Quantitative ¹⁶¹Tb SPECT/CT imaging for dosimetry using a ring-based digital CZT camera

¹⁶¹Tb-labeled compounds are emerging as promising alternatives to ¹⁷⁷Lu-labeled compounds for radioligand therapy (RLT). The assessment of RLT safety and efficacy rely on accurate dosimetry, which necessitates quantitative activity measurement from multiple SPECT acquisitions. However, no studies to date have assessed ¹⁶¹Tb quantification using the ring-based cadmium-zinc-telluride (CZT) digital SPECT/CT systems, nor have optimal reconstruction parameters been established. This study aimed to optimize ¹⁶¹Tb SPECT reconstruction parameters for a CZT-based system.

A phantom study was performed using the NEMA imaging quality phantom with the standard sphere set (diameters 10-37 mm), with the background prepared either with a 10.5:1 activity ratio or without activity. Imaging was conducted on a StarGuide (GE Healthcare) ring-based CZT digital SPECT/CT system using the manufacturer's recommended protocol. Scans were repeated over activity levels up to 2.2 GBq. Three reconstruction methods were evaluated: OSEM, Q.Clear without regularization term (QClearN) and Q.Clear with regularization term (QClearR), each tested with multiple iteration-subset combinations. To compare the reconstruction methods, recovery coefficient (RC), noise levels and spatial resolution were assessed.

RC values increased with sphere size and number of updates. At 864 updates, RC ranged from 0.56 to 0.95 for OSEM, 0.52-0.92 for QClearN, and 0.52-0.92 for QClearR. Noise levels, quantified by the coefficient of variation, in OSEM increased from 0.07 to 1.16 with increasing number of updates, while the noise levels of QClearN and QClearR ranged from 0.07 to 0.40 and 0.07 to 0.38, respectively. Spatial resolution was assessed using the full with at half maximum (FWHM) derived from matched-filter analysis of a digital phantom, with values of 7.9-17.0 mm for OSEM, 8.7-17.7 mm for QClearN, and 8.8-17.8 mm for QClearR. Gibbs artifact appeared in all methods, intensifying with higher number of updates.

This study demonstrated the feasibility of quantitative ¹⁶¹Tb imaging using a ring-based CZT SPECT/CT system. Recommended number of updates are 48-96 for OSEM and approximately 200 for QClearN and QClearR. With these settings, ¹⁶¹Tb achieved higher RC values and improved spatial resolution compared to published ¹⁷⁷Lu data, supporting its potential as a substitute for ¹⁷⁷Lu for image-based dosimetry in RLT.