NEMA NU 2–2007 measurements and GATE Monte Carlo simulations of GE Signa integrated PET/MR for pure and non-pure positron emitters

NEMA characterization of PET systems is generally done with ¹⁸F. However, other PET radioisotopes, such as ⁶⁸Ga and ⁸²Rb are gaining clinical importance as they are of specific interest for oncological applications. However, the physical properties of these PET radioisotope are quite different and there may be a larger interference with the magnetic field of the MR compared to ¹⁸F. Therefore, it is relevant to determine the performance of PET/MR for clinically relevant radionuclide. The aim was divided into three parts: (A) NEMA NU 2–2007 performance measurements for characterizing spatial resolution (SR), sensitivity, image quality (IQ), noise equivalent count rate (NECR) was performed using ¹⁸F. (B) Modelling of a realistic GATE Monte-Carlo model of the GE Signa integrated PET and to validate this model with the NEMA sensitivity and NECR tests for ¹⁸F, ¹¹C, ¹⁵O, ¹³N, ⁶⁸Ga, ⁸²Rb. NEMA tests were validated with measured values, published by GE Healthcare and with a NEMA test acquired on a PET/CT. (C) To investigate the effect of the 3 T MR field on positron range. The NEMA measurements for ¹⁸F (A) resulted in a minor difference system characteristic. The SR is about 1mm larger in the axial direction compared to PET/CT. The impact of this slightly lower resolution is also visible in the recovery coefficients of the smallest spheres in IQ tests. (B) Sensitivity and NECR results obtained with the GATE Monte Carlo simulated model matched with measured and published values. The differences in sensitivity are due to the scale factor from the positron emission fraction of the radioisotopes. The peak NECR was lower than for ¹⁸F and appears at higher activities. (C) The positron range is a tissue-dependent and strongly reduce the positron range by a factor of 3-4 in the transverse plane (x/y) to the magnetic field resulting in significantly increased density distribution in z-direction for higher positron energy emitters. The system performance of GE Signa integrated PET/MR was substantially different, in terms of NEMA sensitivity and NECR compared to ¹⁸F. For some radionuclide a correction in the reconstruction algorithm is necessary. (author)