Multislice Computed Tomography Coronary Angiography – Basics^3–8

Computed Tomography Coronary Angiography – Temporal and Spatial Resolution

A high temporal resolution is essential for the depiction of the coronary arteries, mainly because of the rapid motion of the coronary arteries. The motion velocity of the left anterior descending (LAD) is 22.4±4.0mm/second, of the left circumflex artery (LCx) 48.4±15mm/second and of the right coronary artery (RCA) 69.5±22.5mm/second.^9,10 It is estimated that a temporal resolution of 19–75ms is desirable to capture motion-free coronary images.

The temporal resolution of CT scanners used for the visualisation of coronary arteries is determined by the rotation speed of the gantry around the patient. As the coronary images are reconstructed from data acquired from a 180º gantry rotation, the temporal resolution is equal to half the gantry rotation speed. Current-generation 64-slice or dual-source (DS) CT scanners still have limited temporal resolution, ranging from 83 to 165ms, which may cause image blurring (motion artefacts), particularly during higher heart rates (<70bpm). To decrease the likelihood of the creation of unsharp images, the coronary images are usually reconstructed from data acquired during the relatively motion-free diastolic phase of the heart cycle. During lower heart rates (<65bpm), this relatively motion-free period increases and thus the likelihood of obtaining sharp images increases. Therefore, administration of B-receptor blocking agents 60–90 minutes before CT scanning is recommended to reduce the heart rate to less than 65bpm to prolong the rest period in the diastolic phase. To further reduce motion artefacts a segmented reconstruction mode is used, which improves the nominal temporal resolution by a factor of two to three. Segmented reconstruction selects data acquired during two or more cardiac cycles, which are combined to form one image reconstruction. However, this technique is less robust and requires a very stable heart rate.

A high spatial resolution is necessary to allow assessment of small coronary artery details, such as severity of stenosis or detection of often small non-obstructive plaques. The spatial resolution of current CT scanners in a phantom setting is ~0.4mm, but in clinical CT imaging is estimated to be 0.6–0.7mm. A higher spatial resolution decreases partial volume effects, which allows improved delineation of, for instance, in-stent restenosis or cases of calcified lesions, and so minimises overestimation of the stenosis severity.