In response to demands for evidence that cardiac CT will positively affect patient outcomes, researchers presented results at the RSNA conference Wednesday attesting to its value for identifying which intermediate risk patients are most likely to suffer future catastrophic coronary events.

Dr. James Earls, medical director of Fairfax Medical Consultants in Virginia, demonstrated in a retrospective study of 1692 patients that a zero score on a cardiac calcification test may be close to a guarantee against a future cardiac event. It does not, however, promise that the patient is free from coronary atherosclerosis.

Based on a review of coronary studies, Earls found that 27% of the patients with a calcium score of zero actually had noncalcified plaque that was missed in the original diagnostic interpretation. A substantial number of plaque accumulations were not noted in the initial patient report, he said.

Most of the plaques were nonobstructive and almost always involved positive remodeling of the arterial wall. Two percent of the population (four patients in total) had a zero score and significant obstructive disease involving a 50% blockage.

Earls blamed window and leveling problems for missed disease on the original diagnostic interpretations. Accumulation in the proximal segments of the right coronary artery, left anterior descending coronary artery, and circumflex artery accounted for about 75% of the nonstenotic plaques that were identified.

Even with these cautionary findings, the results overall indicated that the negative predictive value of a zero calcium score for significant coronary disease was greater than 99%.

In another trial, Dr. Joseph Abro of the University of South Carolina in Charleston presented results from 452 patients confirming the findings of several smaller previous studies that the extent of coronary disease identified with 64-slice or dual-source CT angiography is linked with the risk for future cardiac events.

None of the 198 patients with no evidence of significant coronary occlusions on their coronary CTA exams experienced a major cardiac event in the trial's year-long follow-up period.

Contrast-enhanced CTA identified 87 patients with stenosis of greater than 50%. In that group, 26 had a major cardiac event within a year. Abro could not find a correlate between plaque type or global Agatston calcium score and future cardiac risk.

Coronary CTA may be particularly well suited for stratifying the risk of future cardiac events for diabetic patients, a group known to carry at least twice the risk of coronary artery disease than the general population, according to Dr. Anselmo Palumbo, a radiologist at the University of Palma in Italy.

Her group followed 180 prospectively selected patients with type 2 diabetes and 303 nondiabetic patients who received contrast-enhanced 64-slice CTA to rule out the presence of coronary artery disease. None of the patients had a prior history of coronary artery disease, but CCTA identified obstructive disease in 52% of the diabetic and 37% of the nondiabetic patients.

At 20-month follow-up, 8% of diabetic patients had incurred so-called hard events, such as a myocardial infarction, compared with a 2% hard event rate for the nondiabetic population.

A strong correlation was established between the extent of coronary artery disease observed with CCT and the likelihood that a diabetic or nondiabetic patient would incur a major future coronary event, according to Palumbo.

"The prognosis is similar between diabetics with no history of coronary artery disease and patients without diabetes who have previously been diagnosed with coronary artery disease," she said.

The presence of significant coronary disease involving at least one 50% or greater stenosis was the strongest predictor of a future major coronary event, she said. In diabetic patients, however, a relatively high incidence of coronary segments containing nonobstructive plaque accumulations predicted a high likelihood of a major cardiac event.

Preliminary results from Dr. Jung-Ho Kang from Yonsei University in Korea suggest that 64-slice coronary artery CTA is superb for excluding the presence of coronary artery disease for intermediate risk patients whose treadmill stress testing is positive or equivocal.

Based on consecutive 441 patients, the ECG stress test was 49% sensitive and 82% specific for significant disease involving greater than 50% stenosis. In comparison, the sensitivity and specificity of CT were 84% and 96%, respectively.

CT was especially adept at identifying false positives from the treadmill test, Kang said. About 94% of patients who erroneously tested positive during the treadmill test received a negative CCT exam. The results were confirmed with x-ray angiography.

CCT may be more accurate than treadmill tests, but it is also about 10 times more costly, according to session moderator Dr. David Levin.

"I don't see coronary CTA replacing ECG treadmill tests any time soon," he said.