# Revisiting the segmentation threshold for Lu‐177 SPECT

## TL;DR

This study finds that using a 50% threshold instead of the traditional 42% improves accuracy when analyzing Lu-177 SPECT images of tumors.

## Contribution

The study derives and validates a new 50% segmentation threshold specifically for Lu-177 SPECT, improving tumor volume accuracy.

## Key findings

- The optimal threshold for Lu-177 SPECT increases with lower sphere volume or SBR, converging to 56% for FBP and 50% for OS-EM.
- The new 50% threshold significantly improved tumor segmentation compared to the 42% threshold, with higher Dice scores and lower volume errors.
- The derived threshold was validated on multiple radiopharmaceuticals and showed consistent improvements in segmentation accuracy.

## Abstract

The mostly used threshold‐based segmentation method for SPECT, i.e., 42% of the maximum intensity, was derived from 99mTc and may not be directly applicable to 177Lu.

This study aims to revisit the optimal segmentation threshold for 177Lu SPECT.

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 99mTc and 177Lu, 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 177Lu threshold was further validated on 177Lu‐PSMA‐617 (n = 6), 177Lu‐DOTATATE (n = 5), 177Lu‐FAP‐2286 (n = 5) and 177Lu‐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.

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 177Lu 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).

An optimal 177Lu‐specific threshold (∼50%) was derived and clinically validated, differing from the conventional 42% threshold used for 99mTc. The new threshold improved segmentation accuracy across different therapeutic radiopharmaceutical distributions.