![]() Sheets of lead cut to fill holes or surround the part and metallic shot and liquid absorbers are often used as masks. Shutters or masks can be used to reduce undercut. Scattering from within the walls of the part also contributes to undercut, but research has shown that scattering within the detector is the primary cause. Using film, it should also be noted that the faster the film speed, the more undercut that is likely to occur. The high level of radiation intensity reaching the detector results in a high level of scattering within the detector. At the edges of a part, or areas where the part transitions from thick to thin, the intensity of the radiation reaching the detector is much greater than in the thicker areas of the part. Undercut occurs due to scattering within the detector. This results in a loss of resolution or blurring at the transition area. It appears as a brighter ghosting near the edges in a digital image. This work characterises the in-room leakage and scattered radiation for the Varian Halcyon linear accelerator. Undercut appears as a darkening of the film radiograph in the area of the thickness transition. Unintended sources of secondary radiation resulting from photon beams in linear accelerators are patient scatter, collimator scatter, scatter from surfaces within the bunker, and head leakage. Parts with holes, hollow areas, or abrupt thickness changes are likely to suffer from undercut if controls are not put in place. UndercutĪnother condition that must often be controlled when producing a radiograph is called undercut. It is a common practice in industry to place a 0.005" lead screen in front and a 0.010" screen behind the film. The control of backscatter radiation is often achieved by backing the film in the cassette with a sheet of lead that is at least 0.010 inch thick. The image of the "B" is often quite non-distinct because the backscatter source is diffuse. If the letter "B" shows as a "ghost" image on the film, a significant amount of backscatter radiation is reaching the film. Industry codes and standards often require that a lead letter "B" be placed on the back of the film cassette to verify the control of backscatter. They are NOT important in digital detectors. ![]() Control of side scatter can be achieved by moving objects in the room away from the detector, moving the x-ray tube to the center of the vault, or placing a collimator at the exit port, thus reducing the diverging radiation surrounding the central beam.īackscattered x-rays can be significant when using film as a detector. Side scatter originates from walls, or objects on the source side of the detector. Often secondary radiation is thought of as radiation striking the detector reflected from an object in the immediate area, such as a wall, or from the table or floor where the part is resting. The scattered photons create a loss of contrast and definition. Secondary or scatter radiation must often be taken into consideration when producing a radiograph. The result is still higher patient dose.> Secondary (Scatter) Radiation and Undercut Control Secondary (Scatter) Radiation and Undercut Control Secondary (Scatter) Radiation ![]() Therefore, the relative number of x-rays that undergo Compton interaction increases.Īlso, fewer x-rays reach the image receptor at low kVp-a phenomenon that is usually compensated for by increasing the mAs. Three primary factors influence the relative intensity of scatter radiation that reaches the image receptor: kVp, field size, and patient thickness.Īs x-ray energy is increased, the absolute number of Compton interactions decreases, but the number of photoelectric interactions decreases much more rapidly. X-rays of types c and d are called image-forming x-rays. X-rays that arrive at the image receptor are those transmitted through the patient without interacting (c)and those scattered in the patient (d). Others interact with the patient and are absorbed (b). Some x-rays interact with the patient and are scattered away from the image receptor (a). Three primary factors influence the relative intensity of scatter radiation that reaches the image receptor: kVp, field size, and patient thickness. X-rays that exit from the patient are remnant x-rays and those that exit and interact with the image receptor are called image-forming x-rays.Īs scatter radiation increases, the radiograph loses contrast and appears gray and dull. Scatter radiation is a secondary radiation produced when an x- ray interact with the patient and the energy is not enough for an x- ray to reach the IR. ![]()
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