Scatter Control

Scatter ControlScatter Control&&

Grid UseGrid Use

Denise Ogilvie Denise Ogilvie October 2007October 2007

ObjectivesObjectives

Identify factors that affect the amount of Identify factors that affect the amount of scatter radiation producedscatter radiation produced

Describe methods used to control the Describe methods used to control the amount of scatter radiationamount of scatter radiation

Describe the effect of beam restriction on Describe the effect of beam restriction on image quality and patient doseimage quality and patient dose

Compare advantages and disadvantages of Compare advantages and disadvantages of different beam restricting devisesdifferent beam restricting devises

ObjectivesObjectives Describe the purpose of a gridDescribe the purpose of a grid Explain the construction of a grid, including Explain the construction of a grid, including

materials used, grid ratio and grid frequencymaterials used, grid ratio and grid frequency Differentiate between parallel and focused grids, Differentiate between parallel and focused grids,

stationary and moving gridsstationary and moving grids Calculate changes in technical factors to Calculate changes in technical factors to

compensate for changes in grid selectioncompensate for changes in grid selection Be able to identify common errors made when Be able to identify common errors made when

using a grid on an imageusing a grid on an image Know when to use a grid and when not to use.Know when to use a grid and when not to use.

Scatter RadiationScatter Radiation

Scatter is radiation which is changed in Scatter is radiation which is changed in direction as a result of interaction with some direction as a result of interaction with some medium.medium.

Some of the photon’s energy is absorbed, Some of the photon’s energy is absorbed, leaving the resultant photon with a change leaving the resultant photon with a change in its direction and with less energyin its direction and with less energy

These scattered photons are detrimental to These scattered photons are detrimental to contrast of the image and also increase the contrast of the image and also increase the patient dosepatient dose

Scatter RadiationScatter Radiation Other sources of scatter Other sources of scatter

– materials beyond the – materials beyond the image receptor – table image receptor – table top – may cause scatter top – may cause scatter to go back to the image.to go back to the image.

Two primary factors Two primary factors affecting the amount of affecting the amount of scatter produced –kvp scatter produced –kvp and the irradiated and the irradiated materialmaterial


KvpKvp

Affects the penetrability of the beam.Affects the penetrability of the beam. Higher kVp, more photons go through Higher kVp, more photons go through

patient to the IR, less absorbed by patient, patient to the IR, less absorbed by patient, higher scatter and less contrast on imagehigher scatter and less contrast on image

Lower the kVp, increase in dose absorbed Lower the kVp, increase in dose absorbed by patient, less fog on film, more contrasty by patient, less fog on film, more contrasty imageimage


Irradiated MaterialIrradiated Material

Amount of scatter affected by volume and Amount of scatter affected by volume and atomic number of irradiated materialatomic number of irradiated material

Volume is controlled by field size and patient Volume is controlled by field size and patient thicknessthickness

Increase in volume if field size increases Increase in volume if field size increases and patient thickness increases. and patient thickness increases.


To reduce scatter – smallest field size, To reduce scatter – smallest field size, compression of body partcompression of body part

The higher the atomic number of the The higher the atomic number of the material the greater the absorption of material the greater the absorption of photons and the less scatter eg bone photons and the less scatter eg bone compared to soft tissuecompared to soft tissue


Beam RestrictionBeam Restriction

Aperture diaphragms, cones/cylinders, Aperture diaphragms, cones/cylinders, collimators – 3 types of beam restricting collimators – 3 types of beam restricting devices to control scatter and reduce patient devices to control scatter and reduce patient dosedose

Scatter RadiationScatter Radiation Aperture DiaphragmAperture Diaphragm Simplest, low costSimplest, low cost Flat piece of lead with Flat piece of lead with

hole (of different sizes)hole (of different sizes) Slides into slot at bottom Slides into slot at bottom

of collimatorof collimator Some resultant Some resultant

penumbrapenumbra

Scatter RadiationScatter Radiation Cones and CylindersCones and Cylinders Similar to diaphragm Similar to diaphragm

with extension cone or with extension cone or cylindercylinder

Slides into slot bottom Slides into slot bottom of collimatorof collimator

Reduces penumbraReduces penumbra


CollimaterCollimater More complex, most More complex, most

commonly used form of commonly used form of beam restrictionbeam restriction

Set of adjustable lead Set of adjustable lead shuttersshutters

Light & mirror to show Light & mirror to show area of beam and area of beam and collimationcollimation


The bevelled edges of The bevelled edges of lead diaphragm lead diaphragm compared to vertical compared to vertical edge.edge.

Radiographic GridsRadiographic Grids A device to absorb scatter radiation before it A device to absorb scatter radiation before it

strikes the IRstrikes the IR Made of thin Pb strips interspaced with Made of thin Pb strips interspaced with

radiolucent material – usually aluminiumradiolucent material – usually aluminium Frequency – number of lines per inch or cm Frequency – number of lines per inch or cm

eg 60 lines per incheg 60 lines per inch Grids with higher frequency have thinner Pb Grids with higher frequency have thinner Pb

strips – better for stationary grids so you strips – better for stationary grids so you don’t see the linesdon’t see the lines

The more Pb the better the scatter reductionThe more Pb the better the scatter reduction

Radiographic GridsRadiographic Grids

TypesTypes Parallel – Pb & interspace running parallel to Parallel – Pb & interspace running parallel to

one anotherone another Focused – central strips parallel, then Focused – central strips parallel, then

become more angled as you move away become more angled as you move away from the centre – angle matching that of from the centre – angle matching that of divergent rays – allows more transmitted divergent rays – allows more transmitted photons to reach the IRphotons to reach the IR


Crossed grid – 2 Crossed grid – 2 parallel grids on top of parallel grids on top of each other.each other.

May be parallel or May be parallel or focusedfocused


Focal range – Focal range – recommended SID for recommended SID for that particular grid.that particular grid.

For parallel grid focal For parallel grid focal range is from certain range is from certain SID to infinity – SID to infinity – function better at function better at longer SIDlonger SID


Grid RatioGrid Ratio Ratio of height of Pb Ratio of height of Pb

lines to distance lines to distance between thembetween them

Grid ratio increases, Grid ratio increases, contrast increasescontrast increases


The higher the grid The higher the grid ratio the more ratio the more exposure is requiredexposure is required


Potter Bucky – moving Potter Bucky – moving grid for better scatter grid for better scatter clean up and improved clean up and improved image qualityimage quality

Grid is moved during Grid is moved during the exposure to blur the exposure to blur out grid lines.out grid lines.

Movement must Movement must commence before commence before exposure can be madeexposure can be made

Radiographic GridsRadiographic Grids Air gap techniqueAir gap technique Between patient and filmBetween patient and film Eliminates need of gridEliminates need of grid Gap of at least 15cm – Gap of at least 15cm –

increase SID to reduce increase SID to reduce magnificationmagnification

The scatter from the body The scatter from the body does not hit the IRdoes not hit the IR


Grid ErrorsGrid Errors Upside down grid – Upside down grid –

peripheral grid cut off peripheral grid cut off with a focus gridwith a focus grid

Check front of grid – Check front of grid – upper side has line upper side has line down centre indicating down centre indicating direction of grid linesdirection of grid lines


Off centre – tube not Off centre – tube not centred to middle of centred to middle of grid. grid.

Result in decrease in Result in decrease in exposure across entire exposure across entire image and visible grid image and visible grid lineslines

The greater the The greater the decentering the decentering the greater the grid cut offgreater the grid cut off


Off level grid – tube Off level grid – tube angled across long angled across long axis of Pb stripsaxis of Pb strips

Show grid lines with Show grid lines with decrease in exposure decrease in exposure on imageon image

ReferencesReferences

Burns, E, Burns, E, Radiographic imaging a guide for producing Radiographic imaging a guide for producing quality images,quality images, Saunders 1992 1 Saunders 1992 1stst edn edn

Carlton, R, Adler, A, Principles of radiographic imaging an Carlton, R, Adler, A, Principles of radiographic imaging an art and a science, 4art and a science, 4thth edn edn

Fauber, T, Fauber, T, Radiographic imaging & exposure,Radiographic imaging & exposure, 2000 2000 Kodak, The fundementals of radiography,11Kodak, The fundementals of radiography,11thth edn edn Stockley, S, A manual of radiographic equipment,1Stockley, S, A manual of radiographic equipment,1stst edn, edn,

19861986

Scatter Control

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