  Hyperion-IID is a positron emission tomography (PET) insert which allows
simultaneous operation in a clinical magnetic resonance imaging (MRI) scanner.
To read out the scintillation light of the employed LYSO crystal arrays with a
pitch of 1 mm pitch and 12 mm in height, digital silicon photomultipliers (DPC
3200-22, Philips Digital Photon Counting) (DPC) are used. The basic PET
performance in terms of energy resolution, coincidence resolution time (CRT)
and sensitivity as a function of operating parameters, such as the operating
temperature, the applied overvoltage, activity and configuration parameters of
the DPCs, were evaluated on system level. The measured energy resolution did
not show a large dependency on the selected parameters and is in the range of
12.4-12.9% for low activities and degrades to ~13.6% at activities of ~100 MBq.
The CRT strongly depends on the selected trigger scheme (trig) of the DPCs. We
measured approximately 260 ps, 440 ps, 540 ps and 1300 ps for trig 1-4,
respectively. The trues sensitivity for a NEMA NU 4 mouse-sized scatter phantom
with a 70-mm-long tube of activity was dependent on the operating parameters
and was determined to be 0.4-1.4% at low activities. The random fraction stayed
below 5% at activities up to 100 MBq and the scatter fraction was evaluated as
~6% for an energy window of 411-561 keV and ~16% for 250-625 keV. Furthermore,
we performed imaging experiments using a mouse-sized hot-rod phantom and a
large rabbit-sized phantom. In 2D slices of the reconstructed mouse-sized
hot-rod phantom ({\O} = 28 mm), the rods were distinguishable from each other
down to a rod size of 0.8 mm. There was no benefit of the better CRT of trig 1
over trig 3, where in the larger rabbit-sized phantom ({\O} = 114 mm), we could
show a clear improvement of image quality using the time-of-flight information.
