Properties of XRays and Instrumentation

Several properties of x-rays warrant discussion in any consideration of radiation protection. X-ray photons travel in a straight line, diverging from the source. These photons comprise the primary x-ray beam and should be restricted to only the area of the object under examination. At no time should any part of any living individual be exposed to the primary beam.

The primary beam can be restricted by one of two types of devices. The older method employed a metallic apparatus known, because of its shape, as a cone. Cones were manufactured in different lengths, shapes, and sizes. A cone would slide into a track or slot in front of the "window" of the x-ray tube where the primary beam would emerge. The diameter at the opposite end of the cone varied depending on the body part under examination and the size of the film used to record the image. With the exception of mammography, cones have not been employed routinely in medical imaging for probably 30 years.

The more "modern" device used to restrict the primary beam is known as a collimator. The device is "cube shaped" and fixed permanently over the "window" of the x-ray tube. The collimator contains a light bulb, a mirror, and two pairs of lead shutters. The light from the bulb is reflected off the mirror and indicates the area that will be exposed by the primary beam. With the light on, the lead shutters are adjusted to a size that is slightly less than the area of the image receptor. The collimator is considered a precision instrument and, therefore, must be carefully packed prior to transport to a field setting. If the collima-tor is jarred sufficiently to alter the angle of the mirror, the area illuminated by the light will not match the area that will be irradiated. The mirror adjustment will need to be done by a service person authorized by the x-ray tube manufacturer.

The principal disadvantage of the collimator is that if the light bulb burns out, it will not be possible to "view" the area that will be exposed and restrict the area irradiated. Since these bulbs are not the type commonly found in any ordinary hardware store, it is always a good practice to bring extra bulbs into the field, particularly when working in a remote area.

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