JACC: CT myocardial perfusion compares well with gold standard SPECT
A semi-quantitative assessment of the extent and severity of myocardial perfusion defects by CT was comparable to the gold standard automated assessment by SPECT myocardial perfusion imaging (MPI), according to a study in the October issue of Journal of the American College of Cardiology: Imaging.

Researchers noted that stenosis on coronary CTA (CCTA) does not reliably predict myocardial ischemia, which would then require further testing. Combining CCTA and CT MPI could be a way to overcome this drawback, but automated quantitative CT MPI software does not yet exist.

In the study to measure CT MPI accuracy, Balaji K. Tamarappoo, MD, from Cedars-Sinai Heart Institute in Los Angeles, and colleagues compared ECG-gated CT MPI in 30 patients who also had a SPECT MPI scan within 60 days. Eighteen patients had stress-rest CT MPI, while 12 had stress-only CT MPI.

Researchers assessed the extent and severity of perfusion defects on SPECT MPI using a 5-point scale and the standard 17-segment model, and quantified the total perfusion deficit by automated software. Two observers visually assessed regions of perfusion defects on CT MPI using the same grading scale.

For the detection of segments of stress perfusion abnormalities, Tamarappoo and colleagues found good agreement between CT and SPECT (kappa of 0.71). Myocardial segments that were abnormal on CT MPI but normal by SPECT MPI were often found in the apex, distal left ventricle or adjacent to segments that were unequivocally abnormal by SPECT and CT.

On a per-segment basis, the sensitivity, specificity, positive-predictive value and negative-predictive value of CT MPI compared with SPECT were 92, 86, 71 and 96 percent, respectively.

The total perfusion deficit measured by automated SPECT MPI was similar to visual semi-quantitative assessment from CT MPI. Researchers noted excellent agreement between two expert independent readers in identifying regions of myocardium with stress perfusion abnormalities using CT-MPI, with a kappa of 0.88.

Investigators concluded that the use of a commercially available semi-automated software program (Siemens Healthcare) for CT MPI assessment closely matched the automated assessment for SPECT MPI.

The semi-automated quantification required two experienced readers to manually assign regions with perfusion abnormalities.

"This measurement is, therefore, tedious and dependent on reader interaction and bias; however, our analysis shows that, despite this limitation, there is minimal bias and strong similarity between this software-based assessment of the magnitude of perfusion deficit on CT MPI and an automated SPECT MPI–based assessment of total perfusion deficit at stress and rest," they wrote.

One limitation of the study was an average of 18 mSv of effective radiation dose in the patients who underwent combined stress and rest CT MPI, according to the study.

"Compared to other studies reported in the literature, our effective radiation dose was considerably higher," the researchers wrote. "That higher dose was in part due to the inclusion of patients who had coronary artery bypass graft surgery, which required greater scan length (22 cm to 11 cm, median 18 cm) for visualization of the aortic arch and the internal mammary artery."

They added that newer generation CT scanners and "aggressive dose modulation" would produce an effective radiation dose comparable to that for SPECT MPI.