X-Ray Unit Health Article

Definition

An x-ray unit is the equipment used to produce x rays. Because of the risk of over-exposure to x rays, the x-ray unit includes both the machine used for collecting x rays and the protective room within which the x rays are taken and developed.

Purpose

Film radiographs, or x rays, are the most widely used means of medical imaging. Radiographs are used to examine bones for fractures, growth abnormalities, and joint dysfunctions. X rays are also used to find abnormal growths in the breasts (mammography), other organs and soft tissues; problems in the gastrointestinal tract; circulatory problems such as clogged arteries and blood clots; and a variety of other ailments. Additionally, radiation therapy to treat cancer is generally performed with x rays.

Description

The production of an x-ray image (radiograph) involves three distinct steps: the generation of an x-ray beam, the interaction of that beam with the structures of the patient to be imaged, and the development of the image.

Generation of an x-ray beam

Visible light is electromagnetic energy that has characteristics that allow it to be seen by humans. There are many other familiar forms of electromagnetic energy that are not visible to humans. These include radio waves, which permit the transmission of radio signals and the operation of cellular phones; microwaves, which are often used to heat food; and x rays. Each of these forms of light has a characteristic size (wavelength) and speed (frequency) range that defines it. An x-ray beam is an invisible form of light that has a wavelength that is much smaller than visible light and a frequency that is much faster than visible light.

Because an x-ray beam is a beam of light, just like visible light, it is generated in a type of light bulb that resembles a camera flash bulb. A flash bulb is used to increase the amount of visible light available for a photograph during the brief time that the camera is actually taking the picture (creating the visual image). An x-ray bulb is used to provide x-ray light during the brief time while the radiograph is being imaged.

The major differences between an x-ray light bulb and a visible flash bulb are the amount of energy required to produce the light and the energy characteristics (wavelength and frequency) of the light produced. Also, a flash bulb is not "tunable": a visible light bulb produces light anywhere within the visible light range. An x-ray bulb is "tunable" in that only x rays with the exact wavelength and frequency characteristics desired for the production of the radiograph are allowed to contact the patient. An xray bulb uses a filter system to produce light only in a specified x-ray range determined either by the filter system being used, or, in more advanced settings, by the xray unit operator through a variable control system.

Interaction of the x-ray beam with the patient

When visible light from a flash bulb strikes the skin of a human arm, that light is reflected back to the lens of the camera to which the flash bulb is attached, producing an image of a human arm on the film within the camera. The camera lens and film are designed to be able to image visible light. They generally cannot create an image from light outside the visible range.

Because x rays travel much faster than visible light, and because they have a much smaller wavelength, they have more "penetrating power" than visible light. This means that when x rays strike the same human arm, they are not stopped (reflected) by the skin and soft tissues, which are composed primarily of liquids. Instead, these x rays continue to travel through the skin and soft tissues until they meet a relatively dense material, such as bone. It is the "penetrating power" of light in the x-ray range that allows an x-ray image to "see inside" the human body.

An x-ray beam passes through sparse materials and only interacts with (becomes reflected by) dense materials. For this reason, x rays are most often thought of as being useful for the observance of dense tissues, such as bone. But, because an x-ray bulb is tunable, what is "sparse" and what is "dense" can often be defined by the particular type of x ray used. For instance, x-ray imaging of the breast (mammography) does not rely on a very large discrepancy in densities between the tissues being imaged and the tissues being ignored. The breast is largely composed of fat tissue and vessels of the circulatory and lymphatic systems, which are relatively dense when compared to skin and other non-fatty tissues. It is possible to tune an x ray to image the fatty tissues, blood vessels and lymphatic vessels of the breast in preference to the non-fatty tissues of the breast. Also, because abnormal growths (tumors) in the breast are denser than the typical breast tissue, radiographic mammography is an excellent diagnostic tool for the discovery of such breast abnormalities.

Often it is desirable to selectively image certain structures that are not sufficiently more dense than their surrounding tissue. This may often be accomplished through the use of a tracer, or dye, material that is dense that is administered to the type of tissue that is to be imaged. Examples of this type of x-raying include the use of barium to coat the lower gastrointestinal tract (barium enema) and the use of iodine compounds to coat the linings of blood vessels (angiograms). The introduction of barium or iodine tracers makes the gastrointestinal tract or the blood vessels appear to be more dense than the surrounding tissues.