Computed Tomography and Magnetic Resonance Imaging for Noninvasive Coronary Angiography and Plaque Imaging: Current and Potential Future Concepts

Abstract

A therothrombosis is a systemic disease of the vessel wall that causes distinct clinical manifestations, depending on the affected circulatory bed and the characteristics of the individual lesions. 1 These lesions may be quite heterogeneous. 1 Thus, the clinical manifestations of atherothrombosis of the coronary arteries, of the arteries supplying the central nervous system, of the aorta, and of the peripheral circulation can be significantly different.Disruption-prone plaques in the coronary arteries, the so-called "vulnerable plaques," tend to have a thin fibrous cap (cap thickness Ϸ65 to 150 m) and a large lipid core (American Heart Association [AHA] plaque type IV-Va).Acute coronary syndromes often result from disruption of a modestly stenotic vulnerable plaque, not visible by x-ray angiography, which results in a thrombotic complication (AHA plaque type VI).During its evolution, a type Va plaque may also become fibrotic (AHA plaque type Vc) or calcified (AHA plaque type Vb). 2,3In contrast to coronary artery vulnerable plaques characterized by high lipid content and a thin fibrous cap, high-risk plaques of the carotid arteries tend to be fibrotic and severely stenotic. 3 Imaging of Atherothrombotic DiseaseBecause there is striking heterogeneity in the composition of human atherothrombotic plaques, even within the same individual, reliable noninvasive imaging tools that can detect early atherothrombotic disease in the various regions and characterize the composition of the plaques are clinically desirable. 3Such imaging tools would improve our understanding of the pathophysiological mechanisms underlying atherothrombotic processes and allow us to better risk-stratify the disease.Additionally, such tools may permit optimal tailoring of treatment and allow direct monitoring of the vascular response.Presently, a number of imaging modalities are employed to study atherosclerosis; most identify luminal diameter or stenosis, wall thickness, and plaque volume. 3Two noninvasive imaging modalities, computed tomography and MRI, have been introduced to the study of atherothrombosis.They allow identification of flow-limiting coronary stenoses, calcified plaques, imaging of the atherothrombotic lesions directly, measurement of atherosclerotic burden, and characterization of the plaque components. 3Together, by revealing the degree of stenosis and the plaque composition, they provide information that may predict cardiovascular risk, facilitate further study of atherothrombosis progression and its response to therapy, and provide for assessment of subclinical disease. Computed Tomography MethodsThe cardiovascular system can be imaged with the use of two different computer tomography (CT) modalities: one employs nonmechanical movement of the x-ray source (ie, electron beam CT) and the other involves the motion of the x-ray source and table, combined with multiple detectors to acquire the data in spiral or helical fashion (ie, multidetector-row CT). Electron-Beam CTThe necessity for very short image acquisition times to virtually freeze cardiac motion urged the development of a cardiac-dedicated CT system in 1982 on the basis of a nonmechanical movement of the x-ray source and fixed detector arrays.The electron-beam CT (EBCT) uses a single, curved anode with 4 tungsten targets underneath the patient, and a focused electron beam that is rapidly swept across these targets to produce an x-ray fan beam detected by 2 detector rows above the patient (Table 1). Multidetector-Row CTMechanical multidetector-row CT (MDCT) systems were introduced in 1998, and allow for scanning with one x-ray tube and 4 detector rows in a single gantry rotating twice per second around the patient.Continuous gantry rotation and table movement causes the projection data to be obtained along a spiral or helical path (Table 1).