New Procedures and Therapies

Site Map

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Coronary Artery CT Angiography: A new method for non-invasive evaluation of coronary artery stenosis

Robert Optican, MD

Introduction:
Non-invasive evaluation of coronary artery stenosis has until recently consisted of indirect analysis. Such methods include nuclear stress imaging, stress echocardiography, and other less accurate (stress treadmill) and less available (stress MRI) methods. Until the advent of the 21st century, reliable direct imaging of the coronary arteries required catheter-based coronary angiography. New developments in computed tomography over the past few years have enabled direct visualization of coronary stenosis.

Background:
The ability to non-invasively image the coronary arteries directly has been hampered by inherent difficulties with individual imaging modalities. The size of the coronary arteries requires excellent spatial and contrast resolution while the rapid motion of the coronary arteries during the cardiac cycle demands excellent temporal resolution. Echocardiography, a useful tool for the evaluation of structural abnormalities of the myocardium and valves, lacks the spatial and contrast resolution for coronary artery visualization. CT and MRI have much better spatial and contrast resolution but until recently have suffered from lack of temporal resolution sufficient to image the coronary arteries.

State of the art multislice CT now allows temporal resolution in the range of 160-180ms. In patients with regular heart rates below about 60 beats per minute, this degree of temporal resolution allows the stoppage of coronary motion.

Fig. 1-3

48 y/o female with atypical chest pain and indeterminate nuclear stress test. No stenosis demonstrated in the right, left anterior descending, or circumflex coronary arteries. The portions of these arteries 2mm or larger are well visualized. Patient's heart resting heart rate was 72 beats per minute; she required beta-blockade to achieve a heart rate of 58 which was suitable for scanning.

Fig. 4-5

62 year old male 4 years s/p CABG with three saphenous vein grafts. Patient with recurrent pain with equivocal nuclear stress imaging. The SVG graft to the LAD demonstrates moderate stenosis at its origin from the ascending aorta, later confirmed at catheter angiography. Images are shown in surface rendering (arrow demonstrates stenosis) and in curved planar reformat. Patient required beta-blockade to achieve heart rate of 60 beats per minute prior to imaging.

Accuracy:
Data from the literature describes sensitivity between 80 and 90% for the detection of native coronary stenosis, with specificity also between 80 and 90%. Unfortunately, most studies also describe “failure” rates (lack of adequate images due to variations in cardiac rate during the scan) of up to 25%. To achieve even this degree of accuracy, patient selection tends to include only patients with low pretest probability. Current studies typically exclude patients with known coronary disease and patients with typical anginal symptoms. They also exclude patients with markedly elevated coronary calcium scores (above 400).

Interestingly, accuracy for evaluating CABG graft patency is significantly better than native coronary analysis. This is because cardiac motion is much less of a problem with graft analysis.

Patient preparation:
Since adequate images from coronary CTA require regular heart rates 60 beats per minute or lower, many patients require pharmacologic intervention prior to the exam. Patients must have no contraindications to iodinated contrast, and must be able to tolerate temporary beta-blockade.

Technical considerations:
Accurate coronary CTA exams require state of the art CT technology, which although very expensive, is becoming more available in both academic and private practice settings. Highly efficient computer workstations are necessary to analyze and display disease. Physicians performing these exams require substantial knowledge of both the clinical aspects and implications of coronary disease and the technical and anatomic aspects of thoracic computed tomography. Given the paucity of physicians with such training, many centers have developed successful collaborations between cardiology and radiology.

Current Indications/Contraindications:
Although current indications continue to be worked out in large academic centers, there are a few indications which seem to enjoy widespread agreement:

Evaluation of CABG grafts in patients who are suspected of graft closure or complication
Evaluation of patients without known coronary disease, but with indeterminate nuclear or echo stress testing.

Current contraindications include:

Patients with arrythmias
Patients unable to tolerate beta-blockade
Patients with known coronary calcium scores above 400 (CT artifacts preclude accurate analysis in these patients.)
Patients with known coronary disease (CTA is not considered accurate enough to follow known coronary stenoses.)

Future developments:
As CT technologies improve, emphasis must be placed on achieving improved temporal resolution. This would diminish the “technical failure” rate which currently is still about 25%. Improved temporal resolution would also allow the ability to more successfully visualize branch vessels such as large diagonal and large obtuse marginal branches.

There has been recent interest in the literature to use CTA in plaque characterization. As temporal resolution improves, CTA may become a useful tool in distinguishing lipid laden (potentially vulnerable) plaque from predominately calcified or fibrous plaque.

About the author:
Dr. Robert Optican is the director of cardiothoracic radiology at Baptist Memorial Hospital - Memphis. He completed fellowship training in the subspecialty of cardiac radiology at The Cleveland Clinic. His areas of expertise include cardiac MRI and cardiac and pulmonary CT.