Development and evaluation of a new high-TOF-resolution all-digital brain PET system.

Objective: Time-of-flight (TOF) capability and high sensitivity are essential for brain-dedicated Positron Emission Tomography (PET) imaging, as they improve the contrast and the signal-to-noise ratio (SNR) enabling a precise localization of functional mechanisms in the different brain regions. 
 Approach: We present a new brain PET system with transverse and axial field-of-view (FOV) of 320 mm and 255 mm, respectively. The system head is an array of 6 x 6 detection elements, each consisting of a 3.9 x 3.9 x 20mm3 LYSO crystal coupled with a 3.93 x 3.93 mm2 SiPM. The SiPMs analog signals are individually digitized using the multi-voltage threshold (MVT) technology, employing a 1:1:1 coupling configuration.
 Main results: The brain PET system exhibits a TOF resolution of 249 ps at 5.3 kBq/mL, an average sensitivity of 22.1 cps/kBq, and a noise equivalent count rate (NECR) peak of 150.9 kcps at 8.36 kBq/mL. Furthermore, the mini-Derenzo phantom study demonstrated the system's ability to distinguish rods with a diameter of 2.0 mm. Moreover, incorporating the TOF reconstruction algorithm in an image quality phantom study optimizes the background variability, resulting in reductions ranging from 44% (37 mm) to 75% (10 mm) with comparable contrast. In the human brain imaging study, the signal-to-noise ratio (SNR) improved by a factor of 1.7 with the inclusion of TOF, increasing from 27.07 to 46.05. Time-dynamic human brain imaging was performed, showing the distinctive traits of cortex and thalamus uptake, as well as of the arterial and venous flow with 2 s per time frame.
 Significance: The system exhibited a good TOF capability, which is coupled with the high sensitivity and count rate performance based on the MVT digital sampling technique. The developed TOF-enabled brain PET system opens the possibility of precise kinetic brain PET imaging, towards new quantitative predictive brain diagnostics.