Can Advances in Nuclear Cardiology Hardware Overcome the Challenges of Imaging Obese Patients?

Abstract

Worldwide, the estimated total number of obese people is 671 million, with the United States having the largest proportion. Although there is an urgent global need to combat obesity, the prevalence of this disease— which is a well-recognized risk factor for both cardiovascular disease and cancer—continues to increase. Adding to the burden of this disease are the complexities and costs related to the diagnostic evaluation of obese individuals with suspected or known coronary artery disease (CAD). All non-invasive imaging techniques— including, stress echocardiography, cardiac CT, and SPECT myocardial perfusion imaging—have inherent limitation related to imaging obese individuals. To address these issues, there have been various technological advances aimed to improve image quality among obese individuals. Arguably, one of the most important such advances in the field of nuclear cardiology has been the introduction of a new generation of cameras using solid-state detectors. In particular, the cadmium zinc telluride (CZT) detector directly converts gamma radiation to an electronic signal and thus avoids the need for a scintillating crystal or photomultiplier tubes. Using this technology, two vendors (GE and Spectrum Dynamics) have introduced cameras with CZT detectors, with each having significant differences in collimator and camera design. Studies using these cameras have shown that in comparison to conventional sodium iodide detectors, CZT detectors offer improved efficiency, superior count sensitivity, and increased spatial resolution, as also detailed in the Table 1. To date, a small study of mostly non-obese patients suggests that these characteristics also translate into improved diagnostic accuracy vs conventional SPECT, and one study showed modest concordance between ischemia detected by a CZT scanner with fractional flow reserve. However, do the aforementioned advances in nuclear cardiology hardware also provide superior image quality and diagnostic accuracy when imaging obese patients? While the reduced image quality observed in obese patients is most often due to anthropometric parameters which result in excessive soft tissue attenuation, the lower myocardial fraction of injected activity found in obese individuals may also be due to tracer kinetic parameters such as the myocardial extraction fraction. It, therefore, follows that the ideal camera to image obese individuals would have a high sensitivity, as well as robust methods to correct (or correctly identify) attenuation artifacts. Despite the improved image quality observed with CZT cameras, it has been reported (not surprisingly) that when a fixed tracer dose is administered using the multipinhole collimator, obese individuals were more likely to have degraded image quality and lower photon counts. Importantly, this difference was eliminated when using a weight-dependent tracer dose. Gimelli et al evaluated the diagnostic accuracy of 148 patients (mean BMI 39 kg; 87% male) who underwent SPECT MPI using the multipinhole detector and observed ‘‘very good’’ or ‘‘excellent’’ image quality in all patients. Among 103 patients who underwent Reprint requests: Ron Blankstein, MD, FACC, Non-Invasive Cardiovascular Imaging Program, Departments of Medicine (Cardiovascular Division) and Radiology, Brigham and Womenos Hospital, 75 Francis St, Boston, MA 02115; rblankstein@partners.org J Nucl Cardiol 2015;22:276–8. 1071-3581/$34.00 Copyright 2014 American Society of Nuclear Cardiology.