Coronary Computed Tomography Angiography: General Considerations

Coronary computed tomography (CCTA) is currently considered to be a technique for high-resolution computed tomography (CT) imaging of the heart. It has become generally accepted that patients with negative CT cardiac results will not need to undergo cardiac catheterization. The radiation dose for the CCTA with modern techniques is much lower than even a few years ago and in some cases, even lower than for cardiac catheterization.

The 64-Slice CT scanner is considered to be the high-end CT system and is preferable for CCTA. However, even earlier generation CT systems are suitable for this task.

In some research studies, there has been an attempt to determine the most beneficial use of cardiac CT scanners, and the consensus appears to be that the high negative predictive value of the multislice CT scanner, used to filter out those patients who do not require further investigatory tests for coronary disease, is where it is currently used most effectively.

CCTA is a non-invasive test, requiring only a contrast injection and for some patients, beta blockers. Therefore the CT scan can be performed as an outpatient examination. The disadvantage of CCTA is that there is a limitation with respect to the image quality for patients with irregular or high heart rates. In such cases, patients may be medicated by beta blockers.
CCTA is also limited where there is extensive calcification, as the high contrast of the calcium leads to "blooming" artifact.

The implementation of X- ray tube current modulation resulted in a dramatic decrease in radiation. In addition, the use of CT scans with ECG -triggering of the radiation resulted in average doses around 3 MSV.

The cost of premium-class, 64-slice CT scanning can be $1.2 - $1.5 million. A dual-source, 64-slice CT scan system costs $1.9 - $2.5 million.
The running costs including: a service contract, three full- time staff members and contrast media are $700 - $850k. In terms of potential revenue, the CCTA exam costs about 10% to 12% of the cost of cardiac catheterization.

Technical Considerations

CCTA requires an accurate imaging of coronary arteries, which move with the cardiac and respiratory cycles. Therefore, high spatial resolution and high temporal resolution are essential.

High contrast-to-noise ratio is of primary importance. To achieve a good enough ratio, it is possible to either increase the X-ray tube output and use thicker slices, or to increase the contrast. The spatial resolution will be impaired by increasing the slice width and filtering the signals.

To eliminate cardiac motion artifact, high-rotation speed system is necessary. Current cardiac CT scanners have 3-3.3 R.P.M speed.

For higher temporal resolution, half the gantry rotation time is used for the reconstruction. To assure the best temporal resolution, it is important to acquire the image in the most stationary phase of the cardiac cycle. Therefore, ECG signal is used to enable the reconstruction of the CCTA image in the cardiac phase of least motion.

The typical scan length of a CCTA examination is about 14 CM. It is essential that the CT scan is completed well within a patient's breathhold, so as to avoid respiratory motion artifacts.

Imaging of the coronary arteries can be achieved on a CT scanner in a variety of different ways. Where the complete heart cycle is scanned, greatest flexibility is available in selecting the optimal phase for coronary artery image reconstruction. This approach enables the best functional analysis. However, the dose to the patient is high. Where only a selected part of the cardiac cycle is CT scanned, the radiation is only during a short interval. In this situation, the radiation is low. There is also the merging of the two approaches: all phases of the heart are CT scanned with reduced radiation, and only the stationary phase window is fully illuminated. Full image quality is obtained for the coronary artery image required phase, whereas the images for functional analysis have lower quality.

In general, the CCTA systems can be classified in 4 groups:

1. Axial scan, Prospective ECG Triggered: this is the most commonly used mode, due to the resultant lower dose. Modern systems typically require 3-4 rotations. It is suitable for patients with heart rates below 70 BMP.

2. Axial scan, Retrospective ECG Gated: in this mode, the axial scan is performed over the full cardiac cycle, over a number of gantry rotations (depending on the detector coverage) so the patient is exposed to high-dose rates.

3. Helical Scan, Retrospective ECG Gated: in this mode the radiation is continuous, while the heart is covered with a low-pitch helical scan. The involved radiation dose is high. This is the mode of choice for patients with high or unstable heart rates.

4. Helical scan, Prospective ECG Triggered: in this mode, the radiation is triggered by ECG, as it is with the axial triggered systems. Systems with dual source (two x-rays tubes and detectors which are positioned at 90 degrees each to other) can be operated at high pitch. A full scan can be performed in less than 0.5 seconds.

Low-pitch prospective systems are rare nowadays.

Performance of CCTA
CT scanning results in a plain-scan resolution of approximately 0.5 mm. With a typical reconstruction field of view of 25 cm, and a reconstruction matrix of 512×512, the pixel size is approximately 0.5 mm, which is a good match for the resolution.

The longitudinal axis resolution of modern, quality CT scanner systems approaches 0.5 mm.