareshowninTable1.Themodi?edimagingtransducerhasanelevationfocallengthof60mm,whichwasspeci?callydesignedtoprovidebetterimageresolutionatdepththanacom-moncommerciallyavailabletransducer.The?ve-DOFrobotsys-tempreciselymanipulatestheHIFUandimagingtransducerstoperformthescanningfordiagnosisandthepositioningfortherapy.InconventionalUSdiagnosis,theimagingprobeusuallycontactstheskinsurface,andtheUSattenuationisarrestedbycouplinggelstoensuregoodimaging.However,theimagingprobeinoursystemisheldbytherobotsystem,notbythephysician.Therefore,regulartracescanningwiththeprobetouchingtheskinisinconve-nient.Moreover,iftheimagingprobecontactstheskin,thefocus-ingoftheHIFUbeamisimpeded.Therefore,inoursystemdesign,theimagingproberemainsseparatefromtheskinofthepatient.Thecouplingproblemisresolvedbydesigninganunderwaterworkspaceforourrobotsystem.Astheacousticattenuationcoef-?cientofwaterisverylow($0.0022dBá(MHzácm)à1[9]),weobtainedgoodUSimagesevenat120mm(thefurthestdistancebetweentheboundaryoftheUSimageandthesurfaceoftheimag-ingprobe).Anotherbene?tofnotcontactingtheprobewiththeskinsurfaceisthatthebreastdoesnotdeformunderthescanningprocess.Breasttissueisverysoft;however,providedthattheproberemainsdetachedandthe2Dreal-timeUSimageisreadilyavailableduringthetreatmentprocess,deformationshouldnotbeaprobleminoursystem.Duringthetreatment,thepatientliesproneonabedabovethewatertank,andthebreasttobetreatedisinsertedintotheworkspace.ArubbermembranebetweenthebreastandthetankwaterpreventsdirectcontactbetweenthepatientandthewaterwithoutimpedingtheUSwavepropagation.TheoverallcomponentsandconnectionsofthesystemareshowninFig.2.The?rstpersonalcomputer(PC1)setsthefocusdatafortheHIFUtransducerdriver.Thecommandissentthroughauniversalserialbus(USB)-to-universalasynchronousreceiver/transmitter(UART)cable.Thesecondpersonalcomputer(PC2)coordinatestheoverallHIFUsystem.PC2setstheoutputleveloftheHIFUtransducerviaaUSB-to-analoginput–output(AIO)unit(providedbyContecCo.,Ltd.,Osaka,Japan),andreceivesthe2DUSimagesfromtheUSimagingsystemthroughavideograbberwithadigitalvisualinterface(DVI)-to-USB3.0interface(providedbyEpiphanSystemsInc.,Ottawa,ON,Canada).PC2alsocommuni-cateswiththerobotcontrolleroveralocalareanetwork(LAN)cableinordertosetandtracktheposeoftherobot.Therobotcontrollerisconnectedtoamotorthatcontrolstherotationangleu(seeSection3.1)byaUSB-to-RS-232-Ccable.TheotherfourmotorsareconnectedtotherobotcontrollerthroughaUSB-to-controllerareanetwork(CAN)cable.704T.Tangetal./Engineering4(2018)702–713Fig.2.Overallcomponentsandconnectionsofthehardwaresystem.2.2.SoftwareofthenavigationsystemAsuccessfulUS-guidedHIFUtreatmentrequiresseveralpro-cessingsteps.First,a3DUSimageisreconstructedfromthescansperformedbytherobot.Inordertoassociateeachvoxelwiththepositioninformationrelativetotherobotworkspace,theposeinformationoftherobotiscollectedalongwiththe2Dframes.Second,thetargetregion(i.e.,thetumor/malignanttissue)isdetectedandsegmentedoutbythesurgeon.Third,theHIFUtreatmentspotsaredistributedbasedonthesegmenta-tionresults,andthepathoftherobottoseteachtreatmentspotiscalculated.Finally,thepathinformationandtheothersettingsaresenttotherobot,whichexecutesthetreatment.Fig.3isablockdiagramofthenavigationsystemsoftware.Thework?owissummarizedasfollows:(1)3DUSimagereconstruction(andvisualization).(2)Targetde?nition:①3DUSimageexploration(byuserinteraction);②segmentation(byuserinteraction).(3)Pathgeneration(sequenceofrobotposes).(4)OutputoftheplannedpathtorobotsystemandHIFUexcitationinformationtoHIFUtransducerdriver.3.Materialandmethods3.1.ControloftherobotsystemThede?nitionsoftheinputandoutputspacesofthe?ve-DOFrobotsystemaredisplayedinFig.4(a)and(b),respectively.Therobotsystemhas?veinputshafts,whosespeci?cationsarelistedinTable2.ShaftsJ1andJ2controltherotationangleuandthetranslationalongtheyaxis,respectively,andJ3,J4,andJ5collectivelycontrolthetranslationsalongthexandzaxesandtherotationangleh.Therefore,theinputparametersJ1andJ2areone-to-onemappedtotheoutputparametersuandy,respec-tively,whileinputparametersJ3,J4,andJ5aremany-to-manymappedtotheoutputparametersx,z,andh.Wenowdescribethedesignprinciplesoftherobotsystem.First,therobotsystemisdesignedtominimizetheoverallsizeofT.Tangetal./Engineering4(2018)702–713705Fig.3.Blockdiagramofthenavigationsystemsoftware.Fig.4.(a)Inputspaceand(b)outputspaceofthe?ve-DOFrobotsystem.Table2
Inputshaftspeci?cationsoftherobotsystem.InputshaftJ1J2J3J4J5Motionrangeà157°to243°à100to100mmà160to180mmà230to110mmà160to180mmMaxspeed200ráminà145.2mmásà145.2mmásà145.2mmásà145.2mmásà1Precision±0.004°±50lm±50lm±50lm±50lmMotorresolution36000pulseárà1666.3nmápulseà1666.3nmápulseà1666.3nmápulseà1666.3nmápulseà1thesystem(includingthewatertank)whilemaintainingthenecessary?exibilityforvariousapproachanglestothetreatmenttarget.Second,inpracticalbreastcancertreatment,itisusuallypreferabletoplacethetargetregionneartherotationaxiszandsethsuchthattheHIFUbeamisorthogonaltotheskinsurface.Thiscon?gurationreducestheriskofskinburn.Byusingadedicatedmotor(J1)tocontrolrotationaboutthezaxis,wecanspatially?xtheHIFUfocuspointonthezaxiswhilecausingtheapproachingHIFUbeamtorotateaboutit.Thisenablesef?cienttreatmentwhilefurtherreducingtheriskofskinburn[10].Third,theparallellinkscontrollingx,z,andhensurethatwhenirradia-tionisperformedfromanuprightandfromaside-to-sideposition,thezcoordinateoftheHIFUtransducerislowandhigh,respec-tively,asisrequiredfortheactualtreatment.ThejointsandlinksoftherobotsystemareillustratedinFig.5.JointsJ34andJ35aresituatedatthecenteroflinksA–J4andB–J5,respectively.Fromthegivendesignparameters(Table3),wecalcu-latedidenticalxcoordinatesforJ3andjointA,andforJ03andjointB.ThepositionsofjointsJ3andJ03arecontrolledbyshaftJ3,whilethepositionsofjointsJ4andJ5arecontrolledbyshaftJ4andJ5,respectively.TherobotsystemcontrolsthepositionandorientationoftheHIFUfocuspointbymanipulatingtheHIFUtransducer.Thespatialinformationofthefocuspoint(denotedasFinFig.5)intheoutput706T.Tangetal./Engineering4(2018)702–713Fig.5.Jointsandlinksoftherobotsystem.Table3
Designparametersoftherobotsystem.ParameterValueParameterValuel180mml5200mml280mml35100mml380mmfx40mml4200mmfy164mml34100mmX90°XistheanglebetweentheHIFUbeamdirectionandlinkA–C.spaceisspeci?edbytheparameterset(h,x,y,z,u).TheanglebetweentheHIFUbeamdirectionandlinkA–CisdenotedasX.Theinversekinematicsofthesystem(i.e.,determiningtheinputparametersfromthedesiredoutputparameters)aregivenbyEq.(1):26J132u366J2766y7767766J3776e1T4J477?66566qAx??????????????7762?7J54A2xàql??????????????4àAz?775Bxàl25àB2zwhere??2Ax??q??????????????3?64xàqf22???????????????xtfy?ácosehàXt\\FACT7Ae2Tzyàf2tf2hàXt\\FACT5xyásine??B\#x??B?q??????????????????????????????J3tl3B?C2e3Tzzàl2àexàCxT2and??Cx????tl1ácosehàXT??C?AxzAztl4T1ásinehàXTewherethesubscriptsxandzdenotethexandzcoordinates,respec-tively,ofpointsA,B,orCintheoutputspace.J1,J2,J3,J4,andJ5inEq.(1)aretheinputvaluesforeachofthe?veshafts.Theangle\\FACinEq.(2)wasdeterminedfromtan\\FAC?fy=fx,wherefyandfxareknowndesignparameters(Table3).3.2.CalibrationCalibrationisimportantforensuringaccurateHIFUtreatmentundertheUSimageguidance.Tolocatethetargetregionintheworkspaceofthetreatmentsystemfromthediagnosticimage,weneedthespatialrelationbetweentheUSimagespaceandtherobotworkspace.DenotingthecoordinatesystemoftheUSimageas{U}(inpixels)andthoseoftherobotbaseas{B}(inmm),weseektherelativeposeBnUdescribing{U}withrespectto{B}.ThisrelationisgivenbyEq.(5):2B32u36x6B6y747BBz7?nUá6566v747075e5T11whereBp$?Bx;By;Bz;1??Tisthehomogeneouspositionrepresenta-tionofanarbitrarypointontheimageplanein{B},andUp$?u;v;0;1??Tisthehomogeneouspositionrepresentationofthesamepointona2DUSimage.ThetransformationmatrixBnUisresolvedbythefollowingprocedures:(1)Tosimplifythecalculation,theoriginalimage-coordinatesystem{U}(inpixels)istransformedintoanewimage-coordinatesystem{I}(inmm).ThetransformationInUscales{U}alongtheuandvdirectionswithfactorssuandsv,respectively,shiftstheoriginbyatranslation,andadjuststheaxialdirectionbyarotation.(2)Theimage-coordinatesystem{I}istransformedintoarefer-encecoordinatesystemina3Dspacethatis?xedwithrespecttoaworldcoordinatesystem.Thetransformationisperformedbyimagingaphantomwithknowngeometricproperties.Thephan-tomcoordinatesystemisdenotedas{P}.(3)Theimage-coordinatesystem{I}isregisteredwiththerobotend-effectorcoordinatesystem{E}.Theposeoftheend-effectorintherobotbase{B}isreadfromtherobotencodersoutput.{I}and{E}arerelatedthrough{P}andanexternaltrackingsystemthatprovidesaworldcoordinatesystem{G}asthereference.Therefore,theproblemissolvedasfollows:BnU?BnEáEnGáGnPáPnIáInUe6TThescalingfactorssuandsvinInUaredeterminedfromthedis-playinformationofthediagnosticUSinstrument,speci?cally,bycountingthenumbersofpixelsalongtheuandvdirections,respectively,correspondingtothelengthindicatedbythescalebar.Thetranslationandrotationtransformationsareperformedonlyforcomputationalconvenience,sotheseparametersarealreadyknown.Consequently,wecanobtainInU.Astheposeoftheend-effectorisreadfromtheencoders,BnEisalsoknown.Theproblemthensimpli?estoEq.(7):EnI?EnGáGnPáPnIe7TPnIisdeterminedbytheN-?ducialphantommethoddescribedinRefs.[11]and[12].Thecustom-madephantomisshowninFig.6.The‘‘N”shapewasformedbya?shing-linewirewithdiam-eter/=0.2mm.Theworldcoordinate(reference)system{G}isprovidedbyanexternalopticaltrackingsystem(PolarisSpectraò,
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