3 NEMA Performance Measurements of Positron Emission Tomographs

__________________________________________________________________________________ The latest developments in PET technology have allowed for its integration with MR scanners as a superior alternative to PET/CT imaging, especially useful in instances such as detection of low-grade tumours and prostate cancer resurgence. However, the magnetic field inherent to the MR introduces some challenges and uncertainty regarding the scanners’ performance. Furthermore, novel radiopharmaceuticals that feature a wide range of radioisotopes with different properties have been introduced in the market and are being used more and more in clinical contexts. Thus, it is relevant to study how PET/MR systems perform when using such radioisotopes, and the impact the magnetic field has on the performance, as a way to better understand these scanners and optimize clinical practices. Recently, total-body PET scanners that entirely cover the patients, or large portions of the body, have also been introduced. In this dissertation, NEMA sensitivity and count rate statistics tests are performed on a realistic model of the GE Signa PET/MR scanner through GATE Monte Carlo simulations, using the prescribed F, as well as using other radioisotopes such as C, N, O, Ga and Rb, which feature positron energies from 633 keV do 3.3 MeV. The same studies are performed on a conceptual 1.04 m long PET scanner to study the potential performance of such a scanner. Also, a study of the positron range of the same radioisotopes in tissues of varying density was done in order to estimate the impact of the magnetic field. On the GE Signa PET/MR, the sensitivity test results are in line with published values and show a clear dependence on the positron branching ratio, with the pure β emitters having the highest sensitivity values, 21.50 cps/kBq for N in the presence of a 3 T MR field. The magnetic field increases sensitivity up to 5% for the higher energy radioisotopes, due to constraints on positron range preventing positron from escaping the phantom tube. The move to TB-PET shows a tremendous increase of up to 7 times the sensitivity, peaking at 177.87 cps/kBq for N. Regarding the NECR test, the simulation results on the GE Signa PET/MR are confirmed by the published values regarding F, with the other pure β emitters showing comparable peak NECR values. However, Ga and Rb have much lower peak NECR values, due to the 3.0% and 14.2% additional prompt-gamma emission, which contaminate the acquisition and contribute to detector busyness, effectively lowering NECR. Studies on positron range show strong dependence on the tissue density. The 3 T magnetic field introduces significant constraints on positron range in the transversal plane which can mean up to 4 times smaller range in the x and y directions when compared with the z direction, the direction of the MR field, which shows differences only in terms of density distribution but not absolute values. The results obtained in this dissertation point out the need to re-evaluate the different image reconstruction algorithms for PET/MR imaging, given the discrepancies between the transversal and axial ranges under a magnetic field, as well as the adaptation of the NEMA performance measurements protocols for scanners with long axial fields of view, and, to some extent, to be performed with different radioisotopes. However, there are some limitations to the conclusions drawn from the work presented, such as the fact that the method used to estimate NECR and the related measures can underestimate them due to uncertainty regarding dead time set-up. The work developed in this thesis and the achieved outcomes have real world applications in more than one area. Learning how scanners perform under difference circumstances, i.e. using different