NEMA NU 2–2007 performance characteristics of GE Signa integrated PET/MR for different PET isotopes

Background: Fully integrated PET/MR systems are being used frequently in clinical research and routine. National Electrical Manufacturers Association (NEMA) characterization of these systems is generally done with ¹⁸F which is clinically the most relevant PET isotope. However, other PET isotopes, such as ⁶⁸Ga and ⁹⁰Y, are gaining clinical importance as they are of specific interest for oncological applications and for follow-up of ⁹⁰Y-based radionuclide therapy. These isotopes have a complex decay scheme with a variety of prompt gammas in coincidence. ⁶⁸Ga and ⁹⁰Y have higher positron energy and, because of the larger positron range, there may be interference with the magnetic field of the MR compared to ¹⁸F. Therefore, it is relevant to determine the performance of PET/MR for these clinically relevant and commercially available isotopes. Methods: NEMA NU 2–2007 performance measurements were performed for characterizing the spatial resolution, sensitivity, image quality, and the accuracy of attenuation and scatter corrections for ¹⁸F, ⁶⁸Ga, and ⁹⁰Y. Scatter fraction and noise equivalent count rate (NECR) tests were performed using ¹⁸F and ⁶⁸Ga. All phantom data were acquired on the GE Signa integrated PET/MR system, installed in UZ Leuven, Belgium. Results: ¹⁸F, ⁶⁸Ga, and ⁹⁰Y NEMA performance tests resulted in substantially different system characteristics. In comparison with ¹⁸F, the spatial resolution is about 1 mm larger in the axial direction for ⁶⁸Ga and no significative effect was found for ⁹⁰Y. The impact of this lower resolution is also visible in the recovery coefficients of the smallest spheres of ⁶⁸Ga in image quality measurements, where clearly lower values are obtained. For ⁹⁰Y, the low number of counts leads to a large variability in the image quality measurements. The primary factor for the sensitivity change is the scale factor related to the positron emission fraction. There is also an impact on the peak NECR, which is lower for ⁶⁸Ga than for ¹⁸F and appears at higher activities. Conclusions: The system performance of GE Signa integrated PET/MR was substantially different, in terms of NEMA spatial resolution, image quality, and NECR for ⁶⁸Ga and ⁹⁰Y compared to ¹⁸F. But these differences are compensated by the PET/MR scanner technologies and reconstructions methods.