Production and Control of Scatter Radiation

8/6/2019 Production and Control of Scatter Radiation

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Production and Control of Scatter RadiationProduction and Control of Scatter Radiation

Moderator :Moderator : Mr. Ram SinghMr. Ram SinghLecturerLecturer

Deptt of Radio diagnosis & ImagingDeptt of Radio diagnosis & Imaging

PGIMER, ChandigarhPGIMER, ChandigarhPresenter :RUNISHAPresenter :RUNISHA

BOMRAHBOMRAH

B.Sc.M.T.(XB.Sc.M.T.(X--Ray)Ray)Final Year StudentFinal Year Student


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When photon beam interacts with matter,When photon beam interacts with matter,some of its part get absorbed, some getssome of its part get absorbed, some gets

deflected to a new direction and rest of itsdeflected to a new direction and rest of itspart is transmitted to produce radiographicpart is transmitted to produce radiographicimage.image.

The part which is deflected from itsThe part which is deflected from itsoriginal path to a new direction is known asoriginal path to a new direction is known asscattered radiation.scattered radiation.

IntroductionIntroduction


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DefinitionDefinition

The secondary radiation which makes noThe secondary radiation which makes no

favorable contribution to the formation of imagefavorable contribution to the formation of image

and produce an over all blackness on film thusand produce an over all blackness on film thus

reducing overall radiographic contrastreducing overall radiographic contrastEffects of scatterEffects of scatter1.1. Increase overall density of film which is not useful inIncrease overall density of film which is not useful in

production of image thus produces fogproduction of image thus produces fog

2.2. Reduces contrastReduces contrast

3.3. Reduces light transmitting ability of filmReduces light transmitting ability of film

4.4. Result in formation of noiseResult in formation of noise


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Characteristics of scatterCharacteristics of scatter

More oblique in natureMore oblique in nature

Produced by matter in all directionProduced by matter in all direction

Can travel through longer path in body thanCan travel through longer path in body thanprimary beamprimary beam

Less energy than primary beamLess energy than primary beam

With Increase in energy of primary radiationWith Increase in energy of primary radiation

more scattering in forward directionmore scattering in forward direction Amount of scatter depends upon volume ofAmount of scatter depends upon volume of

tissue exposed, built of pt and Kvp usetissue exposed, built of pt and Kvp use


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Interaction of XInteraction of X--Rays with matterRays with matter

There are five basic ways that an XThere are five basic ways that an X--ray photonray photoncan interact with mattercan interact with matter

Absorption includeAbsorption include PhotoelectricPhotoelectric

Pair productionPair production

Photo disintegrationPhoto disintegration

Scattering includeScattering include Coherent scattering andCoherent scattering and

Compton scatteringCompton scattering


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Coherent ScatteringCoherent Scattering

DefinitionDefinition

ThoseThose interactioninteraction inin whichwhich radiationradiationundergoesundergoes aa changechange inin directiondirection withoutwithout

aa changechange inin wavelengthwavelength.. ItIt isis alsoalso calledcalledunmodifiedunmodified ,classical,classical scatteringscattering

ProcessProcess

LowLow energyenergy radiationradiation encountersencounters thetheelectronselectrons ofof anan atomatom andand setssets themthem intointovibrationvibration atat thethe frequencyfrequency of of thetheradiationsradiations.. AA vibratingvibrating electronelectron emitsemits

radiationsradiations


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Coherent ScatteringCoherent Scatteringcont.cont.

TheThe processprocess maymay bebe envisionedenvisioned asas

absorptionabsorption ofof radiation,radiation, vibrationvibration ofof atomatom

andand emissionemission ofof radiationradiation asas thethe atomatom

returnsreturns toto itsits undisturbedundisturbed statestate..

NoNo energyenergy isis transferredtransferred andand nono

ionizationionization occursoccurs withwith coherentcoherent

scatteringscattering.. TheThe onlyonly effecteffect isis thethe changechange

inin thethe directiondirection ofof incidentincident radiationradiation..


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Types of coherent scatteringTypes of coherent scattering--TwoTwo

ThomsonThomson && RayleighRayleigh ScatteringScattering

ThomsonThomson waswas ableable toto determinedetermine thethe

amountamount ofof energyenergy scattedscatted byby freefree electronelectron

AA singlesingle electronelectron isis evolvedevolved

RayleighRayleigh resultsresults fromfrom coco--operativeoperative

interactioninteraction ofof allall thethe electronelectron ofof thethe atomatom


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Compton ScatteringCompton Scattering

AllAll mostmost allall thethe scatterscatter radiationradiation thatthat wewe encounterencounter inin

DiagnosticDiagnostic RadiologyRadiology comecome fromfrom ComptonCompton scatteringscattering..

ProcedureProcedure

WhenWhen anan incidentincident photonphoton withwith relativelyrelatively highhigh energyenergystrikesstrikes aa freefree outerouter shellshell electron,electron, itit givesgives somesome ofof itsits

energyenergy toto thethe electronelectron ,, ejectingejecting itit fromfrom itsits orbitorbit.. TheThe

photonphoton itselfitself getsgets deflecteddeflected inin newnew directiondirection retainingretaining restrest

partpart ofof itsits originaloriginal energyenergy..

ComptonCompton processprocess cancan bebe analysedanalysed inin termsterms ofof collisionscollisions

b/wb/w aa photonphoton andand anan electronelectron..


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Dependence of Compton EffectDependence of Compton Effect

60kev60kev--90kev PE+CE and 200kev90kev PE+CE and 200kev--2Mevonly CE2Mevonly CE

Independent of atomic no.Independent of atomic no.

It depends upon electron density of atom.It depends upon electron density of atom.

It gradually decreases as photon energy increases.It gradually decreases as photon energy increases.SummarySummary

Involves photon and free electron interactionInvolves photon and free electron interaction

In each collision some energy is absorbed and restIn each collision some energy is absorbed and rest

deflected depending upon angle of collisiondeflected depending upon angle of collision

KE of incident photon =KE of recoil electron+KE of deflectedKE of incident photon =KE of recoil electron+KE of deflected

photonphoton


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Two factors determine amount ofTwo factors determine amount of

energy retained by deflectedenergy retained by deflectedphotonphoton1.Initial energy and1.Initial energy and

2.angle of deflection2.angle of deflection


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Sources of Scatter RadiationSources of Scatter Radiation

Glass walls of XGlass walls of X--ray tuberay tube

Patient himselfPatient himself

Table topTable top

CassetteCassette

Back scatter from floorBack scatter from floor

Back scatter from wallBack scatter from wall


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Factors affecting quantity of scatterFactors affecting quantity of scatter

radiationradiation

Size of fieldSize of field

Shape of fieldShape of field

Thickness of body PartThickness of body Part

KV usedKV used


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Methods of PreventionMethods of Prevention

Prevent the production of scatter radiation by usingPrevent the production of scatter radiation by using

following devicesfollowing devices

Beam limiting devicesBeam limiting devices

Compression bandsCompression bands

Prevent the already formed scatter radiation reaching thePrevent the already formed scatter radiation reaching the

film by using secondary radiation grids.film by using secondary radiation grids. GridsGrids


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Beam limiting DevicesBeam limiting Devices

These are devices which regulate the shape andThese are devices which regulate the shape and

size of beam.These aresize of beam.These are

Cones and CylindersCones and Cylinders

Aperture DiaphragmsAperture Diaphragms

CollimatorsCollimators


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CONESCONES

Tapered metal structure which can be fitted toTapered metal structure which can be fitted toxx--ray tube at the beam exit port.ray tube at the beam exit port.

Made up of Brass or Steel,open at both ends.Made up of Brass or Steel,open at both ends.

Cones come in variety of sizes and shape.Cones come in variety of sizes and shape.

Field Size from a particular cone depends onField Size from a particular cone depends on

Anode film Distance(A)Anode film Distance(A)

Length of cone (L)Length of cone (L)

Distance b/w narrow end of cone and focalDistance b/w narrow end of cone and focalspot (f)spot (f)

Diameter of cone(D)Diameter of cone(D)

Field size =(A/L+f)DField size =(A/L+f)D


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PrecautionsPrecautions

PersonPerson shouldshould bebe wellwell awareaware ofof thethe fieldfield

sizesize irradiatedirradiated byby aa particularparticular conecone..

ProperProper calculationscalculations shouldshould bebe mademade toto

findfind outout thethe conecone toto bebe usedused..

DisadvantagesDisadvantages

ProblemProblem ofof changingchanging thethe conecone dependingdepending

onon thethe typetype ofof examexam..

ProblemProblem ofof fittingfitting circularcircular fieldfield ofof radiationradiation

toto nonnon circularcircular ptspts andand filmfilm..


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CylinderCylinder

SometimesSometimes aa radiographicradiographic conecone isis aa steelsteel

cylindercylinder whichwhich maymay bebe extensibleextensible inin lengthlength..

ItIt cancan bebe equippedequipped withwith extensionextension toto givegive betterbetter

beambeam restrictionrestriction

CylinderCylinder isis betterbetter beambeam restrictorrestrictor thanthan aa conecone


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Aperture DiaphragmAperture Diaphragm

TheThe problemproblem ofof fittingfitting thethe circularcircular fieldfield ofof

radiationradiation.. isis overover comecome byby usingusing thisthis..

ThisThis isis simplesimple tablettablet ofof heavyheavy metalmetal andand

hashas centralcentral apertureaperture forfor xx--rayray beambeam.. ItIt cancan bebe slottedslotted intointo aa fittingfitting onon thethe tubetube

partpart inin aa similarsimilar mannermanner toto conecone..


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Advantages:

Simple.

Lead being soft can be altered

to any desired shape

Disadvantage:

Produces a large penumbra at

periphery of x-ray beam


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CollimatorsCollimators

TheseThese areare twotwo pairspairs ofof movablemovable leavesleaves ofof metalmetal

usuallyusually mademade upup ofof leadlead situatedsituated inin xx--rayray beambeam..

TheseThese pairspairs movemove atat rightright angleangle toto eacheach otherothertoto clearclear penumbrapenumbra

EachEach pairpair isis operatedoperated independentindependent ofof otherother byby

meansmeans ofof aa controlcontrol knobknob.. XX--rayray fieldfield cancan bebe seenseen withwith helphelp ofof lightlight bulbbulb inin

thethe unitunit..


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AdvantagesAdvantages

ProvidesProvides infiniteinfinite varietyvariety of of squaresquare andand

rectangularrectangular fieldsfields..

AlreadyAlready fittedfitted intointo thethe beambeam soso nono needneed of of

puttingputting oror removingremoving againagain andand againagain soso savessaves

timetime..

SameSame collimatorcollimator cancan bebe usedused forfor differentdifferent fieldfield

sizesize..


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Compression bandsCompression bands

It reduces the amount of scatter by diminishingIt reduces the amount of scatter by diminishing

the volume of tissue through which xthe volume of tissue through which x--ray beamray beam

passespasses

It displaces the adipose tissue side ways thusIt displaces the adipose tissue side ways thus

reducing the volume of part to be Xreducing the volume of part to be X--rayed andrayed and

lowers the kilovoltage to be used .lowers the kilovoltage to be used .

AdvantagesAdvantagesIt acts as immobilizing device. Helps in reductionIt acts as immobilizing device. Helps in reduction

of scatter thus improving contrastof scatter thus improving contrast ..


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Secondary Radiation GridSecondary Radiation Grid

HistoryHistory :: RapidRapid adoptionadoption ofof newnew techniquetechnique

andand devicedevice DrDr.. GustaveGustave BuckyBucky inin 19131913

demonstrateddemonstrated scatterscatter radiationradiation andand itsitsremedyremedy byby usingusing grids,grids, workingworking inin fieldfield ofof

RadiologyRadiology..

IntroductionIntroduction:: itit isis aa radiographicradiographicaccessoryaccessory whichwhich isis designeddesigned toto minimizeminimize

thethe effecteffect ofof scatterscatter radiationradiation reachingreaching thethe

filmfilm..


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StructureStructure

GridGrid isis composedcomposed ofof thinthin stripsstrips ofof leadleadwhichwhich areare separatedseparated fromfrom eacheach otherother bybyinterinter spacespace materialmaterial whichwhich isis penetrablepenetrable bybyprimaryprimary radiationradiation..

PrinciplePrinciple TheThe scatteredscattered radiationradiation movemove obliquelyobliquely soso

thesethese obliqueoblique raysrays areare absorbedabsorbed byby thethe

leadlead stripsstrips whilewhile thethe primaryprimary radiationradiationwhichwhich movemove inin forwardforward directiondirection willwill passpassthroughthrough interspaceinterspace andand reachreach thethe filmfilm..


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Grid functionGrid function


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Ideal GridIdeal Grid It is the one which absorbs all the scatterIt is the one which absorbs all the scatter

radiation and allows the primary radiation toradiation and allows the primary radiation to

pass through, but no grid is ideal.pass through, but no grid is ideal.

UsesUses AbdomenAbdomen

SkullSkull

SpineSpine

PelvisPelvis

Penetrating chest viewPenetrating chest view


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Parameters of GridParameters of Grid

Grid ratioGrid ratio

Grid latticeGrid lattice

Nature of inter space materialNature of inter space material

Lead contentsLead contents


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Grid ratio = h/D =2/.25

=8:1


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SignificanceSignificance

ItIt isis thethe parameterparameter whichwhich determinesdeterminesthethe abilityability ofof aa gridgrid toto removeremove scatterscatter

radiationradiation.. ItIt measuresmeasures thethe narrownessnarrowness ofof thethe

slitsslits..

ItIt determinesdetermines thethe extentextent toto whichwhichobliquelyobliquely movingmoving scatterscatter radiationradiation isisabsorbedabsorbed..


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Grid LatticeGrid Lattice

Number of pb strips per inch in a grid.Number of pb strips per inch in a grid.

No. of lines per inc=25.4/D+dNo. of lines per inc=25.4/D+d

D= thickness of inter spaceD= thickness of inter spacematerialmaterial

d=thickness of pb stripsd=thickness of pb strips

A grid with large no.of lines per inch orA grid with large no.of lines per inch orwith a high ratio is preferred for betterwith a high ratio is preferred for betterquality radiographsquality radiographs..


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Inter space materialInter space material

Holds the lead strips apartHolds the lead strips apart

Must be rigid enough to maintain thinMust be rigid enough to maintain thinstrips in position but should notstrips in position but should notabsorb primary radiation.absorb primary radiation.

Material usedMaterial used

MetalMetal --:AI:AI

Organic materialOrganic material--: Plastic and wood: Plastic and wood


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Lead contentLead content

It is the amount of lead per unit surfaceIt is the amount of lead per unit surface

area.area.

It is expressed as gram per cmIt is expressed as gram per cm22

SignificanceSignificance

Amount of lead in a grid is a goodAmount of lead in a grid is a good

indicator of its ability to absorbindicator of its ability to absorbscattered radiation provided grid is wellscattered radiation provided grid is well

designed.designed.


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Types of gridsTypes of grids

Linear gridLinear grid

Crossed gridCrossed grid

Parallel gridParallel grid

Focused gridFocused grid

Pseudo focussed gridPseudo focussed grid


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Linear gridLinear grid

LeadLead stripsstrips areare parallelparallel toto eacheach otherother inin

theirtheir longitudinallongitudinal axisaxis.. AdvantageAdvantage--:: AllowAllow usus toto angleangle thethe xx--rayray

tubetube alongalong thethe lengthlength thethe lengthlength ofof thethe

gridgrid withoutwithout thethe lossloss of of primaryprimary

radiationradiation..


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Crossed gridCrossed grid

MadeMade upup of of twotwo superimposedsuperimposed linearlinear

gridsgrids..

GridGrid ratioratio == sumsum of of gridgrid ratioratio ofof twotwo

linearlinear gridsgrids..

CannotCannot bebe usedused inin thethe techniquestechniques

requiringrequiring tubetube angulationsangulations

UsedUsed inin BiplaneBiplane AngiographyAngiography


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Parallel gridParallel grid

Pb strips are parallel when view inPb strips are parallel when view in

cross sectioncross section

These strips are focused at infinityThese strips are focused at infinity

Uses : Flouro and spot film device.Uses : Flouro and spot film device.

Advantages:Advantages:

Simple in structureSimple in structure

Any side of grid can face the xAny side of grid can face the x--ray tuberay tube


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Geometrical cut offGeometrical cut off--Allow central ray toAllow central ray to

pass but peripheral rays get absorbedpass but peripheral rays get absorbed

which is oblique in naturewhich is oblique in nature It results in loss of density alongIt results in loss of density along

edges.edges.

Projection of pb strips wider than theyProjection of pb strips wider than theyare.are.


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Malposition of GridMalposition of Grid

TiltingTilting ofof gridgrid resultsresults inin lossloss ofof densitydensity

alongalong centralcentral bandband andand oneone sideside ofof thethe

filmfilm..

IncorrectIncorrect centeringcentering producesproduces underunder

exposedexposed radiographsradiographs atat thethe edgeedge awayaway

fromfrom tubestubes displacementdisplacement ..


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Focused GridFocused Grid

PbPb stripsstrips areare arrangedarranged toto formform radiiradii ofof

circlecircle toto whichwhich xx--rayray tubetube isis centeredcentered

meansmeans pbpb stripsstrips areare slightlyslightly angledangled soso

thatthat theythey focusedfocused inin spacespace..

ItIt reducesreduces geometricgeometric cutcut offoff inin thethe areasareas

awayaway fromfrom beambeam centercenter


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RangesRanges ofof FFDFFD inin whichwhich itit cancan bebe usedused

AnodeAnode filmfilm distancedistance shouldshould notnot bebe tootoolonglong oror tootoo shortshort..

ManufacturingManufacturing isis difficultdifficult

HighHigh ratioratio gridsgrids soso highhigh KvKv requiredrequired..

OnlyOnly oneone aspectaspect whichwhich facesfaces thethe tubetube..


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Pseudo focused gridPseudo focused grid

PbPb stripsstrips areare arrangedarranged inin parallelparallelfashionfashion andand atat thethe samesame timetime isisdesigneddesigned toto produceproduce effecteffect ofof focusfocusgridgrid ..

HenceHence thethe namename pseudopseudo meansmeans falsefalse..

DesignDesign

HeightHeight of of pbpb stripsstrips decreasesdecreases fromfrom

centercenter towardstowards peripheryperiphery ..ThusThusreducingreducing thethe gridgrid ratioratio fromfrom centercenter totoperipheryperiphery andand givinggiving thethe advantageadvantage ofoffocusfocus asas wellwell linearlinear gridgrid..


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Primary TransmissionPrimary Transmission

Measurement of percentage of primaryMeasurement of percentage of primary

radiation transmitted through grid.radiation transmitted through grid.

TTpp=(I=(Igg/I/Igg00)100)100

IIgg=Intensity with grid=Intensity with grid

IIgg00=Intensity without grid=Intensity without grid


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Bucky FactorBucky Factor

ItIt isis thethe ratioratio of of incidentincident radiationradiation

fallingfalling onon thethe gridgrid toto thethe transmittedtransmitted

radiationradiation passingpassing throughthrough thethe gridgrid..

B=IncidentB=Incident radiationradiation /Transmitted/Transmittedradiationradiation

TransmittedTransmitted radiationradiation areare measuredmeasured

withwith gridgrid andand incidentincident photonsphotons arearemeasuredmeasured withoutwithout gridgrid..


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Cont.Cont.

AdvantageAdvantage--Higher the bucky factorHigher the bucky factor

better the quality of image.better the quality of image.

DisadvantageDisadvantage--Higher the bucky factorHigher the bucky factor

more exposure to ptmore exposure to pt


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Factors affecting BFFactors affecting BF

Grid RatioGrid Ratio--Higher the grid ratio higherHigher the grid ratio higher

the bucky factor.the bucky factor.

BeamBeam energyenergy--higher the Beam energyhigher the Beam energy

greater the bucky factorgreater the bucky factor


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Contrast improvement factor (k)Contrast improvement factor (k)

K= Contrast with grid / contrast withoutK= Contrast with grid / contrast without

gridgrid

Depends upon : Field size, thickness ofDepends upon : Field size, thickness ofphantom and Kv.phantom and Kv.

K factor measures grids ability to improveK factor measures grids ability to improve

contrast.contrast. It is related to lead content and grid ratioIt is related to lead content and grid ratio

so higher the grid ratio higher the k.so higher the grid ratio higher the k.


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Types of Grid MovementsTypes of Grid Movements

Single strokeSingle stroke

ReciprocatingReciprocating

OscillatingOscillating

CatapultCatapult


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Single StrokeSingle Stroke

GridGrid movesmoves continuouslycontinuously acrossacross thethe filmfilm..

UsuallyUsually itit isis springspring loadedloaded andand requiresrequires

manualmanual cockingcocking priorprior toto eacheach exposureexposure.. TotalTotal gridgrid movementmovement isis usuallyusually 22..55 toto 55cmcm..

PitfallsPitfalls::

DifficultDifficult toto useuse asas theythey requirerequire cockingcockingbeforebefore eacheach exposureexposure..

SeldomSeldom usedused inin modernmodern equipmentequipment..


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ReciprocatingReciprocating

GridGrid isis drivendriven continuouslycontinuously tootoo andand frofro fromfrom oneone

sideside ofof thethe tabletable toto anotheranother duringduring exposureexposure..

GridGrid isis propelledpropelled inin oneone directiondirection byby thethe

combinationcombination ofof twotwo springssprings andand speedspeed isis keptkeptconstantconstant withwith anan oiloil dashpotdashpot..

Function of oil dashpotFunction of oil dashpot::

ToTo controlcontrol andand smoothsmooth thethe speedspeed ofof gridgrid.. ToTo enableenable adjustmentsadjustments ofof thisthis speedspeed toto bebe mademade

fromfrom timetime toto timetime..


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OscillatingOscillating

CanCan bebe obtainedobtained byby movingmoving thethe gridgrid onon

springspring atat eacheach cornercorner andand givinggiving itit

pushpush fromfrom timetime toto time,time, causingcausing itit toto

vibratevibrate uponupon springssprings..

AtAt firstfirst gridgrid movesmoves rapidlyrapidly afterafter thatthat itit

slowsslows downdown andand eventuallyeventually stopsstops..


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Catapult grid MovementCatapult grid Movement

GridGrid makesmakes oneone movementmovement acrossacross filmfilm.. ItsIts

initialinitial speedspeed isis highhigh thanthan itit slowsslows downdown inin anan

exponentialexponential fashionfashion.. TheThe exposureexposure isis initiatedinitiated shortlyshortly afterafter thethe

gridgrid beganbegan toto movemove..

ShortShort exposureexposure whilewhile gridgrid movingmoving veryvery fastfastandand longerlonger exposureexposure extendsextends intointo timetime whenwhen

gridgrid movingmoving veryvery slowlyslowly..


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Catapult grid Movement cont.Catapult grid Movement cont.

This type of movement ensuresThis type of movement ensures

whether the exp is short or longwhether the exp is short or long

It is motor drivenIt is motor driven


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Disadvantages of moving gridsDisadvantages of moving grids

CostlyCostly

Subject to failureSubject to failure May vibrate the xMay vibrate the x--ray tableray table

Put a minimum limit on exposure timePut a minimum limit on exposure time

Increase patient dose 15% in compareIncrease patient dose 15% in compare

to stationary grid.to stationary grid.

Mammography GridMammography Grid


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g p yg p y

TheThe xx--rayray beambeam thatthat existexist thethe breastbreastcontaincontain scatteredscattered radiationradiation..

TheThe amountamount ofof scatteredscattered increasesincreases withwiththicknessthickness andand densitydensity ofof breastbreast..

SoSo focusedfocused gridgrid areare usedused toto absorbabsorb scatterscatterradiationradiation andand toto improveimprove qualityquality ofof imageimage..

UseUse of of gridgrid allowsallows greatergreater visualizationvisualization ofofbreastbreast structurestructure micromicro calcificationcalcification inin

densedense breastbreast.. GenerallyGenerally 44::11 toto 55::11 focusedfocused gridgrid ratioratio withwith

3030 toto 5050 lineslines /cm/cm usedused..


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AdvancementAdvancement


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AdvancementAdvancement

HTCHTC (high(high transmissiontransmission cellularcellular

gridgrid))::

ToTo overcomeovercome thethe problemproblem ofof absorptionabsorption ofofthethe primaryprimary radiationradiation byby interspaceinterspace

materialmaterial thethe LORADLORAD companycompany ofof medicalmedical

equipmentequipment introducedintroduced HTCHTC gridgrid for formammographymammography..


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StructureStructure::

CellularCellular gridgrid isis aa honeyhoney combcomb shapedshaped gridgrid..ItIt hashas aa sqsq.. porepore withwith coppercopper septasepta andand airairinterinter spacespace..

ItIt hashas aa rigidrigid kindkind ofof thatthat isis selfself supportingsupportingsoso nono needneed ofof interinter spacespace materialmaterial..

SoSo nono primaryprimary radiationradiation absorptionabsorption takestakesplaceplace byby interinter spacespace..

TheThe HTCHTC gridgrid hashas coppercopper asas gridgrid stripstrip

materialmaterial PhysicalPhysical dimensiondimension resultsresults inin aa 33..88::11 gridgrid

ratioratio


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G id t ffG id t ff


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Grid cut offGrid cut off

ItIt isis thethe proportionproportion ofof primaryprimary beambeam

whichwhich thethe leadlead stripsstrips ofof gridgrid absorbabsorb.. ItIt

isis affectedaffected byby thicknessthickness ofof leadlead strips,strips,

heightheight andand spacingspacing..

MoreMore withwith highhigh ratioratio gridgrid andand shortshort gridgrid

focusfocus distancedistance


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Grid cut offGrid cut offCont..Cont..

ThereThere areare fivefive differentdifferent situationsituation thatthatproduceproduce gridgrid cutoffcutoff::

1.1. FocusedFocused gridgrid upsideupside downdown2.2. LateralLateral decenteringdecentering

3.3. FocusFocus gridgrid distancedistance decenteringdecentering

4.4.C

ombinedC

ombined laterallateral andand focusfocus gridgriddistancedistance decenteringdecentering

5.5. Off Off levellevel errorerror


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Focused Grid upside DownFocused Grid upside Down

AllAll focusedfocused gridsgrids havehave tubetube sideside whichwhich

isis thethe sideside ofof focusfocus ofof pbpb stripsstrips..

WhenWhen itit isis upsideupside downdown therethere isis severesevere

peripheralperipheral cutcut offoff withwith aa darkdark bandband ofof

exposureexposure inin thethe centercenter ofof thethe filmfilm andand

nono exposureexposure inin thethe peripheralperipheral..

Focus grid upside downFocus grid upside down


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Focus grid upside downFocus grid upside down


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Lateral DecenteringLateral Decentering

ResultsResults fromfrom xx--rayray tubetube beingbeing

positionedpositioned laterallateral toto convergentconvergent lineline

butbut atat thethe correctcorrect focalfocal distancedistance.. SoSo therethere isis uniformuniform lossloss of of radiationradiation

overover entireentire surfacesurface ofof gridsgrids ,producing,producing

uniformlyuniformly lightlight radiographsradiographs.. ThisThis isis commoncommon typetype ofof gridsgrids cutoffcutoff..


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Lateral DecenteringLateral Decentering cont.cont.

There are three Factors which affect itThere are three Factors which affect it

1.1. Grid ratioGrid ratio

2.2. Focal DistanceFocal Distance3.3. Amount of decentiringAmount of decentiring

L=(rb)x100/fL=(rb)x100/foo

where L=Loss of primary radiation%where L=Loss of primary radiation%r=grid ratior=grid ratio

f=focal distancef=focal distance

b=Lateral decentiring distanceb=Lateral decentiring distance


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F d id di tF d id di t


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Focused grid distanceFocused grid distance

discenteringdiscentering InIn thisthis targettarget ofof xx--rayray tubetube isis correctlycorrectly

centeredcentered toto gridgrid butbut itit isis positionedpositioned

aboveabove oror belowbelow thethe convergentconvergent lineline.. IfIf targettarget isis aboveabove convergentconvergent lineline itit isis

calledcalled Far Far focusfocus gridgrid distancedistancedecenteringdecentering..

IfIf itit belowbelow thanthan itit isis calledcalled nearnear focusfocusgridgrid distancedistance decenteringdecentering..


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Off level errorOff level error

GridGrid mustmust lielie inin aa planeplane perpendicularperpendicular

toto thethe centralcentral axisaxis ofof beambeam..

WhenWhen linear linear gridgrid isis tiltedtilted asas itit

frequentlyfrequently happenhappen inin casecase ofof portableportable

radiographyradiography therethere isis uniformuniform lossloss of of

primaryprimary radiationradiation acrossacross entireentire surfacesurface

ofof filmfilm..


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Grid SelectionGrid Selection

Selection of Grids depends onSelection of Grids depends on

interrelated Factors like:interrelated Factors like:

Types of exam performedTypes of exam performed

KvKv

Pt. dosePt. dose

Grids should be selected so that goodGrids should be selected so that good


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Grids should be selected so that goodGrids should be selected so that goodquality radiograph obtain keeping ptquality radiograph obtain keeping pt

dose low eg.dose low eg. In mammography Breast dose with andIn mammography Breast dose with and

without gridwithout grid

Generally for 50%grandular and 50%Generally for 50%grandular and 50%adipose tissue with average thicknessadipose tissue with average thicknessof breast of 4.2 cmof breast of 4.2 cm

Without grid dose isWithout grid dose is

1 to 1.5mGy(100 to 150mrad)per film1 to 1.5mGy(100 to 150mrad)per filmWith gridWith grid

3mGy (300mrad) per film3mGy (300mrad) per film


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Cont.Cont.

TheThe dosedose seemsseems toto bebe doubledouble ,yes,yes thethe

concernconcern aboutabout exposureexposure toto ptpt isis

importantimportant.. ButBut simultaneouslysimultaneously gridgrid selectionselection

howeverhowever shouldshould notnot bebe compromisedcompromised

soso far far thatthat thethe lossloss of of contrastcontrastenhancementenhancement interferesinterferes withwith diagnosticdiagnostic

InterpretationInterpretation..


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Air Gap TechAir Gap Tech

ItIt isis anan alternativealternative techniquetechnique toto reducereduce

thethe scatterscatter radiationradiation byby leavingleaving thethe gapgap

b/wb/w thethe patientpatient andand filmfilm duringduring

radiographyradiography andand enhancingenhancing

radiographicradiographic contrastcontrast..

AimAim :: ToTo reducereduce affectaffect of of scatter scatter

radiationradiation onon thethe filmfilm..


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Advantage Over gridAdvantage Over grid

AsAs nono absorptionabsorption ofof primaryprimary radiationradiation

takestakes placeplace soso nono needneed ofof increasingincreasingthethe exposureexposure..

DisadvantageDisadvantage

DueDue toto gapgap betweenbetween patientpatient andand filmfilm

sharpnesssharpness ofof thethe imageimage decreasesdecreases..


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Lead foil screensLead foil screens

These can be incorporated at the back of the xThese can be incorporated at the back of the x--rayray

cassette holder for the purpose of protecting thecassette holder for the purpose of protecting the

film from back scatter from the walls and floor.film from back scatter from the walls and floor.

Lead sheetLead sheet

To avoid the back scatter reaching to film pbTo avoid the back scatter reaching to film pb

sheet are also fitted at the back of the cassettes.sheet are also fitted at the back of the cassettes.

ConclusionConclusion


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ConclusionConclusion

AlmostAlmost allall thethe scatteredscattered radiationradiation inindiagnosticdiagnostic radiationradiation comescomes fromfrom thethe

comptoncompton processprocess..

ThereThere isis nono methodmethod toto stopstop thetheproductionproduction ofof scatteredscattered radiationradiation butbut

wewe cancan reducereduce thethe formationformation ofof scatterscatter

andand alsoalso wewe cancan preventprevent themthem toto reachreach

thethe filmfilm..

SecondarySecondary radiationradiation gridgrid isis consideredconsidered

effectiveeffective wayway toto controlcontrol itit..

Conclusion contd.Conclusion contd.


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Conclusion contd.Conclusion contd.

ThereThere isis nono simplesimple rulerule toto choosechoose gridgrid

butbut somesome compromisecompromise alwaysalways involvedinvolved

SoSo betterbetter qualityquality radiographradiograph shouldshould bebe

obtainobtain withwith moralmoral obligationobligation toto keepkeep

ptspts dosedose minimumminimum asas muchmuch asaspossiblepossible..


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Date post:	08-Apr-2018
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Production and Control of Scatter Radiation

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