The Time-of-Flight Positron Emission Tomography (ToF-PET) is a powerful imaging technique employed in a variety of medical fields, from oncology to neurology. A radiotracer labelled with a - radioactive isotope (a positron emitter) is injected into the patient and then it is attracted to areas of high metabolism, such as tumours. The isotope undergoes radioactive decay, releasing a positron which then annihilates with a surrounding electron producing two back-to-back γ photons with 511 keV energy. These γ-rays are then sensed by scintillating detectors placed around the patient's body and used to reconstruct the image of the radio-labelled tissue. One of the limiting factors of this technique is the detectors ability to discriminate in time the arrival of the γ-photons pairs, i.e. the coincidence time resolution CTR. The CTR should be as low as possible to improve the instrument sensitivity and the image signal-to-noise ratio at low radiotracers concentration. Much effort is now put in the design and development of new scintillators aiming at a CTR < 50 ps, because none of the existing monolithic scintillators shows simultaneously the high efficiency and fast emission necessary to reach it. In order to overcome this obstacle, the concept of scintillating heterostructure has been proposed. Here the scintillator is fabricated by coupling heavy crystalline scintillators to a fast-emitting plastic scintillators to realize a multicomponent material. The dense component stop the -rays, and the fast emission of the plastic component is activated by the recombination of diffusing highly energetic recoil electrons generated by photoelectric effect in the dense component. This energy sharing mechanism is the key to possibly obtain the desired CTR. Unfortunately, scintillating plastics have typical densities of ρ≈1 g cm-3, thus the presence of the fast emitter decreases the global density of the heterostructure reducing its stopping power of -rays. To mitigate this effect, we developed a composite polymeric scintillator which density has been artificially increased by loading it with high density nanoparticles. In an optimized composition, we achieved a 300% improvement of the nanocomposite scintillation yield thanks to the presence of heavy nanoparticles surpassing the performance of several commercial plastic scintillators. The composite has been used to fabricate a multilayer heterostructure. The prototype has been investigated by means of photoluminescence and scintillation spectroscopy experiments. We observe the occurrence of energy sharing and a net improvement with respect to the reference monolithic detector achieving a time resolution below 200 ps. The results obtained demonstrate therefore that a controlled loading with dense nanomaterials is an excellent strategy to enhance the plastic scintillators light yield as well as the scintillating heterostructure light output to maximize the scintillating heterostructure timing performance to meet the technological requirements for implementation in ToF-PET scanners.
Monguzzi, A. (2023). Scintillating heterostructure based on fast emitting nanocomposites for ToF-PET imaging. Intervento presentato a: E-MRS Fall 2023, Varsavia.
Scintillating heterostructure based on fast emitting nanocomposites for ToF-PET imaging
Monguzzi, AM
2023
Abstract
The Time-of-Flight Positron Emission Tomography (ToF-PET) is a powerful imaging technique employed in a variety of medical fields, from oncology to neurology. A radiotracer labelled with a - radioactive isotope (a positron emitter) is injected into the patient and then it is attracted to areas of high metabolism, such as tumours. The isotope undergoes radioactive decay, releasing a positron which then annihilates with a surrounding electron producing two back-to-back γ photons with 511 keV energy. These γ-rays are then sensed by scintillating detectors placed around the patient's body and used to reconstruct the image of the radio-labelled tissue. One of the limiting factors of this technique is the detectors ability to discriminate in time the arrival of the γ-photons pairs, i.e. the coincidence time resolution CTR. The CTR should be as low as possible to improve the instrument sensitivity and the image signal-to-noise ratio at low radiotracers concentration. Much effort is now put in the design and development of new scintillators aiming at a CTR < 50 ps, because none of the existing monolithic scintillators shows simultaneously the high efficiency and fast emission necessary to reach it. In order to overcome this obstacle, the concept of scintillating heterostructure has been proposed. Here the scintillator is fabricated by coupling heavy crystalline scintillators to a fast-emitting plastic scintillators to realize a multicomponent material. The dense component stop the -rays, and the fast emission of the plastic component is activated by the recombination of diffusing highly energetic recoil electrons generated by photoelectric effect in the dense component. This energy sharing mechanism is the key to possibly obtain the desired CTR. Unfortunately, scintillating plastics have typical densities of ρ≈1 g cm-3, thus the presence of the fast emitter decreases the global density of the heterostructure reducing its stopping power of -rays. To mitigate this effect, we developed a composite polymeric scintillator which density has been artificially increased by loading it with high density nanoparticles. In an optimized composition, we achieved a 300% improvement of the nanocomposite scintillation yield thanks to the presence of heavy nanoparticles surpassing the performance of several commercial plastic scintillators. The composite has been used to fabricate a multilayer heterostructure. The prototype has been investigated by means of photoluminescence and scintillation spectroscopy experiments. We observe the occurrence of energy sharing and a net improvement with respect to the reference monolithic detector achieving a time resolution below 200 ps. The results obtained demonstrate therefore that a controlled loading with dense nanomaterials is an excellent strategy to enhance the plastic scintillators light yield as well as the scintillating heterostructure light output to maximize the scintillating heterostructure timing performance to meet the technological requirements for implementation in ToF-PET scanners.
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