MRI sequences evolve to assess flesh and flow

Radiologists use a number of MR imaging techniques to evaluate cardiac morphology and function: free-breathing and breath-hold methods, black blood imaging, standard contrast-enhanced MR angiography, and a fast version called time-resolved MRA.

Breath-hold turbo spin-echo techniques with double inversion pulse-to greatly reduce signal from the blood pool-are the workhorses to evaluate morphology, according to Dr. Charles Higgins, vice chair of radiology at the University of California, San Francisco. These sequences are performed with multiple slices going through the great vessels and the heart. Contrast-enhanced 3D MRA also can be used to evaluate morphology. In the chest or heart applications, 3D MRA images are generally acquired in either a coronal or sagittal plane.

Techniques for evaluating function have evolved from cine gradient-free-breathing and breath-hold versions-to hybrid echo-planar imaging sequences that allow a more rapid acquisition in cine sequence to steady-state free precession techniques (FFE, TFE, TrueFISP), which balance the gradients in all three planes to produce homogeneous signal from the blood pool during the entire cine acquisition period. These sequences are used to assess left and right ventricular function, regional function, and valvular function.

Regional ventricular function is assessed by wall motion evaluation, but cardiac MR also reveals wall thickening, which is a better way to assess regional function and wall motion, Higgins said. Cardiac MR has surpassed transesophageal echocardiography for evaluating valvular function. By acquiring images as rapidly as 40 frames per cardiac cycle (within a 12-second breath-hold period), one can see mitral and aortic valve motion.

To measure blood flow, Higgins and colleagues use quantitative phase imaging, or velocity-encoded cine MR. They initially used this technique to quantify the amount of valvular regurgitation. Forward flow can be measured in systole and backward flow in diastole when images are acquired in about 16 frames per cardiac cycle, evenly spaced.

"This is one of the few techniques that can accurately measure the amount of valvular insufficiency," he said.

Referrals to assess patients with congenital heart disease are common, particularly to evaluate the severity of pulmonary insufficiency in patients who have had tetralogies of Fallot repair. The velocity-encoded cine MR technique measures blood flow in the direction of the pulmonary arch.

To assess tumors, Higgins suggests a wide field-of-view, a morphologic technique that is ECG-gated to every heartbeat, and the acquisition of axial images before and after gadolinium administration.

"We always do a cine MR sequence, which can tell us whether a mass within the heart is a blood clot or a tumor-perhaps one of the biggest differentials one has to make," he said.