Registration of Optical Navigator in Staged Musculoskeletal Procedures

1MarconiMedicalSystemsFinland,Inc.,Ayritie4,Vantaa,Finland;2UniversityofTurku,DepartmentofDiagnosticRadiology,Turku,Finland;3UniversityHospitalofOulu,DepartmentofRadiology,Oulu,Finland;PurposeToinvestigatepracticabilityofusingafastregistrationmethodforanoptical navigator with musculoskeletal procedures performed in theclosevicinityofanMRmagnet.IntroductionInsomemusculoskeletalprocedures,thepatientneedstobetakenoutof the magnet for a better access to the target area (a “staged”procedure).Suchcasesarise,forexample,withobesepatients,andinthecaseofbonebiopsieswhereforceneedstobeappliedtotheinstruments.Needleartefactbasedguidanceisnotpossibleoutsidethemagnet, but other methods, e.g., optical tracking [1], can be usedinsteadforimprovedaccuracyininstrumentaligningandtargeting[2].Movingthepatientwill,however,shifthiscoordinatesystemfromthatofthemagnet.Trackingcanstillbedonewiththeopticalnavigator,provided that it is re-registered to the images and the patient.Typically,thishasbeendonewithapoint-and-clickmethod,wherethree or more points are located on theimagesand retraced on thepatientwiththenavigator.Themethodistime-consuminganderrorprone.Wehaveinvestigatedanalternativewheretwoopticalreferenceframesareused[3,4].Inthismethod,themagnethasafixedreferenceframe,providedbyanintegraltrackerplatevisibletothenavigator.Thenavigatorreferstheorientationandpositionofanyothertrackertothe “magnet tracker”. Assuming that the magnet tracker has beencalibrated to the image coordinate system and the patient has notmoved after the image acquisition, software is able to calculatepositionoftheothertrackerswithrespecttotheimageset.Thepatientmovement is compensated with a second tracker plate which isattacheddirectlytothepatient:Whenthenavigatorhasaline-of-sightto both the magnet and patient tracker, the calibration data can beautomatically converted to a coordinate system where the patienttracker functions as the reference frame. Assuming that the patienttrackerisrigidlyattachedtotheanatomyfromwhichtheimagesetistaken,itisthenpossibletocalculatethepositionofothertrackerswithrespecttothefixedanatomy,irrespectiveoftherelativepositionofthenavigatorandthepatient.Thepatientcanbemovedoutofthemagnetandtheinstrumenttracked,aslongasthenavigatorhasaline-of-sightbothtotheinstrumentandpatienttracker.MethodsAsimplefixationdeviceforattachingapatienttrackerplatetotheextremities was built (See Figure 1). It consisted of two L-shapedsupportswithstraps.Theblockscouldbedockedtoaplatethatfittedthe patient couch of the 0.23 T open magnet (Proview, MarconiMedicalSystems,Ohio).Thetrackerplatewasattachedtooneoftheblocksandcommercialopticalnavigatorsystem(iPath200,MarconiMedical Systems, Ohio) used for navigating with the device. Thesystemwasdemonstratedwithacasewherealargesofttissuetumourontheposteriorsideofthepatient'sleftarmwasbiopsiedunderpatienttrackerguidance:Thetargetareawassupportedwiththeblocksfrombothsidesandstrappedimmobile.AFE3D-setwasacquiredfromthetarget volume. The patient tracking was enabled through the userinterfaceandthepatientwasmovedoutofthemagnet,approx.0.8m,forabetteraccess.Thetargetpointwasselectedandtheneedleguidedtothetarget.Nearrealtime2Dimageswerecalculatedfromthe3Dsetusingamultiplanarreconstruction(MPR)algorithmandupdatedonthe display of the navigator system. The accuracy during thedemonstration was ascertained by moving the patient back to themagnet.VerificationimageswereobtainedwithfastFE2Dsequences.The images contained the needle artefact, which could be used forestimatingtheaccuracyoftheguidance.Alsotheshapeandpositionofthetumourcouldbeinspectedtoconfirmthelevelofimmobilisation.Figure1.Afixationdeviceforimmobilisation.Alsodisplayingthepatienttrackerplatewiththreereflectivespheres.ResultsThe coordinate conversion from the magnet to patient tracker wassuccessfulandnavigatorcontrolledMPRimagesprovideddatathatallowed guiding the needle to the target. Verification imagesconfirmed success in needle positioning. According to the images,tumourwasimmobileduringthewholeprocedureanddidnotdeform.Figure2showstheneedlepositioninanMPRimagejustbeforethetargetwasreached.Italsoshowsthecorrespondingverificationimage,where the displayed overlaid graphics closely match the needleartefact.Figure2.AnMPRimage(left)andsubsequentverificationimage(right).Instrumentisshownasathickline,itsextensionlineasathinline.Thetargetpointismarkedwithasmallsphere.DiscussionUsingtworeferenceframesprovidedafastmethodforcalibratingthesysteminstagedprocedures.Asimplefixationdevicewassufficientforprovidingthenecessaryrigidityforthissofttissuebiopsy. Thesystem could be further improved with more sophisticated fixationmethods [5]. Performing the biopsy next to the magnet allowedfrequent rescansandrapidre-registration,whichdecreasedtherisksassociatedwithtissuemovementanddeformation.References1. Silverman SG, Collick BD, Figueira MR, et al. 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