The purpose of the grid is to absorb scattered x-rays between the patient and the film. The scattered x-rays are created within the patient by an x-ray tissue interaction that results in the conversion of the incoming x-ray to an electron and a "new" x-ray with somewhat less energy moving in a new direction. In essence, the x-ray is deflected off its original straightline course. If this redirected "new" x-ray successfully exits the patient, it delivers exposure to the film that is untrue relative to the anatomical structure from which it originated.
A fundamental assumption in the formation of the x-ray image is that x-rays travel in straight lines from the origin in the x-ray tube through the patient to the film. Most grids are focused grids. This means that the lead bars are angled in the same plane as the x-rays coming from the tube. The grid is placed between the patient and the film. Most often in small animal systems, the grid is incorporated into the table positioned just above the film tray. Grids can also be purchased that are laid on top or independently affixed to the cassette.
Grids are quite effective in removing the scattered x-rays from the thickness greater than 10cm. However, if you decide to use a grid for the area in question, it may be easier to use it for all thicknesses rather than to have to remember to activate it or deactivate it.
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