Whenever x-rays pass through matter, some of the x-ray photons will interact with the atoms of the matter. When you take an X-ray, many incoming x-ray photons are absorbed by the bone, a high attenuation material, while other tissues absorb less. This produces an image of dark and light on the x-rayfilm. The effect is like holding up paper to a light to see the writing on the other sied. However, absorption is not the end. As part of the absorption, a second photon with less energy will be produced, flying off in a different firection than the photon that was absorbed. These “scattered” photons will create a random grayness on the image, reducing the contrast between body tissues and making it hard to read the image clearly. There are five qays x-rays interact with matter: photo electronic (PE), Compton Scattering(C), Pair Production, Tomson Scattering(R), and Photodisintegration (PD). In low energy x-rays below 100keV, only Photolelectric absorption and Compton scattering are significant.
Grids are divided into four types based on their applications. There are parallel types, focused types, crisscross types, and tapered types. A single grid may be of more than one type, but this is rare. The majority of grids used in general radiography are focused grids, focused for 40″, 40-72″, or 72″ distance from the x-ray tube, but other applications require different grids.
A grid where the absorving strips are parallel to each ohter in their longitudinal axis. Most linear grids are also focused, i.e. their strips are slightly tilted, converging at a line in space (the convergent line). A non-focused linear grid will have strips that are parallel when viewed in crosssection; this is called a parallel grid. Many X-ray tables are equipped with linear, focused grids, and the strips in these grids are parallel with the long axis of the table, allowing the X-ray tube to be tilted in this direction without changing the effectiveness of the grid.