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Grids Many facilities utilize a grid for thicker body part radiography. It is suggested that a grid be used for every body part that measures more than 13 cm, the size of a large knee, if the kilovoltage is greater than 70. (Fuchs 1986) The following definitions should be helpful in explaining the purpose and use of grids. GRID: A device used in radiography to absorb scatter radiation, This scattered radiation has no diagnostic value because it has strayed or been scattered from its original course through the body. If this stray radiation was allowed to reach the film, the image would lose much of its sharpness (contrast). A grid "cleans up” the radiographic image, but requires an increase in patient dose. STATIONARY GRID: In 1913 Gustav Bucky introduced the stationary radiographic grid. It consisted of wide strips of lead arranged in two parallel series that intersected at right angles. The modern stationary grid consists of closely spaced lead strips that are extremely thin, and are separated by radiolucent material, usually plastic or aluminum. The grid may be either a parallel grid, in which the lead strips are parallel to each other, or a focused grid, in which the lead strips are canted or angled toward the center. One disadvantage of the stationary grid is that the lead strips leave “blank” or white lines on the film. These lines are referred to as “grid lines." Newer grids reduce the visibility of grid lines and require less of an increase in patient dose than conventional grids. However, this type of grid is very expensive and has found widespread use only in mammography. MOVING GRID: One of the disadvantages of using a grid is that a pattern of clear (white) lines is cast on the film. This pattern of grid lines can be distracting to the radiologist. One solution to this problem, first conceived by Dr. Hollis Potter in 1920, was to move the grid sideways across the film during the exposure. The shadows of the grid strips are blurred out and are, therefore, not visible. Over the years, various mechanisms, from cocked springs to electric motors, have been utilized to achieve this movement. Two physical factors responsible for grid efficiency are the grid ratio and grid frequency, GRID RATIO: The height of the lead strip in relationship to the distance between them, Example: An 8:1 grid has lead strips 8 mm tall and these strips are | mm apart. The strips are eight times as tall as the distance between them. 
