Scatter Radiation - karary.edu.sdkarary.edu.sd/site/assets/files/25171/08_effects... · Radiographic Grids: • An extremely effective device for reducing the level of scatter radiation

Scatter Radiation

Dr. Ahmed Alsharef Farah

• The two principal characteristics of any imageare spatial resolution and contras t resolution.

• Some refer to these together as image detail orvisibility of detail.

Introduction


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• Spatial resolution refers to the ability to imagesmall objects that have high subject contrast,and is determined by focal-spot size and otherfactors that contribute to blur.

• Contrast resolution is the ability to distinguishanatomical structures of similar subject, and isdetermined by scatter radiation and othersources of image noise.


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Production of scatter radiation

Two types of x-rays are responsible for the opticaldensity (OD) and contrast on radiographic image:

• Those pass through the patient withoutinteracting.

• That are Compton scattered within the patient.


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• X-rays that exit from the patient are remnant x-rays and those that exit and interact with theimage receptor are called image-forming x-rays.


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Some x-rays interact with the patient and are scattered away from theimage receptor (a). Others interact with the patient and are absorbed(b). X-rays that arrive at the image receptor are those transmittedthrough the patient without interacting (c) and those scattered in thepatient (d). X-rays of types c and d are called image-forming x-rays.

1. Reducing patient radiation dose by restrictingthe volume of irradiated tissue.

2. Improves image contrast.

• As scatter radiation increases, the radiographicimage loses contrast and appears gray and dull.

Proper collimation of the x-ray beam:


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1. Kvp.2. Field size.3. Patient thickness.

Three primary factors influence the relativeintensity of scatter radiation that reaches theimage receptor:


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• As x-ray energy is increased, the absolutenumber of Compton interactions decreases,but the number of photoelectric interactionsdecreases much more rapidly.

• Therefore the relative number of x-rays thatundergo Compton scattering increases.

kVp:


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Percent interaction of x-rays by photoelectric and Comptonprocesses and percent transmission through 10 cm of softtissue.

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• It would be easy enough to say that allradiographs should be taken at the lowestreasonable kVp because this technique wouldresult in minimum scatter and thus higherimage contrast.


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• Increasing the mAs usually generates enough x-rays to provide a satisfactory image but mayresult in an unacceptably high patientradiation dose.

• On the other hand, a much smaller increase inkVp is usually sufficient to provide enough x-rays, and this can be done at a much lowerpatient radiation dose.


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• Unfortunately, when kVp is increased, the levelof scatter radiation also increases, leading toreduced image contrast.

• Collimators and grids are used to reduce thelevel of scatter radiation.


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• Most radiologists would accept any of these radiographs.• Notice that the patient dose at 90 kVp is approximately

one third that at 70 kVp. In general, because of thisreduction in patient dose, a high-kVp radiographictechnique is preferred to a low-kVp technique.

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• Another factor that affects the level of scatterradiation and is controlled by the radiologictechnologist is x-ray beam field size.

• As field size is increased, scatter radiation alsoincreases.

Field Size:


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Collimation of the x-ray beam results in less scatterradiation, reduced patient radiation dose, and improvedcontrast resolution.

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Lumbar spine radiography The full- field technique results in reduced image contrast.A, Full- field technique. B, Preferred collimated technique.

• Imaging thick parts of the body results in morescatter radiation than does imaging thin bodyparts.

Patient Thickness:


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• Compare a radiograph of the bony structures inan extremity with a radiograph of the bonystructures of the chest or pelvis.

• Even when the two are taken with the samescreen-film image receptor, the extremityradiograph will be much sharper because of thereduced amount of scatter radiation.


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Posteroanterior view of the handExtremity radiographs appear sharp because of less tissueand, hence, less scatter radiation.


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• The types of tissue (muscle, fat, bone) andpathology, such as a fluid-filled lung, also playa part in the production of scatter radiation.


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• Normally, patient thickness is not controlled bythe radiologic technologist.

• A high-quality radiograph produced bychoosing the proper technique factors and byusing devices that reduce scatter radiation tothe image receptor, such as a compressionpaddle.


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When tissue is compressed, scatter radiation is reduced,resulting in a lower patient radiation dose and improvedcontrast resolution.


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• Compression devices improve spatialresolution by reducing patient thickness andbringing the object closer to the image receptor.

• Compression also reduces patient radiationdose and improves contrast resolution.

• Compression is particularly important duringmammography.


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Control of scatter radiation

• Image contrast is the visible differencebetween the light and dark areas of an image.

• Contrast is the degree of difference in ODbetween areas of a radiographic image.

• Contrast resolution is the ability to image anddistinguish soft tissues.

Effect of Scatter Radiation on Image Contrast:

Dr. Ahmed Alsharef Farah 25

• Even under the most favorable conditions, mostremnant x-rays are scattered.


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When primary x-rays interact with the patient, x-rays arescattered from the patient in all directions.

Dr. Ahmed Alsharef Farah 27

If you could image a long bone in cross-section using onlytransmitted, un-scattered x-rays, the image would be verysharp. The change in OD from dark to light, correspondingto the bone–soft tissue interface, image contrast would behigh.


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If the radiograph were taken with only scatter radiation andno transmitted x-rays reached the image receptor, theimage would be dull gray. The radiographic contrastwould be very low.


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In the normal situation, image-forming x-rays consist ofboth transmitted and scattered x-rays.If the radiograph were properly exposed, the image wouldhave moderate contrast. The loss of contrast results fromthe presence of scattered x-rays.


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Two types of devices reduce the amount of scatterradiation that reaches the image receptor:

• Beam restrictors.• Grids.


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• Basically, three types of beam-restrictingdevices are used:The aperture diaphragm.Cones or cylinders.The variable-aperture collimator.

Beam Restrictors:


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Three types of beam-restricting devices.

Dr. Ahmed Alsharef Farah33

• An aperture diaphragm is the simplest of allbeam restricting devices.

• It is basically a lead or lead-lined metaldiaphragm that is attached to the x-ray tubehead.

• The opening in the diaphragm usually isdesigned to cover just less than the size of theimage receptor used.

1. Aperture diaphragm:


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Aperture diaphragm is a fixed lead opening designed for afixed image receptor size and constant source to-imagereceptor distance (SID). SDD, source-to-diaphragmdistance.

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• Radiographic extension cones and cylinders areconsidered modifications of the aperturediaphragm.

• The useful beam produced by an extension coneor cylinder is usually circular.

• Cones are routinely used in dentalradiography.

2. Cones or cylinders:


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Radiographic cones and cylinders produce restricteduseful x-ray beams of circular shape.


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• The light-localizing variable-aperturecollimator is the most commonly used beam-restricting device in radiography.

• Light localization in a typical variable-aperturecollimator is accomplished with a small lampand mirror.

• The radiologic technologist should manuallycollimate more tightly to reduce patientradiation dose and improve image quality.

3. Variable-aperture collimator:Dr. Ahmed Alsharef Farah

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A variable-aperture light localizing collimator.

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Radiographic Grids:

• An extremely effective device for reducing thelevel of scatter radiation that reaches the imagereceptor is the radiographic grid.

• The grid is a carefully fabricated section ofradiopaque material (grid strip) alternating withradiolucent material (interspace material).

• The grid is positioned between the patient andthe image receptor.


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• The grid is designed to transmit only x-rayswhose direction is on a straight line from the x-ray tube target to the image receptor.

• Scatter radiation is absorbed in the gridmaterial.


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The only x-rays transmitted through a grid are those thattravel in the direction of the interspace. X-rays scatteredobliquely through the interspace are absorbed.

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• A typical grid may have grid strips 50 μm

wide that are separated by interspace material350 μm wide.

• Consequently, even 12.5% of x-rays transmittedthrough the patient are absorbed.


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1. The thickness of the grid strip (T).2. The width of the interspace material (D).3. The height of the grid (h).

• The grid ratio is the height of the grid dividedby the interspace width.

A grid has three important dimensions:


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Grid ratio is defined as the height of the grid strip (h)divided by the thickness of the interspace material (D). (T),width of the grid strip.


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• High-ratio grids are more effective in reducingscatter radiation than are low-ratio grids. Thisis because the angle of scatter allowed by high-ratio grids is less than that permitted by low-ratio grids.


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High-ratio grids are more effective than low-ratio gridsbecause the angle of deviation is smaller.

• In general, grid ratios range from (5: 1) to (16:1); higher-ratio grids are used most often inhigh-kVp radiography.

• An (8: 1) to (10: 1) grid is frequently used withgeneral-purpose x-ray imaging systems.

• Whereas a (5: 1) grid reduces approximately85% of the scatter radiation, a (16: 1) grid mayreduce as much as 97%.


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• In all cases the use of a grid increases patientdose.

• An alternative to use of a grid is the air-gaptechnique, in which the image receptor ismoved 10 to 15 cm from the patient.

• The air gap allows much of the scatterradiation to miss the image receptor.


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Increasing the source-to-image receptor distance (SID) to300 cm from 180 cm improves spatial resolution with noincrease in patient dose.


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Scatter Radiation - karary.edu.sdkarary.edu.sd/site/assets/files/25171/08_effects... · Radiographic Grids: • An extremely effective device for reducing the level of scatter radiation

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