Evaluation of silicon photomultipliers: A promising new detector for MR compatible PET

Abstract

The Silicon Photomultiplier (SiPM) is a compact detector which shows great promise for use in MR compatible PET systems. SiPMs are insensitive to magnetic fields, have a high intrinsic gain (about 10 6) and operate at low voltages (40 V), eliminating the need for sophisticated shielding and low noise preamplifiers. A prototype 1×1 mm 2 detector currently under investigation at the University of Toronto consists of an array of 556 independent microcells operating in Geiger mode. Each microcell thus produces a pulse of constant amplitude with the detection of one or more electrons. All microcells are connected in parallel through integrated quenching resistors, and thus the output of the SiPM is the sum of the individual standardized signals and is proportional to energy provided the number of detected light photons is less than the number of microcells. The detector was operated without a preamplifier, and using a 2×2×6 mm 2 LSO crystal and 22Na source, the energy resolution was measured to be 25.6±0.4% FWHM and timing resolution for a detector pair was 1.9±0.1 ns FWHM. LED measurements and measurements using 133Ba (356 keV) and 137Cs (662 keV) verify that the detector is operating linearly at 511 keV. The major noise contribution is the dark current, which at −44 V results in a peak dark count rate of about 16 kHz that reduces to negligible levels above a 90 keV lower energy threshold. In the next generation of compact multi-element SiPM detectors with improved optical coupling the energy resolution should improve to below 15% FWHM, making them well suited for use in a high performance MR compatible depth of interaction PET scanner.