Introduction
Cardiovascular disease is currently the leading cause of death in the United States. American Heart Association statistics show that in 2003, over half-a-million Americans died from coronary artery disease, more than one million had an acute cardiac event, and more than 12 million experienced symptomatic coronary artery disease. Although risk factor profiles may provide an estimation of risk, they are insensitive predictors of actual disease development because of the multitude of causative factors involved. Studies over the last decade have shown that many acute coronary events are due to spontaneous rupture of lipid-rich noncalcified plaques known as “vulnerable plaques.” Accurately determining which patients have these non-calcified or “soft” plaques is helpful for diagnosing and treating patients with sub-clinical atherosclerosis as well as for monitoring their response to medical therapy.
Until recently, definitive diagnosis of coronary artery disease required invasive diagnostic testing including coronary artery catheterization (CAC) with or without intravascular ultrasound (IVUS) examination. CAC, although invasive, was thought to be a very reliable predictor of coronary artery stenosis and occlusion. The development of IVUS revealed that, in fact, the overall plaque burden within the coronary arteries was often significantly greater than the amount identified at CAC. This is due to the fact that the coronary arteries absorb cholesterol-rich deposits in their walls and accommodate them by expanding their outer vessel diameters. This expansion, known as “positive remodeling” results in enlargement of the overall vessel size while maintaining a stable inner luminal size. Because the luminal diameter remains unchanged, the accumulation of these non-calcified plaques is typically undetected on CAC. Although IVUS is useful for identification and quantification of noncalcified plaques, it is an invasive procedure invariably performed in conjunction with CAC. Risks associated with CAC and IVUS include vascular perforation or intimal tear, hemorrhage, embolization due to dislodgement of atherosclerotic plaque by the traversing catheter with possible resultant stroke or infarction, induction of cardiac arrhythmias, risk of infection, and groin hematoma at the catheterization site. Patients who undergo invasive diagnostic testing are routinely immobilized and observed to prevent bleeding from the arterial puncture wound, and thus spend additional time in the hospital.
Noninvasive imaging of the coronary arteries is challenging because of their small size and their constant motion due to the beating of the heart. Coronary artery imaging with computed tomography (CT) requires submillimeter spatial resolution and a high temporal resolution of at least 250 msec. That was not possible until the development of multidetector helical CT scanners (MDCT). In 1998, scanners with four detector rows and subsecond gantry rotation speeds were produced, which provided a promising technology for the non-invasive visualization of coronary arteries. The introduction of MDCT with 16 detector rows in 2002 dramatically improved noninvasive CT coronary imaging. Further improvements occurred with the recent arrival of even higher systems (40-MDCT and 64-MDCT). By allowing submillimeter collimation (0.6 mm) and fast gantry rotation times (as low as 330 msec) these scanners can provide isotropic resolution (similar resolution along all axes) of 0.4 mm for the first time. This, in turn, yields excellent cardiac anatomic definition and the ability to reconstruct the heart and cardiac vessels in any imaging plane. In addition, because of the increased speed of these newer MDCT, cardiac imaging can be accomplished in one short breathold (typically 10–20 seconds).
Coronary CT Angiography (CCTA) (Fig. 1, 2) of the coronary arteries provides several advantages over routine CAC and IVUS. It is a fast, noninvasive examination that requires no sedation and consequently no post-procedure monitoring or recovery, unlike CAC and IVUS. In addition, since contrast administration is via an intravenous catheter inserted in the arm instead of an intra-arterial catheter inserted in the groin, there are fewer potential risks associated with CCTA. A disadvantage is that the radiation exposure for CCTA, (10 to 20 mSv on average), is higher than for CAC (approximately 7–10 mSv).
Fig. 1: Anterior view of the heart rendered from axial CT images showing the coronary arteries and right ventricle.
To begin the CCTA examination, the patient is placed on the CT scanning table, and ECG electrodes are positioned on the chest to monitor the heart rate. CCTA utilizes cardiac gating techniques to improve effective temporal resolution and to minimize imaging artifacts from cardiac motion. Retrospective cardiac gating (in which partially overlapping helical MDCT data is continuously acquired throughout the cardiac cycle while simultaneously recording the ECG tracing) is currently employed. Because image reconstruction necessitates utilization of data acquired during specific segments of the cardiac cycle, significant cardiac arrhythmias are a contraindication to CCTA. In addition, because high heart rates would require a higher temporal resolution, the diagnostic information of the study is significantly compromised with heart rates greater than 70 beats per minute. For optimal imaging heart rates should be in the 50 to 60 beat per minute range. This frequently requires patients to be premedicated with beta-blockers. Nonionic intravenous contrast material (70–100cc Visipaque 320 (Iodixanol) at a 4cc/sec injection rate) is administered via an antecubital intravenous catheter, and scanning is performed through the heart (and bypass grafts if present). Following this scan acquisition, the patient’s vital signs are checked, and the patient is discharged. The patient can immediately resume normal daily activities. The acquisition data are then transferred to a 3-D workstation where the physician and/or technologist creates reconstructed images of the heart and each coronary artery in multiple projections. Typically, the arteries will be evaluated with a variety of volume rendering techniques including multiplanar, MIP, and curved reconstructions. Three-dimensional movies can be created and functional CT analysis can also be performed on the 3-D workstation at this time.
Comparative studies of CCTA, CAC, and IVUS are ongoing, but have so far shown that CCTA is accurate for diagnosing coronary artery stenosis of 50% or greater in >= 2.0 mm diameter segments, with sensitivities ranging from 92–95%. In addition, CCTA has proven to have a high negative predictive value of 97% or greater, thus potentially making it a valuable modality for screening examinations.
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