Diagnostic Imaging
March 2003

SPECIAL SECTION: CARDIOVASCULAR IMAGING

Cardiac imagers assess clinical value of MR, CT

CTA shows potential in coronaries, while MRI excels with infarctions

A more complete understanding of arteriosclerosis has created a need for more powerful ways to examine the coronary arteries.

Coronary artery MR angiography and multidetector CT angiography are leading candidates to supplement diagnostic cardiac catheterization, whose role is limited mainly to diagnosing coronary atherosclerosis and measuring the degree of stenosis.

In the past five years, MRA and multidetector CTA have become far more versatile than catheterization. Researchers have overcome MRI's poor temporal resolution with faster pulse sequences, parallel signal processing techniques, and cardiac and respiratory gating techniques that freeze heart motion. Although users still have trouble visualizing distal vessels with MR, they are gaining easier access to the modality's exquisite contrast resolution. Multidetector design is the key to CT's increasing prominence in cardiac imaging. The modality's performance improved between 1999 and 2002, as dual-detector scanners capable of two slices per second were replaced by 16-detector systems that acquire more than 30 slices per second.

To gain a sense of each modality's merits, organizers of the 2002 RSNA meeting invited three early adopters of coronary MRA and CTA to compare notes. Dr. Michael V. McConnell, director of cardiovascular MRI at Stanford University, described the expanding applications of MRI in the coronary arteries. Dr. Christoph R. Becker, CT section chief at the University of Munich's Grosshadern Clinic, recounted his institution's experience with cardiovascular CT. And Dr. Richard D. White, section head of cardiovascular imaging research at the Cleveland Clinic, described how his group uses both modalities.

MRI's attractiveness, according to McConnell, stems from its versatility, noninvasiveness, and lack of ionizing radiation. Blood flow, blood velocity, or traditional T1 and T2 contrast mechanisms can be used to differentiate blood, fat, muscle, and arteriosclerotic plaque in the coronary arteries.

A single-vendor multicenter trial conducted in 2001 by Dr. Warren Manning's group at Beth Israel Deaconess Medical Center concluded that MRA is good enough to reliably evaluate the proximal coronary arteries in most cases, McConnell said. (Kim WY, Danias PG, Stuber M, et al. Coronary magnetic resonance angiography for the detection of coronary stenoses. NEJM 2001;345[26]:1863-1869).

The results from 109 patients scanned before elective coronary angiography revealed sensitivity and specificity rates of 83% and 73%, respectively, for MRA in detecting coronary stenoses. The negative predictive value was 81% for any disease, but 100% for left main or three-vessel disease.

The investigators visualized an average of 8 cm of the right coronary artery (RCA), 5 cm of the left anterior descending artery (LAD), and 3 cm of the left circumflex coronary (LCx), McConnell said. All four patients whose disease was missed by MRI had single-vessel disease, and two of these stenoses were located in the left circumflex artery.

These results suggest that coronary MRA may not be ready to replace cardiac catheterization for definitive diagnosis of coronary artery disease, but they appear to be good enough to exclude severe, multivessel disease, McConnell said.

"It will also be particularly valuable for patients who present with cardiomyopathy, where the diagnosis is typically severe coronary versus no coronary disease. Based on these data, one should be able to distinguish between these patient subsets," he said.

ASSESSING CORONARY CTA

Although CTA differs from MRA in its strengths and weaknesses, the two modalities have a similar ability to detect coronary artery disease, according to Becker. Several studies have found that the negative predictive value of MDCT to rule out coronary artery stenoses is between 96% and 98%, sufficient to identify patients who do not require cardiac catheterization. Its positive predictive values range between 60% and 85%, suggesting that more refinements are needed before it could replace catheterization.

Those improvements may come soon, with protocol improvements and the diffusion of 16-slice scanners into the market, Becker said. His group learned that even with the clinic's new 16-slice scanner, the patient's heart rate must fall below 60 beats per minute to achieve excellent imaging quality during coronary CTA. To this end, Becker administers 50 to 200 mg of the beta-blocker metoprolol before imaging.

To minimize the radiation dose, Becker's group uses ECG gating and slow table speed to produce a low pitch of 0.3. A continuous scan is performed between diastole and systole, but reconstruction is limited to the images acquired during the diastolic phase. Tube current is ramped up during diastole and down during systole to reduce unnecessary exposure.

Scan times on the clinic's new 16-slice scanner are between 10 and 20 seconds, twice as fast as its older four-slice system, Becker said. Full 360 degrees detector rotation is completed in 0.42 second. To compensate for higher image noise, the amperage was increased from 400 to 500 mAs, producing 5 mSv in effective radiation exposure.

The University of Munich staff uses the added speed to gain more control over the contrast bolus. The right coronary artery, for example, may be examined while it is brilliantly enhanced and no contrast resides in the right ventricle or atrium to degrade the study.

Projection planes are selected to mimic the views seen with cardiac catheterization. Slice reconstructions along the long axis of the heart are also performed to create a spider view that reveals the branches of the left coronary system.

EVALUATING PLAQUE STABILITY

The interrogation of coronary plaque density with CTA has factored into several cases at the Grosshadern Clinic. Early experience suggests the presence of atheroma when there is a mismatch between the results of the CTA study and the angiogram and when the plaques are homogeneous, well-defined, and have intermediate density of about 50 HU, Becker said.

Stable fibrocalcified plaques are characteristically homogeneous, well-defined, and very dense (about 100 HU). Becker showed an example that revealed a layer of calcium in the border between the intima and the medium.

So-called calcified nodules constitute another class of coronary artery abnormality that may be examined with CTA. These small, hard nodules in the coronary artery wall were thought to be harmless until Becker's group saw one evolve into a high-grade stenosis. A typical calcified nodule was found next to a soft, irregular, inhomogeneous mass in the left anterior descending artery.

CLEVELAND CLINIC EXPERIENCE

Radiologists at the Cleveland Clinic regularly use CTA and MRA to evaluate patients in a caseload that leads to 7900 open-heart surgeries annually. CTA routinely supplements cardiac catheterization, while MRI is employed for left ventricular quantification and myocardial perfusion studies. Both modalities help physicians interrogate structural abnormalities, White said.

CTA is increasingly favored at the Cleveland Clinic for patients with classic angina or other symptoms suggesting coronary artery involvement. Coronary CTA is used to investigate the components of arteriosclerotic plaque and to find evidence of arterial remodeling. It aids the diagnosis of coronary artery fistulas, aneurysms, and sarcomas, and is the modality of choice for diagnosing pulmonary emboli in ventricular chambers.

MRI's greatest potential contribution lies in the assessment of ventricular functional and myocardial infarction, he said. Ventricular assessments with MR are regularly performed instead of conventional imaging. Three-D MRI is used for global exams of left ventricular function.

"We map out the flow from the ascending aorta to get two separate measures of stroke volume at the ventricular and aortic levels," he said.

Although CT shows promise for quantifying ventricular function, MR is easier to use, according to White. MR also has an edge over CT in measuring regional myocardial dysfunction.

MR myocardial assessments are routinely performed in place of conventional imaging, White said. Cine MR, first-pass perfusion, and delayed enhancement are used to outline regions of myocardial necrosis. MR delineates transmural and subendocardial infarction.

"We do some scoring with a 17-segment model to rate the severity of scarring. This can be useful because we can show that with more than 50% scarring, the likelihood of recovering function by revascularizing the area is very low," he said.

Fusion imaging combining coronary CTA and myocardial perfusion MRI is performed daily at the Cleveland Clinic. It takes about an hour to superimpose CTA images of the coronary arteries over a myocardial viability map acquired with MRI. The presentation enables the radiologist to attribute myocardial damage directly to the coronary artery that is causing the ischemia, White said.

The experience of White, Becker, and McConnell underscores the progress in the development of CT and MR as mainline cardiac imaging tools. Radiologists made a major contribution to the original adaptation of x-ray angiography for coronary artery imaging, White said, and they should continue to play an important role in inventing better ways to image the heart.