Abstract
BACKGROUND: The mostly used threshold-based segmentation method for SPECT, i.e., 42% of the maximum intensity, was derived from (99m)Tc and may not be directly applicable to (177)Lu. PURPOSE: This study aims to revisit the optimal segmentation threshold for (177)Lu SPECT. METHODS: A cylindrical Jaszczak phantom containing six spheres (2-113 mL) was imaged via simulation and physical experiments using a clinical dual-head NaI SPECT/CT system. The spheres were filled with (99m)Tc and (177)Lu, with different sphere-to-background ratios (SBRs). One hundred and twenty projections were acquired and reconstructed using filtered back-projection (FBP) and 3D ordered subset expectation maximization (OS-EM) algorithms with attenuation and scatter corrections, followed by Gaussian filtering (σ = 3.8 mm). Thresholds from 1% to 99% (1% interval) of peak intensity were applied to minimize the absolute volume error (AVE) of the spheres. The newly derived (177)Lu threshold was further validated on (177)Lu-PSMA-617 (n = 6), (177)Lu-DOTATATE (n = 5), (177)Lu-FAP-2286 (n = 5) and (177)Lu-DOTA-IBA (n = 4) SPECT images, comprising 45 tumors with manual segmentations used as reference. Mean Dice, HD95, and AVE were calculated for all tumors and compared between the conventional threshold (42%) and the newly derived threshold using the Mann-Whitney U test. RESULTS: The optimal threshold increased along with the decrease in sphere volume or SBR. For SBR ≥ 3.5:1 and volume ≥ 16 mL, the mean optimal threshold of (177)Lu converged to 56% for FBP and 50% for OS-EM. The derived 50% threshold significantly improved tumor segmentation performance compared to the 42% threshold, with a higher Dice score (0.5999 ± 0.1589 vs. 0.6694 ± 0.1361) (p < 0.05), lower HD95 (2.0070 ± 1.1508 mm vs. 1.7392 ± 1.0643 mm), and lower AVE (130.72% ± 101.87% vs. 69.21% ± 63.49%) (p < 0.05). CONCLUSIONS: An optimal (177)Lu-specific threshold (∼50%) was derived and clinically validated, differing from the conventional 42% threshold used for (99m)Tc. The new threshold improved segmentation accuracy across different therapeutic radiopharmaceutical distributions.