Cardiac MRI

Cardiac MRI is gaining proponents thanks to its finesse in determining cardiac viability, assessing heart muscle damage after a heart attack, measuring left and right ejection fraction, and determining true ischemia - and rivaling nuclear stress tests and cardiac cath procedures.

Clinical experience is showing that cardiac MRI scans can take less time to evaluate heart function and anatomy compared to a series of other cardiac imaging exams.

"We can measure both left and right ejection fraction; we can look for evidence of blocked arteries; and we can do an assessment of previous heart attacks," says Tony Fuisz, director of cardiac MR at Washington (D.C.) Health Center. "The best attribute of MRI in this setting is the ability to combine multiple types of information in one study, so the patient has one 40-minute exam."

Washington Health Center is a tertiary care facility, which annually performs between 12,000 and 13,000 invasive cardiac procedures and approximately 1,500 cardiac studies. Most of the exams involve coronary artery disease and left ventricular function and may include viability imaging using the gadolinium contrast agent techniques and profusion imaging using Adenosine.

The most common use for MRI is to determine cardiac viability - to tell if a patient has any coronary lesions and to determine treatment. In this area, most practitioners consider MRI the gold standard.

"What is outstanding about cardiac MRI - which the other technologies have not offered so far - is the ability to very correctly predict how much of the heart muscle has been completely damaged," notes Deepika Gopal, MD, medical director for the cardiovascular MRI program at the North Texas Heart Center at Medical City Dallas Hospital. "This is the best test we have today for identification of completely dead myocardium, which is scarred heart muscle from prior damage."

The North Texas Heart Center began its foray into cardiac MR imaging last November. The facility, which performs five to 10 studies per week, also is using MRI with adenosine stress tests for perfusion imaging to detect how much of the heart muscle is dead, alive or at-risk.

Gopal prefers the adenosine stress test, adding that it is "superior to stress nuclear perfusion studies, because the MRI has increased spatial and temporal resolution in any kind of perfusion imaging in cardiology."


DETECTING PLAQUE

The North Atlanta (Ga.) Diagnostic & Cardiovascular Center (NADCC) has utilized a cardiac MRI unit for about two years, having performed some 500 studies over that time.

Stephen Frohwein, MD, NADCC's medical director for cardiovascular imaging, says some encouraging work is being done on MRI's potential role in evaluating coronary artery disease. Early research has found that 60 to 70 percent of patients get what Frohwein describes as an "adequate evaluation" of his or her coronary anatomy.

"But, that's not good enough when it comes to obstructive disease," he adds. "When you have a procedure, such as heart catheterization, that can tell nearly 100 percent of the time what is going on with the cardio vessels, a procedure that is not up to that par is not good enough - especially when it concerns something as serious as coronary artery disease."

If MRI does become a valuable tool in evaluating obstructive coronary disease and identifying plaque in arteries - perhaps with the help of the MR contrast agent gadolinium - Frohwein says physicians "may have the best test out there to evaluate the whole cardiac condition."


FIELD STRENGTH

To some degree, the jury is still out on the most optimum magnet field strength to perform cardiac MRI. Right now, 1.5 tesla (T) magnets are in the market in great numbers and physicians are more than satisfied with image quality.

"The 1.5 tesla MRI has been the mainstay," says Sungkee Ahn, MD, director of MRI at American Radiology Services Inc. (ARS). "Just because of the small size and difficulty of the anatomy of heart - because it is moving - we have to image very fast. That precludes any low-field MR, because it cannot get the images fast enough."

ARS, based in Baltimore, Md., is about to launch its cardiac MRI service this summer. The company has more than 30 medical imaging sites in Maryland and Delaware.

ARS is adding cardiac MRI because of the modality's non-invasive nature and its non-radiation method to image the heart and evaluate the heart's anatomy and function non-invasively, Ahn says.

Lurking on the not-so-distant horizon are higher-field, 3T MRI systems. While 3Ts are expected to become more of a presence, Washington's Fuisz believes it is "still unclear" what the optimum field strength will be for cardiac work.

"We know already that for neurology work where you have a static organ, the higher field strengths give you an advantage in signal and the potential for better resolution," he adds. "In cardiac, with the very rapid imaging that is necessary and the larger fields-of-view, there may be an optimal field strength that may not simply be higher than the existing [1.5T]."

Part of that reasoning is because of a 1.5T system's ability to collect more than enough relevant patient information that, in Fuisz says, "we don't need some dramatic change to make it a useful thing."


EXPANDING APPLICATIONS

Northwestern Memorial Hospital in Chicago has been collaborating with Siemens Medical Solutions on the development of a 3T MRI system - the Magnetom Trio - and additional 1.5T hardware for cardiac applications.

Northwestern currently is investigating how best to take advantage of the added magnet strength - 3T vs. 1.5T - in terms of more-detailed images and/or more rapid imaging.

"We potentially could start imaging the heart with much higher spatial resolution to see structures we weren't able to see before," says James Carr, MD, director of the cardiovascular imaging section in radiology at Northwestern, "or we can image much faster so we can complete the studies in a much shorter period of time."

The potential benefits are in myocardial perfusion [or stress] imaging of the heart, where nuclear medicine stress tests traditionally have been the norm.

"The advantage of the 3T system is, because we get a much higher signal from the heart, we may be able to get the same number of slices, but with much higher detail within the single heartbeat," Carr adds.

Northwestern and Siemens also will work on multi-channel, multi-coil cardiac MR technology. The facility is scheduled to receive a 1.5T MRI unit - the Magnetom Avanto - this month, as one of three sites in the Siemens' project. The system is expected to produce images faster than conventional scanners, as the additional channels help acquire images from the heart in less time. Carr says the advantages could be in functional and cine imaging of the heart.

"The advantage of the multi-coil design is that you could start off doing a study of the head and you see a stroke in a young person," he adds. "You can shift down to the heart and image the heart. If you need to evaluate the blood vessels in the body, you can do a whole-body MRA from head to toe in 30 or 40 seconds with this system."

The system has been FDA-cleared for clinical work and Northwestern will use it as a dedicated cardiovascular scanner.


MRI'S POTENTIAL

Cardiac MRI also may become a viable test to evaluate coronary obstructive disease and eliminate invasive heart catheterization procedures, but studies to-date have been inconclusive.

While MRI may help determine coronary artery disease in 60 to 70 percent of patients, Stephen Frohwein, MD, medical director for cardiovascular imaging at the North Atlanta (Ga.) Diagnostic & Cardiovascular Center (NADCC), says that a low percentage is "not clinically applicable" in the evaluation of obstructive coronary disease.

"When you have a procedure, such as heart catheterization that can tell nearly 100 percent of the time what is going on with the cardio vessels," Frohwein adds, "a procedure that is not up to that par is not good enough - especially when it concerns something as serious as coronary artery disease. If it ever does become a valuable tool in evaluating obstructive coronary disease, we might have the best test out there to evaluate the whole cardiac condition."

Medical City's Gopal notes that MRI is becoming a fine technique to detect microvascular angina where there may be no blockages, but patients still suffer chest pains due to decreased blood supply in the inner most areas of the heart, which cannot be seen by other imaging modalities.

"These patients most likely have underlying hypertension, diabetes and other cardiac risk factors," Gopal says. "They don't have the occlusive disease of the major arteries, but they have what we call microvascular disease. Cardiac MRI has shown - especially within the last year - that this technique is able to demonstrate true ischemia, meaning decreased perfusion with adenosine. I think it will be one of the best techniques to help guide patient management and care."


MARKET EFFECTS

If MRI continues on its current path of more expanded use in cardiac applications, some practitioners believe the imaging modality could supplant nuclear medicine in some traditional imaging roles, particularly in the area of stress tests.

"I think MR will prove to be much more accurate and easier for the patients, because the studies will be shorter in duration and more comprehensive," says Robert Edelman, MD, chairman of the department of diagnostic radiology at Evanston (Ill.) Northwestern Healthcare.

Radionuclide studies in the brain were replaced by CT and MR, and Edelman thinks the same switch can happen between MR and nuclear medicine.

"I don't necessarily think we will replace stress echo, because cardiologists will be reluctant to give up a procedure they do themselves," Edelman adds. "And, they'll be reluctant to give up stress radionuclide studies, but [the studies] are quite good anyway."

Gopal sees positron emission tomography (PET) as the "next best tool" cardiologists have to assess viable heart muscle.

"For viability, as well as for stress perfusion imaging, as a cardiologist and as a person who reads PET, I would do a PET scan, if I could not do an MRI," she says. "PET has its excellent assessment of metabolism and viability and is as good as MRI for assessment of live myocardium. For scar [tissue], MRI is superior to the PET scan."

In comparing MR and CT in cardiac imaging, Edelman describes the two modalities as "quite complementary," with the strength of CT being anatomy and calcium viewing. Evanston uses CT for patients with coronary anomalies, to help determine the risk of coronary stenosis and on patients undergoing RF ablation therapy of the pulmonary veins.

On the other hand, MRI can show in-depth structure and function of the heart muscle, such as the injection fraction or pumping function.

Despite the perceived benefits of cardiac MRI in some applications traditionally reserved for other imaging modalities, Edelman says doctors still will need a "pretty convincing argument to change the standard practice to something new - and that will only happen when large-scale studies show superior accuracy and reduction in study time."


IMAGING AGENTS

Cardiac MRI contrast agents will be one key factor in the transition to increased use of MR in heart imaging, but Edelman does not foresee the influence of imaging agents for another three to five years.

MR also has an advantage because the contrast agent gadolinium is virtually non-toxic and there is no radiation with MR, as there is with radionuclides.

"The major problem with MR," Carr says, "is still claustrophobia."

With current research underway on the development of new MR contrast agents which are tagged with various antibodies or materials designed to localize in certain tissue structures and identify disease processes, MRI also appears poised for the anticipated rise in molecular imaging.

"This will certainly have applications within the heart," says Carr. "What is done now is at the basic research level in animal models. I think down the line you could potentially have a contrast agent which may localize in areas of ischemic myocardium and may be able to direct a more targeted study toward an angiogram."


SMART DECISIONS

So, the bottom line for cardiac imaging and MRI may be just that - what is the best imaging modality for the patient's best interest?

"When you talk about all these tests, what comes into my mind are dollar signs," says NADCC's Frohwein. "I get worried that we spend too much money to identify the same old problems. It is very important that we emphasize good history or exam skills. Unfortunately, those are some of the things that are falling by the wayside when all this technology meets old-fashioned doctoring."

Frohwein says physicians need to determine where studies are most helpful and most cost-effective. If a patient has chest pain and a doctor orders an echo and a stress test, total costs for testing could be as much as $3,000.

"If we can get all that information from either a cardiac MRI or the PET-CT by using one test for $1,500," Frohwein adds, "that's one way of utilizing these tests in a cost-effective manner."