The Stationary Wafer Grid

The stationary wafer grid is composed of alternate strips of lead and a radio-translucent (x-ray transparent) substance, such as wood, aluminum, or a synthetic material. The strips are placed parallel to each other and form a very thin, rectangular sheet the same size as the x-ray film. The grid is inserted between the patient and the film so that the strips are lined up vertically beneath the patient's body. The primary radiation emitted from the focal spot of the tube passes between the lead strips to reach the film. Since the SR is unfocused and is traveling in all directions, most of it strikes the lead strips at large angles and is absorbed, thus having little effect upon the film (figure 1-23). As much as 90 percent of the SR can be absorbed by the stationary wafer grid.

ANODE

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Bucky Grid

Figure 1-23. The Potter-Bucky focused diaphragm. Extension of the directions of the tilted lead strips are indicated by broken lines which intersect at the imaginary point called the grid focus.

Figure 1-23. The Potter-Bucky focused diaphragm. Extension of the directions of the tilted lead strips are indicated by broken lines which intersect at the imaginary point called the grid focus.

a. The use of such a grid, therefore, greatly increases the diagnostic value of the radiograph, because it greatly improves the quality of the image. Even though an image of the lead strips in the grid appears on the radiograph as evenly spaced thin white lines, they are not a disadvantage from the diagnostic standpoint. At the normal viewing distance of about 30 inches, the lines can seldom be seen.

b. Normally, wafer grids are focused--that is, the lead strips are not parallel. Special care must be exercised in the use of this type of grid. If it is used at too short or too long SID, the outside edges of the radiograph will be underexposed because the outer rays will be absorbed by the lead strips. The beam must also be exactly perpendicular to the center of the grid, because tilting the beam across the lead strips will lead to uneven absorption.

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