Bismuth Organic Liquid Detector with Cherenkov Light and Charge Readout for Positron Emission Tomography – BOLD-PET

When a gamma ray interacts with the TMBi liquid, it produces a charged particle and light. By measuring the charge and light, the location and time of the gamma-ray interaction could be determined. To detect the light signals, the team developed an optical prototype using a micro-channel-plate photomultiplier known for its excellent time resolution and specially designed high-speed analogues and digital electronics.
Detecting the charged particle produced by the gamma-ray interaction is more challenging and requires a high level of purity of the liquid and precautions to avoid the risk of electrical breakdowns. The additional difficulty comes from an unexpectedly low amount of ionization, which makes it difficult to detect the charged particles and requires using dedicated low-noise electronics, developed for this project. The different methods to reduce the presence of impurities in the liquid has been tested, but performance still needs to be improved to accurately measure the electric current produced by charge particle.

The French-German collaboration of Paris-Saclay University and the University of Münster for the first time demonstrated that TMBi liquid can detect gamma rays from Positron Emission Tomography (PET) scans with great efficiency. By using Cherenkov optical photons, gamma rays could be detected with high time precision, and these results open the way to develop more accurate PET detectors. To achieve precise localization of the interaction and detect even tiny amounts of ionization, our collaboration has developed a secure method for using liquid under high voltage and developed high-performance, low-noise electronics.

The results obtained with the optical prototype have demonstrated the potential benefits of utilizing Cherenkov light for photodetection in PET and achieving high time resolution. The developed approach, which employs MCP-PMT based read-out systems and fast digitization electronics, could be utilized in other crystal-based systems.
While detecting the ionization signal in TMBi presents a challenge, the purification techniques, stable operation, and read-out using extremely low-noise ASIC-based systems developed for this technology can also be beneficial for low-signal ionization particle detection systems.

Since the beginning of the project, the team has been publishing four papers in international peer-review journals, and an extra publication is under preparation. In addition, three PhD have been completed on this program. The results are also been presented at the international conference and several local meetings.

The aim of the BOLD-PET project is to develop a fast-response, high-efficiency gamma detector with fine grained spatial resolution for positron emission tomography (PET) based on recent developments using the liquid TriMethyl Bismuth (TMBi). This liquid with a bismuth mass fraction of more than 80% allows for a very efficient and accurate detection of 511 keV photons originated from positron annihilation. Bismuth has the highest nuclear charge (Z = 83) and thus the largest photoelectric cross section of all stable isotopes. The gamma energy of 511 keV is transferred to electrons in this material in nearly 50 % of the cases through the photoelectric effect. Both the Cherenkov light emitted by the resulting relativistic photo-electron and the secondary charge carriers produced during multiple scattering interactions are detected in a liquid ionisation chamber, supplemented by photodetectors.

Based on previous studies of liquid TMBi, we intend to develop and evaluate a novel PET detector with simultaneous detection of Cherenkov light and ionisation in a common effort of four research partners. Excellent imaging resolution is anticipated with the proposed detector by cancelling out Depth Of Interaction effects while allowing for placement of the detector close to the body which is increasing the detector’s solid angle. The TMBi’s coincidence photoelectric efficiency is the highest available, with twice the value of LSO/LYSO crystals. The new detector should be able to use accurate time-of-flight (TOF) information through Cherenkov light detection in order to improve the contrast of the reconstructed image.

To achieve a breakthrough in this challenging project, the expertise of the existing French CaLIPSO group (CEA-IRFU, CNRS-LAL) will be supplemented by the expertise on high-resolution PET imaging and detector development (WWU-EIMI group), and ultra-purification as well as light and charge detector readout (WWU-PHYSICS group), both from University of Munster. The main objective of this collaborative project is to develop a novel detector system for PET imaging (e.g. human brain PET, small animal PET) with a projected efficiency of 30%, high spatial precision of 1 mm3 , and high time of flight resolution of 100 ps (FWHM).

In order to achieve these objectives the project will focus on the following work areas:
(1) ultra-purification of TMBi and further characterisation of TMBi for gamma radiation detection,
(2) development of an ionisation detector prototype,
(3) study of the Cherenkov photon detection in liquid TMBi,
(4) Monte Carlo simulation and image reconstruction of a full PET scanner employing the new technology, and
(5) evaluation of a final PET detection demonstrator, merging charge and optical readout.