Periodic Reporting for period 4 - PETALO (A positron emission tomography apparatus based on liquid xenon with time of flight applications)
Période du rapport: 2023-01-01 au 2024-06-30
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 is 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.
The prototype built in this project pointed to an excellent energy resolution, very competitive with the current commercial PET scanners, and a time of resolution which makes it possible to use Time-Of-Flight in the device. Both these caractheristics (energy and time resolution) contribute to a reduction of the noise in the image reconstruction, therefore providing better images in less time.
The prototype built in this project pointed to an excellent energy resolution, very competitive with the current commercial PET scanners, and a time of resolution which makes it possible to use Time-Of-Flight in the device. Both these caractheristics (energy and time resolution) contribute to a reduction of the noise in the image reconstruction, therefore providing better images in less time.
The aim of the protoype built in the project 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 read out the scintillation light produced by a Na-22 calibration source placed in the middle. While some components of the prototype were purchased as commercial solutions, for instance, the cryocooler that liquefies xenon or the gas filter, most of them were 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 was also developed, which reads the output of the ASICs that digitize the signal and distribute and synchronize signals from different ASICs. During the duration of the project, we tested the system, proving its excellent stability in the different phases of operation (filling, recirculation, data taking, recovery) and we characterised the full electronic chain and evaluated different kinds of SiPMs. After that, we measured the energy resolution using only scintillation light, achieving very good results for 511-keV gammas in liquid xenon. While this measurement needs to be refined, the first results already point to an excellent performance for liquid xenon, compared to the current state-of-the-art.
We have also measured the time resolution of liquid xenon, obtaining a result which is very competitive with the current commercial PET scanners. Excellent time and energy resolutions improve the quality of the images, allowing for achieving better images in less time.
At the same time, software tools for simulation, reconstruction and analysis were developed and Monte Carlo simulations were carried out, to guide the decisions on the design of the prototype and to study large-scale PET scanners to demonstrate the potential of the concept.
The performance of a full-body PET scanner was also studied, in particular we started to apply image reconstruction algorithms to the Monte Carlo simulation of a body-size PET. A first study was published, which assesses the attainable precision in the determination of the gamma interaction with a detector based in the PETALO concept. Moreover, we obtained a spatial resolution in the reconstructed image of around 1 mm for one point, a first proof the feasibility of the technology.
We have also measured the time resolution of liquid xenon, obtaining a result which is very competitive with the current commercial PET scanners. Excellent time and energy resolutions improve the quality of the images, allowing for achieving better images in less time.
At the same time, software tools for simulation, reconstruction and analysis were developed and Monte Carlo simulations were carried out, to guide the decisions on the design of the prototype and to study large-scale PET scanners to demonstrate the potential of the concept.
The performance of a full-body PET scanner was also studied, in particular we started to apply image reconstruction algorithms to the Monte Carlo simulation of a body-size PET. A first study was published, which assesses the attainable precision in the determination of the gamma interaction with a detector based in the PETALO concept. Moreover, we obtained a spatial resolution in the reconstructed image of around 1 mm for one point, a first proof the feasibility of the technology.
The results of this project demonstrate that liquid xenon has the potential to compete with scintillating crystals, both in terms of energy and time resolution. Therefore, it is an interesting alternative, which presents some advantages (such as lower complexity and higher homogeneity), and some drawbacks (such as the current market price for xenon, severly affected by geopolitical circunstances).