case studies

Cardiac CT

A new and challenging application for CT imaging is the non-invasive diagnosis of coronary artery disease. New advances in technology, both in reconstruction software and with the advent of multi-detector CT mean that cross-sectional imaging of the heart is now not only possible with just Electron Beam CT (EBCT). Multi-detector CT also allows the non-invasive detection of high-grade stenosis and non-invasive follow-up examinations such as stent and bypass patency control.

Referral
This particular study involved a referral for a patient from a nearby cardiac centre who was having increasingly bad angina but in whom an invasive coronary angiogram had failed due to a very large aortic root aneurysm. The referring physicians wanted to assess the coronary arteries prior to aortic root surgery.

Protocol
The patient preparation prior to cardiac CT is probably the most important aspect. Although the 16 slice CT scanners are extremely fast i.e. a maximum rotation time of 0.42 seconds, the true temporal resolution is 230 – 1000 ms, still considerably slower than that of EBCT (50 – 100ms). This means that for effective cardiac imaging the patient needs a regular resting heart rate of between 60 and 80bpm. If the heart rate is higher than this, or is irregular, the likelihood of good, diagnostic 3D reconstructions is reduced. The routine use of oral beta-blockers 1-2 hours before the scan is therefore recommended. The patient is then placed supine on the scanner and the ECG electrodes positioned to achieve an acceptable trace. A pre-contrast coronary artery calcium-scoring scan is initially carried out to access the arteries. If the walls of the coronaries were full of calcium the angiogram would be useless. Next, a bolus of 100mls of contrast is injected at 5mls per second followed by 50mls of saline. A spiral scan range of 1mm slices with 0.5mm overlap is carried out from the aortic root to the base of the heart. The bolus tracking software is used and the ROI is placed in the ascending aorta. The significance of the saline is to clear the right ventricle of contrast so the right coronary artery is visible. The resultant images can then be reconstructed during different phases of the cardiac cycle to find the best images for each of the coronary arteries.

Figure 1
Figure 1 shows how the aortic root is markedly dilated measuring 6.9cm in its maximum diameter. The left main coronary artery is widely patent, and the left anterior descending artery appears not to have any significant narrowing.

Figure 2
Figure 2 shows that there is some irregularity of the left circumflex artery, but when viewed in conjunction with the axial images and MPR’s it was reported as no significant stenosis.

Figure 3
Figure 3 shows that within the aortic root, there is a linear, partially calcified, filling defect. Its appearance is that of a short dissection of the aortic wall. The right coronary artery is probably non-dominant being a very small, somewhat irregular vessel, possibly with some significant stenosis.

In conclusion, ECG-gated multi-slice spiral CT offers new possibilities for non-invasive CTA of the coronary arteries. The fast volume coverage allows continuous coverage of the heart with 1mm slices within a single breath-hold (10cm in 20-25sec). 3D reconstructions with nearly isotropic z-resolution are possible for visualisation of the coronary arteries. Initial studies show that ECG-gated multi-slice spiral CT is able to provide motion-free 3D images of the heart and the coronary arteries for moderate heart rates. Due to the high spatial resolution reliable diagnosis is possible also for smaller side branches of the coronary artery tree.

Sean Keefe
AMICL

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