Optimal whole-body PET scanner configurations for different volumes of LSO scintillator: a simulation study

Abstract

The axial field of view (AFOV) of the current generation of clinical whole-body PET scanners range from 15-22 cm, which limits sensitivity and renders applications such as whole-body dynamic imaging or imaging of very low activities in whole-body cellular tracking studies, almost impossible. Generally, extending the AFOV significantly increases the sensitivity and count-rate performance. However, extending the AFOV while maintaining detector thickness has significant cost implications. In addition, random coincidences, detector dead time, and object attenuation may reduce scanner performance as the AFOV increases. In this paper, we use Monte Carlo simulations to find the optimal scanner geometry (i.e. AFOV, detector thickness and acceptance angle) based on count-rate performance for a range of scintillator volumes ranging from 10 to 93 l with detector thickness varying from 5 to 20 mm. We compare the results to the performance of a scanner based on the current Siemens Biograph mCT geometry and electronics. Our simulation models were developed based on individual components of the Siemens Biograph mCT and were validated against experimental data using the NEMA NU-2 2007 count-rate protocol. In the study, noise-equivalent count rate (NECR) was computed as a function of maximum ring difference (i.e. acceptance angle) and activity concentration using a 27 cm diameter, 200 cm uniformly filled cylindrical phantom for each scanner configuration. To reduce the effect of random coincidences, we implemented a variable coincidence time window based on the length of the lines of response, which increased NECR performance up to 10% compared to using a static coincidence time window for scanners with a large maximum ring difference values. For a given scintillator volume, the optimal configuration results in modest count-rate performance gains of up to 16% compared to the shortest AFOV scanner with the thickest detectors. However, the longest AFOV of approximately 2 m with 20 mm thick detectors resulted in performance gains of 25-31 times higher NECR relative to the current Siemens Biograph mCT scanner configuration.

Figure 1

The total qualified singles rate at each bucket after multiplexing for the simulated model and actual Siemens Biograph mCT using the 20 cm diameter NEMA count-rate cylindrical phantom.

Figure 2

The simulated model and measured results for the 20 cm diameter NEMA count-rate cylindrical phantom on the Siemens Biograph mCT. The trues+scatters (top left), randoms (top right), NEC 1R (bottom left) and scatter fraction (bottom right) were calculated using the NEMA NU 2–2007 method.

Figure 3

NEC 1R plotted as a function of maximum ring difference and activity concentration for the ~10 l—4 axial blocks mCT-like scanner (left) and the ~90 l—36 axial blocks scanner (right).

Figure 4

Peak NEC 1R results using the variable coincidence timing window method for the set of scanners with 10 and 15 l of scintillator volume.

Figure 5

Peak NEC 1R results using the variable coincidence timing window method (a). Peak NEC 1R results using the variable coincidence timing window method at 1 kBq/cc activity concentration (b). The relative gain ratio is calculated by normalizing the NEC 1R values with respect to the 10 l (mCT-like) scanner result.

Figure 6

Optimal activity concentration where peak NECR occurs is plotted as a function of axial length for scanners with 20 mm thick detector blocks. Injected activity is calculated assuming a half life of 109.7 min, 20% excretion, and 1 h uptake.

Figure 7

Maximum block singles rate and total system singles rate at the optimal activity concentration where peak NECR occurs is plotted as a function of axial length for all scanners with 20 mm thick detector blocks. The singles rate was characterized with dead time and no dead time to show the singles count loss rate.

Figure 8

Total system coincidence rate at the optimal activity concentration where peak NECR occurs is plotted as a function of axial length for all scanners with 20 mm thick detector blocks.