Cardiac computed tomography (CT) has evolved greatly over the last 20 years. This article reviews its current clinical uses, and describes some of the potential for even greater utility in the near future. Electron beam tomography (EBT) was developed 20 years ago specifically for cardiac imaging. Although the technique can quantify ventricular anatomy and function as well as myocardial perfusion it is currently best known for defining and measuring coronary calcified plaque (coronary artery calcification (CAC)). Over the past decade, there have been well over 1,000 articles published regarding EBT and coronary artery imaging. Standardized methods for imaging, identification and quantification of CAC using EBT have been established. The scanner is operated in the high-resolution, single slice mode with continuous, non-overlapping slices of 3mm thickness and an acquisition time of 100msec per tomogram. Electrocardiographic triggering is carried out during end-systole or early diastole at a time determined from the continuous ECG tracing carried out during scanning.
Since 1999, multidetector CT has been utilized to evaluate the heart. Advances in temporal resolution and imaging protocols have allowed cardiac applications to become more routine.The newer generations of multi-detector-row (MDCT) systems are capable of acquiring 16-64 sections of the heart simultaneously with ECG gating in either a prospective or retrospective mode. In the current 16-row MDCT systems, 16 sections can be acquired at either 0.5-0.75mm or 1-1.5mm section widths or eight sections 2.5mm thick. Thin slices (sub-mm) as well as rapid acquisition allow for cardiac imaging with reduced motion artifacts.
Coronary Calcium Assessment
During the past decade, there has been a progressive increase in the clinical use of EBT scanners to identify and quantify the amount of calcified plaque in the coronary arteries. CAC can be quantified and calcium scores can be related to extent and severity of atherosclerotic disease and improving coronary heart disease (CHD) risk prediction. A positive EBT study (presence of CAC) is nearly 100% specific for atheromatous coronary plaque. Since both obstructive and non-obstructive lesions have calcification present in the intima, CAC is not specific to obstructive disease; thus, a positive EBT does not always imply significant stenosis. However, the presence of CAC is extremely sensitive for obstructive (more than 50% luminal stenosis) coronary artery disease ((CAD) 95% to 99%). For clinicians, evidence of CAC is highly sensitive but less specific for obstructive CAD. Importantly, EBT studies of over 5,500 symptomatic patients demonstrate negative predictive values (NPVs) of 96% to 100%, allowing physicians a high level of confidence that an individual with no coronary calcium (score=0) has no obstructive angiographic disease.