Periodic Reporting for period 3 - PETALO (A positron emission tomography apparatus based on liquid xenon with time of flight applications)
Période du rapport: 2021-07-01 au 2022-12-31
This project presents a new technology for detectors used in positron emission tomography (PET), based on liquid xenon instead of current scintillator crystals. The xenon scintillation light will be read out by silicon photomultipliers and low power, low noise customized integrated circuits for time of flight applications. Liquid xenon presents several potential advantages compared to the current technology, such as an excellent energy resolution at the energy relevant for PET, high light yield, fast scintillation time, the possibility of providing a more homogeneous response and cost reduction. This technology can improve the reconstructed image quality, therefore bring a reduction of the exposure time of the patient. Moreover, a reduction in the cost would make a full-body PET – very expensive – more affordable and make it possible to have more full-body scanners in hospitals.
PETALO proposes to build a set of prototypes to evaluate the performance of this technology.
PETALO proposes to build a set of prototypes to evaluate the performance of this technology.
The aim of the first protoype is to perform the measurement of energy and time resolution achievable in a liquid xenon PET scanner. It consists of an aluminum box filled with LXe, with two arrays of SiPMs on opposite sides, which will read out the scintillation light produced by a Na-22 calibration source placed in the middle. While some components of the prototype have been purchased as commercial solutions, for instance, the cryocooler that liquefies xenon or the gas filter, most of them have been designed specifically for this application. They include the gas system for the recirculation and purification of the gas, the vacuum vessel for thermal isolation, and the thermal links that connect the cryocooler with the xenon container and keep the temperature gradients under control. Especially crucial are the feedthroughs, which must bear high thermal stress and provide tightness to vacuum and liquid xenon pressure. A dedicated DAQ system has been also developed, which reads the output of the ASICs that digitize the signal and distribute and synchronize signals from different ASICs. The prototype has been assembled and has been taking data for the last year. During this time, on the one hand, we have tested the system, proving its excellent stability in the different phases of operation (filling, recirculation, data taking, recovery), we have characterised the full electronic chain and evaluated different kinds of SiPMs. On the other hand, we have measured the best energy resolution achieved so far in liquid xenon, using only scintillation light, that is 4.2% FWHM for 511-keV gammas. This result improves dramatically on the current knowledge of the field and suggests that no intrinsic fluctuations are present in the scintillation production in liquid xenon, contrary to what has been assumed so far. This result opens the possibility of building a PET with unprecedented energy resolution, with a better performance than the current state-of-the-art, especially for full-body PET applications.
The results we achieved on the energy resolution have been obtained in a small volume of xenon, with the aim of assessing the best performance attainable in liquid xenon. In the next months we want to measure the energy resolution in a larger volume, more similar to that of a real PET scanner. Moreover, we expect to assess the time resolution of the prototype using the timestamp provided by the ASICs, since a good time resolution would also improve the quality of the reconstructed images.
The thickness of xenon is also a crucial parameter to be studied. 3 cm of liquid xenon stop around 60% of 511-keV high energy gammas. A larger thickness would bring to a better sensitivity, but the position and the time resolution would worsen. We want to study a thickness of up to 4 cm with the current prototype.
Finally, SiPMs from a different provider will be used in the set-up to study their performance: having more than one possible provider is crucial for the future scalability of the project and for a possible industrial exploitation.
The thickness of xenon is also a crucial parameter to be studied. 3 cm of liquid xenon stop around 60% of 511-keV high energy gammas. A larger thickness would bring to a better sensitivity, but the position and the time resolution would worsen. We want to study a thickness of up to 4 cm with the current prototype.
Finally, SiPMs from a different provider will be used in the set-up to study their performance: having more than one possible provider is crucial for the future scalability of the project and for a possible industrial exploitation.