Scatter Radiation
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Image Production
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As scatter radiation increases, the radiograph loses contrast and appears gray and dull. Three primary factors influence the relative intensity of scatter radiation that reaches the image receptor: kVp, field size, and patient thickness.
Some x-rays interact with the patient and are scattered away from the image receptor (a). Others interact with the patient and are absorbed (b). X-rays that arrive at the image receptor are those transmitted through the patient without interacting (c)and those scattered in the patient (d). X-rays of types c and d are called image-forming x-rays.
Three primary factors influence the relative intensity of scatter radiation that reaches the image receptor: kVp, field size, and patient thickness.
kVp
As x-ray energy is increased, the absolute number of Compton interactions decreases, but the number of photoelectric interactions decreases much more rapidly. Therefore, the relative number of x-rays that undergo Compton interaction increases.
Also, fewer x-rays reach the image receptor at low kVp—a phenomenon that is usually compensated for by increasing the mAs. The result is still higher patient dose.
With large patients, kVp must be high to ensure adequate penetration of the portion of the body that is being radiographed. If, for example, the normal technique factors for an AP examination of the abdomen are inadequate, the technologist has the choice of increasing mAs or kVp.
Increasing the mAs usually generates enough x-rays to provide a satisfactory image but may result in an unacceptably high patient dose. On the other hand, a much smaller increase in kVp is usually sufficient to provide enough x-rays, and this can be done at a much lower patient dose. Unfortunately, when kVp is increased, the level of scatter radiation also increases, leading to decreased image contrast.
Collimators and grids are used to reduce the level of scatter radiation.
Field Size
Another factor that affects the level of scatter radiation and is controlled by the radiologic technologist is x-ray beam field size. As field size is increased, scatter radiation also increases.
Collimation of the x-ray beam results in less scatter radiation, reduced dose, and improved contrast resolution.
Compared with a large field size, radiographic exposure factors may have to be increased for the purpose of maintaining the same OD when the exposure is made with a smaller field size. Reduced scatter radiation results in lower radiographic OD, which must be raised by increasing technique.
Patient Thickness
Imaging thick parts of the body results in more scatter radiation than imaging thin parts does. Compare a radiograph of the bony structures in an extremity with a radiograph of the bony structures of the chest or pelvis. Even when the two are taken with the same screen-film combination, the extremity radiograph will be much sharper because of the reduced amount of scatter radiation.
Patient Thickness
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5 comments:
hi. i want to ask can scatter radiation produce an image at image receptor. for example, when we expose a spine phantom, and then we put another cassette with a hand phantom in front of the cassette,at varying distance away from the expose spine phantom, does a hand phantom image can be produced using only the scattered radiation by the expose spine phantom? tq :)
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