Towards a metamaterial approach for fast timing in PET: experimental proof-of-concept

Abstract

Achieving fast timing in positron emission tomography (PET) at the level of few tens of picoseconds of picoseconds is limited by the photon emission rate of existent materials with standard scintillation mechanisms. This has led to consider quantum confined excitonic sub-1 ns emission in semiconductors as a viable solution to enhance the amount of fast-emitted photons produced per gamma event. However the introduction of nanocrystals and nanostructures into the domain of radiation detectors is a challenging problem. In order to move forward along this line, the standard bulk detector geometry and readout should be updated to allow for the implementation of new materials and within others, compensate for some of their intrinsic limitations. In this paper we will cover two study cases in which a fast emitter is combined with state-of-the-art scintillators in a sampling geometry designed to provide better timing for a fraction of the 511 keV events. For this test, we use a fast plastic scintillator BC-422 able to deliver a detector time resolution (DTR) of 25 ps FWHM (equivalent coincidence time resolution CTR of 35 ps) and we combined it with LYSO or BGO 200 m thick plates building a sampling pixel composed by two active scintillating materials. We develop a new proof of concept readout that allows for the identification of different types of events, carrying standard or improved timing information. Results are showing a DTR of 67 ps FWHM (equivalent to a CTR of 95 ps) for one third of the events depositing 511 keV in the BGO + BC-422 mm3 sampling pixel. The other two third of the 511 keV events perform like standard bulk 3 mm long BGO crystals with a time resolution of around 117 ps (equivalent to a CTR of 165 ps). For the case of LYSO + BC-422 sampling pixel, shared 511 keV events reach a DTR of 39 ps (CTR of 55 ps) in comparison to 57 ps (CTR of 83 ps) for 511 keV events fully contained in LYSO of the same size. This work is a step forward in the integration of fast semiconductor nanocrystals and nanostructures with present detector technologies.

On the use of CdSe scintillating nanoplatelets as time taggers for high-energy gamma detection

“…A sampling pixel geometry with optically separated scintillators has been previously implemented and proven to bring better timing to a fraction of the 511 keV events. 12 The experimental proof-of-concept readout has been implemented with BC-422, a plastic scintillator in the shape of 200 μm thin plates combined with 200 μm thin plates of LYSO or BGO. This plastic scintillator presents a very low density of 1.0 g/cm 3 (no-photopeak), a minimum decay time of 1.3 ns, light yield of 10,000 ph/MeV and therefore an excellent CTR of 35 ps FWHM.…”

“…35 The CTR measurements were performed in the state-of-the-art experimental setup previously described in. 7,12 This bench uses a LSO:Ce:Ca 2 × 2 × 3 mm 3 crystal as reference detector with a single time resolution of 41 ps FWHM. The SiPMs in used are FBK NUV-HD with 40 μm SPADs size 36 and measured single photon-time resolution of 70 ps.…”

“…11 In order to increase the prompt photon emission yield, we propose to replace the usual bulk scintillator pixel by a sampling configuration, so that the recoil electron from a photoelectric conversion created in the dense host scintillator can eventually reach a fast emitting nano-scintillator layer. 12 In this way, prompt or relatively prompt emission from nanocrystals could be added to the standard scintillating signal and help to decrease the overall timing jitter associated to particle detection. 4 In this approach, each component brings its own functionality, the fast material is driving the timing improvements and the heavy/standard scintillator provides the stopping power and energy resolution.…”

“…Drop-casted films can be considered as a random distribution of "unconnected"-emitting centers, in the sense of overall energy distribution, in a given volume and they impede the application of a readout method that guarantees the ultimate performance in terms of combined energy and time resolution previously demonstrated in ref. 12…”

Exploring Cherenkov Emission of BGO for TOF-PET

“…While pulse shape discrimination is well established, the idea of separating events based on the signal rise time of the SiPM signal was introduced in [16]. A similar approach applied to BGO allows to classify events based on their timing performance and further to correct the time walk introduced by different number of detected Cherenkov photons [17].…”