This article from Korea, published in the February issue of AJR, by Shim SS, shows how the use of beta-blockers on a 16-slice CT (12 x 0.75mm) improved the image quality, especially in the RCA. The study was not performed on the same patient population, but on two separate groups.
This is something that is now an accepted fact, though there are very few papers showing a head-to-head comparison with and without beta-blockers. It would be difficult and probably unethical to perform such a study.
Having said this, on the new 64-slice CT, though we insist on heart rates below 65 for excellent image quality, if the heart rate is in the 80s, using 2-segment reconstruction, we still get excellent images.