New imagers watch blood flow


July 21, 1992|By Jonathan Bor | Jonathan Bor,Staff Writer

It produces elegant pictures of slipped discs and brain tumors, reveals the inner structure of the eye and ear, captures torn ligaments and cartilage -- even displays the neurological decay of multiple sclerosis.

Magnetic resonance imaging, a technique performed by cylindrical machines the size of pickup trucks, has revolutionized medical diagnosis ever since it first appeared in the medical marketplace a decade ago.

The imagers, which have sprouted in dozens of hospitals and private clinics throughout Baltimore, have always done an excellent job producing high-resolution pictures of organs and tissues. But now, radiologists are exploring a new capability: vivid pictures of blood flow.

The latest generation of imagers also boasts more powerful magnets and more sophisticated software, advances that present radiologists with an enviable choice: better pictures, or less time spent by the patient lying motionless inside the machine's claustrophobic confines.

At Sinai Hospital, which installed its new $2 million machine two months ago, procedures that once took 45 minutes now take about 20 minutes. Looming software updates promise to shorten the procedure even further.

Pictures of blood flow are made possible by a technological advance called magnetic resonance angiography, or MRA. Put simply, it highlights movement rather than differences in the consistency of tissues.

On an MRA picture the intensity of the glow is directly related to the degree of blood flow. Blood coursing through a wide-open vessel may show up as a glowing white line snaking through blackness. A narrowed vessel may appear gray and fuzzy; a totally blocked vessel may not show up at all and simply blend into the blackness that surrounds it.

If the vessel is the carotid artery, the white glow could mean blood is flowing freely on its way to the brain. Anything less could be troubling evidence that episodes of dizziness or blurred vision were small strokes caused by restricted blood flow to the brain.

Dr. Jerry Patt, chief radiologist at Sinai, said the technology is gradually replacing the use of angiograms -- a riskier method for imaging blood flow.

An angiogram is obtained by inserting a catheter into a vessel at the groin, then snaking it to a distant region. Through the catheter, doctors inject a dye, called a contrast medium, that highlights the area to be studied. Then, a rapid series of X-rays is taken.

"We will evolve into doing less angiography," noted Dr. Patt, who said it is still too early to gauge the full potential of the new magnetic resonance machine put into service two months ago at Sinai.

Unlike angiography, magnetic resonance spares the patient ,X potentially harmful X-rays. It also does its work non-invasively, in contrast to angiography which exposes patients to a 1 percent to 2 percent risk of stroke.

Dr. Peter Janick, Sinai's MRI director, said magnetic resonance can help determine whether a patient with stroke symptoms is a candidate for surgery.

Someone with a narrowed artery might undergo surgery to open the vessel. On the other hand, surgery on a thoroughly blocked artery might be too risky. Instead, that patient might be placed on blood-thinning medication.

Dr. Edward Steiner, assistant chief of radiology at Sinai, said pictures of blood flow are being used in conjunction with -- not instead of -- pictures of tissue to help diagnose patients with stroke symptoms.

"Let's say the patient had strange symptoms, lost vision in one eye," he said. "All of a sudden the patient is dizzy."

First, the radiologist images the brain in a search for stroke damage or a tumor. If stroke damage is found, a subsequent MRA -- done by the same machine -- will isolate the precise area where blood flow is restricted. If a tumor is the problem, MRA will reveal the vessels supplying that tumor, providing a detailed road map for surgery.

An article in the June 4 New England Journal of Medicine found that magnetic resonance is more effective than the traditional X-ray technique in mapping bypass surgery of the leg. Researchers at the University of Pennsylvania said it gave doctors a better chance of finding a healthy vessel to re-route blood.

Weighing in at 10 tons, MRI uses powerful magnetic fields and radio waves to produce its detailed pictures.

First, the patient slides into the hollow of an electromagnet with a field thousands of times stronger than that of the Earth. The magnet makes protons in the body's hydrogen atoms line up parallel to each other, as if in formation. When the body is stimulated with radio waves, the protons flip back -- sending radio signals to a receiver.

The signals are interpreted by a computer that produces cross-sectional pictures of the body.

"As these machines get better they do more with the information they have -- so you can have better pictures in the same amount of time [as before] or the same pictures faster," Dr. Patt said.

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