Assessment of MR-compatibility of SiPM PET insert using short optical fiber bundles for small animal research

Simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) can provide new perspectives in human disease research because of their complementary in-vivo imaging techniques. Previously, we have developed an MR-compatible PET insert based on optical fibers using silicon photomultipliers (SiPM). However when echo planar imaging (EPI) sequence was performed, signal intensity was slowly decreased by −0.9% over the 5.5 minutes and significant geometrical distortion was observed as the PET insert was installed inside an MRI bore, indicating that the PET electronics and its shielding boxes might have been too close to an MR imaging object. In this paper, optical fiber bundles with a length of 54 mm instead of 31 mm were employed to minimize PET interference on MR images. Furthermore, the LYSO crystals with a size of 1.5 × 1.5 × 7.0 mm³ were used instead of 2.47 × 2.74 × 20.0 mm³ for preclinical PET/MR applications. To improve the MR image quality, two receive-only loop coils were used. The effects of the PET insert on the SNR of the MR image either for morphological or advanced MR pulse sequences such as diffusion weighted imaging (DWI), functional MRI (fMRI), and magnetic resonance spectroscopy (MRS) were investigated. The quantitative MR compatibility such as B₀ and B₁ field homogeneity without PET, with `PET OFF', and with `PET ON' was also evaluated. In conclusion, B₀ maps were not affected by the proposed PET insert whereas B₁ maps were significantly affected by the PET insert. The advanced MRI sequences such as DWI, EPI, and MRS can be performed without a significant MR image quality degradation.