Technical Factors of Radiology

mAs primarily controls density, but changes in kVp contribute to image density as well. The 15% rule states that a 15% increase in kVp is similar to doubling ...   Chapter7 TechnicalFactorsofRadiology   OBJECTIVES:     * Technical factorsofx-rayproduction:kVp,mAs,exposuretime,and distance.     *Contrast: subject,imageandradiographic.     *Theinverse squarelaw.   Bytheendofthissection,thestudent shouldbeableto: 1.Defineradiographictechnique. 2.DefinekVpanditsinfluenceona radiographicimage. 3.DefinemAsanditsinfluenceona radiographicimage. 4.Describehowexposuretimeeffectsthe radiographic image. 5.Identifytherelationshipbetweenthe inversesquarelaw andradiographictechnique.   Thegoalofeveryradiologictechnologistshouldbeto obtainhighqualityfilmsthatyieldmaximumdiagnostic information.Thereare manytechnicalfactorswhichaddtoortakeawayfromtheoverall technical qualityofeachfilm.Thischapterofthecoursedealswiththese variables. Thethreemaincomponentsofanyx-ray exposurearekVp,mA, andtime.Whenatechnologistpreparestomakeanexposure,kVp, mAandtime mustbesetonthecontrolpanel.Selectionofthesesettingsis referredtoas settingtechnique(sometimesspelledtechnic),orsetting exposurefactors.  The threemain componentsofanyx-rayexposurearekVp,mA,andtime.    *kVp: thepowerandstrengthofthex-raybeam(qualityofthe x-rays).     *mAs:the numberofx-rayphotons producedbythex-raytubeatthesettingselected(quantityof x-rays).     *time:how longtheexposurelasts.   UnderstandingTechnique kVpstandsforkilovoltagepeak.116,117  Thisisthehighest voltage(measuredin thousandsofvolts)thatwillbeproducedbythex-raymachine duringan exposure.Forexample,if60kVpisselected,60kilovolts (60,000volts)is themaximumstrengthofx-raysproducedinthisexposure.If70 kVpis selected,70kilovolts(70,000volts)willbethemostenergetic photons produced. kVpcontrolsthepenetratingstrengthofan x-raybeam(beam quality).130 Whenever an exposureismade,thex-raysmustbeenergetic(strongenough) toadequately penetratethroughtheareaofinterest.ThehigherthekVp,the morelikelythe x-raybeamwillbeabletopenetratethroughthickerormore densematerial. LowkVpphotonsareweakandeasilyabsorbedbybodytissuesor filtersthat havebeenplaced.HigherkVpphotonswilllikelypenetrate throughthe patient'stissuesandmakeitallthewaytothex-rayfilm. Mostx-raysused inmedicalimagingarebetween50and120kVp(50,00to120,000 volts).130 AdecisionismadeonwhichkVptoselect basedonhowthick thebodypartisandwhattypeoftissuewillbeexposed.For example,a finger,handorwristrequireonlyalowkVpsettingbecausethe bodypartis thinandthereisnotmuchbodytissueforthex-raysto penetratethrough.A settingofbetween55and60kVpistypicallyselected.A shoulderorkneeis thickeranddenserthanthefinger,andtherefore,morekVpis neededto adequatelypenetrate.Asettinginthe65-75kVprangeis usuallyselectedfor thesebodystructures.Thehip,abdomen,andpelvisareeven thickeranddenser thanthekneeorshoulder.AdditionalkVpisrequiredfor adequate penetration. Settings inthe75-80kVp rangemaybeused.Thelumbarspineisextremelythickand dense,requiring settingsinthe90-100kVprange.   Calipers(shownbelow)areusedtomeasure thethicknessof thebodypartbeingimaged.  Typically, anincreaseof2kVpforeveryadditionalcentimeteroftissue thicknessis requiredtoensureadequatepenetration. Above: Radiographic calipers   Forexample,ifanacceptableimageofabody partthat measured10centimeterswasobtainedusing60kVp,anincrease of2kVpwould beneededtopenetrateabodypartthatmeasured11cm(a2kVp increasefor everyadditionalcmofthickness,or62kVp).WhatkVpshouldbe selectedifa bodypartmeasures12cm? We shoulduse 64kVpbecausewehaveaddedanothercminthickness. kVpalsocontrolstheamountofcontrast(the difference betweenwhitesandblacksonanimage)seenonanx-ray.131 The amountofcontrastvisibleinanimageisreferredtoasgray scale. -Ifanimagehashighcontrast,therewill bewhiteand blackareasonthefilm,butveryfewgraysinbetween. -Whenafilmhaslowcontrast,thereare numerousshadesof graysrangingfromverylighttoverydark.Lowcontrastimages havesubtle changesineachgrayscalestep. Below:Anx-rayofanaluminumstepwedgeis shown.Step wedgesdemonstratethatthethickerthebodypart,theless x-raysareableto penetratethrough.Italsoshowshowtheamountofcontrast changesaccording tothekVpselected.Highcontrastimageshavewhitesand blacks,butfewgrays (lowkVp)whilelowcontrastimageshavemanyshadesofgray (higherkVp).This stepwedgewasexposedusingthreesettings:40kVp(farleft), 70kVp(center) and100kVp(farright).Differencesincontrastcaneasilybe seen. Asheetofx-rayfilmisplacedunderthe aluminumstep wedgeandanexposureismade.Eachstair-stepchangeindensity isrecorded usingasensitometer,ameterthatmeasurestheamountof darkeningonthe film. AskVp goesup, radiographiccontrastgoesdown.132 IncreasingkVpalso contributestotheoverall density(darkness)oftheimage.Afairlysmalladjustmentin kVpcanhavea significanteffectontheimage.Justa15%increaseinkVpis roughly equivalenttodoublingthemAs.133 Conversely,a15% decreaseinkVpisroughly equivalenttocuttingthemAsinhalf. HigherkVpsettingsproducemorescatter radiation.131-133 Increasedscatterreducesimagedetailandincreasespatient dose.Thereis alwaysadecisiontobemadeonwhatkVpsettingcanbeusedfor anygiven exposurethatwillproducethefollowingresults:     *adequatebody partpenetration    * lowestamountofscatterproduction possible    * highestamountofradiographic contrastpossible    *highestpossibleimage detail   TechniqueChart Techniquechartsaredevelopedtolist averagekVp,mA, time,distance,andfilmtypeusedforvariousexams. Essentiallyitisa referencetoaidtheradiologictechnologistinproducingan optimalimageon thefirstexposure,ratherthantakingafilmthatistoodark, toolight,or under/overpenetrated.  Inmanyhospitalsandclinics,technique chartsaresetup sothatanoptimal"fixed"kVpisassignedtoeachbodypartto ensurethebestpenetrationandcontrastisachievedandthemAs isadjusted accordingtothesizeofthepatient.ThisiscalledafixedkVp system.Some modernequipmenthasbuiltinexposureparameters,making techniquechartsless likelytobeutilizedbytechnologists. Above: X-raycontrol consolewithatechniquechartdisplayed.   Chartshowsatechniquechart(basedona180 poundaverage buildmale) *Detail:100speedfilm/screensystem, *Rareearth:400speedfilm/screensystem *Gridisan8:1focusedgridwith103lp/inch    Exam  kVp  mA  time  mAs  distance film/screen*  grid UpperExtremity   Finger 50 100 .02 2 40" detail no   Hand 50 100 .03 3 40" detail no   Wrist 54 100 .02 2 40" detail no   Forearm 54 100 .025 2.5 40" rareearth no   Elbow 60 100 .03 3 40" detail no   Humerus 72 100 .06 6 40" rareearth yes   Clavicle 64 100 .04 4 40" rareearth yes   Shoulder-AP 76 200 .04 8 40" rareearth yes   Shoulder-axillary 76 300 .08 24 40" rareearth yes   Shoulder-Yview 76 300 .10 30 40" rareearth yes LowerExtremity   Toe 50 100 .016 1.6 40" detail no   Foot 54 100 .032 3.2 40" detail no   Calcaneous 64 200 .04 8 40" detail no   Ankle 62 100 .04 4 40" detail no   LowerLeg 64 100 .025 2.5 40" rareearth no   Knee 66 200 .03 6 40" rareearth yes   SunrisePatella 66 100 .04 4 40" rareearth no   Femur 70 200 .04 8 40" rareearth yes   Hip-AP 75 300 .10 30 40" rareearth yes   Hip-x-tablelateral 80 400 .20 80 40" rareearth  yes Spine   Cervicalx-tablelat 76 200 .10 20 72" rareearth yes   Cervical-AP 74 100 .10 10 40" rareearth yes   Odontoid 70 100 .04 4 40" rareearth no   Swimmer'slateral 80 500 .30 150 40" rareearth yes   T-spineAP 74 200 .07 14 40" rareearth yes   T-spinelateral 66 300 .50 150 40" rareearth yes   L-spineAP 75 400 .10 40 40" rareearth yes   L-spinelateral 90 400 .40 160 40" rareearth yes Chest   ChestPA 110 500 .02 10 72" chestdetail yes   Chestlateral 110 500 .080 40 72" chestdetail yes   Chestnewborn 55 100 .01 1 40" rareearth no   Chest1yearold 60 100 .01 1 40" rareearth no   Chest6yearold 60 100 .03 3 40" rareearth no Abdomen   KUB 70 300 .10 30 40" rareearth yes   Lateraldecubitus 80 400 .05 40 40" rareearth yes   KUB2yearold 66 100 .05 5 40" rareearth no     Half ValueLayer Thepenetratingabilityofanx-raybeam (quality)is dependentonthekVpselectedfortheexposure.134Ahalfvalue testcanbe performedtodeterminethethicknessofmaterialwhichis requiredtoreduce thenumberofx-rayphotonstransmittedthroughthematerialto one-halftheir originalnumber.Thematerialgenerallyusedtodeterminehalf valuelayeris aluminum.Onlyathinlayerofadensematerial,suchas aluminum,lead,barium oriodine,wouldreducethehalfvaluelayer.Athickerlayerof lessdense material(wood,glass,paper,cardboard,etc.)wouldbeneeded toproducethe sameeffectonthex-raybeam. Inamodernimagingdepartment,the half-valuelayerhastwo importantapplications. 1-Thefirstisthehalf-valuelayerofthe primaryx-ray beamusedinpatientdiagnosis.Adiagnosticx-raybeamproduces awiderange ofenergies.Althoughweonlymentionthemaximumenergyofthe beam(for example,wesayweareusing80kVpforanexposure),thebeam ismadeupof photons80kVpandlower. We don'twant lowerenergyphotonsinourbeam-wereallyjustwantthosewith anenergyof 80.Thereisnowaytogetaperfectx-raybeamwithevery photonpossessing80 kVp.However,thehalf-valuelayerhelpsfilteroutlowenergy photons(less than80kVpinourexample). Basically,ifthehalf-valuelayerfora givenx-raybeamis low(thinpieceofaluminumfiltration),thenthex-raybeam containsmorelow energyphotonsthatarelessthan80kVp.Theyalsohaveless penetratingpower becauseoftheirlowerenergy.Ifthehalf-valuelayerishigh (thickaluminum added),thex-raybeamcontainsmorehighenergyorhighly penetrating radiationbecausethelowerenergyphotonscouldnotpenetrate throughthe thickaddedaluminum. Thisisimportantbecausexraysusedfor medicalx-raymust haveenoughenergytopenetratethebodypartofinterestand exposethefilm sufficiently.Lower-energyradiationisabsorbedintothe patient'stissuesor scatteredbythebodyandmaynotreachthefilm,contributing nothinguseful totheimage.Addingmorefiltrationtothebeam,whichis typicallydoneby themanufactureroftheunitpriortoinstallation,willremove theundesirable low-energyxrayswhileallowingthedesirablehigher-energyx raystopass throughthepatienttothefilm. Ontheotherhand,ifthereistoomuch filtrationinthe beam,thereisalossofcontrastinthex-rayimage (differentialabsorption isreduced).Thisiswhyaphysicistevaluatesallx-ray equipmentonaregular basis,typicallyonceayear,andmeasurehalf-valuelayeras partofthat testing. Asecondapplicationofhalf-valuelayerin animaging departmenthastodowithroomshielding.Roomsthatcontain x-rayequipment aretypicallyshieldedwithlead-linedwallstoreducethe radiationexposure toworkersandthepublicfromtheuseofxrayswithinthe department.When designingtheshieldingforaroom,thephysicistwillperform calculations basedonthehalf-valuelayerofthex-raybeam.Ingeneral,the designwill callforenoughhalf-valuelayersofshieldingtoreduce radiationexposure outsidetheroomtoacceptablelevels.   CONTRAST kVpcontrolsradiographiccontrastwhichis definedasthe numberofdifferencesbetweenshadesofwhite,blackandgrayin theimage.133,134  Totheeye,contrastisthenumberof differentshadesseenbetweenthedarkestblackandthelightest whiteinthe image.Toproduceanoptimalimage,acertainamountofcontrast isnecessary sothatdifferencesinvarioustissuescanbedistinguished, alongwith abnormalitiesorpathologicprocesses.     *Ifcontrastis high,therewillbeawhitesandblacksontheimage,butvery fewshadesof grayinbetween.Thisisreferredtoasshort-scalecontrast,or narrow contrastlatitude.     *Ifcontrastis low,therearenumerousshadesofgrayandverylittle differencebetweeneach shade.Theimagelooks"flat".Thisisreferredtoaslong-scale contrast,orwidecontrastlatitude. Imagesbelow: Toprow:comparesthecontrastdifferencesinthreeblack andwhiteimages. Bottomrow:comparescontrastdifferencesinthreecolor pictures. Lowcontrast(longscale)  Moderatecontrast Highcontrast(shortscale) Lowcontrast(longscale) Moderatecontrast HighContrast(short-scale)   Intheimagesabove,differencesbetweenhigh andlow contrastcaneasilybeseen.Thedensity(darkness)wasnever changed-onlythe contrastwasadjustedineachpicture.     *Theleftimage inbothrows(lowcontrast)ismadeupofdensitieswithsimilar shadesofgray andverylittledifferencebetweenoneshadeofgraytothe next.     *Themiddleimage inbothrowsshowsmoderatecontrast.     *Therightimage (highcontrast)showshighcontrast,withlargedifferences betweenthe lightestanddarkestareas,butveryfewshadesofgrayin between.     Below: Inthex-ray imagesofafinger,differencesincontrastcaneasilybe seen.      A                                                 B     *ImageA(left) hashighcontrast.Thedifferencesbetweenlightanddarkareas onthefilmare great.Thereareveryfewshadesofgray.     *ImageB(right) isalowcontrastimage.Theimagelooksflatandgray.There arefew differencesinthelightestanddarkestareasofthisimage.Low contrast imagesareusuallytheresultofusinghigherkVpsettingsor duetothick, densebodytissues.   Thereareseveraltypesofcontrastwhich influencethe radiographicimage:135 1.Filmcontrast 2.Radiographiccontrast 3.Subjectcontrast   -FilmContrast:Thefilmandscreens,or settingsonthe digitalplatesystemareusedtomakearadiographicimage controlthisportion ofthecontrastspectrum.Certaincombinationsoffilm/screens ordigital settingsaredesignedtoproducehighcontrastimages(blackand white,butfew grayshades),whileothersystemsaredesignedtoyieldimages with "flat"contrast(lotsofgrayshades,butfewblacksorwhites). -RadiographicContrast:Theamountof radiographiccontrast isdeterminedbyacombinationoffilmcontrast,subject contrastandthekVp selectedfortheexposure.Radiographiccontrastisdefinedas theoverall differencesinopticaldensitiesseenintheradiographicimage. Radiographic contrastenablesustoseefinedetailswithintheimage clearly. -SubjectContrast: Numerous variationsintheintensityofthex-raybeamoccurasx-ray photons pass throughthepatientduetodifferences inhowthepatient'sbodytissuesaffectthebeam(attenuation). Thisis referredtoassubjectcontrast.Forexample,aradiographic imageobtainedon averyheavypatientwillhavelesssubjectcontrastthana patientwhoisthin becauseadiposetissuedecreasessubjectcontrastsignificantly. Therearefourfactorswhichtogether determinetheoverall amountofsubjectcontrastinanimage:     *thestructure andcompositionofthesubject(patient)     *kVpselected     *theamountof beamfiltration(inherentplusadded)     *theamountof Comptonscatterproducedintheexposure   Time Thelengthoftheactualx-rayexposurecan besetbythe technologistonmostx-rayconsoles.Timeplaysanimportant roleinproducing aqualityimage.Timedirectlyaffectsthedensityofafilm becauseit determineshowlongtheimagewillbeexposed.     *Ifthetime selectedistoolong,theimagemaybetoodark;Longerexposure timeshavethe drawbackthattheremightbemotionontheimagethatwouldhave been eliminatedhadashortertimebeenused.     *Conversely,if theexposuretimeistooshort,theimagemaybetoolight. Anexposuretimeof10milliseconds(.010)or lessis recommendedformostroutinex-rays.47 Generally,the shortestexposuretimein combinationwiththehighestmAshouldbeusedforexposuresto reducemotion artifacts.134   mAs mAsstandsformilli-ampere-second.135  Itdetermineshow manyx-rayphotonsare produced(quantity). It hasnoeffecton thestrength(penetratingpower)ofthex-rayphotons.mAsisa productof multiplyingtwofactorstogether:timeandmilliamperage(mA). mAx time=mAs  For example,a typicaltechniquesettingusedforimaginganinjuredhandmay be60kVp,100 mA,and1/100ofasecond(0.01).Inthisexample,100mA multipliedby0.01 secondsequals1mAs(or1milliampere-second). Inanotherexample,ifwewereimagingthe shoulder,our technicalfactorsmaybesetat70kVp,200mA,and1/10ofa second.Tofind mAs,wemultiply200x1/10(0.10),whichequals20mAs.        Calculating mAs -Thegreencolumnshowssmallfocalspot settings:50,100, 150,200,and300mA. -ThebluecolumnshowslargefocalspotmAsettings:400,500, and600mA -Theredcolumnistimeoftheexposure(insecondsor fractionsofasecond) Timeinsec 50mA 100mA 150mA 200mA 300mA 400mA 500mA 600mA .003sec .15mAs .3mAs .45mAs .6mAs .9mAs 1.2mAs 1.5mAs 1.8mAs .006sec .3mAs .6mAs .9mAs 1.2mAs 1.8mAs 2.4mAs 3mAs 3.6mAs .010sec .5mAs 1mAs 1.5mAs 2mAs 3mAs 4mAs 5mAs 6mas .016sec .8mAs 1.6mAs 2.4mAs 3.2mAs 4.8mAs 6.4mAs 8mAs 9.6mAs .020sec 1mAs 2mAs 3mAs 4mAs 6mAs 8mAs 10mAs 12mAs .025sec 1.25mAs 2.5mAs 3.75mAs 5mAs 7.5mAs 10mAs 12.5mAs 15mAs .032sec 1.6mAs 3.2mAs 4.8mAs 6.4mAs 9.6mAs 12.8mAs 16mAs 19.2mAs .040sec 2mAs 4mAs 6mAs 8mAs 12mAs 16mAs 20mAs 24mAs .050sec 2.5mAs 5mAs 7.5mAs 10mAs 15mAs 20mAs 25mAs 30mAs .064sec 3.2mAs 6.4mAs 9.6mAs 12.8mAs 19.2mAs 25.6mAs 32mAs  38.4mAs .048sec 4mAs 8mAs 12mAs 16mAs 24mAs 32mAs 40mAs 48mAs .10sec 5mAs 10mAs 15mAs 20mAs 30mAs 40mAs 50mAs 60mAs .12sec 6mAs 12mAs 18mAs 24mAs 36mAs 48mAs 60mAs 72mAs .14sec 7mAs 14mAs 21mAs 28mAs 42mAs 56mAs 70mAs 84mAs .16sec 8mAs 16mAs 24mAs 32mAs 48mAs 64mAs 80mAs 96mAs .18sec 9mAs 18mAs 27mAs 36mAs 54mAs 72mAs 90mAs 108mAs .20sec 10mAs 20mAs 30mAs 40mAs 60mAs 80mAs 100mAs 120mAs .25sec 12.5mAs 25mAs 37.5mAs 50mAs 75mAs 100mAs 150mAs 150mAs .32sec 16mAs 32mAs 48mAs 64mAs 96mAs 128mAs 160mAs 192mAs .40sec 20mAs 40mAs 60mAs 80mAs 120mAs 160mAs 200mAs 240mAs .50sec 25mAs 50mAs 75mAs 100mAs 150mAs 200mAs 250mAs 300mAs .64sec 32mAs 64mAs 96mAs 128mAs 192mAs 256mAs 320mAs 384mAs .80sec 40mAs 80mAs 120mAs 160mAs 240mAs 320mAs 400mAs 480mAs 1sec 50mAs 100mAs 150mAs 200mAs 300mAs 400mAs 500mAs 600mAs 1.6sec 60mAs 160mAs 240mAs 320mAs 480mAs 640mAs 800mAs 960mAs 2sec 100mAs 200mAs 300mAs 400mAs 600mAs 800mAs 1000mAs 1200mAs 3.2sec 160mAs 320mAs 480mAs 640mAs 960mAs 1280mAs 1920mAs 1920mAs 5sec 250mAs 500mAs 750mAs 1000mAs 1500mAs 2000mAs 4800mAs 3000mAs   mAsisadjustedaccordingtothesizeand tissuetypeofthe bodypartbeingexamined.mAscanbechangedbyalteringeither theamountof timeusedinmakingtheexposure,orthemilliamperage(mA) setting.Thereare manycombinationsofmAandtimethat,whenmultipliedtogether, equalthesame mAs.Forexample,allofthefollowingcombinationsofmAand timeequal100 mAs.   mAselected  Timeselected mAmultipliedbytime=mAs    50mA    2seconds   100mAs    100mA    1second   100mAs    200mA    1/2second(0.5)   100mAs    400mA    1/4second(0.25)   100mAs    500mA    1/5second(0.20)   100mAs   Anyoftheaboveexposureswillproducethe exactsame densityontheresultantx-rayfilm.Fasterexposuretimesare neededifthe patientishavingtroubleholdingstill,asinthecaseofsmall children.When usingashortexposuretimeandahighmAsetting,besureto refertothetube ratingcharttoverifythatthetubewillbeabletostandupto theextreme temperaturesgeneratedbysuchanexposure.Regardlessofwhich ofthefive possiblecombinationsisselectedintheexampleabove,each willproduce exactlythesamenumber(quantity)ofx-rayphotons.Animage producedbyany ofthesecombinationswillhavethesameradiographicdensity (amountofimage blackening).mAsisthemostimportanttechnicalfactorin controllingthe density(darkness)ofanimage. Rules AboutmAs *mAsisdirectlyproportionaltopatient exposure rate.  *IfmAsisdoubled,twiceasmanyx-rayphotonsareproduced, andthepatient receivestwicetheradiationdose. *Inordertomakeaperceivablechangeinanimage,mAsmustbe adjustedbyat least30% *mAsisresponsibleforthedensityofthefilmandcontrols howmanyx-ray photonsareproducedbythex-raytube. *mAshasnoinfluenceonthestrength(penetratingpower)of x-rayphotons. Inordertomakeaperceivablechangeinan image,mAsmust beadjustedbyatleast30%.136Anadjustmentofat least50%in exposureisrequiredifasignificantdensitychangeisdesired. Ifyoujust obtainedanimageofahip,using20mAs,andthefilmistoo light,increasing themAswillmakethefilmdarker.Butincreasingtoonly24mAs willnotmake anappreciabledifferenceintheappearance,becauseyourfirst increasemust beatleast30%(whichinthisexamplewouldbeabout27mAs). Iftheimagewasvery light,youmayneedto doublethemAs(40mAs)fromtheoriginalexposure.   The15%Rule mAsprimarilycontrolsdensity,butchanges inkVp contributetoimagedensityaswell.The15%rulestatesthata 15%increasein kVpissimilartodoublingmAs.133-136Conversely,decreasing kVpby15%is similartocuttingthemAsinhalf. RememberthatanychangeinkVPaffectsimage density,but primarilyitchangesimagecontrast.TheprimaryroleofkVpis tocontrol strengthandpowerofthex-raybeamandcontrolcontrast. Wheneverpossible,mAs shouldbeusedto increaseordecreasetheoveralldensity(darkness)ofanimage- notkVp.When adjustingtechniquesettings, initiallyonlyoneortheother(kVpormAs)shouldbeadjusted ratherthan makingchangesinboth. *Ifyoumakeanexposurewheretheoverall image,including thebackground,istoolight,increasemAsonly. *Ifthefilmisoveralltoodark,reducemAs. *Whenthefilmisunderpenetrated(can'tseethroughtissues), increasekVp. Thebackgroundofanunderpenetratedimageandsometissues willhave acceptabledensities,buttheareaofinterest(forexample,a bone)willlook washedoutwithlittleornodetail. *Whenafilmisoverpenetrated,theentirefilm,includingthe background, willhavefeworevennowhiteorblackareas-everythingwill begray.Insuch animage,thex-raysweresostrongthatmostpenetratedright throughthe patient,andnodifferentialabsorptiontookplace. ReducekVp.   ChangingDensityofanImage *Ittakesaminimumof30%changeinmAsto makea discernibledifferenceinthedensityofanimagetobe recognizedbythehuman eye. *Aminimum50%changeinmAsshouldbemadeifasignificant changeindensity isdesired. *A100%(doubleorcutinhalf)changeinmAsisrequiredto correctthe exposureofanoverorunder-exposedfilm.Whentakingarepeat filmforover orunderexposure,theminimumamountofchangeintechnique shouldbeas follows:      - AtleastdoublethemAsforan imagethatisbeingrepeatedbecausethefilmwastoolight      - AtleastcutinhalfthemAsfor afilmbeingrepeatedbecausethefilmwastoodark.   QuantumMottle Quantummottleisagrainyappearanceona radiographthat isseenmostoftenwhenusinghighspeedorfastfilm/screen systems.Screens andfilmaremadefasterbymakingthephosphororsilver crystalslarger(or thelayersofcrystalsthicker/packingcrystalsinmore tightly).This increasesthechanceofapassingincidentx-raytohavean interactionwith thecrystal.Alargercrystalwillgiveoffmorelightthana smallcrystal, therebydarkeningagreaterareoffilm. UsinglowmAsforanexposuremayalsolead toquantum mottle.IfmAsissettoolow,therearenotenoughx-ray photonsinthebeam tointeractwithasufficientnumberofcrystals.Itissimilar tocomparinga lightraintoabigthunderstorm. DISTANCE Distancefromx-raytubetofilm(sourceto imagedistance, orSID)isoneofthekeycomponentsinproducingaquality x-ray.Distancehas adirectrelationshipwithmAs.Sincex-raysobeyallofthe lawsoflight,we canbetterunderstandhowdistanceaffectsx-raysbyconducting asimpleexercise. Takeaflashlightintoadarkenedroom.Holdthelightabout12 inchesfroma wall.Youwillseethattheareacoveredbythelightisavery small,well definedcircle.Youwillalsoseethattheintensityofthe lightisvery strongandbright.Nowslowlyincreaseyourdistanceawayfrom thewall, keepingyoureyeontheareailluminatedbythelight.Alsopay attentionto theintensityofthelightasyouincreaseyourdistance.As distanceis increased,intensityofthelightdecreases.Atthesametime, thearea illuminatedbythelight"fansout"andmoreofthewallis covered bylight.    X-raysbehaveintheverysamewayaslight inouranalogy above.Whenthedistancefromthesourceofx-rays(x-raytube) totheimage receptor(film)isincreased,twothingshappen: *intensityofthex-raybeamdecreasesthe fartherthe x-raystravel. *thex-raysfanout(divergence),coveringawiderandwider fieldthefurther theytravelfromthesource. Thedistancebetweenthesourceofthex-rays (x-raytube) andtheimagereceiver(film)isreferredtoasthesourceto imagedistance, orSID;alsocalledfocalfilmdistance(FFD).   INVERSESQUARELAW Thenaturallawthatgovernstherelationship between intensityanddistanceisreferredtoastheinversesquarelaw. Itstatesthat "theintensityofenergyisinverselyproportionaltothesquare ofthe distance".Thedrawingbelowmayhelpsimplifythisrather complex concept. Noticethatattheshorterdistance(40"), the intensityofthex-raybeamishigh.Eventhoughthex-raybeam hasn'ttraveled veryfar,ithasalreadystartedtofanoutslightlyandalready covers4 squares.Bythetimethex-raybeamhastraveledroughlytwice asfar (72"),theintensityhasdroppedoffconsiderablyandthebeam hasfanned outwideenoughtocover16squares.Thatisessentiallythe inversesquare law! Noticeat40”,only4 squaresarecoveredbythedivergent x-raybeam.   Doublingthedistance to80”,weseethatnow16squaresare covered.Doublingthedistancefromthe x-raytubetotheimagingcassette resultedin4timemoresquaresbeing coveredbythedivergingbeam.   Inourexample,let'ssaythattheintensity at(40") is4mR.Laterwetakeanotherimage,butthistime,our distanceisnearly doubledto72". According tothe inversesquarelaw,wewillneedfourtimesmoremAstomaintain thesame densityaswasseeninthefirstimage.Therefore,Itwouldnow take16mRto maintainthesamedensity. Inanotherexample,let'ssaythatweplaced aninstrument thatmeasuresradiationexposure(inmR)andwepositionediton thepatient's skinwhereourcentralbeamwascentered.Wetookanexposure usingaSIDof40". Themeasurementreads200mR.Nowweusedthesametechnique factors(wedon't changekVp,Ma,ortime),butwenearlydoubleourdistancefrom 40"upto 72".Usingtheinversesquarelaw,whatwouldthemRreadingbe now?Since wedoubledthedistance,thenewintensityofthebeamwouldbe 1/4ofthe original.Therefore,wedivide200mRby1/4(or25%),which equals50mR. Keep theserulesin mindwhenchangingyoursubjecttoimage(SID):     *Whenyoudouble thedistance(SID),youmustincreasetheamountofmAsby4 timestomaintain thesamedensityontheimage.     *Example:Wetake aportablechestx-rayonapatientwhoislyingonagurney. Supinefilmsare generallytakenusingaSIDof40".Wesetourtechniqueat2 mAsandget awellexposedfilm.Nowthedoctortellsyouthatheneedsan uprightviewso thathecanbetterseelayeringoffluidinthepatient'slungs. Younearly doubleyourSIDdistancebecauseuprightfilmsaretakenat72 inches.Using theinversesquarelaw,youknowthatbecauseyouhavedoubled thedistance, youneed4timestheintensity.Yousetyourtechniqueat8mAs (2x4)andget aperfectuprightfilm!  * Whenyoucutthe distanceinhalf,youwillneedtodecrease(mAs)by4times (use1/4ofwhat youusedbefore). Example:Wetakeaportablechestfilmona patientwhois sittinguprightonagurney.WeuseaSIDof72"andselect10 mAs.Our x-rayiswellexposedandwearepleasedwiththeresults.Later thatday,the patientsuffersaserioussetbackandhisconditionrapidly declines.Nowyou areaskedtoobtainanotherchestfilm.Thistimethepatient cannotsitupas before,andsoyoutakeasupinefilmusingaSIDof40".Using the inversesquarelaw,youknowthatifyourmAswas10at72",and nowyou havecutyourdistanceinhalfto40",youwillneedtodecrease the intensityby4times.Youdivide10by1/4,whichequals2.5 mAs.Yourfilm turnsoutperfect!  Inreallifesituations,wedon'talways changeourdistance bydoublingorcuttingitinhalf.Often,weonlychangeour distancebyafew inches.Thefollowingchartprovidesmultiplicationfactors whichcanbeused todeterminewhatnewexposurefactorsshouldbeusedwhen distanceischanged. Howto usethechart (below):     *Findonthe chartyouroriginalSID(redrow).     *FindyounewSID (bluerow).     *Theconversion factoristhenumberwherethetwocolumnsmeet.     *MultiplythemAs youusedfromyouroriginalSIDbytheconversionfactornumber.     *Thisgivesyou thecorrectmAstouseatthenewSID. Forexample,youtookafilmat48"using10 mAs.Later inthedayyouhavetorepeatthefilmbutbecauseyouhaveto usedifferent equipment,youcanonlygetaSIDof40".  Intheredcolumn,(originalSID),find48". Followthiscolumn downuntilitintersects withthebluerow(newSID)for40".Theconversionfactorwhere thetwo rowsmeetis0.7.TakeyouroriginalmAs(10)andmultiplyby 0.7 (10x.7=7).Your newmAsat40"shouldbe 7. MaintainingFilmDensityWhenChangingDistance   OriginalSID 36" (91cm) 40" (102cm) 42" (107cm) 44" (112cm) 48" (122cm) 60" (152cm) 72" (183cm) 100" (305cm)   NewSID| 36" (91cm) 1 .8 .7 .7 .6 .4 .3 .1 40" (102cm) 1.2 1 .9 .8 .7 .4 .3 .2 42" (107cm) 1.4 1.1 1 .9 .8 .5 .3 .2 44" (112cm) 1.5 1.2 1.1 1 .8 .5 .4 .2 48" (122cm) 1.8 1.4 1.3 1.2 1 .6 .4 .2 60" (152cm) 2.8 2.3 2 1.9 1.6 1 .7 .4 72" (183cm) 4 3.2 2.9 2.7 2.3 1.4 1 .5 100" (305cm) 7.7 6.3 5.7 5.2 4.3 2.8 1.9 1   ChangeinExposurefromFiberglassorPlasterCasts Whencastsareplacedonextremities,an increaseof techniqueisrequired,bothtopenetratethroughthedensecast material,and becausethereismorematerialthatwillabsorbphotons. Castmaterial&size Increasetechnique    Fiberglasscast    add3-4kVporincreasemAs25-30%    Smalltomediumplastercast    add5-7kVporincreasemAs50-60%    Largeplastercast    add8-10kVpordoublemAs   Collimatingthex-raybeamtotheareaof interestreduces thevolumeoftissueirradiated.Asaresult,patientexposure isreducedand imagequalityimprovesbecausethereislessscatterradiation exposingthe film.Becausecollimation(alsoknownasreducingfieldsize) reducesthe amountofscatterexposingthefilm,filmdensityisalso reduced.Therefore, anadjustmentinmAsorkVpisrequiredwhenfieldsizeis reducedtomaintain adequatefilmdensity. Inotherwords,ifyoumakeanappropriately exposedimage oftheabdomenusing80kVpand40mAsona14x17"film,and thenneed tocoveraspecificareaoftheabdomenusinga10x12"film, thefilm willturnouttoolightifyouusethesametechniqueof80kVp and40mAs.By collimatingdowntoasmallerfieldsize,younowneedto increaseyourfilm density. Thefollowingtablecanbeusedtocalculate howmuchofa changeisnecessarywhenreducingorenlargingthefieldsize. Exposurefieldsize ChangeinmAsrequiredto maintaindensity Multiplicationfactor   Abdomen: Reducing fieldsizefrom14x17"film downto10x12"  IncreasemAs25-35%  multiplymAsby1.25to1.35   Abdomen: Reducing fieldsizefrom14x17" downto8x10"  IncreasemAs50-75%  multiplymAsby1.5to1.75   Abdomen: Increasing fieldsizefrom10x12" upto14x17"  ReducemAs25-35%   multiplymAsby.25to.35   Abdomen: Increasing fieldsizefrom8x10" upto14x17"  ReducemAs50-75%  multiplymAsby.50to.75   Forexample,ifyouused80kVpand40mAs fora14x 17"imageoftheabdomenandnowtakea10x12"imageofthe same area,youwouldincreasetechniqueby25-35%.  Take40mAstimes1.25,whichequals50.  Youshouldmakethe10x12"exposureusing50mAs.   CompensatingforExposureChangeWhentheCentralRay isAngled Anotherfactorthatinfluencesdensityofan imageoccurs whenthepositionrequiresthatthecentralraybeangledfora specificview. SupposeyouhaveaSIDof40"andobtainanadequatelyexposed imageof theclavicleusing70kVpand8mAs.Nowforthesecondviewin theseries,you angletheCR20degreescephalic.Asyoucanseeinthedrawings below,by anglingthetube,youhavedonetwothings.Youhaveincreased yourSIDandyou arenowpenetratingthroughmorebodytissue.Youhavetwo choices:     *Keepthesame exposurefactors(70kVpand8mAs)bydecreasingSIDbackto 40".     *Leavethe distanceasitisandincreasetheexposuretocompensatefor theadditional distanceusingthechartabove.       Thebasicrulethatappliestomaintaining densitywhen anglingthetubeisthatSIDisdecreased1"forevery5degrees of angulation.Thisruleappliestoanyincreaseofmorethan2" (increasing from40"to42").   CRAngle        DecreasingSIDtomaintaindensity 5-10degrees 1/2"  15degrees 1"  20degrees 2"  25degrees 3"  30degrees 4"  35degrees 6" 40degrees 8"                                                                                     or CRAngle IncreaseinmAsisneeded tomaintaindensityw/ochangingSID 5-10degrees    increasemAsby10-20% 15-25degrees    increasemAsby30-50% 30-35degrees    increasemAsby75-100% 40-45degrees    increasemAsby100-150%   Key PointstoRememberFrom ThisChapter: -Theabilityofaphotontopenetrateabody partis determinedlargelybythecompositionofthatpart(bone,soft tissue,air, etc)aswellasanypathologythatmaybepresent.Some conditionsresultina decreaseinthedensityofbodytissue(suchasosteomalacia, whichcavitates orerodes).Insuchcases,x-rayspenetratemoreeasily.In otherconditions, bodytissuedensityincreasesbecauseacertainpathologyis present,suchas ascites.Inthesecases,x-rayscannotpenetratethroughthe tissueaseasily. -RadiographicdensityiscontrolledbymAs. Ifanexposure iscutinhalf,theradiographicdensitywillalsobecutin half. -kVpalsoaffectsdensity.IfkVpis decreasedby15%,the densityoftheimagewillbecutinhalf.However,changingkVp alsochanges imagecontrastandthepenetratingpowerofthex-raybeam, Therefore,density changesshouldbemadetoanimagebyalteringmAs,notkVp. -TheformulatodeterminemAs(which regulatesradiographic density)ismAxexposuretimeinsecondsorfractionsof seconds=mAs. -Theinversesquarelawstatesthatthe intensityof radiationisinverselyproportionaltothesquareofthe distancetheradiation travels.Therefore,asdistancefromthex-raytubeincreases, intensityofthe x-raybeamdecreases. TableofContents