In: Physics
1-A satisfactory radiographic image was obtained using 300 mA,
20ms at 90 kVp, 182cm SID, 20cm OID, 1mm FSS, 8:1 grid, 400 IP
(CR), and no pathological condition.
The basic function of a grid is to absorb radiation that is moving
along a path that is not aligned with the grid interspaces. It is
desirable that the primary radiation from the x-ray tube focal spot
pass through the grid with a minimum of absorption. Maximum grid
penetration by primary radiation can occur only if the x-ray tube
focal spot is located at the grid focal point. If these two points
are not properly aligned, as shown in the figure below, the
direction of the primary radiation might be such that the radiation
does not adequately penetrate certain sections of the grid.
A number of factors must be considered when selecting a grid for a specific application. In most cases, a grid is selected that provides a reasonable compromise between contrast improvement and patient exposure, machine loading, and positioning.
The advantages of a 5:1-ratio grid are that it is easy to use and does not require critical positioning. Its use must be restricted, however, to situations in which the amount of scattered radiation is relatively small (thin body section, low KV) or in which maximum image contrast is not necessary. On the other hand, a 16:1-ratio grid produces high-contrast recovery but significantly increases patient exposure. With a high-ratio grid of this type, there is very little latitude in positioning. Many applications are best served by grid ratio values between these two extremes. Such grids generally represent compromises between image quality and the other factors discussed.
Some grids have strips running at right angles to each other, generally designated crossed grids. This design generally increases contrast improvement but cannot be used in examinations in which the x-ray tube is tilted.
In stationary grid applications in which lines in the image are undesirable, grids with a high spacing density (lines per centimeter) can be used. An increase in the spacing density generally requires a higher ratio grid to produce the same contrast improvement.