what is radiographic grids?

Answer 1

Grids
One of the problems in getting a sharply defined image in clinical radiology is the presence of scattered or secondary radiation. These photons are created in the body of the patient or closely surrounding objects by the interaction of that material and the primary "x" photons coming from the x-ray tube. Several possible interactions occur in the diagnostic energy range. At relatively low energies, the photoelectric effect is probable. The photoelectric effect is actually the desirable, photon/tissue interaction because there is complete absorption of the photon with no production of a secondary photon. The more common tissue interaction at the photon energies used for the majority of clinical procedures is called the Compton effect or coherent scattering. In this interaction, a secondary photon is produced at the site of interaction. The secondary photon will always have lower energy than the primary photon and will be going in an altered direction. These secondary photons, if allowed to reach the film, will actually produce erroneous information by recording gray tone variation (and therefore indicating relative tissue densities) at some distance from the site at which the photon/tissue interaction actually occurred. The net result of allowing a significant number of secondary photons to reach the film is a reduction in image sharpness. There will always be a loss of spatial resolution.

Several methods have been devised to reduce the problem of scattered radiation. The simplest and most direct is to simply limit the field of exposure. If a small image area is adequate to make the clinical diagnosis, the image area should be "coned down" to that small size. For instance, if you want to image the gallbladder, you will get a much sharper picture if you bring the shutters down to include an area only the size of the gallbladder instead of including the entire upper abdomen on the image. Just remember that the smaller the area of the x-ray beam the fewer scattered photons you will produce.

In the typical clinical imaging situation, the most common method of reducing scatter is to use a radiographic grid. The grid looks like a flat metallic plate the size of the x-ray film if you look at it directly. However, it is more complicated than that. It actually is composed of alternating radiopaque (lead) and radiolucent (aluminum) strips. These are arranged on edge, sort of like looking at the strips of a venetian blind which is arranged to let light come between the strips. The edge of these strips is turned towards the source of x-rays and in the most commonly used grid, the focused grid, the anglulation of the strips is arranged to match the divergence of the x-ray beam.

This arrangement of the radiographic grid will give the highest probability for primary "x" photons passing between the lead grid strips and reaching the film, while the off-focus or secondary photons are likely to interact in the lead strips and never reach the film.

The use of this radiographic grid will greatly improve image sharpness when a relatively thick body part is being imaged. Unfortunately, there is always a trade off. Since the grid does stop some of the photons which would contribute to film blackening, if you just add a radiographic grid without changing the tube settings, the film will be greatly underexposed. If you decide to use a grid, you will have to increase the number of photons produced by the x-ray tube in order to get the correct film exposure. This will result in giving the patient increased radiation exposure. Remember, the position of the grid is between the patient and the film.

The third method of reducing scatter or at least reducing the probability that scattered photons will reach the film is to use an air gap. This is infrequently used in clinical radiography but can still, sometimes be used to an advantage particularly when magnification of the image might be helpful. Ordinarily we would have the film positioned as close to the patient's body as possible for the radiography of any body part. With an air gap technique, the film is moved several inches away from the patient's body. That separation, (because secondary photons are likely to be lower energy and moving at a greater angle than primary photons) will result in a decreased probability of the secondary photon hitting the film. From the diagram below, you will be able to understand that creating the air gap will also result in magnifying the radiographic image. Remember the x-ray beam is produced from almost a point source and it diverges as it goes towards the patient.