Speaker
Description
The use of Time-of-Flight (TOF) information has become a standard in Positron Emission Tomography (PET). It allows a more precise determination of the radioisotope location, resulting in an improvement of the reconstructed images and better diagnosis of pathologies in patients. The high-performance photodetectors used in clinical scanners are one of the key components for TOF-PET, playing a key role in achieving the precise timing resolution required for accurate imaging. As a result, there is an increasing demand for photodetectors capable of reaching timing resolutions in the order of the few hundreds of picoseconds.
In this work we have investigated the timing capabilities of the new MAPD-3NM-II silicon photomultiplier (SiPM) for PET. These new SiPMs utilize advanced technology with buried pixel structures, enabling a pixel density of up to 40,000 pixels/mm², resulting in high photodetection efficiency and 100% fill factor. The MAPD-3NM-II photodiode used in this experiment has the following parameters: PDE-35% (450nm), gain~3*105, operation voltage-54-56.5 V, capacitance-150 pF, number of total pixels of 66500 pixels and a sensitive area - 3.7 x 3.7 mm2. Such a design allows for a much broader dynamic range. Although the MAPD-3NM-II photodiode area is 52% larger than its counterpart, its capacitance is 3.4 times smaller. Despite the MAPD-3NM-II photodiode's area being 52 % larger than that of its counterpart, its capacitance is 3.4 times smaller, making it particularly well-suited for timing measurements.
To evaluate their timing performance, a coincidence setup was tested using the SIPMs coupled to a PETSYS TOFPET2 evaluation kit for data readout, inside of a light tight box kept at a constant temperature of about 21 ºC. As scintillator was used LYSO(Ce) crystal with dimensions of 3 × 3 × 5 mm. All sides of the crystal were wrapped in Teflon, except for one side, which was attached to the photodiode using optical grease. An energy resolution of 6 % was achieved on the 511 keV photopeak of a Na-22 source. The best achievable coincidence timing resolution (CTR) was determined by measuring this parameter across a range of different biases and thresholds. Through this method a value of 304±3 ps was achieved at an overvoltage of +4.5 V. By correcting time walk effects correlated to different rise times in the used energy window it was possible to achieve a timing resolution of 301±5 ps. More studies will be done to: characterize the temperature coefficient of these SIPMs and its effect on the CTR, fully calibrate the system to correct possible non-linearities that may be affecting the energy resolution and to improve the time walk correction to achieve the best possible CTR, using the same setup configuration.
A similar setup with the S14160-3015PS SiPMs from Hamamatsu was used for comparison. These SiPMs were chosen because they have the same pixel pitch size of 15 μm as the MAPD-3NM-II. At the same overvoltage these were able to achieve a CTR of 308±10 ps and an energy resolution of 10 %.
This study presents an overview of the timing capabilities of a new technology of SiPMs. Results are promising and will be shown and discussed. Further development of these photodetectors with a larger pitch size holds potential for achieving state-of-the-art CTR values in TOF applications.
