Revisiting the segmentation threshold for Lu-177 SPECT.

Area of Science:

• Nuclear Medicine
• Medical Imaging
• Radiopharmaceutical Therapy

Background:

• The standard 42% threshold for SPECT segmentation, derived from ^99mTc, may not be optimal for ¹⁷⁷Lu.
• Accurate tumor segmentation is crucial for effective radiopharmaceutical therapy monitoring.

Purpose of the Study:

• To determine the optimal segmentation threshold for ¹⁷⁷Lu SPECT imaging.
• To validate a new ¹⁷⁷Lu-specific threshold against the conventional ^99mTc threshold.

Main Methods:

• Simulated and experimental SPECT/CT imaging of phantoms with ^99mTc and ¹⁷⁷Lu.
• Image reconstruction using FBP and OS-EM algorithms with corrections and Gaussian filtering.
• Systematic evaluation of thresholds (1-99%) to minimize volume errors, followed by clinical validation on 45 tumors across four ¹⁷⁷Lu radiopharmaceuticals.

Main Results:

• The optimal ¹⁷⁷Lu threshold varied with sphere volume and SBR, converging around 50-56%.
• The derived ~50% threshold significantly improved segmentation accuracy (higher Dice, lower HD95 and AVE) compared to the 42% threshold (p < 0.05).

Conclusions:

• An optimal ¹⁷⁷Lu-specific threshold of approximately 50% was established and clinically validated.
• This new threshold enhances segmentation accuracy for various ¹⁷⁷Lu radiopharmaceuticals, improving therapeutic monitoring.