X-Ray Unit

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X-ray unit


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.


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.


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.

Development of the x-ray image

Only about 1% of the x rays that strike a patient's body emerge from the body to produce the final image. The radiographic image is formed on a radiographic plate that is similar to the film of a camera. The other 99% of the x rays are either absorbed by the body or scattered by the tissues of the body.

Those x rays that are scattered (reflected) by the tissues of the body are generally scattered in a random pattern. If these x rays reach the radiographic plate, they tend to obscure the radiographic image. Therefore, an anti-scatter grid, which is similar to a set of partially closed window blinds, is used to prevent these scattered x rays from reaching the plate. X rays that have passed through the body without being scattered will reach this grid traveling perpendicular to the grid. X rays that have been scattered will reach this grid traveling, for the most part, in directions other than perpendicular. Any x rays that are traveling perpendicular to the grid will pass

through and strike the radiographic plate, helping to create the final radiographic image. Those x rays that are not traveling perpendicular to the grid will strike the grid and be absorbed, such that they do not contribute to the final radiographic image.

After passing through the anti-scatter grid, x rays strike a radiographic plate that works almost identically to a photographic plate, or film in a camera. In recent years, modifications in the development process of radiographic plates have been achieved that allow the necessary clarity of the radiograph with much lower x-ray exposures to the patient.

Those x rays that are absorbed by the body do not reach the radiographic plate. Therefore, they present no difficulty in the production of a clear radiographic image. However, these absorbed x rays have been shown to be a cause of cancer in those individuals who are over-exposed to them, either over time or during periods of intense radiation.

Components of the x-ray unit

To prevent exposure to the operator and to prevent the unnecessary leakage of x-ray radiation to the rest of the facility where x rays are performed, the x-ray unit is generally enclosed in a room that has walls made of, or reinforced with, a dense material (usually lead) that will absorb any x rays that are scattered during the x-ray process.

Additionally, the operator of an x-ray unit generally turns the x-ray equipment on and off from behind a protective wall that is lined with lead. Lead is extremely dense to x rays and even a one-quarter inch thickness of lead will prevent all x rays emitted from current x-ray machines from being able to pass.

A lead-impregnated smock or apron is also provided to patients while they are being x-rayed to prevent unwanted exposure of their bodies to x rays.


Anti-scatter grid —A grid that is placed between the patient and the radiographic plate to prevent x rays that have been reflected from reaching the plate. Without the use of this grid, the resulting xray image (radiograph) would be unreadable, or would appear severely "out-of-focus."

Radiograph —An image formed on a radiographic plate (similar to the film in a camera) by x rays. This is the final image produced by an x-ray unit.

Tracer —A chemical that is relatively dense to x rays that is added to the body to make that part of the body imagable with x rays. Examples include barium, used to image the gastrointestinal tract, and iodine, used to image blood vessels. Without the use of a tracer, these structures would be difficult, or impossible, to differentiate from surrounding tissues.

X ray —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 of these properties of x rays, they can be used to image dense structures within the human body.


Modern x-ray equipment is automated. An x-ray technician, or other licensed radiographer, properly positions the patient between the x-ray source and the radiographic plate. Then the technician goes into a separate room and pushes a button to turn on the x-ray beam. The reason for leaving the room while x rays are being taken is to prevent harmful effects in the technician that could occur after repeated x-ray exposure. The length of time that the x-ray beam remains on and the intensity level of the beam are based on the part of the body being imaged. In the newest equipment, these times and intensities are controlled by a computer, but may be manually adjusted, within certain safety limits, by the x-ray technician. After exposure, the technician removes the radiographic plate and places it in a fully automated development chamber where the final image is produced.

The skill and training of the technician comes in the proper positioning of the patient and in the examination of the resulting image. The image is ultimately examined for clinical findings by a radiologist and/or by the physician who ordered the x rays. It is the job of the x-ray technician to exam the radiograph to ensure that a clinically useful image has been produced. When an unsuitable image is produced, the x-ray technician will have to retake the x ray. Unsuitable images may be produced when the patient failed to remain still during the x ray exposure, the positioning of the patient was incorrect, there was an alignment or other problem with either the radiographic plate or the x-ray beam, or the exposure time and/or intensity was incorrect for the part of the body being imaged.


X-ray units are large and expensive pieces of equipment. They are generally covered by maintenance contracts provided by their manufacturers. Medical personnel should not attempt to maintain this equipment. Maintenance contracts generally call for routine maintenance every four months to inspect the equipment, replace any aging or wearing parts, to check for radiation leakage, and to ensure proper operation.

Health care team roles

X rays are generally ordered by primary care, emergency, or other specialized physicians. Most x rays are taken by registered x-ray technologists under the supervision of radiologists. A radiologist is a physician who has completed a minimum of a four-year residency program in radiology after medical school. A registered xray technologist is a person who has received a one year certificate, a two year associate degree, or a four year bachelor degree from a training program that is accredited by the Joint Review Committee on Education in Radiologic Technology and, if required, has received a license from the state in which he or she practices, to perform radiologic measurements or therapies on the general public.


All people who take x rays of patients or perform radiation therapies in the United States are required to be licensed and/or registered under the Consumer-Patient Radiation Health and Safety Act of 1981. This act was designed to protect the general public from unnecessary exposure to medical and dental radiation by ensuring that operators of radiologic equipment are properly trained in the use of such equipment.

Education and training programs in x-ray technology are offered by hospitals, colleges and universities, the armed forces, and vocational-technical institutes. Formal training is offered in radiography, radiation therapy, mammographic imaging, and diagnostic medical imaging (e.g. ultrasound, CT, and MRI). Programs range in length from 1 to 4 years and lead to a certificate, an associate degree, or a bachelor degree. Associate degrees that require 2 years of training are the most common. One year certificate programs are generally pursued by individuals already trained in another health occupation, such as medical or dental technology, or registered nursing, who want to change fields or expand their skill set within the setting in which they are presently employed; or, by experienced radiographers who want to specialize in radiation therapy or medical imaging.

All x-ray technology programs offer classroom and clinical instruction in anatomy and physiology, medical terminology , medical ethics , patient care procedures, positioning of patients for appropriate radiologic imaging, radiation physics, radiation protection (of both the patient and the x-ray technician), and radiobiology. In order to retain his or her licensure or registration as an xray technician or radiation therapist, a person licensed or registered as such must complete and provide documentation to the licensing or registration board of his or her state of 24 hours of continuing education every two years.

With additional training, available at most major cancer centers, radiation therapy technicians can specialize as medical radiation dosimetrists. A medical radiation dosimetrist works with oncologists (physicians specializing in cancer causes and treatments) and health physicists to develop effective treatment plans for patients who require radiation therapy in the treatment of cancerous tumors.

X-ray technologists who are also able to perform medical imaging are projected to have the best job opportunities, at least through 2006. This is because many hospitals, the main employers of x-ray technologists, are attempting to cut costs by merging their radiologic and nuclear medical imaging (MRI) facilities. The need for independent diagnostic imaging centers that specialize in providing radiographic and other medical imaging techniques to medical clinics and private physician practices is also expected to grow extremely rapidly. Advances in technology that lead to lower cost equipment permit more and more radiographic and medical imaging procedures to be performed outside a hospital environment.



Wolbarst, Anthony. Looking Within: How X-Ray, CT, MRI, Ultrasound, and Other Medical Images Are Created. Berkeley: University of California Press, 1999.


American Healthcare Radiology Administrators (AHRA). 111 Boston Post Road, Suite 105, Sudbury, MA 01776. 1-800-334-2472 or 978-443-7591. <http://www.ahraonline.org/contact.htm>.

American Society of Radiologic Technologists (ASRT). 15000 Central Avenue, SE, Albuquerque, NM 87213-3917. 505-298-4500. <http://www.asrt.org>.

Radiological Society of North America, Inc. 820 Jorie Boulevard, Oak Brook, IL 60523-2251. 630-571-2670. <http://www.rsna.org/index.html>.

Paul A. Johnson