[Paper Review] Initial PET Performance Evaluation of a Preclinical Insert for PET/MRI with Digital SiPM Technology

David Schug, Christoph Lerche+9 more|arXiv (Cornell University)|Jul 2, 2015

Medical Imaging Techniques and Applications54 references51 citations

TL;DR

This study evaluates the initial performance of a preclinical PET/MRI insert, Hyperion-IID, using digital silicon photomultipliers (dSiPMs) with LYSO scintillator crystals. It demonstrates stable PET performance across varying operating parameters, with a coincidence resolution time as low as 260 ps (trig 1), energy resolution of 12.4–12.9%, and sensitivity of 0.4–1.4% at low activity, showing significant image quality improvement with time-of-flight (ToF) information in large phantoms.

ABSTRACT

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.

Motivation & Objective

• To evaluate the performance of a preclinical PET/MRI insert using digital SiPMs (dSiPMs) in a clinical MRI environment.
• To assess the impact of dSiPM operating parameters—overvoltage, trigger scheme, and validation level—on PET system performance.
• To determine optimal measurement settings for maximizing sensitivity, energy resolution, and timing performance.
• To investigate the benefit of time-of-flight (ToF) information in improving image quality for different phantom sizes.

Proposed method

• The Hyperion-IID PET insert uses 60 dSiPM sensor tiles with digital photon counting (DPC) technology (Philips DPC 3200-22) to read out LYSO scintillator arrays (1 mm pitch, 12 mm height).
• System-level performance was evaluated using NEMA NU 4-compliant phantoms: a mouse-sized hot-rod phantom and a larger rabbit-sized scatter phantom.
• Raw DPC sensor data were captured and processed using the COG-ACE algorithm for event reconstruction and performance analysis.
• Key parameters varied included overvoltage (2.5 V to 3.0 V), trigger scheme (trig 1–4), and validation level (val 28ph to 37ph), with temperature and activity levels also adjusted.
• Image quality was assessed via reconstructed 2D slices, and signal-to-noise ratio (SNR) and noise-equivalent count rate (NECR) were analyzed.
• Interference with MRI was minimized via MRI-compatible design, and optical Ethernet links transmitted digitized data from inside the bore to the acquisition server.

Experimental results

Research questions

• RQ1 How does the dSiPM trigger scheme (trig 1–4) affect coincidence resolution time (CRT) and image quality in PET/MRI?
• RQ2 What is the impact of overvoltage and validation level on energy resolution, sensitivity, and data saturation in the Hyperion-IID system?
• RQ3 To what extent does time-of-flight (ToF) information improve image quality in small versus large phantoms?
• RQ4 How do temperature and activity levels influence system performance, particularly in terms of sensitivity and NECR?
• RQ5 What are the optimal operating parameters for maximizing PET performance in a preclinical dSiPM-based PET/MRI system?

Key findings

• The coincidence resolution time (CRT) varied significantly with trigger scheme: 260 ps (trig 1), 440 ps (trig 2), 550 ps (trig 3), and 1300 ps (trig 4), with trig 1 providing the best timing performance.
• Energy resolution remained stable at 12.4–12.9% for low activity, degrading to ∼13.6% at 100 MBq.
• Sensitivity ranged from 0.4% to 1.4% at low activity, with the random fraction below 5% up to 100 MBq and scatter fraction ∼6% (411–561 keV) or ∼16% (250–625 keV).
• In the mouse-sized phantom, rods as small as 0.8 mm were distinguishable, but ToF did not significantly improve image quality due to the small size.
• In the rabbit-sized phantom, ToF with trig 1 (260 ps CRT) significantly improved image quality over non-ToF and other trigger schemes.
• The highest NECR was achieved at low temperature (<10 °C), low overvoltage (2.5 V), and trig 1 or 2, with optimal settings depending on activity and object size.