CT and MR compete to visualize vessels

Advances in CT and MR technology have led to increased accuracy in cardiovascular disease detection, spurring unprecedented growth in noninvasive cardiovascular imaging. While each modality offers clear advantages over the other in certain clinical applications, gray areas exist where clinicians continue to debate which is the better approach.

The annual meeting of the North American Society for Cardiac Imaging offered a forum for that debate. In one corner, Dr. Lawrence M. Boxt, chief of cardiovascular medicine at Beth Israel Medical Center in New York City, presented evidence in favor of CT. In the other corner, Dr. J. Paul Finn, a professor of radiological sciences at the University of California, Los Angeles, endorsed MR. Refereeing the event was Dr. Thomas Grist, chief of MRI at the University of Wisconsin Medical School in Madison. At the end of the evening, the only clear winner was technology, which will continue to advance and improve the capabilities of each noninvasive imaging modality.

Both sides agreed that CT angiography and MR angiography have largely replaced conventional catheter angiography for diagnosing vascular disease. CT provides accurate information about the vessel lumen and wall, as well as extravascular information pertinent to the vessel in question, Boxt said. The ease of the exam and rapid patient throughput further increase the value of CTA, and the spatial resolution of multislice CTA rivals that of digital angiography.

CTA is a standard tool for evaluation of acute aortic disease, and it is essential in the diagnosis of acute aortic dissection and the diagnosis and management of patients with aortic aneurysms. It also has been crucial to diagnosing and following patients with acute intramural hematoma and penetrating atherosclerotic ulceration of the aorta.

"Not only can we measure the caliber of the aorta at different levels with CTA and use that as an index of the severity of disease, but we can look at the wall of the aorta," Boxt said. "The problem is not the lumen; it's the wall. Our understanding of the atherosclerotic process relates to the changes in that wall."

Moving down the body, Boxt noted that MSCTA allows for a complete examination of the extremities. It has an accuracy rate of 95% in the diagnosis of stenoses greater than 50% in the lower extremities and high sensitivity and specificity for detecting obstructive disease, although its accuracy below the knee needs improvement. Overall, shorter scan durations with MSCTA allow for more consistent vessel enhancement, use of less contrast material, increased accuracy, and improved safety.

Finn was not daunted by CTA's virtues as chronicled by Boxt. MRA, he said, can confirm or exclude narrowing, occlusion, ulceration, aneurysm, or dissection of arteries in most vascular territories, including the carotids, vertebrals, thoracic aorta, subclavian arteries, abdominal aorta, renal arteries, mesenteric arteries, iliac arteries, and lower extremity arteries. It can also diagnose occlusion or thrombosis of veins and image the pulmonary arteries for chronic thromboembolism, although CTA is more commonly used for suspected acute pulmonary embolism.

The combination of MRA and MRI can often be used to diagnose and grade vascular disease and assess the effect of end organs. For example, severe carotid disease diagnosed on MRA may be responsible for brain infarction seen with MRI. Arterial occlusive disease in a diabetic patient's legs may be diagnosed with MRA, and complicating osteomyelitis may be visualized with MRI. Renal artery stenosis diagnosed with MRA may be associated with a delayed nephrogram suggesting diminished renal perfusion, Finn said.

Speed, spatial resolution, and temporal resolution can be traded off in more or less arbitrary ways. Single-phase, high-spatial-resolution imaging can be combined with rapid, time-resolved 3D imaging in a single sitting. This enables functional angiography to be added to a detailed vascular map.

HEART OF THE MATTER

Generations of CT scanners have had adequate spatial and contrast resolution for diagnosis and characterization of many forms of pericardial, myocardial, and valvular heart disease. But CT's limited temporal resolution limited its clinical value to showing incidental calcification, pericardial effusion and hemorrhage, and the occasional cardiac or paracardiac mass.

With recent advances in CT technology, however, scanners can produce excellent diagnostic images of the beating heart. CT has since been evaluated for assessment of the myocardium, myocardial perfusion and viability, cardiac function and wall motion, heart valves, and cardiac tumors. It has considerable competition, however, from echocardiography and MR for these applications, Finn said.

CT has no noninvasive competitors in imaging the coronary artery tree. With both speed and high resolution, it can identify, characterize, and quantify atherosclerotic lesions and total disease burden within the coronary arteries.

Finn countered Boxt's presentation by noting that MR is not only excellent for assessing cardiac function, myocardial thickness and composition, and myocardial scar, but that it is becoming increasingly competitive for imaging myocardial anatomy and perfusion. Several studies have endorsed the accuracy of dynamic myocardial perfusion imaging with MR, mostly in combination with adenosine or dipyridamole stress. Chiu et al showed a 90% accuracy in detecting angiographically significant coronary artery disease with first-pass perfusion imaging using inversion recovery steady-state free precession (Radiology 2003;226:717-722).

Although its role in imaging the coronaries is still evolving, MR has proved effective for evaluating anomalous coronary artery anatomy.

The current state of CT and MR enables clinicians to image cardiovascular morphology and function simultaneously. Cardiac and vascular questions can be answered in one sitting, replacing invasive diagnostic methods while improving the quality of information. The future of cardiovascular imaging most likely will include both CT and MR, as well as methods currently being researched and others not yet even on the drawing board.