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Feasibility of Dual-Isotope Coincidence/Single-Photon Imaging of the Myocardium

Medical Imaging Research Laboratory, Department of Radiology, University of Utah, Salt Lake City, Utah

Hybrid PET scanners offer the possibility of obtaining myocardial viability information from coincidence imaging of the positron emitter ¹⁸F-FDG and perfusion measurements from a single-photon tracer—potentially simultaneously. This new approach is less costly and more readily available than dedicated PET and offers potential for improved FDG resolution and sensitivity compared with SPECT with 511-keV collimators. Simultaneous imaging of the coincidence and single-photon events offers the further advantages of automatic image registration and reduced imaging time. However, the feasibility of simultaneous coincidence/single-photon imaging or even immediately sequential imaging is unknown. In this study, the potential of using standard low-energy high-resolution (LEHR) collimators with hybrid PET to obtain coincidence and SPECT data was assessed. Methods: Phantom and human studies were performed to investigate the effect of LEHR collimators on FDG coincidence imaging with a hybrid PET system, the effect of the presence of ^99mTc during FDG coincidence imaging with LEHR collimators, and the effect of the presence of FDG during ^99mTc SPECT imaging. Results: FDG images were somewhat degraded (a measure of myocardial nonuniformity increased 10%) with LEHR collimators. With 148 MBq (4 mCi) ^99mTc present during FDG imaging of a phantom, image quality was maintained and the number of detected coincidences changed by <5%. With ^99mTc/¹⁸F whole-body ratios of 7:1, crosstalk from ¹⁸F photons accounted for the majority of counts in the ^99mTc SPECT images and resulted in severe artifacts. The artifacts were decreased with a simple crosstalk correction scheme but remained problematic. Conclusion: ^99mTc/¹⁸F ratios of at least 9:1 and state-of-the-art reconstruction and crosstalk correction are likely to be required to perform immediately sequential coincidence/single-photon imaging of the myocardium with clinically useful results. Additional challenges remain before simultaneous imaging of coincidence events and single photons can be realized in practice.

Key Words: hybrid PET • SPECT • dual isotope • cardiac imaging