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New xray tube performance in computed tomography by introducing the rotating envelope New xray tube performance in computed tomography by introducing the rotating envelope

New xray tube performance in computed tomography by introducing the rotating envelope - PDF document

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New xray tube performance in computed tomography by introducing the rotating envelope - PPT Presentation

We report on a new tube technology satisfying all these demands by making use of a novel cooling principle on one hand and of a novel beam control system on the other hand Nowadays tubes use a rotating anode disk mainly cooled via radiation The Stra ID: 29807

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Newx-raytubeperformanceincomputedtomographybyintroducingtherotatingenvelopetubetechnologyPeterSchardt,JosefDeuringer,JorgFreudenberger,ErichHell,WolfgangKnupfer,DetlefMattern,andMarkusSchildSiemensMedicalSolutions,VacuumTechnology,Erlangen,GermanyReceived30January2004;revised16June2004;acceptedforpublication28June2004;published27August2004Thefuturedemandsofcomputedtomographyimagingregardingthex-raysourcecanbesumma-rizedwithhigherscanpower,shorterrotationtimes,shortercooldowntimesandsmallerfocalspots.Wereportonanewtubetechnologysatisfyingallthesedemandsbymakinguseofanovelcoolingprincipleononehandandofanovelbeamcontrolsystemontheotherhand.Nowadaystubesusearotatinganodediskmainlycooledviaradiation.TheStratonx-raytubeisthe®rsttubeavailableforclinicalroutineutilizingconvectivecoolingexclusively.Itisdemonstratedthatthiscoolingprinciplemakeslargeheatstoragecapacitiesoftheanodediskobsolete.Theunprecedentedcoolingrateof4.8MHU/mineliminatestheneedforwaitingtimesduetoanodecoolinginclinicalwork¯ow.Moreover,anelectronicbeamde¯ectionsystemforfocalspotpositionandsizecontrolopensthedoortoadvancedapplications.Thephysicalbackgroundsarediscussedandthetechnicalrealizationispresented.Fromthisdiscussionthesuperiorsuitabilityofthistubetowithstandg-forceswellabove20gcreatedbyfastrotatinggantrieswillbecomeevident.Experiencefromalargeclinicaltrialisreportedandpossiblewaysforfuturedevelopmentsarediscussed.AmericanAssociationofPhysicistsinMedicine.DOI:10.1118/1.1783552Keywords:x-raytubes,anodecooling,electronbeam,cardiacCTI.INTRODUCTIONMedicalx-raydiagnosticsmakesuseoftwotypesofx-raytubes.Forlowpowerapplicationsstationaryanodetubesarethebestcompromisebetweentechnicaldesigneffortandper-formance.Butformostmidrangeandhighperformanceap-plicationsintermsoffocalspotpowerthereisaneedtoutilizerotatinganodetubes.Thesetubesprovideafocalspotpowerof60±100kWforcomputedtomographyaswellasangiography.EspeciallyinCT,largeamountsofdissi-patedenergyareproducedbypatientscansintheorderofsometenseconds.Incontrasttothestationaryanodestheheathastobestoredintheanodediskandhastobetrans-ferredbyradiationathightemperaturestothecoolingme-dium.ThemaincomponentsofsuchatubecanbeseenfromFig.1takenfromRef.1.Asaconsequenceofthisbottle-neckfortheheat¯ow,theanodehastostorenearlythecompleteamountofenergyuptoseveralMJorexpressedinthemorecommonunitMegaHeatUnit,MHU,where1MJequals1.34MHU.ThereasonisillustratedinFig.2.Thetemperatureinthefocalspotislimitedbythematerialusedandformedicaltubessomewherebelowthemeltingpointoftungsten.Thistemperatureforrotatinganodesisacomposi-tionoftheanodedisktemperatureandtheshorttemperatureriseofthematerialpassingthetargetspotoftheelectronbeam.Thetemperatureriseisafunctionofmaterialproper-ties,focalspotsize,focalspotpowerandthevelocityoftheforrotatinganodesafunctionofradiusandrotating.Incontrastthedisktemperatureisdeterminedbytheheatcapacityandtheappliedcoolingmechanism.Manyimprovementstocoolthediskdealwithenhancedemissivityorincreasedareasofradiationemittingandabsorbingoptimizedforx-raytubes,e.g.,inRef.2.Thegoalistomaximizetheheat¯owgivenbytheStefan±Boltzmannradiationlaw,beingthetemperatureoftheanodeandthetem-peratureofthehousing.FewattemptshavealsobeenmadetointroduceconvectivecoolingbyusingliquidmetaltobeartheanodediskandachieveamodestimprovementoftheCoolingratesofupto1.4MHU/minhavebeenHowever,forafurtherincreaseofcoolingratesonepossibilityistogettheanodediskitselfincontacttothecoolingmedium.Onesolutionforthatisadesignwheretheanodediskispartofthetubeenvelope.Thisimpliesrotationoftheentiretubewithrespecttotheanodeaxis,whichwerefertoastheclassofrotatingenvelopetubescontrasttotherotatinganodetubes,wherethetubeenvelopeisstationary.TheStratontubeSiemensMedicalSolutions,Erlangen,Germanyisthe®rsttubebasedonthistechnologyintroducedintothehighperformanceclassofcomputedto-mographywithoutstandingpowerreserves.PhysicaldetailsofconvectivecoolingandbeamcontrolarediscussedinSec.II:theimpactonthetechnicalrealiza-tionispresentedinSec.III.Wereporttheresultsfromalargeclinical®eldtrialinSec.IVbeforeanoutlooktofuturedevelopmentsisgivenintheconcludingsectionSec.VII.PRINCIPLEANDPHYSICALASPECTSAveryearlyappearanceoftheideatorotatethetubeenvelopeandde¯ectanelectronbeambystationarymeansisMed.Phys.31,September20040094-2405$22.002004Am.Assoc.Phys.Med. ®rstdocumentedinapatentof1917.ThisdesignisinfactveryclosetotherealizedStratontube.Withinthisprinciple,theelectronbeaminthetubeisshapedandcontrolledbyamagnetic®eldsimilartoaminiatureelectronbeamCT.InFig.3onecanrecognizethevacuumhousingrotatinginbearingsoutsidethevacuumanddrivenbyanexternalmo-tor.Thetubehousingitselfconsistsofacathodeshaft,aceramichighvoltageinsulator,atubeenvelope,anannularx-raywindowandtheanodediskmountedonasecondshaft.Forrunningthetube,adrivemotorisattachedtotheanodeshaftandamagneticde¯ectionsystemispositionedintheplaneofthetubewaisttode¯ecttheelectronbeamontothefocalspotposition.Inthefollowingwediscussthephysi-calaspectsofthetwomajordifferencesbetweenrotatingenvelopetubesandrotatinganodetubes,namelythethermo-dynamicsinSec.IIAandtheelectronbeamdynamicsinSec.IIB.A.ThermodynamicsInthisconceptthecoolingsurfaceoftheanodeisindi-rectcontactwiththecooling¯uid.Theheattransferforthesocalledconvectivecoolingisgivenbyisthetransferredpower,theheattransfercoef-thecoolingsurfaceandthetemperaturedif-ferencebetweencoolingsurfaceandcooling¯uid.Thelarg-estvaluesofareachievedforturbulent¯ow,becausetheturbulencecausesadditionalmaterialtransportinthedirec-tionperpendiculartothecoolingsurface.Withthehelpofnumericalcomputersimulationsusing®-niteelementmethodsonecanderivevaluesof36.000Kforananodediskof60mmradiusandarotatingspeedof150Hz,asillustratedinFig.4.Toobtainaroughestimationfortheheat¯ow,onecaninsertnumbersinEq.forthecoolingsurfacearea85cm,thetemperaturedifference250Kandameanof30.000W/mKtoendupwithaheattransferrateofmorethan60kWdirectlytrans-ferredtothecooling¯uid.Thisvaluecorrespondstoanun-precedentedcoolingrateof4.8MHU/mincomparedtostateoftheartperformancesof0.8±1.4MHU/min.LookingatFig.5onecanrecognizethatin20stheanodeiscompletelycooleddowntotheoiltemperature.Thecooldowntimeforaradiationcooledanodeisalsovisible.Withthisexampleit .1.Mainpartsofamedicaldiag-nosticx-raytube1:cathode,2:anode,3:rotor,4:ballbearings,astakenfromRef.1.Thelargeanodediskhastostoretheheatgeneratedduringtheex-posureandradiatesatveryhightem-peraturestocooldownintheinterme-diatetimes. .2.Temperaturesofanodediskandfocalspotforaspeci®cappliedelectricalpoweratthefocalspot.ThecomparisonexhibitsthedifferenceintemporalbehaviorforarotatinganodetubeandacorrespondingrotatingenvelopetubeconvectivecoolingNumericalvaluesareforillustrationonly. .3.PrincipleofoperationandmainpartsofaREThousingassembly.2700Schardtetal.:Stratonx-raytubeMedicalPhysics,Vol.31,No.9,September2004 becomesclearthatthereisnoneedforalargeheatstoragecapacityattheanodeanylonger.However,asmallamountisstillneededfortheanodetoactasaheatspreaderfromtheareaofthefocalspottothelargeareaofthecoolingsurface.Usingthissetup,therotationoftheanodeservesadual-purpose:insideofthetubetherotationmaintainsatolerablefocalspottemperaturerise,outsideofthetubetherotationcausesturbulentoil¯owtomaintainlargeheattransferco-Nevertheless,therequirementsofturbulenceandrotationspeedresultinasubstantialamountofdissipatedfrictionalpowerwhichhastobeprovidedbytheexternalmotor.Inthefollowingwedescribeawaytoreducethedrivepowerbygeometricalimplications.Thefunctionaldependenciesofthefrictionalpowerongeometricandphysicalpropertiesoftherotatingpartcanbeexplainedbyisthedrivingpower,thedensityandcinematicviscosityofthe¯uidandtherotationalfrequency.hasbeenderivedforasolidcylinderofradiusandheightrotatinginastationarycylinder.Thero-tatingcylinderandthe®xedsurroundinghousingde®neanintermediatevolumebetweenthecylin-dersofthickness,wherethefrictioninthe¯uidisgener-ated.FromEq.itbecomesclearthatthereisastrongdependencyonthetuberadiusandtherotationalfrequency.Theviscosityofthe¯uidandthewidthoftheslithaveonlyaminorimpact.Amajortaskforkeepingdrivepowerlowisthereforeanoptimizationoftheradialdimensionsofthege-ometry.AsaresultofthiswefoundaverycompactdoubleconeshapeddesigndescribedindetailinSec.III.Inparticu-larwefoundothersolutionstorealizearotatingenvelopeforexample,atubewithabearedandmagnetically®xedcathodetoexhibitmuchlargerdrivepowerforagivenanodediameterandrotatingfrequency,becausealargehastobemaintainedoveralongheightB.ElectronbeamdynamicsThesecondphysicalaspectcoverstherequirementsfortheelectronbeamde¯ectionandfocusing.Toachieveanx-rayfocalspot®xedinspacewhilethewholetubeisrotat-ing,astationarybeamde¯ectionsystemisnecessary.Intheeasiestwaythisisachievedbyadipolemagnetsystemde-¯ectingtheelectronbeamgeneratedonthetubeaxissuchthatthebeamhitstheanodediskexactlyatthefocalspotpositionofthetubehousingassembly.Inthetubevoltagerangeusedformedicalimagingofabout40kVto150kV,theelectronsgainaspeeduptoabout50%ofthespeedoflight.Therefore,theLorentzforcehastobecalculatedusingtherelativisticelectronmassandisnonlinearinthisrange.Thebasicformulaforthede¯ectionoftheelectronbeamwillbefoundbythebasicequationsfortheLorentzforceandthecentrifugalforce.Theresultisthecurvatureradiusoftheelectrons, beingtherelativisticmassofanelectronacceleratedbythevoltage.UsingasimplemodelseeFig.6thedistanceofthefocalspotfromtherotationcenterfocaltrackradiusdependsonlyonthegeometricdataand .4.Localheattransfercoef®cientasafunctionofdistancefromtheaxiscalculatedfortheStratongeometrywithatuberadiusof60mm.Theradiusofthefocaltrackis48mm.Dataaregivenfortworotatingfrequen-ciesandareobtainedbya¯uiddynamicalFEMsimulation. .5.CoolingcurvecomparisonwithdatafromtimeresolvedFEMcal-culations.Uppercurveforrotatinganodetube;lowercurvefortheStraton .6.Simpli®edmodeltocalculatetherequiredmagnetic®eldstrengthBforagivengeometryandacceleratingvoltageU.Themagnetic®eldbendsthebeamwithcurvatureradiusRoverthelengthL1.Afterthe®eldfreesectionoflengthL2theelectronshittheanodeatthefocalspotposition.2701Schardtetal.:Stratonx-raytubeMedicalPhysics,Vol.31,No.9,September2004 S5S12sinSacosSL1 RR1L2 tanSacosL1 UsingEqs.weobtaintherequiredmagnetic®eldforagivende¯ection,whichisplottedasafunctionofthetubevoltageinFig.7.Despitethenonlinearfunctionforthede¯ection®eldanadditionalaspectcomplicatesthesituation.Theelectronbeamlengthwayofelectronsfromemittertothefocalspotisabout115mmandthereforeconsiderablylongerthaninconventionalx-raytubesseeforexampleFig.1,wherethebeamlengthisabout30mm.Thisisduetothefactthatthebeamiscreatedontherotationalaxisofthetube,acceleratedinthedirectionoftheaxisandthende¯ectedtothepositiongivenbythefocaltrackradius.Asaconsequencethereof,spacechargeeffectsplayanimportantrole,becausetheelec-tronsarenotfullyrelativistic.Theelectricandmagnetic®eldswithinalongbeamwithcylindricalsymmetryand «0rE0rr8r~r8!,~6a!Bw5m0 Uponanindividualcharge,these®eldsin¯ictrepulsiveandattractiveforces,respectively,accordingtothebasicequa-tionofmotion, oneendsupwiththeresultingradialforce, whichisdefocusingforallbuthighlyrelativisticparticles,thefractionalspeedoflight.Fromtrajectoryequationcanbederivedundertheassumptionofahomogeneousradialcurrentdensity.Withconstantcurrentandenergy,onecanfurthermoregeneratetheenvelopeequa-tion,whichgivesthebeamradiusasafunctionoftheimplicitform Rz!5 p«0m0~!3•1 ThisdifferentialequationcanbesolvednumericallyandisplottedinFig.8.ItcanbeseenthatforthegivenbeamparametersabeamwithalargebeamlengthlikeinaRETismuchmorespreadthaninaconventionalrotatinganodetube.However,thiscalculationexampleisonlyforillustration,becausenoneofthegivenidealassumptionsisaccomplishedintherealtube.IntheStratontubetheaccel-eration,de¯ectionandfocusingofthebeamarenotsepa-ratedinspace.Propertiesoftherealbeame.g.,beamsizeatfocalspotpositionhavethereforetobecalculatedwithFEMTubecurrentandvoltagearedeterminedbytheapplica-tionandthereforevariable.BothareselectedinordertooptimizeCTimagequalitywithrespecttothegivenattenu-ationpropertiesofthehumanbody.Tomaintainacertainrangeofoperationwithstablefocalproperties,oneneedsadditionalfocusinginordertocompensate.Tosupplythisadditionalforce,aquadrupolecomponentofthemag-netic®eldmaybeusedinadditiontothedipolecomponentofthede¯ection®eld.Thecalculationoftheelectrontrajec-toriesunderthein¯uenceofthesesuperimposedmagnetic®eldcomponents,whiletheelectronsareacceleratedsimul-taneously,iseasilydonebyFEMcalculationsevenunderconsiderationofthespacechargeeffects.AnexampleforsuchacalculationisgiveninFig.9.Ignoringthevacuumenvelopeofthetube,thetracksoftheelectronsarevisiblestartingattheemitterandtravelingthroughthemagneticde¯ectionandfocusing®eldbeforereachingtheanodeatthefocalspot.III.TECHNICALREALIZATIONAlthoughtheideaofturningthetubehousingisveryoldandtheunderlyingphysicsiseasilyunderstandable,thistechnologydidnot®nditswaytoroutinemedicalimagingbefore2003.Thisismainlybecauseofsomeobstaclesin .7.Fieldstrengthformagneticbeamde¯ectionsystemasfunctionoftubevoltageU. .8.Beamspreadforanelectronbeamcalculatedwithaninitialradiusof2.5mmattheemitterasafunctionofbeamlength100%isthebeamdiameterattheemitterincomparisonforrotatinganodetubesandaRET120kVtubevoltageand500mAbeamcurrent2702Schardtetal.:Stratonx-raytubeMedicalPhysics,Vol.31,No.9,September2004 technicalrealization,whichhadtobeovercome.Fromimagequalityrequirementsonfocalspotstabilityaneedforanintelligentbeamcontrolcanbederived.Positionandstabilityinshapearecrucialespeciallyincomputedtomography.Thereforeamicrocontrollerisessential,inparticularconsid-eringthemostlynonlineardependenciesasdiscussedbefore.However,alsomechanicalaspectshavetobetakenintoac-count.Highperformancetubeshavetorunatveryhighro-tationspeedsandonecanimagine,thatthetraditionalglasstubetechnologyisnotadequateintermsofrequiredpreci-sionandstability.Sincetheearlynineties,themetalceramictechnologyisroutinelyusedformedicaltubeswithhighestpowerdemandsandisakeyfactorformanufacturingreli-ablerotatingenvelopetubes.Outofthesereasons,wedescribetherealizationanddis-cusssomepropertiesofthekeycomponentsoftheStratontube,namelythetubeenvelopeinSec.IIIAandthe®lamentforelectronemissioninSec.IIIB.InSec.IIICwedealwiththemagneticde¯ectionsystemandinSec.IIIDwiththecoolingsystem.A.TubeenvelopeInFig.10theStratontubeisshownwithananodediskof120mmdiametercapabletorotateat150Hz.Thetubeen-velopemadeofnonmagneticstainlesssteelisattachedtotheanodediskmadeofatungstenzirconiummolybdenumbodyequippedwithatungstenrheniumfocalspottrackincludinganannularx-raywindowof0.2mmthicknessmadeoutofstainlesssteel.Incontrasttoallotherx-raytubes,thewin-dowisrotatingthroughthex-raybeamandhastobeex-tremelyuniformintermsofx-rayattenuation.Onthecath-odeside,aceramicdiskinsulatorensuresareliablehighvoltageinsulationofabout170kVinthevacuumwellasinthecoolingoil.Thegeometriesofallthecomponentswerechosenverycarefullyinordertokeepthedrivepoweracceptable.Thepowerrequiredtomaintaintherotationandturbulent¯owoftheinsulatingoilatthemaxi-mumrotationalspeedof150Hzdoesnotexceed6±7kW.Tokeepfrictionalpowerlosseslow,thevacuumhousingisac-inabouthalfasecondtothishighspeedonlywhenthemaximumelectronbeampowerisimpingingtheanode.Allotherapplicationsrunatlowerrotatingfrequen-cieswithanintelligentpowermanagement.Therefore,theaveragedrivingpowercanbekeptbelowalevelof500±600Wonly.B.ElectronemissionMuchattentionhastobespentonthe®lament.Ideally,acircularuniformemissionisneededtoavoidunwantedrota-tionalfocalspotsizemodulations,whichmayreduceimagequalityincomputedtomography.Ourtubeusesacircularemittermadeof100mthicktungstensheetmaterial,whichiscutbythelaserinawaytoformmeanderlikepathsforthe®lamentheatingcurrent.InFig.11onecanrecognizethestructureofthe¯atemitterinsidethefocusingdevice.This¯atemittertechnologyhasimportantadvantagesconcerninghighspeeddosevariationsduringfastCT-scans.Thelimitingparameterforfastemissionchangesisthecool-ingtimeoftheemitter.Assumingradiationcoolingonly,athightemperatures2000ÉCtherateofchangefortheemittertemperatureisgivenby .9.FEMcalculationofelectrontrajectoriesfromthecathodethroughthecomplexmagneticde¯ectionandfocusing®eldtothefocalspotatthe .10.TheStratontubewithacathodeandceramicinsulatorattheleftandananodeattheright.Ballbearingsaremountedaswellastheinductive®lamenttransformer.Thelargestdiameterofthetubeis120mm. .11.DetailsoftheStratoncathode.Insidethefocusinghead,theemittermadeoftungstensheetisvisible.Theemitterhasadiameterof5mm.2703Schardtetal.:Stratonx-raytubeMedicalPhysics,Vol.31,No.9,September2004 dt51 denotingthespeci®cheatcapacity,massandtherateofchangeofthethermalenergy.Takingintoac-count,thatthetemporalchangeofiscausedbyradiationandequalsthereforetheradiatedPowergivenbyEq.oneendsupwith « cs•AE beingtheareaoftheradiatingsurfaces,temperaturesoftheemitterandthesurroundingsurfaces,re-spectively.Therateoftemperaturedeclineisdependentonthegeometryfactorforagivenmaterialonly.Insertingvaluesforatypicalhelicaltungstenwireemit-ter(93mm250mg)andcomparingthemwiththenewdeveloped¯atemitter(40mm37mg),afactorofthreeisgainedinvariationspeed.Thisisingoodagreementwithmeasurements.Fordosemanagementinclinicalapplications,thisisacrucialbene®tinachievinganaccuratedosepro®leasafunctionofgantryrotation,inpar-ticularforhighestrotatingspeedsincardiology.Thetuberotatinginlubricatedballbearingshasaninduc-tiveheattransformer,whichtransmitsthe®lamentheatingpowerfromastationaryprimarycoiltotherotatingsecond-arycoilvisibleinFig.10toavoidanydegradationoftheinsulationoil.C.Magneticde¯ectionsystemThede¯ectionsystemhastoperformthreetasks:®rstthede¯ectionofthebeamintheradialdirectionontothefocalspotandtoperforma¯yingfocalspotinthesecondthefocusingofthebeamtodeterminethesizeoftheelectronicfocalspot;andthirdade¯ectiontomakeuseofa¯yingfocalspotinthephi-direction.Forthispurposeamag-netdesignsimilartoaquadrupoleischosenofwhichFig.12givesaschematicsketch.Incontrasttoapurequadrupolemagnet,theStratonmagnetsystemhasthreedifferentsetsofcorrespondingtothethreetasksmentionedabove.-coilsgeneratethemaindipolecomponentofthemag-netic®eldtode¯ectthebeamontothefocalspot,whichisquasistaticintimedisregardingtheswitchingwhenthetubevoltageisturnedonoroff.Thefocusingisperformedby-coilsgeneratingaquadrupolecomponentusedforformingtheelectronicfocalspot.Thethirdsetupareusedtoproduceasmallbutfastdipolecomponentforthe¯yingfocalspotinthetangentialdirection.BecauseallthecoilcurrentsdependonthevalueofthehighvoltageusedinthewayasdescribedinSec.II,amicrocontrollerdrivesin-dividualcurrentpowerampli®ersforeachcoilsetup.Themicrocontrollercalculatestherequiredcoilcurrentinrealtimeasafunctionofthemeasuredhighvoltage.Foreachfocalspotsize,asetofcoilcurrenttablesismanagedbythemicrocontroller.So,anelectronicallyadjustablefocalspotisrealizedhavingtheadvantagesoflowtolerancesinspotsizedimensions,fastswitchingandhigh¯exibility.Forex-ample,ourRETusesthreedifferentfocalspotsizeswithonlyoneemitter,whichcanbeusedtoadapttothebestimagequalitypossible.TableIillustratesthevarietyofthespotsizesforStraton.D.CoolingsystemThecoolingsystemusedfortransferringtheheatfromtheoiltothegantryairisdesignedforthelargecentrifugalforcesatgantryrotationtimesof0.37sperrevolutionandiscapableofcoolingoiltemperaturesofabout100ÉC.Toavoidanycavitationintheoilduetothefastrotatingtube,whichmayresultinareducedhighvoltagestability,thecoolingsystemworksatapressureofabout1bar.Althoughthereisanoilpumpinthesystem,themainoil¯owiscausedbytherotatingtube.Wemeasured25L/minwhenthetubeisrotating,resultinginaveryeffectivecoolingincontrastto8L/minwithonlytheoilpumprunning.Theoiltoairheatexchangerisdesignedtocooldownacontinuouspowerof7kW,whichisappropriateforhighperformanceCT.However,thisheatexchangeristhebottleneckforthemaximumcontinuouspowernow.Whiletheheatfromtheanodeisdirectlytransferredtothecoolingoil,ithastobestoredbytheheatcapacityoftheoilcontainedbythetubehousingassemblyandthecoolingsystem,untilitisagaincooleddownbythegantryair.Inthissituationtheaveragepowerisonlydeterminedbythecoolingcapacityoftheheatexchanger.Therefore,inprinciplemuchlargeraveragepowerwouldbepossiblewithanappropriatedesignofthecoolingsystem. .12.Schemeofthemagnetsystemwithdifferentcoils. I.ThreeelectronicallyadjustedsizesforthefocalspotsofStratonandtheirmaximumloadfor20sFocalspotSpotsize(wIEC60336valuesMaximumload20sscantimeFI0.60.742kWFII0.80.851kWFIII0.81.260kW2704Schardtetal.:Stratonx-raytubeMedicalPhysics,Vol.31,No.9,September2004 IV.PERFORMANCERESULTSANDCLINICALFIELDTheStratontubewasintensivelytestedinaclinical®eldtrialontheSOMATOMSensation16scannerMedicalSolutions,Forchheim,Germany.About25installa-tionsrunningfromFeb/03toOct/03weremonitoredonlineviaremotecontroltoobservetheclinicalperformance.Theinstallationswiththesetubesexhibitedessentiallynotubecoolingtimes,con®rmingthedifferenceinthetubecoolingperformance.Oneclinicisrunningthetubeat45.000scansecondspermonthwithabout50±60patientsperday.Animpressivenumberforthedemonstrationofawork-¯ownolongerlimitedbywaitingtimesduetotubecooling.ThefocalspotFIImediumsize,seeTableIwasusedformorethan90%ofallscans,resultinginastatementthat50kWscanpowerisappropriateforroutineexamination.AnultrahighresolutionmodeutilizingtheFIaswellasthehighpowermodeswereonlyusedinafewcases.TheStratontubeisthe®rsttuberoutinelyusedatagantryrevolutiontimeof0.37s.Thismodewasusedforallcardiacinvestigations,becauseoftheunprecedentedtimeresolutionforarotatinggantryscanneronlysurpassedbyEBT-scannerelectronbeamtomography.Itdemonstratesthebigstepforwardbeingmadeinnoninvasivecardiacimaging.Animpressiveimageofthehearttakenat0.37srevolutiontimegiveninFig.13underlinesthepossibilitiestodayanddemonstratesthestrongdemandforfurtherimprovementoftemporalresolution.The®eldtrialclearlycon®rmed,thattheballbearingsnolongercausetubefailuresevenatthehighcentrifugalforces.Here,thesmalltubewiththelowweightofonly3.6kgfortheentiretubeandthelubricationofthebearingsresultedinnoobservedbreakdownduetobearingfailure.Thishastobeseenindirectcontrasttoconventionalrotatinganodetubetechnology,wherebearingsinsidethevacuumaredif®culttolubricate,becauseofhighoperatingTheoverallincreasedlifetimecapabilitycouldofcoursenotbedemonstratedduringtheshorttrialperiod.However,weobservedasigni®cantlowernumberofhighvoltagemainlybecausetherearenomovingpartsintheandalowerfocaltrackdegradationresultinginamoreconstantx-rayoutputcomparabletoconventionalV.CONCLUSIONSANDFUTUREDEVELOPMENTSFromallthephysicalandtechnicalaspectsdiscussedbe-fore,someconclusionsforfuturedevelopmentscanbeAlthoughthedemandforaclinicalwork¯ownotlim-itedtocooldowntimesatanysituationisastrongdriverfortheconvectivelycooledtubetechnology,theclinicalappli-cationsstillwillaskforfurtherimprovementintemporalresolutionforcardiacscanningandwillneedfasterrotatinggantriesinconsequence.TableIIshowsthecentrifugalforcesforfutureCTmachines,assumingatubeaxisata650mmradiusfromthecenterofrotation.Runningnowatabout20gthenextgenerationofscannerswillperform0.3sperrevolutionbeingnottheendofthescannerdevelopment.Wedemonstratedthattherotatingenvelopetechnologyisthe .13.Cardiacscanningat0.37sperrevolutiondemonstratingthesupe-riorresolutionappliedtothebeatingheartcourtesyofDr.A.Kuttner,Univ.ofTubingen,Germany .14.Flyingfocalspotwiththemagneticde¯ectionsystemnotonlyinphi,butalsointhe-direction.1:anodedisk;2:focalspottrack;3:focalspotposition;4:electronbeam;F1,F2:x-raysfromdifferentfocalspots. II.CentrifugalforcesattubeaxisasfunctionofgantryrotationGantryrevolutiontimeCentrifugalaccelerationontubeaxis650mmunitsof9.81m/s1s30.5s100.4s160.3s300.2s652705Schardtetal.:Stratonx-raytubeMedicalPhysics,Vol.31,No.9,September2004 mostappropriatewaytocardiacCTscanningwithhightimeresolution.Thishastobeseenasanalternativetolargerandheavierrotatinganodescarriedbybearings,whicharestillhardtolubricateinvacuum.MaybetheEBTelectronbeamtechnologyiscomparablewithrespecttotimeresolution,butsufferingfromotherknowndisadvantages.Themicrocontrolleronboardopensnewpossibilitiestoimproveimagequalityincomputedtomography.Incom-binationwiththemagneticde¯ectionsystem,themicropro-cessorcontrolsthebeamintwodirectionsofspaceatthefocalspotposition.Besidesthealreadymentionedfocalpo-sitionstabilization,oneisnowabletocreatea¯yingfocalspotnotonlywithintheplaneofthegantryrotationtheleftsideofFig.14,butalsointheradialdirectiontherightsideofFig.14.Thisopenstheopportunitytoacquiretwicethenumberofslicesperrevolutionwithagivendetector,becauseduetotheanodeanglearadialmovementofthefocalspotcorrespondstoashiftofthefocalspotpositioninpatientaxis.Moreover,themicrocontrollercanmonitortubeperformanceandwearoutofcomponentsinordertopredicttheremainingtubelifetime.Knowingthisnumber,servicecallscanbescheduledforatubeexchangewithoutdisturbingtheclinicalwork¯ow.Intimesofcompactelectronicsandsolidstatedetec-tors,theanodediameterofthex-raytubemainlydeterminesthesizeofaCTgantry.HeretheRETtechnologywithitssmarttubehousingassembliesopensawaytocompactgan-trydesign.Itenableslargerboresandabetterpatientacces-sibilityasafuturebene®tforusers.Electronicmail:peter.schardt@siemens.comNowatBSHBoschundSiemensHausgerateGmbH,ProductAreaRe-frigeration,Giengen,Germany.H.Bittorf,inImagingSystemsforMedicalDiagnostics,editedbyE.SiemensAG,Berlin,1990,p.234.G.LaveringandR.Treseder,VarianAssociates,USPatent5,689,542,C.Gerthsen,H.Kneser,andH.Vogel,in,editedbyH.VogelSpringer-Verlag,Berlin,1989,p.219.T.SchmidtandR.Behling,MedicaMundi,50WebsiteofTOSHIBAMedicalSystems,MegacoolŸX-raytube,http://www.medical.toshiba.com/clinical/radiology/aquilionmulti16-360-360-431.htm,2004.W.Coolidge,GeneralElectricCompany,USPatent1,215,116,1917.K.Stephan,inTaschenbuchfurdenMaschinebau,editedbyW.BeitzandK.-H.GroteSpringer-Verlag,Berlin,1997,p.D29.G.I.Taylor,Proc.R.Soc.London,Ser.A,494±512,499±523,565H.SchlichtingandF.W.Riegels,Grenzschicht-Theorie,Braun-Verlag,8.Au¯.,pp.491±513See,forexample,J.E.Burke,L.Miller,andS.G.Perno,PickerInter-nationalInc.,USPatent5,274,690,1993.J.Groûer,hrungindieTeilchenoptikB.G.Teubner,Stuttgart,,p.124.E.Lenz,P.Rother,andP.Schardt,SiemensAG,patentpendingtobeE.Hell,D.Mattern,andP.Schardt,SiemensAG,USPatent6,115,453,E.Hell,D.Mattern,andP.Schardt,SiemensAG,USPatent6,292,538,E.Hell,D.Mattern,andP.Schardt,SiemensAG,USPatent6,181,771,E.Hell,T.Ohrndorf,andP.Schardt,SiemensAG,USPatent6,084,942,J.Deuringer,P.Schardt,M.Schild,andJ.Freudenberger,SiemensAG,patentpendingtobepublished2706Schardtetal.:Stratonx-raytubeMedicalPhysics,Vol.31,No.9,September2004