Rapid X ray opens window to arteries.

A century ago, a photographer used high-speed photos to prove that a galloping horse lifts all four hooves off the ground.

Today, the same principle of rapid exposures drives a $2 million device known as an ultrafast CT scanner. Capturing 10 X-ray images each second, the scanner enables doctors to peer into pulsing coronary arteries and identify blockages with 95 percent accuracy.

As any camera buff knows, rapid exposures can record an instant so brief that a moving object registers as a still image on film. Standard CT scans, in contrast, require exposures lasting 2 seconds. "When you're shooting with a conventional scanner, you get a blur," says Arthur S. Agatston, of Mount Sinai Medical Center in Miami Beach, Fla., one of 12 researchers at six medical centers nationwide who report their findings in the March 1 Circulation. The new technique depicts fatty deposits, called plaque, which obstruct blood flow. The scans can detect plaque because it is laced with calcium, which X rays cannot penetrate. On standard CT scans, these deposits show up indistinctly; on ultrafast CT scans, they stand out clearly.

Using both ultrafast CT scans and conventional angiography, the researchers studied 710 people with suspected heart disease. The CT scans detected plaque in 404 of 427 patients with blocked arteries. Of the patients in whom the scans failed to detect blockages, only four had two or more blocked arteries. Consequently, the researchers calculate, ultrafast CT scans predict 90 percent of coronary artery disease in people with a single blocked artery and at least 97 percent in people with multiple blocked vessels.

If the technique lives up to its promise, Agatston says, many fewer people may have to undergo angiography. In this invasive diagnostic procedure-performed on an estimated 1 million people each year-doctors poke a catheter into a leg artery and guide it to the heart, where they flood the arteries with dye that shows up on X-ray images.

Angiography has its risks. One in 500 patients suffers a heart attack or stroke caused by blood clots or plaque dislodged by the catheter; 1 in 1,000 dies.

Though noninvasive, ultrafast CT scanning remains controversial. Some cardiologists do not believe the technique portrays the internal anatomy of the arteries in enough detail for physicians preparing to perform surgery. Also, because older people have more plaque, the test becomes increasingly difficult to interpret as patients age, says Donald P. Harrington of the State University of New York School of Medicine at Stony Brook and former chairman of the American Heart Association council on cardiovascular radiology. Agatston counters that ultrafast CT scanning is accurate enough to rule out coronary artery disease in many patients. By a series of calculations that improve the specificity of the findings, he says, overall accuracy can reach more than 95 percent.