First Results From a High-Resolution Small Animal SiPM PET Insert for PET/MR Imaging at 7T

Abstract

We present the initial results from a small animal PET insert designed to be operated inside a 7T MRI. The insert fits within the 114 mm inner diameter of the Bruker BGA-12S gradient coil while accommodating the Bruker 35 mm volume RF coil (outer diameter 60 mm), both used in the Bruker 70/20 MRI systems. The PET insert is a ring comprising 16 detectors. Each detector has a dual-layer offset (DLO) lutetium-yttrium oxyorthsilicate (LYSO) scintillator array read out by two SensL SPMArray4B SiPM arrays. The DLO scintillator has bottom (top) layers of: <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$22\times 10$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$21 \times 9$ </tex-math></inline-formula> ) crystals of size <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.2 \times 1.2 \times 6$ </tex-math></inline-formula> (4) mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for a total of 409 crystals per block, providing an axial extent of 28.17 mm. The detector outputs are multiplexed to four signals using a custom readout board and digitized using the OpenPET data acquisition platform. The detector flood images successfully resolve over 99% of the crystals, with average energy resolution of 12.5 ± 2.0% at 511 keV. Testing of the PET system inside the MRI showed that the PET insert had no effect on MRI image homogeneity and only a small effect on echo planar images (EPI) signal to noise ratio (SNR) (-9%), with neither PET nor MRI images showing obvious artefacts. These acquisitions used the OpenPET operating in “oscilloscope mode” with USB2.0 interface, allowing a maximum total singles event rate of 280 kcps, strongly limiting the count rate capabilities of the system. The PET radial spatial resolution (as measured with a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> Na point source and FBP-3DRP reconstruction) is 1.17 mm at the centre, degrading to 1.86 mm at a 15 mm radial offset. Simultaneous phantom and mouse PET/MR imaging produced good quality images that were free of any obvious artefacts.