Quantitative terbium-161 SPECT/CT imaging: demonstrating the feasibility of image-based dosimetry and highlighting...

Terbium-161 (¹⁶¹Tb) is a promising β⁻-emitter for theragnostics. However, its complex photon emission pattern-including intense X-rays and low-yield, high-energy γ-emissions-may complicate image-based quantification. This study aimed to assess the feasibility of accurate SPECT/CT-based ¹⁶¹Tb dosimetry through a series of phantom measurements using a GE Discovery NM/CT 670 Pro system. Three collimators were evaluated: extended low-energy general-purpose (ELEGP), low-energy high-resolution (LEHR), and medium-energy general-purpose (MEGP), using two separate energy windows: around the 75 keV γ-peak (± 10%), and around the 49 keV γ-peak and nearby X-rays (40.7-62.9 keV). A clinical OSEM reconstruction algorithm was employed.

On average, the SPECT calibration factors (CFs) were 2-fold higher with ELEGP compared to MEGP and LEHR, and 3-fold higher at 49 keV compared to 75 keV. For each collimator, derived CFs varied substantially depending on measurement and volume-of-interest geometry-more so at 49 keV, compared to 75 keV. Measurements of two 3D-printed kidney inserts revealed superior visual image quality with LEHR compared to ELEGP and MEGP. Across all collimators, the 75 keV window provided better spatial resolution and contrast than the 49 keV window. An anthropomorphic phantom study, including a LungSpine phantom with 8 spherical inserts and 3 different background activity levels, demonstrated a greater quantitative accuracy for MEGP compared to LEHR and ELEGP, with statistical significance for both energy windows (p ≤ 0.001). Errors were generally larger at 49 keV compared to 75 keV. For the low-energy collimators, considerable septal penetration (e.g., at 292 and 475 keV) was observed, along with systematic underestimation at high activity levels.

This study demonstrates that highly accurate SPECT/CT-based ¹⁶¹Tb quantification is feasible, further cementing ¹⁶¹Tb as a viable theragnostic alternative. A MEGP collimator, a 75 keV window, and a CF derived from a homogeneous cylinder measurement appears preferable. The 49 keV window could be useful at late imaging time points, given its high sensitivity, if further optimized. Degradation from penetration and subsequent downscatter may be mitigated with a more refined reconstruction. Further investigations into dead-time effects are encouraged.