Beam Restrictions

What is the purpose of restricting the primary beam?

In order to reduce radiation exposure it is necessary to decrease the area of the x-ray beam. Proper beam restriction will reduce the amount of primary photons emitted from the tube and collimator thereby reducing the dose to the patient. Also, beam restriction will keep the total amount of tissue irradiated to a minimum so fewer scattered photons are created consequently the image quality will be improved. It is important to control scatter since it has no useful diagnostic effect. Another principle factor in reducing scatter is kilovoltage or the penetrability of the beam. As kVp is increased, fewer atoms interact with the tissue, and more pass through to end up on the image receptor. In radiography, kilovoltage is selected based predominantly on the size of the part examined. Whenever kilovoltage is increased more scatter will result unless it is accompanied by a reduction in mAs thereby reducing scatter and the dose to the patient. Thus beam restriction along with technical factors are very important aspects of radiation protection by reducing the patient dose and improving the image quality (Bushong, 2008).

What are the different types of beam limiting devices?

Some examples of beam restricting devices are:
  • Aperture Diaphragms
  • Cones
  • Collimators

Aperture diaphragms are the simplest type of beam-restricting device. It is a flat piece of lead containing a hole in the center that attaches to the x-ray tube port. The opening can be made in any size or shape, but rectangular is the most common. The main advantages of aperture diaphragms are there simple design, low cost, and ease of use. The main disadvantage is the increase in the unsharpness around the periphery of the image known as penumbra. Other disadvantages include off-focus radiation and no light field for use in positioning. Aperture diaphragms are used in special procedure angiography studies (Carlton & Adler, 2006).

Cones or cylinders are essentially aperture diaphragms with metal extensions that can be either straight or flared and attach to the x-ray tube housing. Cones are extensions that flare and cylinders are straight but both are routinely called cones. The longest cone with the smallest diameter will provide the greatest beam restriction. The advantage of cones is there low cost and ease of use. One difficulty with using cones is alignment. The physical weight of the cones can sometimes cause the tube to angle slightly when used with a horizontal beam causing cone cutting if the central ray is not checked carefully. Today, cones are reserved for select areas of radiology such as sinuses, L5/S-1 spine, and dental radiographs.

Collimators are the most widely used beam restricting device because they contain a light source to help the radiographer center the x-ray beam. They are composed of a lamp, mirror, and a pair of upper and lower lead shutters that are at right angles to each other and move independently. Upper shutters absorb the off focus radiation before it leaves the tube and the lower shutters further restrict the beam to the area of interest. Collimators permit an infinite number of field sizes using only one device and hence reduce the light field to only the area of interest resulting in reduced patient exposure. Some equipment is supplied with automatic collimators that are electronically interlocked with the Bucky tray so the x-ray beam is automatically restricted to the size of the cassette. These devices are known as positive beam limitation (PBL) devices. Accuracy within 2% of the SID is required with all PBL devices (Forshier, 2009).

Ultimately, it is the responsibility of the radiographer to use proper collimation and under no circumstances should the exposure field exceed the size of the image receptor. The radiographer should always limit the field to the part being examined thereby improving the image quality and minimizing the patient dose.

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Aperature Diaphragms
Cones
Collimators