A digital phoswich detector using time-over-threshold for depth of interaction in PET

Area of Science:

• Medical Imaging
• Nuclear Instrumentation
• Materials Science

Background:

• Positron Emission Tomography (PET) imaging relies on detecting gamma rays from positron annihilation.
• Improving depth of interaction (DOI) determination in PET detectors enhances spatial resolution and reduces parallax error.
• Digital phoswich detectors offer potential for DOI measurement by combining signals from multiple scintillator layers.

Purpose of the Study:

• To evaluate the performance of a digital phoswich PET detector utilizing integrated charge and time-over-threshold (ToT) for DOI determination.
• To assess the effectiveness of ToT measurements in separating events from different scintillator layers and identifying cross-layer-crystal-scatter (CLCS) events.
• To demonstrate practical DOI determination and event separation using prototype LYSO/BGO phoswich detectors.

Main Methods:

• Simulations were conducted to assess the efficacy of ToT for event separation and CLCS identification.
• Prototype LYSO/BGO phoswich detectors with pixilated scintillator arrays were constructed.
• Digital readout of integrated charge and ToT was achieved using a PETsys TOFPET2 ASIC and multi-pixel photon counters.
• Flood imaging, energy resolution, and timing resolution measurements were performed.

Main Results:

• Simulations confirmed that ToT and charge integration provide sufficient information for DOI determination.
• Flood images showed clear resolution of individual crystals with high peak-to-valley ratios (11.8 ± 4.0 for LYSO, 10.1 ± 2.9 for BGO).
• Measured energy resolutions were 9.9% ± 1.3% (LYSO) and 28.5% ± 5.0% (BGO).
• Timing resolutions for LYSO-LYSO, LYSO-BGO, and BGO-BGO coincidences were 468 ps, 1.33 ns, and 2.14 ns, respectively.
• ToT successfully identified the interaction layer and rejected a majority of CLCS events.

Conclusions:

• Time-over-threshold (ToT) is a viable method for determining the interaction layer in digital phoswich PET detectors.
• The developed method effectively separates events from different scintillator layers (LYSO/BGO).
• The prototype detectors demonstrate good performance in terms of crystal resolution, energy, and timing, with ToT enabling CLCS event rejection.