Characteristic of Radiation
Because xrays and visible light are both forms of electromagnetic energy, they share some similar characteristics. Both travel in straight lines, and both have an effect on photographic emulsions. When film is used, the photographic effect of xrays is important in the production of radiographic images. It is also important to remember because accidental exposure can occur when film is placed near xray sources.
Both xray and light have a biologic effect; that is, they can cause changes in living organisms. Because of their greater energy, xrays are capable of producing more harmful effects than light. Unlike light, xray cannot be refracted by lens. The xray beam diverges into space from its source until it is absorbed by matter.
Unlike light, xray cannot be detected by the human senses. This fact may seem obvious, but it is important to consider. If xray could be seen, felt or heard, we would have an increased awareness of their presence and radiation safety might be much simpler. Because they are undetectable, however, safety requires that you learn to know when and where xrays are present without being able to perceive them.
Xrays can penetrate matter that is opaque to light. This penetration is differential and depends on the density and thickness of the matter. For example, xrays penetrate air very readily. There is less penetration of fat or oil, even less of water, which is about the same density as muscle tissue, and still less of bone. Xrays that have passed through the body are referred to as remnant radiation or exit radiation. Remnant radiation has a pattern of intensity that reflects the absorption characteristics of the body. It is pattern that is recorded to form the image.
Xray cause certain crystal to fluoresce, giving off light when they are exposed. Among crystals that respond in this way are barium platinocyanide, barium lead sulfate, calcium tungstate, and several salts consisting of rare earth elements. These crystal are used to convert xray pattern into a visible image that can be viewed directly, as in fluoroscopy, or recorded on photographic film. The use of fluorescent intensifying screens to expose radiographs greatly reduces the quantity of radiation needed. The combination of film and intensifying screens has been the conventional image receptor (IR) for decades, but is now being replaced by filmless technology that produces digital images.
The Primary Xray Beam
Xrays are formed within a very small area on the target (anode) called a focal spot. The actual size of the largest focal spot is no more than a few millimetres in diameter. From the focal spot, the xray diverge into space, forming the cone-shaped primary xray beam.
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| Cross section of xray beam is called the radiation field. imaginary perpendicular ray at its center is called central ray. |
The cross section of the xray beam at the point where it is utilized is called the radiation field. A photon in the center of the primary beam and perpendicular to long axis of the xray tube called the central ray.
The xray beam size is restricted by the size of the port, the opening in the tube housing. Attached to the housing is the collimator, a device that enables the radiographer to further control the size of the radiation field.
Scatter Radiation
When the primary xray beam encounters any solid matter, such as the patient or the xray table, a portion of its energy is absorbed. This results in the production of scatter radiation.
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| Scatter radiation forms when the primary xray beam interacts with matter |
This scatter radiation is generally has less energy than the primary xray beam, but it is not easily controlled. It emanate from the source in all directions, causing unwanted exposure to the image receptor and posing a radiation hazard to anyone in the room. Scatter radiation is the principal source of occupational exposure to radiographers.
The characteristic of primary radiation, scatter radiation and remnant radiation are summarized for comparison below.
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