Monte Carlo Simulations of the GE Signa PET/MR for Different Radioisotopes
2020
NEMA characterization of PET systems is generally based on 18F because it is the most
relevant radioisotope for the clinical use of PET. 18F has a half-life of 109.7 min and
decays into stable 18O via bC emission with a probability of over 96% and a maximum
positron energy of 0.633 MeV. Other commercially available PET radioisotopes, such as
82Rb and 68Ga have more complex decay schemes with a variety of prompt gammas,
which can directly fall into the energy window and induce false coincidence detections
by the PET scanner.
Methods: Aim of this work was three-fold: (A) Develop a GATE model of the GE Signa
PET/MR to perform realistic and relevant Monte Carlo simulations (B) Validate this model
with published sensitivity and Noise Equivalent Count Rate (NECR) data for 18F and
68Ga (C) Use the validated GATE-model to predict the system performance for other
PET isotopes including 11C, 15O, 13N, 82Rb, and 68Ga and to evaluate the effect of a
3T magnetic field on the positron range.
Results: Simulated sensitivity and NECR tests performed with the GATE-model for
different radioisotopes were in line with literature values. Simulated sensitivities for 18F
and 68Ga were 21.2 and 19.0 /kBq, respectively, for the center position and 21.1
and 19.0 cps/kBq, respectively, for the 10 cm off-center position compared to the
corresponding measured values of 21.8 and 20.0 cps/kBq for the center position
and 21.1 and 19.6 cps/kBq for the 10 cm off-center position. In terms of NECR,
the simulated peak NECR was 216.8 kcps at 17.40 kBq/ml for 18F and 207.1 kcps
at 20.10 kBq/ml for 68Ga compared to the measured peak NECR of 216.8 kcps at
18.60 kBq/ml and 205.6 kcps at 20.40 kBq/ml for18F and 68Ga, respectively. For 11C,
13N, and 15O, results confirmed a peak NECR similar to 18F with the effective activity
concentration scaled by the inverse of the positron fraction. For 82Rb, and 68Ga, the
peak NECR was lower than for 18F while the corresponding activity concentrations were
higher. For the higher energy positron emitters, the positron range was confirmed to be tissue-dependent with a reduction of the positron range by a factor of 3 to 4 in the plane
perpendicular to the magnetic field and an increased positron range along the direction
of the magnetic field.
Conclusion: Monte-Carlo simulations were used to predict sensitivity and NECR
performance of GE Signa PET/MR for 18F, 15O, 13N, 11C, 82Rb, and 68Ga radioisotopes
and were in line with literature data. Simulations confirmed that sensitivity and NECR
were influenced by the particular decay scheme of each isotope. As expected, the
positron range decreased in the direction perpendicular to the 3T magnetic field.
However, this will be only partially improving the resolution properties of a clinical
PET/MR system due to the limiting spatial resolution of the PET detector.
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