Simultaneous PET/MRI with 13C magnetic resonance spectroscopic imaging (hyperPET): phantom-based evaluation of PET quantification

Background: Integrated PET/MRI with hyperpolarized C-13 magnetic resonance spectroscopic imaging (C-13-MRSI) offers simultaneous, dual-modality metabolic imaging. A prerequisite for the use of simultaneous imaging is the absence of interference between the two modalities. This has been documented for a clinical whole-body system using simultaneous H-1-MRI and PET but never for C-13-MRSI and PET. Here, the feasibility of simultaneous PET and C-13-MRSI as well as hyperpolarized C-13-MRSI in an integrated whole-body PET/MRI hybrid scanner is evaluated using phantom experiments. Methods: Combined PET and C-13-MRSI phantoms including a NEMA [F-18]-FDG phantom, C-13-acetate and C-13-urea sources, and hyperpolarized C-13-pyruvate were imaged repeatedly with PET and/or C-13-MRSI. Measurements evaluated for interference effects included PET activity values in the largest sphere and a background region; total number of PET trues; and C-13-MRSI signal-to-noise ratio (SNR) for urea and acetate phantoms. Differences between measurement conditions were evaluated using t tests. Results: PET and C-13-MRSI data acquisition could be performed simultaneously without any discernible artifacts. The average difference in PET activity between acquisitions with and without simultaneous C-13-MRSI was 0.83 (largest sphere) and -0.76 % (background). The average difference in net trues was -0.01 %. The average difference in C-13-MRSI SNR between acquisitions with and without simultaneous PET ranged from -2.28 to 1.21 % for all phantoms and measurement conditions. No differences were significant. The system was capable of C-13-MRSI of hyperpolarized C-13-pyruvate. Conclusions: Simultaneous PET and C-13-MRSI in an integrated whole-body PET/MRI hybrid scanner is feasible. Phantom experiments showed that possible interference effects introduced by acquiring data from the two modalities simultaneously are small and non-significant. Further experiments can now investigate the benefits of simultaneous PET and hyperpolarized C-13-MRI in vivo studies.