Bridging physics and practice: evaluating sensitivity, septal penetration, and detector dead time in terbium-161 gamma-camera imaging
Westerbergh F, McDougall L, Ritt P, Fricke JG, van der Meulen NP, Müller C, Schibli R, Wild D, Bernhardt P (2025)
Publication Type: Journal article
Publication year: 2025
Journal
Book Volume: 12
Article Number: 81
Journal Issue: 1
DOI: 10.1186/s40658-025-00792-x
Abstract
Introduction/Aim: Terbium-161 (161Tb) has emerged as a promising therapeutic radionuclide, yet standardized imaging guidelines are lacking. This study aimed to characterize a SPECT/CT system, currently used in an ongoing clinical trial (BETA PLUS; NCT05359146), focusing on sensitivity, septal penetration, and dead-time effects. Methods: Measurements were conducted on a Siemens Symbia Intevo system using two collimators: low-energy high-resolution (LEHR) and medium-energy low-penetration (MELP). Two energy windows were evaluated: 75 keV ± 10% and 48 keV ± 20%. Planar sensitivity and penetration were assessed using a 161Tb-filled Petri dish. Penetration fractions were determined as a function of distance for each collimator-window combination. Dead time was measured intrinsically for each detector using a set of 161Tb point sources. SPECT measurements of a homogenous cylinder phantom were performed to assess count rate performance and predict activity levels at which dead-time effects could occur. To evaluate the potential impact of dead time in patient imaging, SPECT projection data from patients treated with 1 GBq of [161Tb]Tb-DOTA-LM3 (n = 8) was analyzed. Results: Sensitivity was comparable for both collimators at 75 keV (LEHR: 15.7 cps/MBq, MELP: 18.5 cps/MBq) and increased at 48 keV (LEHR: 44.4 cps/MBq, MELP: 67.9 cps/MBq). Maximum penetration occurred at 75 keV with the LEHR collimator (7.5% at 10 cm). In acquired spectra, more than half of the detected counts (51.6%) appeared above the 75 keV window with LEHR, compared to only 12.2% with MELP. Dead-time analyses revealed non-linear detector responses at wide-spectrum count rates exceeding 93 kcps, corresponding to in-field activities of 1.4–2.0 GBq for LEHR and 1.7–2.2 GBq for MELP. The dead-time constant was determined to 0.42 µs for both detector heads, however, the maximum recorded count rate differed significantly (384 kcps vs. 546 kcps). The median and maximum wide-spectrum count rate for patients treated with [161Tb]Tb-DOTA-LM3 was estimated to ~ 20 and ~ 40 kcps per GBq 3 h p.i., respectively, when imaged with LEHR, corresponding to a maximum estimated dead-time loss of 1.7%. Conclusions: While high-quality 161Tb SPECT imaging is feasible, careful consideration is essential; the wide range of photons emitted will produce a higher wide-spectrum count rate as compared to 177Lu. The use of low-energy collimators increases penetration and scatter, impairing quantitative accuracy and elevating the wide-spectrum count rate, which may intensify dead-time effects. At therapeutic activity levels (e.g., 7.4 GBq), dead time should be closely monitored to ensure reliable quantification.
Involved external institutions
Paul Scherrer Institute (PSI)
Switzerland (CH)
Universitätsspital Basel
Switzerland (CH)
ITM Isotope Technologies Munich SE
Germany (DE)
Switzerland (CH)
Universitätsspital Basel
Switzerland (CH)
ITM Isotope Technologies Munich SE
Germany (DE)
How to cite
APA:
Westerbergh, F., McDougall, L., Ritt, P., Fricke, J.G., van der Meulen, N.P., Müller, C.,... Bernhardt, P. (2025). Bridging physics and practice: evaluating sensitivity, septal penetration, and detector dead time in terbium-161 gamma-camera imaging. EJNMMI Physics, 12(1). https://doi.org/10.1186/s40658-025-00792-x
MLA:
Westerbergh, Frida, et al. "Bridging physics and practice: evaluating sensitivity, septal penetration, and detector dead time in terbium-161 gamma-camera imaging." EJNMMI Physics 12.1 (2025).
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