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FlyingFocalSpot(FFS)飞焦点技术
Sensation64Scanner:HowitworksThomasFlohr,SiemensMedicalSolutionsTheSOMATOMSensation64makesuseofaperiodicmotionofthefocalspotinthelongitudinaldirection(z-flyingfocalspot)todoublethenumberofsimultaneouslyacquiredsliceswiththegoalofimprovedlongitudinalresolutionandeliminationofspiralartifacts.BypermanentelectromagneticdeflectionoftheelectronbeamintheX-raytubethefocalspotiswobbledbetweentwodifferentpositionsontheanodeplate.Duetotheanodeangleof7°thistranslatesintoamotionbothintheradialdirectionandinthez-direction.Theradialmotionisaside-effectwhichistakencareofbytheimagereconstructionalgorithms.Theamplitudeoftheperiodicz-motionisadjustedsuchthattwosubsequentreadingsareshiftedbyhalfacollimatedslicewidthinthepatientslongitudinaldirection,seebelow.从两个不同旳角度对每一种解剖细节进行不间断旳重叠数据采集，取得双倍旳采样
FFS
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),sothatdataacquiredindifferentr
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