A Primer on Medical Imaging

Article excerpt

A Primer on Medical Imaging-Part One

When today's physicians need to peer inside the body, they can choose from a variety of radiological techniques, from "traditional" X-rays to computer-assisted systems using the latest in medical high technology. To help patients scheduled for aradiological procedure understand what kind of an "X-ray" or "scan" they're getting, FDA Consumer offers this two-part primer on modern medical imaging. Part one covers techniques that use X-rays, though often in ways far different from the traditional X-ray machine. Part two, which will appear next month, will cover non-X-ray techniques, some of which work without potentially hazardous ionizing radiation.

Imagine back to before 1895, the year the German physicist Wilhelm Roentgen discovered the X-ray and its penetrating powers. How handicapped physicians must have been with no sure way of telling whether a bone had been broken, where a bullet had come to rest inside a gunshot victim, or if a patient with symptoms of gallstones actually had them.

Today, to make accurate diagnoses, doctors can call on not only highly sophisticated X-ray techniques, but also an array of other imaging technologies. These so-called noninvasive procedures often make positive diagnoses possible while sparing patients the pain and trauma of exploratory surgery.

Some of the newer technologies employ widely different types of radiation: ultra-high-frequency sound, radioactive substances, or a radio wave effect of atomic nuclei called nuclear magnetic resonance. Still being explored is the PET scan, based on energy emitted by an atomic particle called a positron.

Traditionally, physicians have used X-rays to spot anatomical signs of disease. But some of the newer technologies can uncover functional and metabolic signs as well. In other words, doctors can see not only how an organ looks, but also how it's working. Functional abnormalities often show up earlier than anatomical signs, making possible earlier diagnosis and treatment.

X-rays Still the Workhorse

Despite the development of newer technologies, the X-ray machine continues to be the workhorse of diagnostic imaging. Skilled radiologists (physicians who specialize in the medical uses of radiation) can gain a great deal of information when they "read" an ordinary chest X-ray, for example. They can identify signs of lung cancer, tuberculosis, heart disease, or silicosis, a lung disease that afflicts some miners.

But the chest X-ray is sometimes subject to varied interpretations. And by the time lung cancer shows up on a chest X-ray, it may be too late to stop it from spreading.

Generally, ordinary X-rays are stopped by the calcium in the skeletal system, so bones show up as distinct shadows on X-ray film. But these same rays pass through soft tissues without differentiating clearly between blood vessels, the gastrointestinal tract, and other organs.

Fortunately, radiologists have found that other elements besides calcium, principally barium and iodine, also stop X-rays. Thus, when doctors want to inspect the condition of the soft tissues that compose the gastrointestinal tract (the gut), the radiologist administers a compound called a "contrast agent" --in this case barium sulfate--by mouth to X-ray the upper GI tract or by enema to image the lower bowel. The barium coats the inner lining of the gut, permitting the doctor to "see" problem areas such as ulcers or potentially harmful outpouchings of the colon. Similarly, contrast agents that contain iodine are injected to help "see" the urinary tract, veins and arteries.

A Study in Contrasts

Some of the more common X-ray examinations that make use of contrast agents include: . Intravenous pyelogram: Contrast material is injected into a vein to visualize the kidneys, the ureters (which connect the kidneys to the bladder), the bladder, and the urethra, which carries urine from the bladder. …