Characterization and Calibration of the iQID Digital Autoradiography System for Direct Quantitative Imaging of Beta-Emitters in Tissue Samples

Autoradiography provides microscale mapping of radionuclide distributions, a promising approach to complement nuclear medicine imaging for small-scale radiopharmaceutical therapy (RPT) research. However, quantitative protocols for β-emitters remain under-established compared to those for α-emitters. In this work, the ionizing-radiation quantum imaging detector (iQID) digital autoradiography system was characterized and calibrated specifically for the theranostic β-emitter 177 Lu. Spatial resolution, detection efficiency, background and minimum detectable activity, and depth dependence were characterized and compared to Geant4 Monte Carlo simulations. A methodology for converting count rates to activity was established, yielding a high linear response (range from 0 to 300 Bq). To validate the system for realistic measurement scenarios, cross-modality benchmarking was performed using a custom stacked multi-layer virtual water phantom to compare iQID performance with preclinical µSPECT/CT. The iQID system demonstrated an effective spatial resolution of ∼43 µm for 177 Lu and achieved total activity estimates of (0.194 ± 0.022) MBq, agreeing within 2% with the dispensed reference (0.197 ± 0.015) MBq. Crucially, iQID exhibited superior quantitative accuracy for small-scale features (0.8 mm to 2.5 mm diameters), resolving activity concentrations in regions where µSPECT/CT performance was severely limited by partial volume effects. This study establishes a validated framework for quantitative 177 Lu digital autoradiography, laying the groundwork for accurate activity estimation in ex vivo tissue samples.