State of the Heart

Advances in imaging are helping cardiologists and physicians see and understand more about the heart than ever before.

by Alicia Di Rado

A woman comes to the hospital complaining of chest pain. The heart specialists run her through a battery of tests, with unremarkable results. She undergoes heart catheterization and has no evidence of significant coronary artery disease. No obstructions. Nothing.

So what now?

Fortunately, cardiologists have available a cutting-edge scanning technology that may reveal reasons behind the pain,

a puzzling condition known as cardiac syndrome X. Through complex magnetic resonance imaging, chemical abnormalities in heart muscle may come to light in many of these women, pointing out previously hidden heart disease.

That is just one of the potential advances wrought by evolving cardiac imaging technology.

“I’m still amazed about all of the information we get about the heart using cardiovascular magnetic resonance imaging, and the field continues to develop,” says Gerald Pohost, M.D., chief of cardiovascular medicine in the Department of Medicine and professor of medicine at the Keck School of Medicine of USC. “In the future, advances in imaging will help us see and understand more about the heart than ever before.”

Pohost led the first study that used magnetic resonance spectroscopy to detect heart disease in women with syndrome X. He did this while he was director of the division of cardiovascular disease and the Center for Nuclear Magnetic Resonance Research and Devel-opment at the University of Alabama at Birmingham. He joined the Keck School in January 2002, bringing several National Institutes of Health-funded research projects and a commitment to making the Keck School and USC University Hospital centers for the advancement of cardiovascular imaging and medicine.

Maintaining the beat

The heart poses a challenge for physicians needing a closer look: It pumps and contracts, in constant motion, deep within the chest.

That may be why it is the final frontier of imaging.

Lives depend on finding better windows to see into the heart—particularly in relation to coronary artery disease, or CAD. One of every two people in the United States dies of cardiovascular disease, and more people die as a result of CAD in the U.S., Europe and other developed regions than from any other disease.

CAD is a disease of the arteries that supply blood to the heart muscle, potentially damaging the heart or making it work inefficiently.

The heart must consume the oxygen delivered by way of the blood to fuel its constant beating. But even as the heart pumps blood through its chambers, that blood cannot nourish it. Instead, an intricate network of blood vessels, called the coronary arteries, wrap around the heart and carry the much-needed oxygen and other nutrients.

Unfortunately, the hearts of CAD patients often do not get enough of that blood.

The root of the problem: atherosclerosis, the accumulation of plaque along the walls of arteries. As fatty materials collect in these vessels over time, the artery walls thicken and narrow the space where blood can flow.

Several factors, including heredity, diet, cholesterol, exercise and smoking, contribute to atherosclerosis risk.

When atherosclerosis progresses to the extent that it deprives tissue of sufficient blood, some patients experience angina (chest pain) or shortness of breath. Angina may increase with exercise and ease with rest, and it can feel like a dull ache or pressure in the chest, with pain radiating up the neck or down an arm.

Yet, for other patients, a myocardial infarction—or heart attack—is their first symptom.

Checking the list

“A cardiologist can usually make a diagnosis of coronary artery disease by establishing the characteristics of the chest pain,” Pohost explains. “The pain can come and go, and might be a single episode or can occur several times. It also might increase in intensity or frequency, or radiate into the left shoulder or arm or elsewhere.”

But to understand each patient’s heart, physicians need a full picture. “It’s useful not only to know a patient has disease, but the extent of the disease. And it’s best to catch it before a patient has a myocardial infarction,” Pohost says.

Cardiologists gather information about five important heart characteristics:

Imaging can tip off physicians to concerns within those areas.

Today, cardiologists can image the heart from outside the body through ultrasound (echocardiography), radioisotope imaging (nuclear cardiology), X-ray computed tomography (CT) and nuclear magnetic resonance imaging (MRI or CMR).

Patients usually start with echocardiography, in which a technologist uses sound waves to produce a two-dimensional image showing the structure of the heart, how it moves and contracts and how its walls thicken. This test can show if defects, disease, valve problems or a heart attack have limited the heart’s function.

A heart attack can happen when plaque ruptures, expressing some of its contents into the arterial lumen, leading to an occlusion that dislodges from an artery wall and cuts off blood flow to the heart muscle. Starved of oxygen and other nutrients—

a condition called ischemia—muscle tissue begins to die and eventually scars. Scarred tissue does not contract, and that can show up on an echocardiogram.

Howard Hodis, M.D., associate professor of medicine and preventive medicine and director of the USC Atherosclerosis Research Unit, says ultrasound also can be a good tool to measure atherosclerosis itself.

He uses the technique to measure the thickness of artery walls in the neck, “which is a good reflection of what’s going on in the coronaries,” Hodis says. “There is no other way to non-invasively assess the arterial walls of the coronaries.”

For other information on their heart checklist, cardiologists may turn to a different technology: radioisotope imaging.

In this technique, technologists inject a substance containing a tiny amount of radioactive tracer into the patient. A special camera then captures images created by detecting the gamma or X-rays emitted by that tracer. Such scans include single photon emission computed tomography, or SPECT, and positron emission tomography, or PET.

Cardiologists can use these studies to find areas where heart tissue gets too little blood.

Hossein Jadvar, M.D., Ph.D., assistant professor of radiology and biomedical engineering, says that cardiac studies account for about 4 percent of patients at the USC PET Imaging Science Center, but the number is growing.

Jadvar noted that PET can point out areas of the heart muscle that are “hibernating.” Dwindling blood flow can force muscle cells to go to sleep, in a way, but not yet to die. The good news: these cells wake up when blood returns.

“This is important because if you can identify these tissues, then you know that their function can be restored by improving blood flow to them,” Jadvar says. “It can help guide treatment decisions.”

Put practically, a CAD patient with areas of hibernating heart tissue likely would benefit from a heart bypass surgery or a coronary angioplasty with stent placement. These methods can restore blood flow to the area. But once tissue is already scarred, restoring blood flow to it does not accomplish much, Jadvar says, so physicians may recommend medication instead.

In the future, Jadvar expects that PET increasingly will be used to examine the heart’s biochemistry. Not only can it make pictures, but it also can provide hard data for physicians about blood flow and other heart characteristics.

Another technology is computed tomography, or CT. Many centers are touting electron-beam CT as the best way to examine the heart non-invasively in healthy patients to discover early signs of disease.

Pohost warns, though, that CT scans alone cannot give the full story of heart health. They may show the presence of calcium in arteries around the heart, and while such calcium suggests CAD, it is hard to draw helpful conclusions about the status of the CAD.

“CAD has several phases, and some coexist in the same artery,” Pohost says. At first, fats gradually accumulate on the innermost lining of artery walls. Then, inflammation and scarring occur within arteries.

“Only then does calcification take place,” he says. “Calcium in plaque indicates that plaque exists, but by then it’s stable. It’s hard. What is believed to make plaque unstable is the amount of softer fat, or lipid, that’s in it, and you won’t see that on a CT scan.

“What people don’t usually know is that it’s the invisible part of the plaque that can be the most devastating.”

Hodis says that calcium points out significant atherosclerosis, and studies indicate calcium might be linked with higher risk for rupture. “But the problem is that if you don’t have calcium, it doesn’t mean you don’t have atherosclerosis,” he says. “And most people don’t even have calcifications.”

CT has exciting potential, he says. New studies are showing that when used with an injected contrast agent, CT will be able to provide physicians with a 3-D, virtual tour inside a patient’s coronary arteries to spot dangerous wall lesions, Hodis says. And today, it provides an excellent window for following up on the health of patients who have had a coronary artery bypass.

Highlighting the heart

Neither echocardiography, nuclear cardiography nor CT alone can give information about all five items on the heart-health checklist that cardiologists need to guide treatment decisions for CAD patients. Getting all the necessary tests sometimes requires several visits to a cardiologist or imaging center, or even an overnight hospital stay.

But that might end in the not-too- distant future.

Pohost and others believe advances in cardiovascular magnetic resonance imaging, known as CMR, will allow cardiologists to completely analyze all five characteristics of the heart with one machine.

“In essence, it will be a one-stop-shop for cardiac imaging,” Pohost says. Patients will be able to have the heart checked out in one visit, saving the patient time and cutting costs.

The imaging technology is the same as that used to look for evidence of tumors in cancer patients, for example. Some call it MRI, short for magnetic resonance imaging.

Unlike the other technologies, CMR uses no radioactivity, no sound waves and no X-rays. Instead, it uses magnetic fields and radiowaves.

An image of the heart can be acquired in 50 milliseconds. The technology is so advanced that high-tech navigation beams can detect any breathing movement in a patient’s chest that could hurt image quality, and stop scanning until the heart moves into a similar position. Imaging the heart in the same position while breathing generates sharp images.

Pohost, who was the first cardiologist to use CMR for the heart back in 1979, is enthusiastic about the potential across the spectrum of heart studies. Coupled with exercise or stress testing, the scans provide lots of information about patients’ heart disease and their likely outcome.

“Because it is three-dimensional, it is the best technique, in theory, to determine the size of the heart and how well it functions,” Pohost says. “No technology can provide such accurate measurements of volumes in the heart.”

Physicians can look at cross-sections of the heart that are thinner than a pencil eraser, watching for subtle abnormalities not seen in echocardiograms.

Researchers also are investigating using CMR together with an injected, water-like dye to look at perfusion: pointing out patches of heart muscle that are not getting enough blood. The bonus, Pohost says, is that 10 to 15 minutes after the dye is injected, the dye settles in dead or scarred areas of the heart—providing more information by highlighting areas of the heart that are not viable.

And in the case of syndrome X, a technique known as magnetic resonance spectroscopy can shed light on the chemistry of the heart muscle, too. When parts of the heart muscle do not get enough oxygen, the level of certain body chemicals called high-energy phosphates plummets. One of the earliest signs of ischemia, such chemical change actually alters the pH of the heart itself, making it more acidic. CMR can also show the changes of pH within the heart muscle.

Amazingly, spectroscopy can measure that without any instrument ever touching the heart.

While few centers possess the machines needed to perform cutting-edge CMR scanning, USC University Hospital will install a new CMR system in early 2003. Its magnet will be twice as powerful as those used almost anywhere else. The stronger magnet will generate more precise images than widely available machines so physicians can get better information about the state of their CAD patients—and enable Pohost and his colleagues to evaluate numerous aspects of heart function, allowing an advance in cardiovascular research.

“I’m excited about what’s ahead for us,” Pohost says. “We’re expecting to do some great things in cardiology.”

For more information about cardiovascular medicine, or to learn more about The Doctors of USC, call 1-800-USC-CARE (1-800-872-2273).

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