Over the past decade, heart disease has emerged as the number one killer in the United States, responsible for more than half a million deaths last year.

At least 40 million Americans -- about one in four adults -- may suffer from some form of cardiovascular disease, estimates the American Heart Association. Many people with a problem may not be aware of it since the first symptom of heart disease is often death.

The most common cardiovascular problem is coronary artery disease -- the blockage of the blood vessels feeding the heart and the major cause of heart attacks. The deadliness and pervasiveness of this disease has spawned a high technology race to create a painless, reliable method of diagnosing the condition. Because it is expected to be a multibillion dollar market, millions of research and development dollars have been pouring in.

"Non-invasive cardiac diagnostics is going to be a whole new industry," says Bobby J. Bowen, general manager, X-ray programs department head of General Electric Medical Systems. "It's sort of the Holy Grail of the diagnostic imaging community."

Currently, the most reliable test for coronary artery disease is the coronary angiogram, a procedure in which a special contrast material is injected through a catheter passed from an artery in the upper leg or arm to the arteries that carry blood to the heart. The contrast material shows up on an X-ray, providing a picture of the arteries and heart. The procedure is invasive (which means an instrument is entered into the body), takes about an hour and typically requires a hospital stay of one day.

Two emerging technologies are battling to be first to offer a non-invasive way to assess the coronary arteries.

The first is cardiovascular mapping using ultrasound. Sound waves are beamed into body and their reflections and distortions are reassembled to construct a picture of what the heart looks like while it is pumping.

In the past, ultrasound -- or echocardiographic -- images have had a fuzzy, indistinct quality that made them inadequate for visualizing the coronary arteries. However, several companies -- such as Microsonics in Indianapolis, Ind. and General Electric -- are programming computers to enhance and refine those images. These new high-resolution ultrasound images offer a valuable way to view the heart in action.

"We'll start to see blips of this technology as early as the next 18 months," says Walter Channing, a venture capitalist with Channing -- Weinberg, a New York-based company specializing in medical technology investments."Maybe we will see it commercially in the next two or three years."

One drawback of ultrasound is that it has difficulty scanning people with certain body builds. Still, some believe it will become the most cost-effective diagnostic tool around at nearly $100,000 per unit initially.

The other technology touted as a potential breakthrough in cardiovascular diagnostics is the nuclear magnetic resonance scanner. The NMR uses a powerful magnet to line up hydrogen atoms in the body. A computer constructs an image of the body's interior from atomic displacement of the hydrogen by a radio pulse. The scan is quick, painless and thorough.

But there are two major problems with NMR. The first is cost. Building an NMR center at a hospital can cost upwards of $2.5 million. The second is that, unlike ultrasound, the NMR scanner doesn't scan in real time. The heart moves inside the chest cavity and this makes it difficult for the static scanner to view it effectively. NMR image processing companies like Resonex, in San Francisco's Bay Area, are examining that problem.

Still, the NMR scanner is new and is better known for its potential than for its practical utility.

Other high-tech techniques are also likely to be further refined and developed to meet the vast economic potential of the marketplace. Among them: thalium scanning, digital subtraction angiography and positron emission tomography.

"There's an enormous priority placed on this disease," says Channing. "Every male over 40, who is overweight by as little as ten pounds, who leads a sedentary life style or is an active exerciser or has elevated blood pressure or a family history of heart disease or is in a high stress occupation is a heart disease candidate.

"All it takes is that one jogger to die and $500 or $1,000 a scan doesn't look that expensive."