The Small Animal Fast Insert for MRI (SAFIR) collaboration is developing novel Positron Emission Tomography (PET) inserts for a (7,hbox {T}) Bruker BioSpec 70/30 Ultra Shield Refrigerated (USR) Magnetic Resonance Imaging (MRI) system [1, 2]. The inserts are specified to enable quantitative, truly simultaneous PET/MRI research in mice and rats, with a spatial resolution of around (2,hbox {mm}) in the center of the Field Of View (FOV), at injected activities extending up to (500,hbox {MBq}). Exploitation of the associated high count rates, specifically the corresponding low noise, enables an exceptional image frame acquisition speed in the order of one diagnostically relevant frame every few seconds [1, 3, cf.]. This high frame rate is required for capturing fast tracer kinetics, which was not possible with existing small-animal PET systems [1, cf.].
A first insert, Small Animal Fast Insert for MRI detector I (SAFIR-I), has successfully been constructed and commissioned [1]. In this study, fundamental performance parameters of SAFIR-I have been determined according to the NEMA Standards Publication NU 4-2008 (NEMA-NU4) protocol.
The SAFIR-I detector design
The complete design and functionality of the detector have previously been described in [1]. Additional material on the data acquisition software and the analysis software can be found in [3,4,5]. The following paragraphs summarize the key information from those sources.
SAFIR-I, shown in Figs. 1 and 2, has a dodecagonal shape with an outer diameter of (198,hbox {mm}), an inner diameter of (114,hbox {mm}) and an axial FOV of (54.2,hbox {mm}). The detector head comprises 24 rings of Lutetium Yttrium OxyorthoSilicate (LYSO) crystals (Sichuan Tianle Photonics) with dimension (2.12,hbox {mm},times ,2.12,hbox {mm},times 13,hbox {mm}), which are separated by Enhanced Specular Reflector (ESR) foils (3 M), assembled into pairs of matrices of (8 times 8) and (7 times 8) crystals (due to spatial constraints) at a pitch of (2.2,hbox {mm}). Three such pairs are mounted in series in axial direction at a pitch of (18.1,hbox {mm}), effectively forming three rings of crystal blocks, and hence summing up to 4320 crystals in the detector head in total. Due to the weakly radioactive (half life of ((4.05 pm 0.09) times 10^{10},hbox {year}) [6]) (^{176})Lu contained in the crystal material, contributions from the intrinsic radioactivity of the detector have to be considered explicitly below.
The crystal matrices are read out by Silicon Photomultiplier (SiPM) arrays (Hamamatsu S13361-2050AE-08 SPL0) matching the crystal matrices’ pitch. The signals are digitized by Application Specific Integrated Circuits (ASICs) (Position-Energy-Timing Application Specific Integrated Circuit, version 6, Single Ended (PETA6SE) [7, 8], 144 in total), subsequently processed by a Field-Programmable Gate Array (FPGA) (Xilinx Kintex-7 XC7K70T) in every detector section and then passed to a Data Acquisition (DAQ) computer through (1,hbox {GBit}) Ethernet connections.
The data processing including all calibration steps is described in [1, 4]. Coincidences are saved in listmode files subsequently used for reconstruction and data analysis according to NEMA-NU4.
