FFS(飞焦点技术)PPT幻灯片

1,FlyingFocalSpot,(FFS),飞焦点技术,2,Sensation64Scanner:HowitworksThomasFlohr,SiemensMedicalSolutionsTheSOMATOMSensation64makesuseofaperiodicmotionofthefocalspotinthelongitudinaldirection(z-flyingfocalspot)todoublethenumberofsimultaneouslyacquiredsliceswiththegoalofimprovedlongitudinalresolutionandeliminationofspiralartifacts.BypermanentelectromagneticdeflectionoftheelectronbeamintheX-raytubethefocalspotiswobbledbetweentwodifferentpositionsontheanodeplate.Duetotheanodeangleof7thistranslatesintoamotionbothintheradialdirectionandinthez-direction.Theradialmotionisaside-effectwhichistakencareofbytheimagereconstructionalgorithms.Theamplitudeoftheperiodicz-motionisadjustedsuchthattwosubsequentreadingsareshiftedbyhalfacollimatedslicewidthinthepatientslongitudinaldirection,seebelow.,从两个不同的角度对每一个解剖细节进行不间断的重叠数据采集，获得双倍的采样,3,FFS,4,Therefore,themeasurementraysoftwosubsequentreadingswithcollimatedslice-width0.6mminterleaveinthez-direction,andeverytwo32-slicereadingsarecombinedtoone64-sliceprojectionwithasamplingdistanceof0.3mm.Withthistechnique,64overlapping0.6mmslicesperrotationareacquired.Thez-coverageis32x0.6mm=19.2mm,andthesamplingschemeisidenticaltothatofa64x0.3mmdetector.Thisfinesamplingisthereasonfortheimprovedspatialresolutionandtheeliminationofspiralartifacts.Theimprovedsamplingisobtainedatanyspiralpitch.Hence,resolutionisimprovedandspiralartifactsareeliminatedatanypitch.Theimprovedsamplingisfurthermorenotrestrictedtotheiso-center,butismaintainedinawiderangeofthescanfieldofview(SFOV),seethefigureabove.Thisisamajordifferencetoconventionalapproacheswhichattempttoimprovelongitudinalsamplingbythechoiceofoptimizedsmallpitchvalues(so-calledHighQualitypitches),sothatdataacquiredindifferentrotationsinterleaveinthez-direction.Inthiscase,asamplingdistanceofhalfthecollimatedslicewidthcanbeachievedclosetoiso-centeronly,seebelow,andimprovedresolutionwillonlybeachievedclosetoiso-center,whichisaclinicaldrawback.Freeselectionofthepitchisnotpossible,andtheuserisrestrictedtospecialmodeswithlowtablefeedandlowvolumecoverage.,32x0.6mm=64x0.3,5,Theclinicalbenefitsofoptimizedz-samplingwiththez-flyingfocalspottechniquearetwo-fold:firstly,longitudinalresolutionisimprovedatanypitchandinawiderangeoftheSFOVbyestablishingnarrow,well-definedslicesensitivityprofiles(SSPs).Secondly,spiralartifactsaresuppressedatanypitch.Typicalspiralartifactspresentashyper-orhypo-densewindmillstructuressurroundingz-inhomogeneoushigh-contrastobjectssuchasbones,whichrotatewhenscrollingthroughastackofimages.UsingconventionalMDCTscanandreconstructiontechniquesspiralartifactscanbereducedbyeitherdecreasingthepitchand/orincreasingthereconstructionslicewidthrelativetothecollimation.Bothapproachesaimatimprovingthespiralsamplingalongthez-direction,butattheexpenseofreducedtablefeedand/orreducedspatialresolution.UsingconventionalMDCTsystems,demandingapplicationssuchasneuroscanningneedlowpitchprotocolstoreduceartifactsandtoimproveimagequality.CTAsofthecarotidarteriesandthecircleofWillisforinstancerequireacarefuloptimizationofthespiralpitchtoensuresufficientvolumecoveragespeedontheonehandandavoidintolerablespiralartifactsontheother.Thez-flyingfocalspottechniquemaintainsalowartifactleveluptohighpitchvaluesevenforcriticalapplications,thusincreasingthemaximumvolumecoveragespeedthatisclinicallyuseful.,成功应用,6,Tosumup:Thez-flyingfocalspottechniqueimproveslongitudinalresolutionandeliminateswindmillartifactsatanypitch.Eventhemostdemandingclinicalapplicationscanbeperformedatmaximumpitchwithoutdegradationofimagequalityorresolution.ThisisamajordifferencetoconventionalCT-systemswhichclaimtohavealargerdetectorcoveragebutarerestrictedintablefeed(HighQualityModes),ifgoodimagequalityisrequired.,飞焦点技