Contrast Kinetics in CT Coronary Angiography

Abstract

Computed tomography coronary angiography (CTCA) is a technically demanding radiological investigation that requires adequate opacification of coronary arteries at peak aortic enhancement phase, with minimal or no contrast in the superior vena cava and right-sided cardiac chambers to avoid streak artifacts of dense contrast. Therefore, it is prudent to know about contrast media, contrast kinetics, and contrast injection protocols. This article attempts to describe the essentials of various aspects of contrast media that should be considered for CTCA along with an in-depth analysis of contrast kinetics that every radiologist should know for obtaining adequate opacification of coronary arteries.

Keywords: CT coronary angiography; bolus-tracking; contrast kinetics; minimum scan delay; peak aortic enhancement; test-bolus; time-density curve.

Fig. 1

Time-density curve representing temporal variation in aortic enhancement following pressure injection. T _ct represents the minimum scan delay, proportionate to the time taken for contrast to reach the aorta from the venous side. Following this, monitoring is done for aortic enhancement in the bolus-tracking technique. TW or temporal window (area within the blue dotted lines) represents the period of acquisition, during which there will be optimal, maximal contrast enhancement in the aorta and coronaries.

Fig. 2

Actual versus ideal bolus geometry. The blue-colored curve represents the actual bolus geometry, which has a short, high peak. The black-colored curve represents the ideal bolus geometry, which has a broad plateau representing the longer temporal window for acquisition (TW). Increasing the contrast injection rate and iodine concentration broaden the peak, optimizing the enhancement during the acquisition time.

Fig. 3

Diagram showing major factors affecting the time-density curve.

Fig. 4

Relationship between contrast enhancement of aorta/coronary arteries and iodine concentration. At constant tube voltage, there is a linear relationship between the iodine concentration and contrast opacification (more the iodine concentration in contrast, more is the attenuation in the aorta/coronary artery).

Fig. 5

Test-bolus technique. For computed tomography coronary angiography (CTCA), the region of interest is placed in the ascending aorta and low-dose scans are acquired following injection of a test dose of contrast. The time-density curve is plotted for the bolus and the scan delay time is automatically displayed based on software calculations.

Fig. 6

Bolus-tracking technique. The region of interest is placed in the ascending aorta, followed by one-time injection of the complete contrast volume. The scan acquisition is automatically triggered when the threshold attenuation (100 Hounsfield units [HU] in this case) is reached.