BEAM RESTRICTORS OR COLLIMATORS

An X-ray beam restrictor is a device that is attached to the X-ray tube housing, to regulate the size and shape of an X-ray beam. They can be classified into three categories, namely, (i) aperture diaphragms (ii) cones and cylinders  and (iii) collimators. Aperture diaphragms consist of a sheet of lead with a hole in the center. The size of the hole determine the size and shape of the X-ray beam. It is simple and the aperture can be altered to any size and shape. The disadvantage of an aperture diaphragm is that it produces large penumbra. The penumbra can be reduced by keeping aperture diaphragm far away from the X-ray target. Aperture diaphragms are used in dental radiography with rectangular collimation. In addition, it is used in trauma and chest radiography. The use of cones and cylinders will reduce the penumbra considerably. Both have extended metal structures that restrict the useful circular beam to the required size. The position and size of the distal end determine the field size. If the X-ray source, cone and film are not aligned properly, then, one side of the film may not be exposed, which is called cone cutting. Cone is a ideal beam restrictor, but the flare of the cone is greater than the flare of the X-ray beam. These systems provide only limited number of field sizes. The collimator is the best X-ray beam restrictor. It defines the size and shape of the X-ray field that emerges from the X-ray tube.The collimator assembly is attached to the tube housing at the tube port. A collimator consists of two sets of shutters, which can be moved independently. Each shutter consists of four or more lead plates of 3 mm thick, which can absorb X-rays completely, to provide a well defined X-ray field. When the shutters are closed, they meet at the center of the X-ray field. The collimator also has a light and mirror arrangement, to illuminate the X-ray field. The light bulb is positioned laterally and the mirror is mounted in the path of the X-ray beam at an angle 45o. The target and the light bulb should be kept at equal distance from the center of the mirror. The collimator provides variety of rectangular X-ray fields and the light beam shows the center of the X-ray field. The light field and radiation field should match exactly with each other. The variation must be with in 4% of TFD. Thealignment of light beam and X-ray beam should be checked periodically. A well collimated beam covers lesser area of the patient, giving less patient dose. Also it generates less scatter radiation, which improves the image quality.Collimators that automatically limit the X-ray field size to the useful area of the detector is also available. These are called positive beam limitation (PBL) collimators. A sensor in the cassette holder, adjust the collimator opening, equal to the cassette dimensions. Thus, PBL collimators limit the irradiated volume and reduce the patient dose.