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Radiology, in medicine, the discipline of medical science that uses electromagnetic radiation and ultrasonics for the diagnosis and treatment of injury and disease.

Radiology originated with the discovery of X rays by German physicist Wilhelm Conrad Roentgen in 1895. Roentgen was awarded the first Nobel Prize in physics (1901) for his work.

Diagnostic radiology, or diagnostic imaging, is the medical evaluation of body tissues and functions-both normal anatomy and physiology and abnormalities caused by disease or injury-by means of static (still) or dynamic (moving) radiologic images.

In the century since Roentgen's discovery, electromagnetic radiation in the form of ionizing radiation (alpha, beta, gamma, and X rays) has been the predominant energy source for diagnostic radiology.

The use of ionizing radiation in diagnostic radiology involves passing a localized beam of X rays through the part of the body being examined. This produces a static image on film. The image, called a radiograph, or X-ray picture, can take several forms. It may be a plain radiograph, such as the common chest X ray; a mammogram, an X-ray image of the female breast used to scan for cancerous tumors; a tomograph, which produces an image of the entire depth of an anatomical structure with a series of X rays; or a computerized axial tomography (CT or CAT) scan, a computer analysis of a cross-sectional image of the body.

Many organs, organ systems, and certain muscular and skeletal structures are not visible with routine radiographic techniques. They become visible with the ingestion, injection, or inhalation of substances called contrast media, which are opaque to radiation. Diagnostic techniques involving contrast media include the upper gastrointestinal (GI) series, barium enema (colon examination), arthrogram (injection of contrast into a joint), myelogram (injection of contrast into the spinal canal), and angiogram (injection of contrast into an artery, vein, or lymph vessel).

Dynamic images, which record movement of organs or organ systems such as the intestinal tract or the flow of contrast material through blood vessels or the spinal canal, may be obtained by fluoroscopy (recording the radiographic image on a movable, radiation-sensitive screen) or cineradiography (recording the image on film or videotape). Both film and videotape are permanent recording media. The fluoroscopic image, analogous to a television image, is transient. Permanent radiographic images (spot films) can, however, be made at any time during a fluoroscopic examination. Another type of diagnostic imaging that identifies biochemical activity in addition to structural tissues is positron emission tomography (PET). In this method, a patient is injected with glucose treated with radioactive tracers. As the body metabolizes the glucose, the PET scan monitors the radioactive particles emitted by the tracers in the glucose. Images are produced that show metabolic reactions, making this method useful to diagnose brain tumors and strokes.

The use of ionizing radiation in the evaluation of disease is similar to the use of drugs in the treatment of disease. Diagnostic radiographic examinations should only be performed for a specific medical indication on the direct request of a physician or other qualified person. Although diagnostic radiation dosage levels involve a small health risk, there is no evidence to show detectable adverse effects of radiation from medically indicated and properly conducted diagnostic radiographic examinations. In the opinion of the American College of Radiology and the Bureau of Radiologic Health of the United States Department of Health and Human Services, with careful patient selection, the risk-benefit ratio clearly lies in favor of the radiographic procedure.

Since the 1970s new imaging procedures that utilize energy sources other than ionizing radiation have become essential in diagnostic radiology. Magnetic resonance imaging (MRI) produces computer-processed views of soft tissue, such as arteries, nerves, tendons, and some tumors, that present little or no shadow on a conventional X ray. During an MRI, powerful electromagnets create a magnetic field up to 30,000 times stronger than the earth's, which influences the alignment of protons in hydrogen atoms in the body. A radio wave, emitted 25 or more times per second, knocks the protons out of this temporary alignment. When each radio pulse stops, the protons realign within milliseconds. MRI scans these differences in the alignment of hydrogen protons to produce the diagnostic images.

Ultrasound utilizes high-frequency sound waves, which are reflected by tissue in the body. The sound reflection is processed by a computer to produce a photograph or a moving image on television. Ultrasound is used to examine many parts of the body; however, its best known application is the examination of the fetus during pregnancy.

Each of these techniques has unique features that, under various conditions, make it more likely to reveal clearly the part of the body to be examined. The radiologist, a physician specializing in imaging techniques, has the opportunity to select the imaging procedure best suited to the diagnostic needs of the patient.

Therapeutic radiology, also referred to as radiation oncology, is based on the use of ionizing radiation in the treatment of cancer. Normal tissues have a greater ability to recover from the effects of radiation than tumors and tumor cells. A radiation dose sufficient to destroy cancerous cells only temporarily injures adjacent normal cells. When the ability of normal tissues to recover from a given amount of radiation is the same as or less than that of the cancerous tissue, the tumor is described as radioresistant. In such cases radiation therapy is usually not considered an appropriate form of treatment.

Treatment with ionizing radiation is often described in terms of the energy of the beam used: superficial (less than 120 kilovolts, or kv); orthovoltage (120 to 1000 kv); and megavoltage (greater than 1000 kv). Superficial radiation therapy is used in treating malignant diseases involving the skin, the eye, or other body surfaces. Orthovoltage therapy has been largely replaced by megavoltage (cobalt, linear accelerator, and betatron) therapy (see Particle Accelerators). Megavoltage therapy provides more efficient and effective delivery of the intended radiation dose to tumors deep inside the body and, at the same time, spares the healthy skin and surrounding normal tissue from excess radiation.

Radiation therapy may be used alone as the treatment of choice in most cancers of the skin; in certain stages of cancers involving the cervix, uterus, breast, and prostate gland; and in some types of leukemia and lymphoma, particularly Hodgkin's disease. In such instances, radiation therapy is intended to effect a cure. When radiation therapy is used in conjunction with cancer-treatment drugs (known as combined modality therapy), however, it may be intended for either cure or palliation (the relief of symptoms). Radiation therapy is commonly employed either before or shortly after surgical removal of certain tumors to destroy tumor cells that could (or may already have) spread beyond the surgical margins. Radiation therapy is also frequently employed in controlling local tumor recurrence after surgery.

Interventional radiology is the nonsurgical treatment of a growing number of diseases using radiologic imaging to guide catheters (hollow, flexible tubes), balloons, filters, and other tiny instruments through the body's blood vessels and other organs.

Common interventional radiologic procedures include: balloon angioplasty, the use of a balloon to open blocked or narrowed arteries; chemoembolization, the delivery of anticancer drugs directly to a tumor; fallopian tube catherization, which opens blocked fallopian tubes, a common cause of infertility in women; and thrombolysis, which dissolves blood clots.

A radiologist is a physician who, following completion of medical school, spends an additional four or five years exclusively studying diagnostic, therapeutic, or interventional radiology. In the United States, an individual is eligible for examination by the American Board of Radiology after completing an approved residency program. Successful candidates are entitled to the designation of diplomate of the American Board of Radiology.
After completing the residency program and either before or after passing the board examination, an individual may train for an additional year or two in a subspecialty of radiology such as neuroradiology or pediatric, skeletal, genitourinary, or gastrointestinal radiology. Individuals may then confine their practice to a particular aspect of radiology or simply demonstrate a special interest in that area while practicing general diagnostic radiology. Career opportunities in radiology include practice in a hospital or private office, or both, as well as teaching, research, or administration.
Radiologists are assisted by radiation physicists, radiation biologists, radiologic administrators, and radiologic technologists. The last are high school graduates who have successfully completed a course of at least two years in a school of technology approved by the American Medical Association through the Joint Commission on Radiologic Technology and who have passed a written examination offered by the American Registry of Radiologic Technologists. As with the radiologist, the radiologic technologist may, following additional training, become certified in an area of special interest. Registered radiologic technologists are qualified to perform certain radiologic procedures and assist in the performance of others, but always under the supervision of a radiologist. Technologists, however, are not qualified by either education or certification to interpret radiographic examinations.



Electromagnetic Radiation
Positron Emission Tomography
Magnetic Resonance Imaging
Particle Accelerators
Hodgkin's Disease