Calcium scoring fills imaging triage role

Coronary artery calcium scoring has been tested at the University of Western Ontario to flag nuclear myocardial perfusion scans that missed the presence of three-vessel coronary artery disease, and to triage patients who need coronary CT angiography or cardiac catheterization.

Dr. Sundip Datta presented findings on behalf of the UWO nuclear medicine division, which is in London, ON, and is led by Dr. Jean-Luc Urbain, at the 2009 Society of Cardiovascular Computed Tomography meeting in July. Results demonstrated how calcium testing can uncover the likely presence of significant coronary artery disease missed by myocardial perfusion imaging.

MPI is susceptible to false-negative findings for patients with balanced three-vessel disease. Clinicians have long known that the condition poses a diagnostic dilemma because the radiotracer administered during myocardial perfusion imaging can be equally distributed in the heart muscle when all three main coronary arteries have an equivalent degree of occlusion.

In such instances, the perfusion images may have a patchy, or what Urbain describes as a “wishy-washy,” appearance. He stressed the need for MPI generally, however, for acquiring physiological data about the myocardium that cannot be appreciated with CTA or invasive angiography.

A positive electrocardiogram or treadmill test can raise suspicion about a negative MPI for a patient with an intermediate or high risk of coronary disease. Urbain's experience with 492 consecutive patients who had normal MPI despite their intermediate risk of coronary disease indicates that coronary artery calcium scoring provides another valuable check for the condition.

The study confirmed a strong correlation between calcium scores and coronary artery stenosis (p<0.0001). Significant disease was identified in 1.1% of patients with a calcium score of zero. It was also unlikely for patients with Agatston scores above zero and below 400, though experience has shown that they may harbor moderate levels of coronary plaque, Urbain said. Patients in both categories were referred back to their physicians without additional imaging.

Patients with a negative MPI and calcium scores above 400 received more substantial workups. Nearly eight of 10 patients (78.3%) with scores from 400 to 1000 were diagnosed with significant coronary artery disease and received immediate coronary CTA.

Patients with Agatston scores greater than 1000 were referred for cardiac cath because extensive calcium would probably degrade the CTA images, Urbain said.

“It is very difficult to read cardiac CTA when so much calcium is present,” he said. “You can't distinguish the calcium from the contrast.”

Urbain and colleagues developed the calcium scoring protocol after a 64-slice CT scanner was installed at the hospital in 2007. More than 2500 coronary CTA exams have been performed on the system.

Over time, coronary calcium scoring evolved into a gate-keeping test that spares patients invasive catheterization and avoidable radiation exposure from unnecessary CTA, Urbain said. To save time, the five-minute test is performed and analyzed by a clinician on the fly to identify who may be released, administered contrast for an immediate coronary CTA, or referred for coronary catheterization.

“It is time-intensive for our clinicians because they have to attend to these scans themselves, but is clinically efficient,” Urbain said.