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 18 F which is clinically the most relevant PET isotope. However, other PET isotopes, such as 68 Ga and 90 Y, are gaining clinical importance as they are of specific interest for oncological applications and for follow-up of 90 Y-based radionuclide therapy. These isotopes have a complex decay scheme with a variety of prompt gammas in coincidence. 68 Ga and 90 Y have higher positron energy and, because of the larger positron range, there may be interference with the magnetic field of the MR compared to 18 F. 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 18 F, 68 Ga, and 90 Y. Scatter fraction and noise equivalent count rate (NECR) tests were performed using 18 F and 68 Ga. All phantom data were acquired on the GE Signa integrated PET/MR system, installed in UZ Leuven, Belgium. Results 18 F, 68 Ga, and 90 Y NEMA performance tests resulted in substantially different system characteristics. In comparison with 18 F, the spatial resolution is about 1 mm larger in the axial direction for 68 Ga and no significative effect was found for 90 Y. The impact of this lower resolution is also visible in the recovery coefficients of the smallest spheres of 68 Ga in image quality measurements, where clearly lower values are obtained. For 90 Y, 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 68 Ga than for 18 F 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 68 Ga and 90 Y compared to 18 F. But these differences are compensated by the PET/MR scanner technologies and reconstructions methods.