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The x-ray beam, as it emerges from the tube head, is composed of a large number of photons that travel in straight lines in all directions unless controlled in some manner. It is important to limit the size of the primary beam in order to decrease the surface area on the patient exposed by the primary beam and to decrease the amount of secondary radiation that is formed when the beam strikes the patient. This limitation:
There are many devices that can be used to limit the area exposed by the primary beam. All of these beam limiting devices function in essentially the same manner; that is, the absorption of a part of the primary beam.
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These devices do not "focus" the x-ray beam or "change its direction" in any manner. They simply absorb radiation and limit the field of exposure.
Aperture Diaphragms (Fig. 6-1) This is a small lead sheet with an appropriate sized rectangular, square or circular opening cut in the center that permits the passage of a part of the primary x-ray beam. This is obviously the simplest type of collimation available and may be the type of collimation offered with small portable x-ray units. The lead sheets are easily cut, using a knife or metal shears; and several different sizes can be cut to match the size of cassettes in use and the appropriate focal-film distance. The disadvantage of this type of beam collimation is that the templates must be physically changed, and often the method of attachment of the lead to the tube housing does not provide for quick interchange.
Cone or Cylinder (Figs. 6-1, 6-2) The base of the cone or cylinder contains lead and is most important in absorption of the beam . The cone or cylinder is often copper lined and assists in absorption. Often the cylinder is of such a type that extends to further limit the size of the field. When fully extended, it also provides a device for centering the small x-ray beam on the part to be radiographed.
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Adjustable Lead Shutter Beam Limiting Device (Figs. 6-1,6-2,6-3) Movable lead sheets may be placed together in such a way as to form a circular field or a rectangular field. The shutters operate together in a circular field and permit the exposure of a round field of varying diameter. The shutters used to produce a rectangular field may operate together or as two separate pairs . It is obvious that separate pairs of lead sheets forming a rectangular field gives the best opportunity to expose only that minimal portion of the patient required for the study. The thickness of the lead sheets varies, and the collimator is thus constructed to be used with kVp settings below a stated maximum kVp setting. Energies above this maximum setting can penetrate the lead shutters and result in ineffective collimation of the x-ray beam. This is usually not a problem in veterinary radiography, but is a point that should be checked before purchase of a new collimator.
Often the adjustable collimator has a light bulb with the light directed toward a mirror and then onto a table top (Fig. 6-4). In such a way, the size of the field to be exposed is indicated by the area illuminated on the table. Often a pair of cross lines are painted on a plexiglass sheet so that the center of the radiographic field is identified. It may be desirable to have switches that permit two intensities of the light, one much lower than the other or to have a rheostat on the switch. This permits the light to be on for a longer period of time without damaging the unit by the heat produced. often the time the light is on is determined by an automatic timer switch which shuts off the light after a few seconds. This limits the heat produced and protects the electrical wiring and keeps the bulb housing cooler. This timer must not make an objectionable noise that excites the animals. Often the time is not long enough to position animals and the collimator light must be rewired so that the light remains on until switched off. In this case, it is important to have a lower intensity light. It is also possible to connect the light switch to a "dead-man's switch," so that the light is on as long as the switch is depressed but turns off when the finger is removed. That ensures that the light cannot be left on long enough to cause damage. It is often desirable to have the room lights controlled by a rheostat, so that the room light can be low enough to permit easy identification of the collimator light on the table top.
It is possible that the mirror may become displaced and the area on the table top that is illuminated by the light is not the same as the field being exposed by x-radiation. A technique is available to check whether the illuminated field and radiation field are the same (Chris tensen, et al., 1972) (Fig. 6-5). A cassette is placed on the table top and a field selected that is smaller than the size of the cassette. The corners of the illuminated field are identified by paper clips or other convenient metal wire. The lower right hand corner of the field is identified by a lead "R." A low intensity exposure is made (40 - 50 kVp, 1- 2 mAs). The collimator is altered to increase the field of exposure and a second, similar exposure is made without altering cassette location or the markers. Evaluation of the
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field with the two exposures will inform you concerning the similarity or dissimilarity of the field illuminated and the field radiated. It is possible to correct the disparity by adjusting a set screw on the side of the collimator that changes the angle of the mirror.
Many collimators also have a light beam directed to the side of the table that can be seen when the cassette tray is pulled out. This light beam identifies the center of the field of exposure and permits more accurate alignment of the cassette with the x-ray beam.
For portable studies, a fixed slot light beam indicator can be of value in determining focal-film distance instead of an adjustable light beam diaphragm. The radiographer knows that the tube film distance is correct when the two light beams are super imposed. Another technique to confirm focal-film distance utilizes the size of the illuminated field. The distance is correct when the light just illuminates the edge of the cassette.
Automatic Beam Limiting Device Automatic collimation is required, effective August 1, 1974, on all new radiographic installations used for human radiography. These standards were established by the U.S. Bureau of Radiological Health as mandated by Public Law 90-602. It has been reported that the genetically significant radiation dose to people from medical diagnostic x-rays can be reduced from 55 millirads per exposure to 19 millirads per exposure if the size of the x-ray beam in the plane of the film is limited by collimation to the film size (Penfil and Brown, 1968). The automatic collimator ensures that the size of the field irradiated will not exceed the size of the cassette. The automatic collimator control system consists of four basic mechanisms:
The system functions by determining cassette size when the cassette is placed in the bucky tray and the cassette holders are fastened tightly around the cassette. This information is passed to the collimator and the signal amplified and the shutters electrically moved to permit coverage of that size field. An additional compensating unit is needed if the focal-film distance is altered. It is obvious that this unit will not function during examinations made on the table top without the use of a grid. It also will not permit automatic setting of field size smaller than the cassette, such as might be desired for examination of the vertebral column in a dog.
This type of automatic collimation will probably become financially attractive to veterinary radiography only as used units are made available for resale. Since the main purpose is to reduce radiation to the patient, it is doubtful if these units will be required in veterinary medicine.
Every diagnostic x-ray machine should have a collimator of some type. The collimator should attach to the tube housing in such a way that, if the field is a square or rectangle, it can be rotated to fit the cassette no matter how it is placed on the table top. It is also important that the collimator can be used if the tube housing is shifted 90° to permit use of a horizontally directed x-ray beam. The collimator may provide a place to hold the aluminum filters, although the filters may be a part of the tube housing. Often the collimator has a metal tape box incorporated so that a measurement of focal-film distance can be easily made.
It is recommended that each film exposed should have some area of unexposed film bordering the margin of the exposed portion of the film.
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Only if you use this technique can you be sure that the field of exposure is smaller than the film and not exceeding the margin of the film by a great distance. It is too easy to adopt the habit of increasing the size of the exposure field to compensate for not having centered the tube correctly. This practice must be avoided if you are to have the best quality radio graphs with the least exposure to the patient.
During routine radiography, the film is exposed by both primary and secondary radiation. Thus, the density of the radiography is greater as a result of the exposure by the secondary radiation. This increase in additional exposure continues to increase with field size up to 30 cm x 30 cm (Christensen, et al., 1972). Exposure factors must be increased slightly if the collimator limits the field of exposure to a small area (less than 10 cm x 10 cm). The increase required is usually about 5 to 6 kVp. This increase is necessary because of the limitation on the production of scatter radiation that would otherwise contribute to the overall film density.
Sometimes it is possible to limit the area of the film to be exposed by use of lead sheets placed over the film rather than by use of a collimator. This is often done when using non-screen film or screen film in cassettes in table top techniques. Use of lead sheets permits several exposures to be made on a single film without altering the size of the collimator (Fig. 6-7). It should be realized that this method of radiography does not produce as high a quality radiograph as limiting the area to be radiographed by collimation. This is because use of the collimator limits the size of the primary beam and decreases the amount of tissue irradiated. Therefore, the amount of secondary radiation produced is decreased. Use of lead sheets limits the portion of the film exposed but does nothing to limit the amount of tissue exposed, and the amount of secondary radiation produced remains unchanged.
Examples of examinations in which small radio graphic fields can be used are generally of two types. The first is the making of additional studies following a complete examination in which a questionable lesion is found. Additional studies of a limited area (coned down) produce a radiograph of the highest technical quality possible centered on the suspect area because of limiting the production of scatter radiation. Examples are radiography of:
In these cases, it is necessary to make the survey radiographs before the cone-down views are
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made. A second type of case in which cone-down studies are of value is in progress studies following earlier radiographic diagnosis. Examples are progress studies of:
It is often possible to combine several small exposures on the same film. If you are using table-top technique, the exposures may be arranged on the film as is most convenient. If you are using bucky technique with a focused grid, the exposures must be centered along the center line of the grid to avoid grid cut.
