Positron emission tomography (PET) is a powerful nuclear imaging technique used widely nowadays in oncology, cardiology and neuropsychiatry.
The PET technology consists in injecting the patient with a radioactive tracer, of interest to probe a biochemical process. The decay of the tracer emits a positron which annihilate with an electron. As a result of the annihilation, two photons with energy 511 keV are emitted back-to-back and registered by the dedicated detectors. The line-of-response connects the two points where photons are detected and allow to reconstruct the tracer distribution when large statistics events have been accumulated.
The CaLIPSO group works on two projects :
The CaLIPSO project (French acronym for Calorimètre Liquide Ionisation Position Scintillation Organométallique) is based on the recent developments of the TriMethyl Bismuth (TMBi). This innovative liquid allows a very efficient and accurate detection of positron annihilation. Once fully developed and integrated with an acquisition system, it may become a key element of PET imagers. In particular, the detector performances would allow efficient 1 mm3 resolution PET imaging of the whole human brain, thus being an excellent complement of magnetic resonance imagers for diagnosis and research on neurodegenerative diseases. The development of such detector is a long term effort pursued at IRFU since 2009, partially funded by the Neuropôle de Recherche Francilien (NeRF), LabEx P2IO, and CEA interdisciplinary program TechnoSanté. This innovation is patent protected. The CaLIPSO detector uses the TriMethyl Bismuth to efficiently convert photons of energy less than 1 MeV, through the photoelectric effect. The photoelectric electron is relativistic in the liquid TMBi. Thus it produces a quasi-instantaneous flash of few tens of Cherenkov photons, as well as free charges in the liquid. Light produced is detected by an efficient photodetector and charges released drift along a strong electric field, pass through a Frisch grid, and are collected by a pixelated charge detector.
The CaLIPSO project is very ambitious. At the project start, little was known about liquid TMBi for particle detection. Key science and technological issues were listed as:
We need to extract electronegative molecules that trap free electrons in order to achieve free electron lifetime larger than 10 μs. This will ensure effective charge drift and collection of the ionization signal. We use molecular sieves for purification because this technology has achieved an impurity level lower than 0.1 ppb O2 equivalent on TetraMethyl Silane in the past. We monitored TMBi purity level by measuring macroscopic ionization current yields, and measured significant increase of the ionization yields with purification. Big efforts are starting to improve the efficiency of this molecular sieve and to upgrade all the systems: vacuum system, distillation system, and purification system. And keep them clean.
“Scintronic” crystal encapsulated within a MCP-MT. The photocathode is deposited directly on the scintillating crystal. The generated photoelectrons are amplified by a micro-channel late. The amplified signals are collected on a densely pixelated anode plane read out by transmission lines and fast electronics.