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A Digital Preclinical PET/MRI Insert and Initial Results

B. Weissler, et al
IEEE Trans Med Imaging, vol. 34, issue 11: 2258-2270, November 2015

Combining Positron Emission Tomography (PET) with Magnetic Resonance Imaging (MRI) results in a promising hybrid molecular imaging modality as it unifies the high sensitivity of PET for molecular and cellular processes with the functional and anatomical information from MRI.

Digital Silicon Photomultipliers (dSiPMs) from Philips Digital Photon Counting (PDPC) were used to develop an MRI-compatible preclinical PET/RF gantry as an insert for clinical MRI scanners. DSiPMs are the digital evolution in scintillation light detector technology, and their usage in PET detectors promise very high SNR. On the other hand, bringing digital electronics directly into the bore of an MRI scanner creates many technological challenges.

With a scintillation crystal pitch of 1 mm, the PET system achieves a high spatial resolution of 0.73 mm3 (FWHM). Furthermore, with a time resolution of up to 260 ps (FWHM), it introduces Time-of-Flight PET to preclinical applications.

Currently, three different PET-transparent RF coils are available: A small 1H coil, dedicated to mouse imaging, a large 1H coil for rabbit-sized objects, and a double-resonant multi-nuclei coil for simultaneous 1H/19F-MR / PET imaging.

Unintended interactions between both systems are minimized: MRI SNR is reduced by less than 13%, and 0.1 ppm (VRMS) B0-homogeneity is kept within a diameter of 96 mm. Neither PET nor MR images show any artifacts when the other modality is in operation.

A longitudinal study of a tumor-bearing mouse has demonstrated the operability, stability, and in-vivo capabilities of the system. Breathing and heart activity were monitored, and the resulting dual-gated cardiac PET/MRI CINE sequences of the mouse heart demonstrated the advantage of simultaneous image acquisition for temporal and spatial image registration.