Medical Imaging

by Food and Drug Administration (FDA)

This month FDA Consumer offers the second of two articles on the wide array of radiological techniques that physicians can use to help them "see" inside the body. The first article, in last month's issue, covered techniques that use X-rays, though often in ways far different from the traditional X-ray machine. Part two covers non-X-ray techniques, some of which work without potentially hazardous ionizing radiation.

Advances in X-ray technology have vastly increased the range and precision of medical imaging. Meanwhile, research scientists have developed entirely new imaging technologies, some of which enable physicians to look inside the body without subjecting it to potentially harmful ionizing radiation, such as X-rays. They also enable doctors to "see" beyond the capabilities of X-rays. And in some instances, they can be used to study body functions such as metabolism as well as anatomical features.

Sound Waves from the Navy

The Navy has long used sonar, a system of underwater detection based on ultra-high frequency sound waves, to locate submarines and other underwater objects. Like sonar, ultrasound medical imaging, or sonography, relies on the echoes of inaudible, high-frequency sound waves.

Medical sonography has been in use in the United States since the early 1960s. To make sonograms (sound "pictures"), ultrasound waves are transmitted from a wand-like probe called a transducer, which is passed back and forth in contact with the skin over target areas such as the liver or a kidney. The ultrasound waves bounce off the internal organs and echo back to the transducer. Other equipment converts the echoes electronically into a picture on a TV screen where they can be monitored, recorded on videotape, or photographed.

Sonograms can't provide fine structural details, but they can show the size and shape of an organ. And they can reveal cysts and tumors as well as abnormalities of the heart.

Ultrasound, however, is ineffective in imaging the lungs, bones or brain. And obesity or large scarred areas pose obstacles to ultrasound imaging.

Obstetricians have turned to ultrasound as a safe alternative to X-rays. However, while no harmful effects on the human fetus have been documented, experts recommend that it be used only in cases of clear medical necessity. For example, some question the use of ultrasound if the sole motive is to predict the baby's sex. From fetal sonograms, obstetricians can often predict twins, locate the placenta, identify abnormalities, help prepare for a Caesarean birth, and tell the position and age of the fetus.

Sonograms are used in other applications: to identify aneurisms — dangerous outpouchings of the aorta or other arteries; to locate blocked bile ducts, gallstones and liver disorders, such as cysts, tumors and abscesses; to identify abnormalities and diseases of the pancreas, kidneys and thyroid.

With a technique called Doppler ultrasound, specialists can detect abnormal rates of blood flow that betray blockages or narrowing of blood vessels and blood clots.

Newer ultrasound probes can be used inside the vagina for a closer look at a fetus, or inside the rectum to detect signs of colon cancer, or to view the prostate gland. Specialized ultrasound probes have also proven useful in diagnosing the cause of female infertility.

Radiation from the Inside Out

Nuclear medicine emerged after World War II when radionuclides (radioactive isotopes, which emit ionizing radiation) became available. At first, radiation from radionuclides was used to destroy cancerous tissue inside the body. In 1963, however, a body scanner using radionuclides was developed. Unlike an X-ray machine, which beams radiation at the body from outside, a nuclear scan places the source of radiation inside the patient.

To prepare for nuclear scans (also known as scintigrams), a very small and virtually harmless amount of a radionuclide is administered by mouth, injection or inhalation. A variety of radionuclides, such as technecium and thallium, are available; each has a special affinity for a different organ or part of the body.

A "camera" or scanning device then picks up the radiation being emitted from the body and transforms it into an image. Nuclear scans lack the clear definition of structure visible on an X-ray. But they can reveal areas of an organ, such as the liver, that are not functioning normally.

Some radionuclides that concentrate in diseased areas, such as tumors, show them as hot spots on a scintigram. Others concentrate in healthy, functioning tissues to reveal areas of disease as cold spots.

Nuclear scans of bones can enable doctors to detect bone tumors long before they show up on X-rays. They also aid in diagnosing bone injury, infection, and arthritis. Nuclear scans are also used to locate blood clots in the lungs, and scans of the liver help to diagnose cirrhosis, hepatitis, tumors, cysts and abscesses. In the brain they can uncover tumors and areas damaged by stroke.

SPECT and Stroke

The single photon emission computed tomography (SPECT) scan is a refinement of nuclear scanning. SPECT employs some of the same radionuclides, but it uses a more sophisticated camera to pick up the radiation. SPECT resembles the CAT scan inasmuch as the signals picked up by the "camera" are fed to a computer, which performs countless computations and transmits the results to a TV screen to produce either a slice-like cross-section or a 3-D image.

While some of the radionuclides used in nuclear scans are employed in SPECT, newer ones have been developed especially for SPECT. One new injectable imaging agent, called SPECTamine, is specifically designed to pass intact through the blood-brain barrier (which keeps many chemicals out of the brain). When used by skilled specialists, the new agent can help make quick, accurate assessments of the effects of a stroke, showing which blood vessels have been affected and the nature and extent of brain damage.

About the Author

www.fda.gov
FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.