Diagnostic Imaging Europe
August/September 2004

Imaging News

Multislice CT challenges MR in plaque detection

CT and MR are both powerful tools in detecting early, subclinical vulnerable plaque. They can prove useful in assessing changes in the blood vessel wall and identifying high-risk patients who may benefit from treatment, thereby preventing atherosclerosis.

Since the discovery that the components of atherosclerotic plaque could be characterized by density and signal intensity, multislice CT (MSCT) and high-resolution MR imaging have emerged as alternatives to intravascular ultrasound in the assessment of the blood vessel wall, as well as its lumen. Imaging can characterize plaque in any vascular location, including the coronary arteries.

The pathophysiology of plaque has important implications for imaging, according to Dr. Gabriel C. Fernandez of the diagnostic radiology department at the POVISA Medical Center in Vigo Pontevedra, Spain. Atherosclerotic disease is a generalized disease that commonly involves the compromise of various areas. Half of patients with intermittent claudication have a coronary disease associated with it, and 9% also have a cerebrovascular disease. If several vascular areas are affected, mortality increases. About 90% of patients who have only coronary disease will survive longer than five years, but if new areas are associated or added over time, the survival rate decreases to around 62%.

MSCT scanners are being evaluated not only for their ability to detect and calculate the level of calcium in the coronaries but also for their ability to detect vulnerable plaques in coronary arteries.

"The next generation of MSCT units will improve spatial and temporal resolution, helping to detect plaque and grading stenosis even in the presence of calcium and in patients with limitations of cardiac rhythm or high frequency," Fernandez said at the European Congress of Radiology in March. "Newer MSCT scanners can obtain high-quality images, permitting us to rule out coronary disease in patients with atypical chest pain and to detect soft plaques."

In studies of heart tissue specimens, MSCT can differentiate lipid-rich plaques from fibrous and calcified lesions on the basis of Hounsfield unit densities, according to Dr. Christoph R. Becker, section chief of body CT at Ludwig Maximilian University in Munich. In the coronary vasculature, contrast-enhanced MSCT can depict the location and extent of a thrombus that may be treated in patients with acute coronary syndrome or find atheromas that may be susceptible to rupture in asymptomatic individuals.

Screening for coronary calcifications emerged in 1990, primarily to identify patients with coronary artery disease. But the detection and quantification of coronary calcification may be better suited to estimating the risk for unheralded cardiac events in asymptomatic patients than to identifying symptomatic patients with ischemic heart disease.

"Screening for coronary calcifications is not an established method because of the lack of prospective randomized cohort studies demonstrating its value to predict cardiac events," Becker said. "It is of particular use in patients with an intermediate risk for a cardiac event-10% to 20% in 10 years. Depending on the presence and the amount of coronary calcium, these patients may be reassigned to either a low- (< 10%) or a high- ( > 20%) risk group. "

Assessment of coronary atherosclerosis by contrast-enhanced CT angiography shows promising results. Initial trials indicate that noncalcified plaques are more commonly found in patients with unstable angina, whereas calcified lesions are found predominantly in patients with stable angina. The predictive value of noncalcified plaques has not been determined conclusively and requires further prospective studies.

Radiologists should report the existence of plaque and the percentage of stenosis, as well as any suspicion of the existence of a lipid core implying the diagnosis of a vulnerable plaque, Fernandez said.

Plaques that are exclusively fibrotic-with or without calcium-may also be broken, probably due to a rheologic, or flow, problem. This leads to a penetrating ulcer that causes more profound damage to the wall and is invariably accompanied by an intramural hematoma. If this flow action is important or persistent enough, it will trigger a dissection or will weaken the wall so much that an aneurysm develops. An ulcer and hematoma followed by dissection or aneurysm may cause the wall to rupture.

The process of plaque formation can be repeated, and a vulnerable plaque that breaks its lipid center and provokes thrombosis may be clinically imperceptible. The thrombus becomes fibrous, building a structure that is re-endothelialized. It produces rapid growth of the plaque and leads to stenosis, which again generates flow disorders that damage the new endothelium, restarting the cycle, he said.

MR imaging has been used experimentally for early diagnosis of arterial disease, introducing concepts such as atheroma plaque in vulnerable or high-risk phase. Early diagnosis is important for characterizing the plaque and selecting treatable patients before the occurrence of ischemic attack. It enables doctors to distinguish lesions that are reversible by medical treatment from those that require surgical or endovascular treatment, he said.

"MRI allows for the evolutionary checkup of the atheroma plaque and its response to treatment. It shows the plaque in an early phase and distinguishes the lipid-rich plaque with higher risk for disruption," Fernandez said.

High-resolution MRI has traced the size of plaque lesions, illuminated their lipid core, and assessed the integrity of their fibrous cap. Because external coils provide only limited signal penetration, however, visualization of the components of the vessel wall has required the use of intravascular coils. MR investigation of ves-sel walls is also time-consuming. Resolution has improved because of advances in dedicated coil technology, but plaque-specific contrast agents will be needed to improve noninvasive detection of vulnerable lesions.