Revisiting the segmentation threshold for Lu‐177 SPECT

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.