CaLIPSO
Development of innovative detectors for Positron Emission Tomography

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 :

  1. The first project, CaLIPSO, is a development of the high spatial resolution PET for the brain and pre-clinical studies.
  2. The ClearMind project is a development of the fast and precise light detection technique using Cherenkov and scinitillation photons to boost the time-of-flight (TOF) performance of PET scanners.
 

Motivations for CaLIPSO

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.

 

Scientific and technological challenges

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:

  • TMBi physical properties: namely light production yield, light absorption length at optical and UV wavelength, charge mobility in TMBi. We measured these properties using tailored devices designed for handling our mildly-pyrophoric liquid.
  • The low light production efficiency does require careful optimization of the optical couplings. The first optical demonstrator, along with its detailed Monte Carlo simulation, allowed us to quantify the issue. The second optical prototype allowed us to demonstrate a very good efficiency of detection of 511 keV photons, especially, when they are converted to the electron via the photo-electric effect.
  • The electronic readout density: Integrating an electronic density approaching one preamplifier per mm2, has already been achieved in the laboratory (ASIC and 3D electronics), in the context of a spaceborne experiment. We have chosen to limit the resolution of our demonstrator to 2 x 2 mm2, in the charge collection plane, in order to avoid having to use 3D electronics and to focus in the short term our efforts on the detector development.
  • The TMBi must be ultra-purified to allow the charge drift and the measurement of gamma energies. We organized the detector cleaning and mounting work at the clean rooms. The materials, from which the detector is made, shall be carefully chosen to be compatible with ultra-high vacuum technologies.
 

Ultrapurification of TMBi

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.

 
CaLIPSO

“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.

ClearMind Project: study of a “scintronic” crystal targeting tens of picoseconds time resolution for gamma ray imaging

In this project we propose and study the concept of a new gamma ray “scintronic” detector targeting a time resolution of the order of 25 ps FWHM, with millimetric volume reconstruction and high detection efficiency. Its design consists of a monolithic large PbWO4 scintillating crystal with an efficient photocathode directly deposited on it. With an index of refraction higher for the photocathode than for the crystal, this design negates the total reflection effect of optical photons at the crystal/photo-detector optical interface, and thus largely improves optical coupling between the crystal and the photodetector. This allows to detect efficiently the Cherenkov light produced by 511 keV photoelectric conversions in PbWO4 , and to optimize the detector time resolution. Furthermore, the low-yield, fast scintillation light produced additionally by PbWO4 increases the detected photon statistics by a factor 10, thus fostering accurate (3 dimensional) localization of the gamma ray interaction within the crystal and providing a fair measurement of the deposited energy.

For more details see: D. Yvon  et al.,  “Design study of a scintronic crystal targeting tens of picoseconds time resolution for gamma ray imaging: the ClearMind detector.” 2020, JINST 15 P07029, arXiv:2006.14855 [physics.ins-det]

 
#3937 - Last update : 09/06 2021

 

Retour en haut