Systematic validation of time-resolved diffuse optical simulators via non-contact SPAD-based measurements

TL;DR

This study systematically validates three open-source time-resolved diffuse optical imaging simulators against experimental data from a SPAD array, providing guidelines for their use in non-contact, clinically relevant scenarios.

Contribution

It offers the first comprehensive experimental validation of TD-DOI simulators with SPAD data, highlighting the importance of virtual source correction and comparing simulator accuracy and speed.

Findings

MMC achieves highest accuracy in spatial and temporal metrics.

Virtual source correction reduces errors by over 34%.

MMC is best for accuracy-critical applications, while NIRFASTer and Toast++ are faster.

Abstract

Objective: Time-domain diffuse optical imaging (DOI) requires accurate forward models for photon propagation in scattering media. However, existing simulators lack comprehensive experimental validation, especially for non-contact configurations with oblique illumination. This study rigorously evaluates three widely used open-source simulators, including MMC, NIRFASTer, and Toast++, using time-resolved experimental data. Approach: All simulations employed a unified mesh and point-source illumination. Virtual source correction was applied to FEM solvers for oblique incidence. A time-resolved DOI system with a 32 $\times$ 32 single-photon avalanche diode (SPAD) array acquired transmission-mode data from 16 standardized phantoms with varying absorption coefficient $\mu_a$ and reduced scattering coefficient $\mu_s'$. The simulation results were quantified across five metrics: spatial-domain (SD) precision, time-domain (TD) precision, oblique beam accuracy, computational speed, and mesh-density independence. Results: Among three simulators, MMC achieves superior accuracy in SD and TD metrics, and shows robustness across all optical properties. NIRFASTer and Toast++ demonstrate comparable overall performance. In general, MMC is optimal for accuracy-critical TD-DOI applications, while NIRFASTer and Toast++ suit scenarios prioritizing speed with sufficiently large $\mu_s'$. Besides, virtual source correction is essential for non-contact FEM modeling, which reduced average errors by > 34% in large-angle scenarios. Significance: This work provides benchmarked guidelines for simulator selection during the development phase of next-generation TD-DOI systems. Our work represents the first study to systematically validate TD simulators against SPAD array-based data under clinically relevant non-contact conditions, bridging a critical gap in biomedical optical simulation standards.