64 slices let CT join MR in a two-stop cardiac shop

November 2, 2005
Lawrence M. Boxt, MD

Progress in the management of patients with heart disease has always been closely tied to radiologists' expertise in cardiac imaging. But we have to move away from lumenology and toward a more cellular and molecular focus to fully appreciate modern noninvasive cardiac imaging.

Progress in the management of patients with heart disease has always been closely tied to radiologists' expertise in cardiac imaging. But we have to move away from lumenology and toward a more cellular and molecular focus to fully appreciate modern noninvasive cardiac imaging.

After Roentgen discovered the x-ray in 1898, radiologists quickly found plain-film examination of the heart useful to diagnose valvular and myocardial heart disease. The ability of angiocardiography to document specific intracardiac defects played a central role in the dramatic growth of cardiac surgery to palliate and cure congenital malformations of the heart. The development of coronary artery bypass surgery and, subsequently, percutaneous angioplasty and stent placement-all of interventional cardiology, in fact-is derived directly from catheter-based, film-recorded cineangiocardiography.

Progress in the diagnosis of ischemic and congenital heart disease reflects ongoing developments and improvements in echocardiography, nuclear cardiac imaging, MRI, and now ECG-gated multislice CT.

Our clinical paradigm for managing patients with atherosclerotic cardiovascular disease is based on the association between reduction of arterial luminal narrowing shown on imaging and relief of clinical symptoms. In other words, we select patients for their chest pain syndromes. Those who have luminal coronary arterial stenoses are traditionally treated by coronary artery bypass graft surgery and more recently by percutaneous angioplasty and stent placement.

This approach brings symptomatic relief. Except in the case of CABG surgery for left main coronary stenosis, however, we obtain no improvement in patient longevity. We improve the quality of life by removing symptoms, but we do not alter the quantity of life.

The focus on luminal narrowing and its relationship with coronary artery blood flow reveals only part of the picture. Our increased understanding of the cellular and molecular biologic basis of atherosclerotic disease has shed a great deal of light on earlier observations made using coronary arteriography. These observations were correct, but they were initially considered less important and only really appreciated in retrospective analysis of a series of angiograms. The physical limitations of angiographic methods were insensitive to the morphologic changes needed to support new and evolving theories of atherogenesis; there was no imaging correlate of the "new" atherosclerotic process.

64-SLICE CT UPS THE ANTE

Current MR and CT scanners sample volumes of space and are very sensitive to calcium and fat deposition. This allows high-resolution data set acquisition that can be displayed in arbitrary tomographic sections, 3D representations, or visualization of an organ. The ability of these scanners to resolve fat and calcium supports their value in plaque characterization. Observations made on MR and CT scans reflect our understanding of fatty plaque formation and the remodeling changes that result in their characteristic non-flow-limiting stenoses.

Medicine is undergoing a paradigm shift in our understanding of atherosclerosis. This is reflected in how we detect and manage atherosclerotic heart disease. The developing concept of the fat-laden plaque at risk helped explain some angiographic observations made in patients with chest pain syndromes. What we now call fatty plaques were probably observed on older cineangiograms but were ignored or considered of minimal consequence. These "lumpy-bumpy" lesions rarely limited flow, so they did not produce flow-limiting stenosis (Figure 1). No regional myocardial ischemia was induced, and no chest pain was experienced. Only at the time of plaque rupture and acute coronary thrombosis do these lesions become symptomatic.

Yet to improve longevity, we must address fatty plaques. The shift from symptomatic relief based on luminal narrowing to prevention and aggressive medical therapy supported by arterial wall (plaque) imaging will mobilize our resources toward identification of patients at risk, methods of early detection, efficient and consistent means of image interpretation, and clinical outcome analysis. At the heart of this shift lies arterial plaque characterization.

The ability of MRI to provide accurate, reliable image and physiologic data in a clinically timely manner will determine its niche in the armamentarium of cardiac imaging modalities. From its early development and application, MRI has been praised as a noninvasive, multifunction cardiac imaging technique. Its image-derived morphologic and physiologic data, invaluable for the evaluation of children and adults with congenital heart disease and adult patients with ischemic heart disease, have been helpful. But cardiac MRI remains a rather sparsely practiced subspecialty that is widely unavailable, and clinical practitioners are not familiar with its utility.

The development of the electron-beam CT scanner began in earnest the application of CT technology to the problem of cardiac imaging. The ability to acquire image data rapidly enough to obtain sharp, clear images of the cardiac chambers and myocardium encouraged further applications of digital computer technology to medical imaging. Both EBCT and multidetector spiral CT produce a data set reflecting the 3D distribution of tissue attenuation of x-ray photons. The ability of MR and CT to identify and characterize atherosclerotic plaque will support their role in our new model of cardiovascular disease.

The great advance of CT over MR, and, I believe, the landmark advance of the 64-detector CT scanner, is reflected in the data set obtained. The x-ray photon exposure a patient receives during an ECG-gated MSCT exam produces an image data set of contrast and temporal resolution equal to or better than that obtained from the microwave exposure in an MRI exam. The isovolumetric voxels generated by 64-slice contrast-enhanced ECG-gated spiral CT, however, provide the substrate for a geometric (fourfold) leap in spatial resolution (Figure 2). More important, the acquisition of cube-shaped image elements allows distortion-free reconstruction in planes away from the axial acquisition (Figure 3). Furthermore, 3D reconstructions will more accurately reflect the appearance of the anatomic structure than does an angiographic projection image or conventional MR tomographic display.

LOOKING FOR LONGEVITY

If this improvement results in management strategies that improve longevity, it will support the shift from organ imaging to organ visualization. The visual display reflects what the organ looks like, rather than a parametric representation of some morphologic or physiologic aspect of the organ. MRI and CT can produce such imagery. Differences in patient throughput, image quality, and confidence in and accuracy of diagnosis will affect clinical indications. The future role of both modalities will depend on their ability to affect clinical outcome as well as on the economic, professional certification, and radiation safety issues related to their practice.

We now have a two-stop shop. Two noninvasive, multifunction cardiac imaging techniques are available for use. Issues concerning clinical experience, cost, patient risk, and effect on clinical outcome will nudge each modality into a clinically relevant, market-based imaging niche. In these defined and supportable arenas, MR and CT imaging of the heart will play an important role in our new approach to cardiovascular disease.

Dr. Boxt is director of cardiac MRI and CT in the cardiology division and radiology department at North Shore University Hospital in Manhasset, NY.