Obese patients should have MPI using conventional SPECT camera
Patients with a body mass index (BMI) of 40 kg/m2 or greater should be scheduled for myocardial perfusion imaging (MPI) on a conventional SPECT camera, as it is difficult to obtain diagnostic image quality on a cadmium-zinc-telluride (CZT) camera, according to a study in the September issue of the Journal of Nuclear Medicine.

Recently, a new generation of cardiac y-cameras emerged using the concept of miniaturized CZT semiconductor detectors serially aligned around the patient. The multipinhole design renders rotation of the camera around the patient unnecessary, as it covers the entire heart throughout the scan, which according to the study authors, has multiplied system sensitivity.

“The first results from CZT cameras have fuelled high expectations with regard to lower activity, shorter scan times and possibly improved image quality,” they wrote. “On the other hand, as a result of this geometry, the heart has to be centered in a much smaller field of view than with conventional cameras, and image acquisition may often be challenging in obese patients even after repositioning of the patient.”

Thus, Michael Fietcher, MD, of the department of radiology at University Hospital Zurich in Switzerland, and colleagues said the aim of this study was to systematically assess the feasibility of CZT MPI scanning in patients with high, very high or excessive BMI.

To achieve this end, the researchers enrolled 81 patients, including 18 with no obesity (BMI less than 30 kg/m2), 17 in World Health Organization obese class I (BMI, 30–34.9 kg/m2), 15 in class II (BMI, 35–39.9 kg/m2), and 31 in class III (BMI 40 kg/m2 or greater), including 15 with BMI of 45 kg/m2 or greater.

They scored image quality as poor (1), moderate (2), good (3) or excellent (4). Patients with BMI of 45 kg/m2 or greater and nondiagnostic image quality (2 or less) were rescanned after repositioning to better center the heart in the field of view.

The receiver-operating-curve analysis resulted in a cutoff BMI of 39 kg/m2 for diagnostic image quality. In patients with BMI 40 kg/m2 or greater, the study authors reported that the image quality was nondiagnostic in 81 percent; after CT-based attenuation correction, this decreased to 55 percent. Repositioning further improved image quality. Rescanning on a conventional SPECT camera resulted in diagnostic image quality in all patients with a BMI of 45 kg/m2 or greater.

“Obesity may often affect the quality of SPECT MPI scans, mainly because of reduction of test specificity by diaphragm soft-tissue attenuation and extracardiac activity,” wrote Fietcher et al. “Our data show that AC [attenuation correction] may improve image quality but fails to solve the fundamental problem of imaging obese patients with CZT cameras. Even careful repositioning frequently remains unsuccessful in some very obese patients, in whom the standard SPECT camera remains the first-choice tool.”

The study authors added that BMI rather than absolute weight is most appropriate for identifying patients who should not undergo CZT scanning.