Hybrid molecular and anatomical imaging devices, especially when simultaneously working, have shown to provide advantages over sequential acquisitions. In particular, we present in this preliminary study, the working performance of a brain positron emission tomography (PET) insert, within a 3T magnetic resonance imaging (MRI) system. To our knowledge, this is the largest PET system based on monolithic LYSO blocks. It consists of 60 scintillator blocks of

$50\ \text{mm} \times 50\ \text{mm} \times 20\ \text{mm}$

arranged in 3 rings of 20 detector modules each. An effective field of view (FOV) of 240 mm in diameter and 154 mm axially is defined. The crystals included a retroreflector layer at the entrance face, and are coupled to custom arrays of

$12 \times 12$

silicon photomultipliers (SiPM),

$3\ \text{mm} \times 3\ \text{mm}$

each. Frontend electronics provide

${X}$

and

${Y}$

projections of the scintillation light by sampling each row and column of the SiPM arrays. The insert is thermally stabilized by using temperature-controlled air to about 27 °C. The PET insert has been installed at the Klinikum rechts der Isar (Munich) and tested within the whole-body Siemens biograph mMR, a 3T MRI combined with a PET scanner. A system sensitivity of almost 7% for an energy window of 350–650 keV was measured using a small size source at the center of the FOV (CFOV). Current system evaluation showed a spatial resolution at the CFOV of 1.7 mm using iterative algorithms, being below 2 mm within a centered diameter of 120 mm. Rods of a small Derenzo phantom of 2.5 mm were clearly resolved, independently of all tested MRI sequences including echo-planar imaging, ultrashort time echo, MPrage, and T2-flair or arterial spin labeling. The PET insert did not show any count rate degradation also under those sequences for a variety of MR imaging acquisitions.

#	Concept	H-Index	Publications	Citations
1	Engineering	1	1	56
2	Health Sciences	1	1	56
3	Biomedical Engineering	1	1	56
4	Neuroscience	1	1	56
5	Biomedical Imaging	1	1	56

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