Hybrid PET/CT scans join clinical mainstream

Before considering revascularization procedures, surgeons want proof of ischemia. While catheter angiography has value in assessing lesions associated with coronary artery disease, it cannot assess the associated ischemia. PET imaging is increasingly being used to provide that information.

With the rise of multislice CT angiography as a first-line test for patients with suspected CAD, researchers have set their sights on integrated PET/CT for combined acquisition of coronary anatomy and perfusion.

"This is clearly an important marriage between nuclear imaging and CT," said Dr. Daniel S. Berman, director of nuclear cardiology and cardiac imaging at Cedars-Sinai Medical Center in Los Angeles, California. "Which patients will need both tests done in the same setting? We don't know the answer yet. What we do know is that there are patients who will need both."

The test still faces several challenges to widespread use, such as defining the appropriate patient population that would most benefit from the combined study and developing more cost-effective radiotracers.

Peer-reviewed evidence defining the appropriate patient population is scant. That is likely to change as more institutions adopt cardiac PET/CT, according to Dr. Marcelo F. Di Carli, a cardiologist and chief of nuclear medicine at Brigham and Women's Hospital in Boston, Massachusetts.

A normal CTA would exclude disease and lead to the appropriate management decision. An abnormal CTA, even one suspected to be obstructive, does not always correlate with a positive PET perfusion scan. In this scenario, only half of the patients would have evidence of ischemia on the PET study.

"If you rely solely on the CT information and disregard perfusion imaging, you will revascularize 50% of patients inappropriately," he said. "The negative predictive value of CT is extremely high, but the positive predictive value to identify patients in need of revascularization is fairly low."

Reconciling these two extremes leads to further questions. Will CT be done on everyone as a first-line test, followed up with perfusion imaging in those with an abnormal scan? Would that be the most cost-effective approach versus the current protocol, which is to perform the perfusion test, make the diagnosis, stratify the patient's risk, and make a management decision?

The cost-effectiveness of either approach depends greatly on the patient group, Di Carli said. The value of CTA for excluding disease in older symptomatic patients with a high probability of calcification, for example, is low compared with younger patients who are not likely to have calcium.

In one of the first published studies assessing the feasibility of integrated cardiac PET/CTA, researchers at the University Hospital of Zurich found the technique not only feasible but accurate to visualize coronary anatomy and simultaneously assess the functional significance of lesion severity. These results came with a four-slice CT scanner. The research team performed contrast-enhanced CTA and rest/adenosine-stress PET myocardial perfusion scanning with nitrogen-13 ammonia on 25 patients with CAD that had been documented by coronary angiography (J Nuc Med 2005;[46]:930-935).

The standard reference of PET perfusion plus conventional angiography identified 11 lesions with corresponding ischemia that qualified for revascularization. The 89 remaining coronary segments did not qualify for revascularization, as they contained either no stenosis and no ischemia or stenosis without ischemia. Integrated PET/CTA correctly classified 82 of these 89 segments, indicating that an invasive diagnostic procedure could have been avoided with the noninvasive test, according to the study. These results are promising but require further confirmation in larger trials.