Comparison between SiPM-based and PMT-based TOF-PET/CT to detect sub-centimeter spheres

Objectives: The new-generation Discovery MI (DMI) has silicon photomultipliers (SiPM) instead of PMT, and thus offers good intrinsic timing resolution. The DMI has been for penalized likelihood reconstruction (PL). Combining the DMI with SiPM and PL might be enable the accurate detection of small lesions. The present study aimed to compare the abilities of SiPM- and PMT-based TOF-PET/CT to detect sub-centimeter spheres representing small lesions.

Methods: We used a DMI and Discovery 710 (D710) PET/CT (GE healthcare) to image a NEMA body phantom comprising six spheres (diameter: 4, 5, 6, 8, 10, 13 mm) containing ¹⁸F solutions with target-to-background ratios (TBRs) of 8. The PET data were reconstructed using a baseline OSEM algorithm +PSF, +TOF, +PSF and TOF, and penalized likelihood (PL). The reconstructions included OSEM applied iteration numbers of 2 - 6 and Gaussian filters of 0, 2 and 4 mm. We investigated β-values of 50 - 300 using the PL

Methods: The duration of acquisition varied from 2 to 100 minutes. The dimensions of the reconstructed image matrices were 192 × 192, 256 × 256 and 384 × 384. Recovery coefficients (RC) and lesion detectability were calculated as the physical evaluation. Results: The RC values were higher in spheres ≤ 10 mm with the PL, than with the other reconstruction models. The trends of D710 and DMI were almost identical. The D710 overestimated the RC, whereas DMI became closer to the true value. Both OSEM+PSF and PL identified RC overestimation at 13 and 8 mm, respectively, due to the overshoot of edge artifacts. The ability to detect lesions was better for the D710 than the DMI. In contrast, the ability of DMI+PL to detect spheres of ≤ 5 mm was best among the reconstruction methods. Longer acquisition periods improved the ability of DMI to detect lesions. The RC and lesion detectability of the DMI and D710 did not vary according to matrix size, although DMI was visually more prominent as a function of increasing matrix size.

Conclusions: The quantitation and detectability of sub-centimeter spheres is better than those of conventional PMT-based PET/CT when SiPM-based PET is combined with PL.

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