Introduction

The GE Discovery MI PET/CT system is a time-of-flight (TOF) PET system based on silicon photomultipliers (SiMs) that was initially available with 3- or 4-rings of PET detectors (with respective axial field of views of 15- or 20-cm \cite{Hsu_2017} ). This system has been recently upgraded with the addition of a 5th ring, which results in a 25-cm large axial FOV and provides increased sensitivity.  
Similarly to the 3- and 4-ring systems, each PET ring uses 136 detector blocks, each of which comprises a 4x9 array of lutetium-yttrium-oxyorthosilicate (LYSO) scintillator crystals coupled to a 3 x 6 array of SiPMs. The crystal elements are 25 x 3.95 x 5.3 mm, and each Hamamatsu SiPM array has 2 x 3 pixels with an active area of 4 x 6 mm. The system incorporates a 128-slice CT.  (Depending on final length, consider adding a tble with summary of physical specifications?)