X-Ray Tech Exposure Calculator, 5 Simple Relationships That ...

Since x-ray exposure goes approximately as kVp5 and we generally don't think in powers of 5 a well known approximation is typically used called ... Skiptocontent Aninteractivecalculatorforchangingmultipletechnicalparameters(kVp,SID,s,BuckyFactor)inandobservingtheeffectonthemAneededtokeeptheexposureontheimagereceptorconstant.GreatforstudyingforyourARRTboardsorbrushinguptosharpeningyourtechnicalparameterskillsforRadiographyexams. X-RayExposureCalculator Paymentoptions Submit Ifyouareahuman,donotfillinthisfield. TableOfContents X-RayExposureCalculatorOverviewofTechnicalParametersmA(tubecurrent)kVp(tubecurrent)s(time)SID(SourcetoImageDistance)BuckyFactor(ScatterGridFactor)GoldenEquation(AllTechnicalFactors) OverviewofTechnicalParameters Asatechnologistyouareoftenfacedwiththesituationwhereyouneedtomakeachangetoaprotocolbutwanttokeeptheexposureonthedetectorthesame.Inthisscenarioitisreallyimportanttounderstandthetechnicalparametersandtheirinfluencetheexposure,sothatwhenchangingparametersyoudonotendupunder-exposingorover-exposingtheimagereceptor(film,CRorDR). Thetechnicalparameterswhichcontroltheexposuremeasuredonthedetectorandalsotheradiationdosedeliveredtothepatientaresummarizedinthetablebelowandinclude(mA,kVp,s,SIDandBuckyFactor).Inthefollowingsectionswewilldescribeeachinabitmoredetailandlinktootherresourceswheretheseconceptswereintroducedandcoveredmorecompletely. Asatechnologistyouneedtocommitthesedependenciestomemorysothatyoucanhaveanintuitiveorgutfeelingaboutthechangesintechnicalparametersandthiswillhelpyoutoprovidethebestacquisitionsforyourpatients.Attheendofthethispostweputtheseintoonebigequation,butyoucanseefromthistablethattherearereallyjustFIVESIMPLERELATIONSHIPSthatyouneedtoknowtomasteryourtechnicalparameters. TechnicalParameterUnitsExposureDependenceTubeCurrentmA(milliAmperes)mATubePotentialkVp(kilovoltpotential)kVp5ExposureTimes(seconds)sSourcetoImageDistance(SID)cm(centimeters)1/SID2BuckyFactor(ScatterGridFactor)unitless1/Bucky mA(tubecurrent) Thetubecurrentinthex-rayexamistheparameterthatismostfrequentlychangedandyoushouldconsiderthisasyourfirstgo-toadjustmentinordertochangethex-rayexposurewithoutimpactingthequalityoftheexaminotherways(i.e.thecontrastandmotionartifactsarenotaffected). Increasingthetubecurrentisactuallyjustprovidingmoreelectronsthatwithsmashintothex-raytargetandthusproducemorex-rayphotons.Formoredetailsonthex-raytubeandinfluenceofmAcheckoutourarticlesonx-raygenerationandbeamquality. kVp(tubecurrent) ThekVpisthemostimportanttechnicalparameterasithasaverystrongdependenceonthex-rayexposure(kVp5).ThisstrongdependenceonthekVpfortheexposureattheimagereceptorisbecauseofthehigherfluxandstrongerpenetrationofhigherkVpimaging.Additionally,thekVpalsochangestheinherentcontrastinthex-rayimages. Sinceitdoesimpactthex-raycontrastchangestothekVpofanexamneedtobeconsideredmorecarefullythanchangingthemA,buttherearedefinitelytimeswhenitisneeded. AsthekVpisincreasedthex-raybeambecomesmorepenetrating.Thisisagoodthingforpassingthroughlargeanatomy.OntheotherhandasthekVpisincreasedtherelativecontrastbetweentissuesinthebodyisreduced(x-raycontrast).AsthekVpisincreasedthereisalsoahighercontributionofComptonscatterevents.Therefore,thegeneralruleisusethelowestkVpwhichwillstillprovideagoodexposureontheimagereceptor. ThisisonereasonwhymammographyexamsareperformedatalowkVpandalargeabdominalx-rayisperformedatahigherkVp. Sincex-rayexposuregoesapproximatelyaskVp5andwegenerallydon’tthinkinpowersof5awellknownapproximationistypicallyusedcalledthe15%ruleofkVpofradiography.Thisrulecomesfromthefactthat1.155~2.0.Therefore,foreachincreaseof15%inkVptheexposurewilldouble.Thus,ifthekVpisincreasedby15%thenthemAmustbedecreasedbyafactorof2inordertomaintainconstantexposureontheimagereceptor. ChangingthekVpofanacquisitionchangesthetubepotentialsothatastheelectronscollidewiththex-raytargettheywillhavemoreenergyandthusproducehigherenergyx-rayphotons(x-raygeneration). s(time) Thetimeoftheexposureisarelativelyeasyconcepttounderstand.Asthetimeoftheexposureisincreasedbyafactorof2,thenumberofx-rayphotonscomingoutofthex-raytubewillalsobeincreasedbyafactorof2.Thisiswhytheexposuredependenceon(s)isalsolinearasitiswithmA. Itisalsointuitivethatastheexposurelengthisincreasedthereisahigherlikelihoodofmotionwithinthex-rayimage.Dependingontheexamtypethismayormaynotbeamajorconcern.Forinstanceduringanx-rayofanextremityofawellinformedpatienttheanatomyofinterestcanremainrelativelystill,whileinachestx-raythelunganatomyneartheheartwillbemovingregularlyalongwiththeheartmuscle. SID(SourcetoImageDistance) TheSourcetoImageDistance(SID)affectstheexposureattheimagereceptorsincethex-raybeamisdivergent(i.e.spreadingoutasitgetsfurtherfromthex-raytube). Sincethex-raysarespreadingouttheexposuredependsontheSIDas1/SID2.Inourpostonthebasicx-raypropertieswedemonstratehowthedivergentbeamleadstothe1/R2dependenceandinthiscasethedistanceRfromthesourcetotheimagereceptorisalsoreferredtoasSID. Sincethisisaninverserelationshipifwemovetheimagereceptorfurtherfromthex-raytubeanothertechnicalparametermustbeupdatedtomaintaintheexposure.Forinstance,iftheSIDisincreasebyafactorof2from50cmto100cmthemAwillneedtobeincreasedby(100/50)2=22=4. BuckyFactor(ScatterGridFactor) Finally,theBuckyfactorisaunitlessquantitywhichistheratioofthex-rayexposurereachingthegridtothex-rayexposurereachingtheimagereceptor.IfnogridisusedthisdefinitionwillleadtoaBuckyfactorof1.0. TheBuckyfactorforgridsisdependentuponthegridratio(gridheight:gridspacing)andthekVp.Ittypicallyrangesfrom2.0–6.0fordiagnosticx-raygrids.MoredetailsontheBuckyFactorintheX-rayScatterpost. Thex-rayexposureontheimagereceptorscalesas1/(BuckyFactor).Therefore,ifyouhaveahigherBuckyFactorforthenewgridyouwillhavetoincreasetheexposurebychanginganothertechnicalparametersuchasincreasingthemA. Forinstanceiftheinitialprotocoldidnotuseanygrid(Bucky=1)andthenewprotocolusesagridwitha(Bucky=2.0).ThenthemAwillneedtobeincreasedby2.0inordertokeeptheexposureconstantontheimagingreceptor. GoldenEquation(AllTechnicalFactors) Nowthatwehaveallpieceswecanputthese5simplerelationshipstogethertomakewhatwecallthegoldenequation.Youcangiveityourowncoolnametoo.Wejustcallitthegoldenequationsinceithasallofthefactorsthatyouneedtoconsidersothatasyouchangeonefactoryoucancompensatetokeepyourexposureconstant. AlsothisisjustacalculatorfordemonstrationpurposessoitdoesnothavelimitsonthemA.Iknowyouwishyouractualsystemwaslikethattoo.SinceourphysicalsystemsoftenhavelimitsonthemAyoumayneedtocompensateinotherwayssuchasincreasingthelengthoftheacquisition.Thatcanstillbeaccomplishedwiththisframeworkandwiththecalculatorabove. WestartbywritingdowneachofthetermsthatareproportionaltotheexposurefromthesectionsabovesothatwegetoneequationcontainingmA,kVp,s,SIDandBuckyFactor. Nowthatwehavetheexposurerelationshipwithourtechnicalparametersweconsideryourrealworldproblemwhereyouhaveanoriginalprotocolandoneormoreofthetechnicalparametersneedstochange.InthiscasethenewexposurewillbedenotedasExposurenewandwewantitequaltotheoriginalExposure. Combiningthetwoequationsaboveweseethatwecanwritethenewexposureoutintermsofallofthetechnicalparameters,andtheoriginalexposureaswell. ThenwecansolveforthemAnew,bydividingbythetermsinthenumeratoronthelefthandside,andmultiplyingbythetermsinthedenominatorontherighthandside.Nowwehavewhatwecallthegoldenequationthatisimplementedinthecalculatorabovesothatwecancompensateforanychangestotechnicalparameterswithoutoverorunderexposingyourimagereceptor. Thereisnomagichere,justoneequationtostudyupon.Onceyoustarthavingdreams(notnightmares)aboutthisgoldenequationthenyouknowthatyouhaveprobablystudieditenough. Obviously,ifoneoftheparametersisnotchangedfromtheoriginaltothenewsetofparametersthenthattermwillcanceloutaswewilljustbemultiplyingby1. SoforinstanceifyouareonlychangingoneparameteryoucanrefertothistabletodeterminehowtochangethemAinordertocompensateforyourparameterchange.Thistableisjustanotherwayofsummarizingthegoldenequation. TechnicalParameterUnitsMultipliertomATubePotentialkVp(kilovoltpotential)(kVp/kVpnew)5ExposureTimes(seconds)(s/snew)SourcetoImageDistance(SID)cm(centimeters)(SIDnew/SID)2BuckyFactor(ScatterGridFactor)unitlessBuckynew/Bucky Nowthatyouhavereadallthewaythroughonthedetailsofthegoldenequationmakesuretowriteitdownyourselfwithapencil/penandpaperandtryseveralexamplessothatyoucangetfamiliarwithit. Postnavigation ←PreviousPostNextPost→ ScrolltoTop