Spinal cord injury (SCI) occurs as a result of damage to neurons in the spinal cord, which causes an interruption of neural signal conduction along the axonal tracts. SCI can result in complete or partial paralysis. Several studies have demonstrated that photobiomodulation therapy (PBMT) has the ability to repair nerves and enhance functionality. PBMT is also known to present a biphasic dose-response pattern. Various PBMT research studies on contusion spinal cord injuries in rats have applied a fixed fluence onto the rats’ skin during the study period without considering the rats’ weight, which can lead to a reduction of the delivered fluence delivered to the SCI site. Therefore, the purpose of this work is to evaluate whether the bodyweight of the rat model, as well as the irradiation parameter, will in fact affect the fluence delivered to the SCI site. The study employed four computational rat models of the same age (11 weeks) and four different weights (250, 270, 290, and 310 g) as well as different irradiation parameters (3 wavelengths ×15 beam diameters × 2 beam shapes) to study their effect on the fluence delivered to the SCI site using Monte Carlo simulation. As the rat’s weight during the study period decreased, more fluence was delivered to the SCI site. Our results showed that the percentage increase of the fluence delivered to the SCI site was highest for the 660 nm large gaussian beam when compared with other parameters. We found that the irradiation parameters and the rats’ weight strongly influenced the dosage delivered to the SCI site in the rat. Therefore, researchers should consider the fluence delivered to the injury site instead of onto the skin when studying the effectiveness of PBMT.
KEYWORDS: Injuries, Spinal cord, 3D modeling, Monte Carlo methods, Tissues, Animal model studies, Tissue optics, Optical properties, Beam shaping, Skin
Photobiomodulation therapy (PBMT) has demonstrated efficacy in various areas of medical practice including pain management, wound healing, inflammation treatment, and treatment for neurological diseases. More recently, animal model studies have reported that near-infrared light irradiation to a spinal cord injury (SCI) site transcutaneously enhances axonal regeneration and functional recovery. Many such studies typically use a variety of irradiation parameters and surface irradiance to calculate the fluence delivered to the injury site, and many times ignoring factors such as tissue optical properties, beam divergence, beam positioning, and tissue thickness to the organ of interest. While these studies show a broad range of treatment outcomes, a comparison of treatment efficacy among these studies with respect to light fluence is many times extremely difficult. Therefore in this study, we use Monte Carlo simulation to provide an overview of the effect of the light source probe parameters and positioning on an injury site using a 3D voxelated SCI rat phantom model with regards to PBMT. In this study, an 810 nm Top-hat beam was simulated for 3 numerical apertures (NA) (0, 0.4, and 0.8), 5 beam diameters (0.04, 0.1, 0.2, 0.4, and 0.8 cm), and 14 different irradiation positions relative to the SCI injury site. Our findings are beneficial for research into understanding the effects of the probe parameters on tissues and organs, which ultimately will aid in reducing the variability in the used fluence and help optimize PBMT outcomes.
Spinal cord injury (SCI) can result in complete or partial loss of sensation and motor function due to interruption along the severed axonal tract(s). SCI can result in tetraplegia or paraplegia, which can have prohibitive lifetime medical costs and result in shorter life expectancy. A promising therapeutic technique that is currently in experimental phase and that has the potential to be used to treat SCI is Low-level light therapy (LLLT). Preclinical studies have shown that LLLT has reparative and regenerative capabilities on transected spinal cords, and that LLLT can enhance axonal sprouting in animal models. However, despite the promising effects of LLLT as a therapy for SCI, it remains difficult to compare published results due to the use of a wide range of illumination parameters (i.e. different wavelengths, fluences, beam types, and beam diameter), and due to the lack of a standardized experimental protocol(s). Before any clinical applications of LLLT for SCI treatment, it is crucial to standardize illumination parameters and efficacy of light delivery. Therefore, in this study we aim to evaluate the light fluence distribution on a 3D voxelated SCI rat model with different illumination parameters (wavelengths: 660, 810, and 980 nm; beam types: Gaussian and Flat; and beam diameters: 0.1, 0.2, and 0.3 cm) for LLLT using Monte Carlo simulation. This study provides an efficient approach to guide researchers in optimizing the illumination parameters for LLLT spinal cord injury in an experimental model and will aid in quantitative and qualitative standardization of LLLT-SCI treatment.
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