Evaluation of PET performance and MR compatibility of a preclinical PET/MR insert with digital silicon photomultiplier technology

In this work we present detailed characterizations of our preclinical high resolution
PET/MR insert based on the Hyperion-IID platform. The PET/MR insert consists of a
ring of 10 singles detection modules, each comprising 2×3 scintillation detector stacks.
Each detector stack features a 30×30 pixelated LYSO crystal array with a height of
12 mm and a pitch of 1 mm, coupled via a slit 2 mm light guide to a digital SiPM tile.
The PET performance is stable under a wide range of operating points. The spatial
resolution is below 1Ä,mm and the CRT reaches 260 or 450 ps depending on trigger settings.
The energy resolution is 12.6% FWHM. The characterization of the MR compatibility
showed no relevant degradation in PET performance during MRI operation. On the MRI
side, we observe a degradation in B0 homogeneity from a VRMS of 0.03 ppm to 0.08 ppm
with active shimming, while observing only minor degradations in the B0 field. The
noise floor is slightly increased by 2-15% without any observable dependence on the
activity. The Z gradients induces an observable eddy current inside the PET inserts
which can lead to ghosting artifacts for EPI sequences. However, we don’t observe
any visible image degradation for widely used anatomical imaging sequences such as
gradient echo and turbo spin echo sequences. To prove the viability of our PET/MR
insert for in vivo small animal studies, we successfully performed a longitudinal
mouse study with subcutaneously injected tumor model cells. The simultaneously acquired
PET/MR images provide a high level of anatomical information and soft tissue contrast
in the MR layer together with a high resolution image of the FDG tracer distribution
in the PET layer.