Since scattered radiation travels in a changed direction from the primary radiation, it is more probable that the scattered photons will be absorbed by the grid strips. If the primary photons and grid strips are aligned, the primary photons will pass through the grid. When the x-ray beam hits the grid, the grid is supposed to let the primary photons through it and absorb the scattered photons. Since the scattered photons are traveling at different angles from the primary photons, most of the scattered photons run into a lead strip and are absorbed before they can reach the image receptor. See Figure 10-4.īecause the lead strips in a focused grid are angled to match the divergence of the beam, most of the primary beam x-rays that are transmitted through the patient pass through the grid without being absorbed. The photons at the edge of the image receptor are at more of an angle. The x-ray beam is pyramid-shaped, and the primary photons near the central ray are not at much of an angle when they reach the image receptor. It’s a pretty smart design, because the grid strips are aligned to match the way the primary photons emerge from the x-ray tube. The angle increases as the strips get closer to the sides of the grid. The strips in the very center of the grid are parallel to each other, but the strips at the sides of the grid are angled. When the grid strips are placed on edge during the construction of a focused grid, they are not all placed parallel to each other. The most common type of grid pattern in use is the focused grid. When viewed from the edge, criss-cross grids look pretty much like parallel grids except you can’t see down the entire length of the grid strips. Criss-Cross grids form a cross-hatch pattern when viewed from the face.Parallel grids have their grid strips aligned perpendicular to the face of the grid, so they appear very similar in both the edge view and the face view.When viewed from the face, the grid strips appear wider at the outer edges of a focused grid because those grid strips are angled more than the strips near the center of the grid. Focused grids have their grid strips aligned to correspond to the divergence of the beam at a particular SID. Grid patterns viewed from the edge and face. The Bucky, which is used to move the grid during the exposure, will also be described. The grid is the device that prevents scatter from reaching the image receptor and is the subject of this chapter. The collimator and several other devices and techniques will be discussed in the next chapter. One device that prevents the production of scatter in the patient’s body is the collimator. The amount of scattered radiation that hits the image receptor can be controlled by either prevention the production of scatter in the patient’s body, or by preventing the scatter from reaching the image receptor after it is produced in the patient’s body.
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