Cardiac Computed Tomographic Angiography - A Diagnostic Revolution

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Since 1999, cardiac computed tomographic angiography (CCTA) has evolved into the most innovative approach to cardiac imaging to emerge in over 30 years. The adoption and application of this remarkable technology represents a true revolution in the evaluation of patients for cardiac problems. The evaluation of patients with potential cardiac problems traditionally uses a variety of non-invasive or invasive procedures. Non-invasive procedures include treadmill or pharmacologic stress testing with or without perfusion imaging with a radioactive isotope. While they have been good prognosticators, a sizable number of falsely positive and falsely negative results typically plague these imaging modalities. Cardiologists resort to invasive coronary angiography to resolve diagnostic dilemmas that cannot be answered by non-invasive techniques. Invasive angiography is accurate but carries a small but significant risk of complication and is costly and time consuming. In addition, invasive angiography images only demonstrate the interior lumen of the coronary artery, thus providing little or no information about atherosclerotic changes within the vessel wall. CCTA offers as a cost-effective non-invasive imaging modality that has the potential to overcome many of the limitations of traditional techniques.

CCTA has its foundation in coronary calcium scoring, which provides a quantification and measurement of calcified coronary plaque. Numerous scientific publications have documented the relationship between coronary artery calcification (CAC), the extent of coronary atherosclerosis and the attendant risks of this disease process. The presence of CAC provides information that is additive to standard risk factors and predictive data regarding future cardiac events. CAC is not, however, specific to obstructive coronary disease. Hence the development of CCTA using multi-detector scanners, which have improved temporal and spatial resolution, allowing non-invasive coronary angiography. Since 1999, multi-detector scanners have increased from four to 64 detectors with 256-detector and flat panel devices on the horizon. This progressive evolution of scanners with higher resolution will make the technology more accessible to a larger patient population and allow its application to an increasing number of clinical scenarios.

Atherosclerotic vascular disease in general and coronary artery disease (CAD) in particular remain the leading cause of morbidity and mortality in the industrialized world. Early detection and treatment remain cornerstones of therapy. CCTA's unique combination of high spatial and temporal resolution allows visualization of not only the coronary lumen but also atherosclerotic plaque and associated vessel stenosis. Furthermore, CCTA allows characterization of atherosclerotic plaque based upon tissue attenuation measured with CT Hounsfield units. Early studies suggest that plaque density as well as volume can be assessed with CCTA.

Numerous clinical studies have documented the accuracy of CCTA. Early systems using 4- and 16- detector scanners were accurate in excluding disease but were limited in their ability to quantify disease. This limitation was primarily the result of motion artifact secondary to prolonged scan time and cardiac movement. Despite this problem, early studies documented a sensitivity from 83% to 99%, a specificity between 93% and 98% and a negative predictive value (NPV) of 95-100% when compared with invasive coronary angiography. Current-generation 64-detector scanners have maintained the excellent negative predictive capacity and have improved upon the positive predictive value (PPV). Sixty-four-detector scanners improve spatial resolution with thinner collimation (0.5mm) and improve temporal resolution with faster gantry rotation (0.33 seconds), thereby providing greater detail of the coronary arteries with limited artifact from cardiac motion. Further developments in scanner technology have included additional detectors (up to 256) and the placement of a second detector array in a single gantry (dual-source CT), which reduces scan times and decreases temporal resolution. These advances will allow for accurate visualization of smaller caliber vessels at higher heart rates without attendant motion artifact. Reduced scan times translates to shorter breath hold, increased patient comfort, decreased need for beta-blockade, and improved throughput.

The confirmation or exclusion of CAD is the primary clinical application of CCTA. In experienced centers, the sensitivity of CCTA in detecting CAD in major epicardial vessels is excellent. Heart rate control and stabilization, coronary vasodilation with nitrates and avoidance of patients with excessive calcium burden (CAC scores in excess of 600 Agatson units) are all important factors in scan acquisition.

Clinical scenarios most appropriate for CCTA include patients with low to intermediate pre-test probability of CAD, equivocal stress-perfusion testing, suspected coronary artery anomaly, and patients with multiple risk factors for CAD. Because of its superior negative predictive power, CCTA is increasingly being utilized as the diagnostic entry point for these types of patients. Patients with a high likelihood of CAD or those with an unstable presentation are unlikely to benefit from non-invasive angiography using CCTA.

Patients with known CAD comprise a subset also suitable for evaluation with CCTA. Coronary artery bypass grafts, due to their large size and relative lack of motion, can easily be visualized non-invasively with CCTA. The precise localization of graft origins, their course and anastomotic sites to native vessels are easily determined from reconstruction of a single data set. Internal mammary grafts can be tracked from their origin at the subclavian to their anastomosis; metallic clips often hamper interpretation at the anastomotic site. Coronary arteries that have been stented can be analyzed with CCTA, but certain limitations must be observed. In-stent restenosis cannot reliably be determined unless the stent diameter is greater than 3.0 mm. Artifacts from metal stent struts impose additional challenges to accurate interpretation, but the next generation of CT scanners will help overcome this problem. Patients who have undergone cardiac transplantation are another group that may benefit from non-invasive coronary angiography. Due to their high risk for development of vasculopathy and lack of symptoms, invasive angiography is often required on a recurring basis to exclude this problem. Recent publications document the ability of CCTA to detect coronary vasculopathy in post-transplant patients without the need for invasive procedures. There is growing interest in the use of CCTA to evaluate patients in the emergency room setting who present with chest pain suggestive of angina. Because of its excellent negative predictive power, CCTA in the emergent setting offers the promise of efficient and accurate diagnosis or exclusion of CAD as an etiology for symptoms of chest pain. Several preliminary clinical trials have documented impressive reductions in time to diagnosis, time to treatment and overall cost for care of this patient population. Large-scale multicenter trials are under way to confirm clinical outcomes in this application of CCTA.

Functional and structural assessment with CCTA is far more comprehensive and detailed than traditional angiography and, in some respects, superior to echocardiography. A host of non-coronary findings can also be identified with CCTA. This comprehensive ability lies in the fact that data is acquired for the whole heart throughout the entire cardiac cycle; often in less than 15 seconds.All of the data sets in a full cardiac cycle can be reconstructed then presented as a cine loop. These images can be displayed in standard angiographic projections (right (RAO) or left anterior oblique (LAO)) or echocardiographic views (four-chamber, short-axis, and parasternal long axis). This ability allows assessment of right and left ventricular function, valvular motion and septal defects. Segmental wall motion analysis can be carried out to identify myocardial hypertrophy, thinning and infarction. Pathologic changes associated with myocardial infarction (MI) or dysplasias are identified as hypodense areas within the myocardium. Ventricular volume over time is used to calculate ejection fraction and provide an estimate of cardiac output.

Valvular heart disease, as well as a variety of septal defects and cardiac masses, can be detected and evaluated with CCTA. Valvular stenosis is suspected based upon the presence of valvular calcification or leaflet thickening. Accurate quantification of valvular area is possible using all of the data sets from a full cardiac cycle. Peak systolic excursion is obtained and valve area can be planimetered with a valve area calculated. Bicuspid configuration of the aortic valve is easily demonstrated with CCTA. Aortic regurgitation is suggested by an incomplete approximation of the valve leaflets, but cannot be quantified by CCTA techniques. By observing the mitral valve carefully throughout the cardiac cycle detection of both prolapse and stenosis are possible. Some centers have explored the use of CCTA as a replacement for invasive angiography pre-operatively in patients preparing for valvular surgery and the results are promising. Intra-cardiac thrombi, tumors, lipomas, pericardial calcification and effusion, and prosthetic valves are also assessable with CCTA.

Conclusion

CCTA continues to evolve as new technological advances are made. As scanners continue to improve, the ability to non-invasively image coronary anatomy will become a reality for most patients. Already a large number of patients can use this modality as their diagnostic entry point for evaluation of possible angina. While not a universal replacement for invasive angiography, CCTA is certainly an alternative for selected patients. CCTA also provides a diagnostic alternative for a wide variety of non-coronary pathologic conditions.With rapid technological advances in CT, the widespread use of this technology in a more general patient population is on the horizon.