Effects of wavelength and spinal cord injury segment on photobiomodulation efficacy: Monte Carlo simulation in a rat model.

Significance: Spinal cord injury (SCI) is a severe condition characterized by complex pathophysiology and challenges in neural regeneration within the central nervous system, presenting substantial difficulties for repair. Although surgery and medication are employed for early rehabilitation and complications prevention, biomaterial transplantation, stem cell therapy, and neuromodulation techniques have been extensively investigated to promote axon regeneration and neural circuit remodeling in SCI, though results remain suboptimal. Aim: Photobiomodulation (PBM), with its strong anti-inflammatory and tissue repair effects, is gaining increasing attention as a noninvasive physical therapy. However, only a limited number of studies have focused on injury site and wavelength selection. This study addresses these issues. Approach: To tackle this issue, we performed low-cost and quantitative comparisons of light distribution in a SCI rat model using Monte Carlo simulations of light propagation. The SCI models encompassed cervical (C2, C4, and C6) and thoracic (T1, T3, T7, and T10) spinal regions, and simulations were performed for four wavelengths (660, 808, 980, and 1064 nm). Results: The cervical spinal injuries benefit more from PBM than thoracic spinal injuries due to higher photon fluence in the cervical spinal cord compared with the thoracic region. Notably, 1064 nm demonstrated deeper penetration than 980, 808, and 660 nm. Conclusions: We present a robust computational framework and empirical insights to inform the optimization of PBM parameters for SCI treatment. Our simulation and comparisons offer valuable reference for researchers and clinicians in performing precise and quantitative PBM treatment for SCI. As further studies are conducted, we aim to develop standardized, personalized optical parameters for clinical PBM in SCI treatment.