Timing limits of ultrafast cross-luminescence emission in CsZnCl-based crystals for TOF-CT and TOF-PET

• Published in: EJNMMI physics Vol. 11; no. 1; pp. 59 - 20
• Main Authors: Herweg, Katrin, Rutstrom, Daniel, Nadig, Vanessa, Stand, Luis, Melcher, Charles L., Zhuravleva, Mariya, Schulz, Volkmar, Gundacker, Stefan
• Format: Journal Article
• Language: English
• Published: Cham Springer International Publishing 09.07.2024; Springer Nature B.V; SpringerOpen
• Subjects: Applied and Technical Physics; Background noise; Barium fluorides; Computational Mathematics and Numerical Analysis; CsZnCl; Decay; Engineering; High-frequency readout; Image quality; Imaging; Light emission; Luminescence; Lutetium; Medical imaging; Medical research; Medicine; Medicine & Public Health; Nuclear Medicine; Original Research; Photons; Positron emission; Radiology; Scintillator; Stopping power; TOF; Scintillator; TOF; High-frequency readout; CsZnCl
• ISSN: 2197-7364; 2197-7364
• DOI: 10.1186/s40658-024-00663-x

Background Good timing resolution in medical imaging applications such as TOF-CT or TOF-PET can boost image quality or patient comfort significantly by reducing the influence of background noise. However, the timing resolution of state-of-the-art detectors in CT and PET are limited by their light emission process. Core-valence cross-luminescence is an alternative, but well-known compounds (e.g. BaF 2 ) pose several problems for medical imaging applications, such as their emission wavelength in the deep UV. CsZnCl-based materials show promise to solve this issue, as they provide fast decay times of 1–2 ns and an emission wavelength around 300 nm. Results In this work, we investigated two CsZnCl-compounds: Cs 2 ZnCl 4 and Cs 3 ZnCl 5 . We validated the previously published decay times on a time-correlated single-photon counting setup with 1.786 ± 0.016 ns for Cs 2 ZnCl 4 and 1.034 ± 0.013 ns for Cs 3 ZnCl 5 . The setup’s high resolution enabled the discovery of an additional prompt emission component with a significant abundance of 98 ± 18 (Cs 2 ZnCl 4 ) and 86 ± 14 (Cs 3 ZnCl 5 ) photons/MeV energy deposit. In a PET coincidence experiment, we measured the best coincidence time resolution (CTR) of 62 ps (FWHM) for Cs 2 ZnCL 4 coupled to FBK VUV SiPMs with silicon oil. To assess the CTR for lower energies, we filtered the energy along the Compton continuum and found a deteriorated CTR that seems to be mainly influenced by photon statistics. Furthermore, this study gave us a rough estimate of e.g. 150 ps (FWHM) CTR at 100 keV energy for Cs 2 ZnCL 4 . From measurements with high activity of 14 MBq to check for pile-up effects we assume that Cs 2 ZnCl 4 is better suited for high-rate time-of-flight applications than lutetium-based oxides. Simulations demonstrated that the stopping power of Cs 2 ZnCl 4 is lower than for LSO:Ce,Ca, meaning that a high amount of material would be needed for TOF-PET applications. However, the stopping power seems acceptable for applications in TOF-CT. Conclusions The fast decay time, state-of-the-art CTR in benchtop experiments and high-rate suitability make CsZnCl materials a promising candidate for time-of-flight experiments. We consider especially TOF-CT a suitable application due to its relatively low X-ray energies (~ 100 keV) and the thusly acceptable stopping power of Cs 2 ZnCl 4 . Currently, further exploration of the prompt emission and its creation mechanism is planned, as well as investigating the light transport of Cs 2 ZnCl 4 in longer crystals.