Coronary CT angiography

Coronary CT angiography (CTA) is the use of computed tomography (CT) angiography to assess the coronary arteries of the heart. The subject receives an intravenous injection of radiocontrast and then the heart is scanned using a high speed CT scanner, allowing physicians to assess the extent of occlusion in the coronary arteries, usually in order to diagnose coronary artery disease.

The positive predictive value of cardiac CTA is approximately 82% and the negative predictive value is around 93%. This means for every 100 patients who appear to have coronary artery disease after CT angiography, 18 of them actually won't have it, and that for every 100 patients who have a negative CT angio test result (i.e. the test says they do not have coronary artery disease), 7 will actually have the disease as defined by the reference standard of invasive coronary angiography via cardiac catheterization. Both coronary CT angiography and invasive angiography via cardiac catheterization yield similar diagnostic accuracy when both are being compared to a third reference standard such as intravascular ultrasound or fractional flow reserve.

The significance of the low radiation doses used in diagnostic imaging is unknown, although the possibility of increasing cancer incidence across a population is of significant concern. This potential risk must be weighed against the competing risk of not diagnosing a significant health problem in a particular individual, such as coronary artery.

Pregnancy is considered a relative contraindication, similarly to many forms of medical imaging in pregnancy. The potential harms to a fetus include the application of X-rays in addition to radiocontrast. Since an iodine-containing contrast agent is used, severe contrast agent allergy, uncontrolled hyperthyroidism or renal function impairment are also relative contraindications. Cardiac arrhythmias, coronary artery stents and tachycardia may result in a reduced image quality.

In this technique, a portion of the heart is imaged during one heart cycle while an ECG trace is recorded. During the next heart cycle, the next portion of the heart is scanned for up to 5 total cycles until the entire heart is imaged. The reconstruction algorithm then combines the images from these different cycles to generate one complete image. The advantage of this method is that each image segment is acquired in less time as compared to acquiring the entire heart in one heart cycle, thus improving temporal resolution. The disadvantages are 1) the potential for image artifacts from fusing the image segments and 2) the requirement of additional X-ray radiation for image acquisition.

The speed advantages of 64-slice MSCT have rapidly established it as the minimum standard for newly installed CT scanners intended for cardiac scanning. Manufacturers have developed 320-slice and true 'volumetric' scanners, primarily for their improved cardiac scanning performance.

The latest MSCT scanners acquire images only at 70-80% of the R-R interval (late diastole). This prospective gating can reduce effective dose from 10-15 mSv to as little as 1.2 mSv in follow-up patients acquiring at 75% of the R-R interval. Effective dose using MSCT coronary imaging can average less than the dose in conventional coronary angiography.