Washington, DC - In patients with an intermediate likelihood of coronary artery disease (CAD), multislice computed tomography (MSCT) offers diagnostic information that is different from but complementary to that of myocardial perfusion imaging (MPI) a new study concludes [1].

The findings support integration of the two technologies, Dr Joanne D Schuijf (Leiden University Medical Center, the Netherlands) and colleagues conclude in their paper, appearing in the December 19, 2006 issue of the Journal of the American College of Cardiology. Indeed, several CT manufacturers have already moved to develop hybrid imaging machines, senior author Dr Jeroen J Bax (Leiden University Medical Center, the Netherlands) told heartwire. But while the rationale for obtaining information on both atherosclerosis burden and ischemia has been around for a while, the relative benefits of MSCT and MPI are unknown, and head-to-head studies are lacking.

Schuijf et al performed both MSCT (using either a 16- or 64-slice scanner) and MPI using gated single-photon-emission computed tomography (SPECT) in 114 patients, most of whom had an intermediate likelihood of CAD. A subset of 58 patients underwent conventional coronary angiography. The authors report that 41 patients were classified as having no CAD on MSCT, 33 with nonobstructive CAD, and 40 with at least one luminal narrowing >50%. On the basis of MPI results using adenosine stress, normal myocardial perfusion was found in 77 patients, while 37 were found to have reversible and/or fixed perfusion defects. In the 58 patients who underwent conventional angiography, nine were found to have no coronary abnormalities, 22 had nonobstructive disease, and 27 had at least one narrowing >50%.

Of note, while 90% of patients with a normal MSCT also appeared to have normal perfusion on MPI, more than half of the patients with evidence of some coronary obstruction on MSCT were found to have normal MPI results, while just 45% had abnormal MPI scans.

"Only half of the observed lesions on MSCT may be of hemodynamic significance," the authors note. "Even among patients with obstructive CAD on MSCT, 50% had a normal MPI."

On the flip side, more than half of patients with a normal MPI exhibited an abnormal MSCT, indicating that a normal MPI scan does not rule out coronary atherosclerosis.

Among patients who also underwent coronary angiography, all of those who had normal MSCT results also had normal coronary angiograms, and the majority also had normal MPI results. Abnormal MSCT results were confirmed by conventional angiography in all 49 patients who underwent both tests.

"These findings highlight the discrepancy between the two tests—namely, that atherosclerosis is not synonymous with ischemia—but also emphasize the complementary information that both tests provide," Schuijf el al write. "These findings confirm that the severity of focal stenosis severity in itself is not sufficient to predict the hemodynamic significance of the coronary plaque burden."

The authors propose that in clinical practice, MSCT could be used as a first-line test, at least among people with intermediate likelihood of disease. If MSCT is normal, CAD can be ruled out, but if atherosclerosis is found, sequential MSCT and nuclear myocardial perfusion imaging using either positron-emission tomography or SPECT may be appropriate. In the setting of a positive MSCT test but normal MPI test, patients could be candidates for aggressive medical therapy and risk-factor modification, whereas patients who also have abnormal MPI results should be referred for invasive angiography with revascularization. Patients deemed to have a high pretest likelihood of having CAD should still be referred directly for conventional angiography, Bax told heartwire.

But in an editorial, Dr Sharmila Dorbala (Brigham and Women's Hospital, Boston, MA) et al point out that it is precisely CT angiography's limited ability to define myocardium jeopardized by ischemia that restricts its potential for predicting benefit from revascularization [2]. As such, "evaluation of ischemic burden by MPI as an initial test, with CT angiography reserved for discordant test results, might be the optimal strategy for patients with intermediate to high pretest likelihood of CAD," Dorbala et al suggest.

A major drawback of doing sequential or hybrid imaging along the lines Schuijf et al propose is the high total radiation dose of using the two, or potentially three, imaging modalities, an exposure described as "considerable" by the study authors. Asked just how much higher this exposure might be, Bax declined to speculate, saying such a question is "not easy to answer."

But Dorbala et al take a stab, noting that multidetector CT ranges from around 7 to 12 mSv, chest dose, while MPI ranges around 15 mSv, whole-body dose.

Many questions persist as to whether CT and MPI provide incremental clinical value over conventional tests, cost efficacy, and what patients would benefit most, the editorial notes. An ongoing prospective registry, Study of Myocardial Perfusion and Coronary Anatomy Imaging Roles in CAD (SPARC), may provide some answers, Dorbala et al add.