RADIATION

CONTROL OFSCATTER RADIATION Scattered x rays are produced in the patient by the Compton effect and serve no useful purpose in diagnostic radiology. The relative intensity of scatter radiation was previously shown to be a complex function of many factors, primaly kVp, beam size, and patient thickness. The amount of scatter radiation is reduced as kVp is lowered because reduction of kVp enhances differential absorption. However, there are a number of factors opposing the of low kVp for making radiographs. The beam-restricting devices discussed in the previous chapter are very efficient in reducing the quantity of scaterred x rays, but their effect is not enough, since even in the most favorable conditions more than half the remnant x rays (all x rays exiting the film side of the patient) will be scattered. Fig. 12-1 illustrates that scattered x rays are emitted in all directions from the patient. The direction of the scatter radiation becomes slightly more forward as the kVp of the primary beam is increased. Effect of scatter radiation on contrast In a later chapter many characteristics that affect the quality of a radiograph will be discussed in detail. One of the most important characteristics of film quality is contrast, the degree of difference between the light and dark areas of a radiograph. If one could radiograph a long bne in cross section using only primary-beam x rays, he image would be very sharp, as shown in Fig. 12-2, A. The change indensity from dark to light corresponding to the bone-soft tissue interface would be high. On the other hand, if the radiograph were taken with only scatterradiation and no primary-beam x raysreached the film, the image would be dull gray, as in Fig. 12-2, B. The radiographic contrast would be very low or perhaps even nonexistent. In the normal situation, however, the x rays arriving at the film consist of both primary and scattered x rays. If the radiograph were properly exposed, the image in cross-sectional view would appear as in Fig. 12-2, C. The image would be neither as sharp as that in Fig. 12-2, A, nor as dull as that in Fig. 12-2, B; it would have moderate contrast. The loss of contrast results from the presence of scattered x rays; the more scattered x rays, the lower the contrast. How a grid works Two general approaches to reducing the amount of scattered x rays in the remnant beam are possible. The film could be treated so that the scaterred x rays could not interact. Scattered x rays have lower energy than primary-beam x rays, but unfortunately radiographic film is nearly equally sensitive to x rays throughout the diagnostic range. To date no additive to the film emulsion has been effective in increasing film sensitivity to primary-beam x rays at the expense of scattered x rays. The use of screens in conjunction with x-ray film increases the cpntrast of the image but not because primary-beam x rays are preferentially absorbed over scattered x rays. The second approach would be to reduce the number of scattered x rays arriving at the film plane. Since scattered x rays have lower energies than primary-beam x rays, a properly designed filter should be an effective device for reducing the amount of scattered x rays. Selective filtration has been investigated but unfortunately has been shown to be largely ineffective. An extremely effective device for reducing the level of scatter radiation is the grid, a carefully fabricated series of sections of radiopaque material (grid material) alternating with sections of radiolucent material (interspace material). The grid is designed to transmit only those x rays whose direction is on a line from the source to the image receptor. X rays that travel obliquely (at an angel) are absorbed in the grid material. Fig. 12-3 is a schematic of how a grid cleans up scatter radiation. This technique for reducing the amount of scatter radiation reaching the film was first demonstrated in 1913 by Dr. Gustave Bucky. Over the years Dr. Bucky’s grid has been improved by more precise manufacturing, but the basic principle is unchanged. All x-ray photons exiting the patient that are incident on the radiopaque grid material are absorbed and do not reach the film. For instance, a typical grid may have grid stripsapproximately 50 µm thick. Consequently, up to 12.5%* of all potons incident on the grid will interact with radiopaque grid material and be absorbed. Primary-beam photons incident on the interspace material are transmitted through to the film. Scattered x-ray photons incident on the interspace material may or may not be absorbed, depending on their angle of incidence and the physical characteristics of the grid. If the deviation of a scattered x ray is great enough to cause it to intersect the grid material, it will be absorbed. If the devition is slight, the scattered x ray will be transmitted like a primary x ray. Laboratory measurements show that high-quality grids can be expected to attuanute 80% to 90% of the incident scatter radiation. Such a grid is said to exhibit good cleanup. Example : When viewed from the top, a particular grid shows a series of lead strips 40 µm wide separated by interspaces 300 µm wide. How much of radiation incident on this grid should be absorbed? Answer : If 300+40 represents the total surface area of absorbing material, then percent absorption is CHARACTERISTICS OF GRID CONSTRUCTION Grid Ratio A number of characteristics of a grid are used to specify its radiographic properties: the grid ratio is perhaps the one most often employed. Grid ratio can best be understood by reference to Fig. 12-4. There are three important dimensions on a grid: (1) the thickness of the grid material (T); (2) the thickness of the interspace material (I); and (3) the height of the grid (h). The grid ratio is the height divided by the interspace thickness: Grid ratio = h/D Grids with high ratios are more effective in cleaning up scatter radiation than are grids with low ratios because the angle of deviation allowed by high ratio grids is less than that permitted by low-ratio grids. Fig 12-5 illustrates this property. Unfortunately, grids with high ratios are more difficult to manufacture than are low-ratio grids.