CT scanners fail to gauge in-stent restenosis fully

Though the latest generation of 64-slice CT scanners often excels, the technology is still not good enough to confidently assess in-stent restenosis, according to Dr. Stephan Achenbach, a professor of medicine at the University of Erlangen in Germany.

Speaking in July at the 2007 meeting of the Society of Cardiovascular Computed Tomography, Achenbach pointed to what he calls a surprising lack of research using CT to image stents, despite their widespread use. About 615,000 stent implants were performed in the U.S. in 2005 alone.

Achenbach showed several images with the stents nicely outlined but cautioned that they are exceptions. Artifacts from calcium and from the stent itself can lead to false-positive diagnoses, but the main reason for nondiagnostic images is the still limited temporal resolution of CT scanners and consequent artifacts caused by motion.

"If you have a data set that is entirely free of motion artifacts, you will often be able to evaluate stents," Achenbach said.

The first consideration is to use a scanner with high temporal resolution and do everything possible to control the heart rate. A second consideration is body mass index; images get harder to interpret as body mass increases. The size of the stent is another area of concern. Smaller stents, 3 mm or less, tend not to image as well as those of 3.5 mm or more.

To maximize spatial resolution, he recommended image reconstruction with the smallest possible slice thickness. Using a sharp reconstruction kernel may be helpful. Sixty-four-slice scanners create fewer blooming artifacts than 16-slice machines.

Stent type also plays a role, but so many types exist that it is difficult to point to one that is the best for imaging, Achenbach said. Several studies testing the efficacy of 64-slice scanners to evaluate in-stent restenosis have shown sensitivities in the high 80% to low 90% range and specificity in the high 90%. But the size of the stents varied from study to study, possibly skewing results one way or the other. One study that evaluated only stents in the left main coronary artery-often larger than 4 mm-clocked a sensitivity of 98%.

The other problem with many of these studies is that the positive predictive value ranged between 50% and 60%. In other words, if the cardiologist or radiologist sees in-stent restenosis, there is a chance of up to 50% that it is not really there.