PET Perfusion Imaging Improves Risk Estimates

Reed Miller

December 06, 2012

BOSTON — New data from a large multicenter registry suggest that positron-emission-tomography (PET) myocardial perfusion imaging (MPI) can greatly improve the accuracy of risk estimation in coronary disease patients compared with a model based on traditional risk factors [1].

Only small single-center studies have demonstrated the prognostic value of PET MPI in predicting which patients are at greatest risk for coronary disease events. So Dr Sharmila Dorbala (Brigham and Women's Hospital, Boston) and colleagues analyzed outcomes from 7061 patients from four centers who underwent a clinically indicated rest/stress rubidium-82 PET MPI test.

Results of the study are published online December 5, 2012 in the Journal of the American College of Cardiology. "The results of the current study are critical to advance the field and guide more effective use of PET MPI in clinical practice," Dorbala et al state.

Median follow-up was 2.2 years. During follow-up, there were 169 cardiac arrests and 570 all-cause deaths. Net reclassification improvement and integrated discrimination analyses showed that the risk-adjusted hazard of cardiac death increases as the percentage of abnormal myocardium increases. A mildly abnormal stress test is associated with a 2.3 times greater risk of cardiac death than a normal test. The hazard ratio for a severely abnormal test is 4.9.

The addition of PET MPI measurements of myocardial ischemia and myocardial scarring to traditional clinical information improves the performance of a risk prediction model based on traditional risk factors (C statistic 0.805–0.839) as well as risk reclassification for cardiac death, with small improvements in risk assessments for all-cause death. The assessment of the magnitude of ischemia and scar added to the reclassification of risk for cardiac death in one in every nine patients who underwent clinical PET MPI in the study.

Unlike computed-tomography (CT) coronary angiography, perfusion imaging provides information about myocardial blood flow and accounts for underlying coronary disease, collateral flow, and myocardial adaptation to wall stress and can be used in patients with renal insufficiency, the authors point out. Compared with single-photon-emission computed tomography (SPECT) perfusion imaging, PET MPI offers better image quality, test specificity for the diagnosis of obstructive coronary disease, and identification of scar and ischemia, according to Dorbala et al, and PET MPI uses a lower effective radiation dose. However, while the prognostic value of SPECT MPI has been described in tens of thousands of patients, the prognostic value of PET MPI has been studied in only a few thousand patients.

Does More Risk Information Help?

The value of the prognostic information offered by PET MPI is not yet clear, according to an accompanying editorial by Drs Paul Schoenhagen and Rory Hachamovitch (Cleveland Clinic, OH) [2]. "Rather than assessing whether a test yields improvement in risk assessment, the focus [should be] shifted to whether a test can identify which patients will gain a benefit from a specific therapeutic approach," they write. "The role of testing [should be] defined in the context of a specific intervention and whether the effectiveness of the intervention is improved by the use of an imaging study to identify optimal candidates for treatment.

"However, this process is neither simple nor inexpensive and will require prospective randomized clinical trials, validating the results and hypotheses generated by observational data," the editorialists conclude.

Commenting on the study, Dr Kavitha Chinnaiyan (William Beaumont Hospital, Royal Oak, MI) told heartwire , "While the details of downstream management of these patients are unclear in this paper, the association of ischemia with mortality is clear, as is the reclassification of risk. The next step in terms of management of ischemic patients is really the question here." She also pointed out that the ongoing ISCHEMIA trial, comparing angiography and revascularization plus optimal medical therapy with optimal medical therapy only, may provide more insights on the best option for patients who show more than mild ischemia on stress studies.

Dorbala has received research grants from Astellas Pharma and Bracco Diagnostics; has served on advisory boards for Astellas Pharma; and has received honoraria from MedXcel. Disclosures for the coauthors are listed in the paper. Schoenhagen and Hachamovitch report that they have no relationships relevant to the contents of this paper to disclose. Chinnaiyan has no relevant disclosures.