Chapter 25

### INTRODUCTION

Traditionally, stress testing is complemented by invasive coronary angiography in diagnosing physiologically significant coronary artery stenoses. However, non-invasive methods of visualizing the coronary arteries are gaining popularity as they do not have the complications1,2 associated with the traditional form of coronary angiography. One such method is computerized tomography (CT), which was first developed by Sir Godfrey Hounsfield in 1971.3 The rapid advancement in CT technique has led to its increasing use in visualizing the cardiac structures, including the coronary arteries. However, the challenge for cardiac CT angiography (CCTA) is to have a spatial and temporal resolution that is comparable to invasive coronary angiography. For now invasive coronary angiography remains the gold standard in evaluating coronary artery stenoses.4

This chapter discusses the technology and technique of CCTA as well as its current indications, controversies, and future direction.

### TECHNOLOGY

The clarity of information received from CCTA imaging depends on its temporal and spatial resolution. The temporal resolution or the time needed to acquire one CT image set (an image set is acquired when the x-ray tube rotates 180°, beyond the fan width, around the gantry) (Fig. 25-1) of the heart must be such as to minimize the effects of cardiac motion. The spatial resolution or the ability to visualize submillimeter structures must be such that small calcified and non-calcified plaques can be assessed. Only then will CCTA images be acceptable. The approach to achieve this is multipronged.

###### FIGURE 25-1

Diagrammatic view of a basic CT scanner (cross-sectional).

Temporal resolution can be improved by pharmacologically reducing the heart rate.5 The images are acquired, using electrocardiographic (ECG) synchronization, during the relatively immobile isovolumic relaxation phase of the cardiac cycle. Alternatively, temporal resolution can be improved by either increasing the gantry rotation speed or using dual source CT (DSCT) or by segmented reconstruction. Increasing gantry rotation speed reduces the time needed to obtain one image set. However, current CT scanners already have gantry rotation speeds of up to 330 ms per rotation that can result in gravitational forces >20 G.6 Using two pairs of x-ray sources and detectors, placed at 90° to each other, DSCT scanners reduce the time to scan by 50% without having to increase the gantry rotation speed. Thus, they can achieve good images even at higher heart rates (Fig. 25-2).7,8 Segmented reconstruction, wherein data from partial scans of multiple cardiac cycles are used to reconstruct the image, is theoretically attractive, in terms of use at higher heart rates and slower gantry rotations.9

###### FIGURE 25-2

Diagrammatic view of a dual source CT (DSCT) scanner (cross-sectional).

Spatial resolution can be improved by ...

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