Frank J Rybicki Co-Director, Cardiovascular Imaging Section and Director, Applied Imaging Science Laboratory, Department of Radiology, Brigham and Womenâ??s Hospital

Originally printed in:
» US Cardiology 2006 - November 2005

Scanning Parameters

As with ECG gating and oversampling, coronary imaging pushes the limits of CT technology with respect to the parameters required to achieve diagnostic images. Typical values of mAs are 550–700mAs with 120kV.The spatial resolution along the Z-axis (the craniocaudal direction) is determined by the image slice thickness and can be as low as 0.4mm.

Although the details of image interpretation are beyond the scope of this article, it is important to point out that one major advantage of coronary CTA in comparison with catheter angiography is the ability to perform multiplanar reconstructed images. The quality of the reconstructed images is inversely proportional to the image slice thickness, and it is beneficial to perform reconstructed images with so called ‘isotropic data’; that is, CT data sets where the spatial resolution is equal in the X,Y, and Z directions. At present, the best isotropic resolution commercially available is 0.4 x 0.4 x 0.4mm.Thus, with perfect ECG gating and no respiratory motion, a 3-mm coronary artery spans seven or eight high-quality pixels (3/0.4) in any direction. This explains why properly performed CTA has a high negative-predictive value but can be limited in characterization of stenoses.

Image Field of View

For imaging the native coronaries alone, the superior border of the field of view should be set at the top of the carina, and the inferior border should include the entire inferior wall of the heart. Ideally, the planned field of view should include several slices of the liver to account for cardiac displacement during breath-holding. Because the CT acquisition is in the craniocaudal direction, obtaining a small amount of CT data inferior to the heart does not affect image quality.As hinted in the introduction, cardiac CT can be extended beyond imaging the coronaries alone. In several cases, the greatest protocol change is the field of view. For example, in the assessment of coronary artery bypass grafts, imaging includes the internal mammary arteries, so the superior aspect of the field of view is extended to the apices of the lungs to ensure that the subclavian arteries are included.

For some cross-sectional evaluations (e.g. cardiac MR imaging (MRI),CT myelography), image reconstruction is performed with a limited field of view. In some cases, such as cardiac MRI, limiting the field of view can be beneficial because the imaging time and potential wrap artifact can be minimized. However, coronary CTA data includes complete imaging of the thorax over the entire field of view, and a full field-of-view reconstruction and interpretation is required to evaluate for findings outside the coronary arteries.

Contrast

Dual injection with iodinated contrast followed by saline at rates of at least 5cc per second are now standard. The purpose of the saline is to avoid dense opacification of the right heart and subsequent artifacts that can limit interpretation of the RCA. The volume of contrast material is determined by the rate of contrast injection and the scan time for the prescribed craniocaudal field of view. For example, given a 12- second scan of the native coronaries alone using an injection rate of 5cc per second, an adequate volume of contrast media would be 60cc (12 seconds x 5cc/sec). Because the timing of the contrast bolus may be imperfect, a slightly larger volume of contrast (e.g. an additional 10cc) may be administered without introducing artifacts.Typically, 50cc of saline following the contrast is adequate to eliminate artifacts that obscure analysis of the RCA. One consequence of the more widespread use of cardiac CT scanners with better temporal resolution (faster rotation time) and more rows (more Z-axis coverage per rotation) is an overall decreased volume of contrast material used.

Conclusion

Because it must overcome cardiac motion, coronary CTA is the most sophisticated CT examination to date. Study requires cardiac gating, high spatial and temporal resolution, and imaging to push the limit of CT technology. However, understanding and careful adherence to CT protocols can assure that clinically useful images are routinely obtained in the vast majority of patients, enabling those patients to benefit from the diagnostic power of CT.