Monte Carlo Dosimetry Simulation of Transcranial and Intranasal Photobiomodulation

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Abstract Submission 

Hannah Van Lankveld¹, Anh Que Mai², Lew Lim³, Nazanin Hosseinkhah³, Paolo Cassano⁴, J. Jean Chen²

¹University of Toronto, Toronto, Ontario, ²Baycrest Health Sciences, Toronto, Ontario, ³Vielight Inc., Toronto, Ontario, ⁴Massachusetts General Hospital, Boston, MA

Hannah Van Lankveld  
University of Toronto
Toronto, Ontario

Anh Que Mai  
Baycrest Health Sciences
Toronto, Ontario

Lew Lim  
Vielight Inc.
Toronto, Ontario

Nazanin Hosseinkhah  
Vielight Inc.
Toronto, Ontario

Paolo Cassano  
Massachusetts General Hospital
Boston, MA

J. Jean Chen  
Baycrest Health Sciences
Toronto, Ontario

Photobiomodulation (PBM) is defined as the application of low levels of red or near-infrared light to stimulate neural tissue [1,2]. Wavelength, optical power density, pulsation frequency, skin colour and light source are commonly known parameters that impact the overall energy delivered to the tissue. Previous in vitro research suggests that the peak PBM response happens when the deposited energy reaches 3 Joules/cm3, but it is unclear how this relates to different stimulation parameters. Despite the many in vivo studies, the PBM stimulation protocols vary amongst studies, and there has yet to be a full characterization of light energy deposition based on the physics, likely leading to the large variabilities in responses [3]. This study will model the extent that local transcranial and intranasal photobiomodulation (tPBM & iPBM) can penetrate through neural tissue using Monte Carlo simulations [4], [5].

The Monte Carlo Extreme (MCX) package was utilized to simulate the near-infrared light propagation through the multi-layer tissues of the human head, using the colin27 brain atlas, in which light propagation through different tissue types is mainly governed by coefficients of (1) absorption, (2) scattering (dispersion), and (3) transmission. We simulated a single optode laser source positioned for transcranial (tPBM) and intranasal (iPBM) simulation (as shown in Figure 1), with 8e9 incidental photons. Wavelengths simulated: 670nm, 810nm and 1064nm; power densities: 100mW/cm2 200mW/cm2 and 300mW/cm2. These are typical values from the literature. Moreover, we incorporated attenuation and scattering coefficients associated with Caucasian (white), African and Asian skin colours [6]. Matlab was used to compute the energy deposition in the brain regions closest to the optodes, as summarized in Fig. 1.

Simulations show that the rostral dorsal prefrontal cortex for tPBM and the ventromedial prefrontal cortex for iPBM accumulate the highest energy (Figure 2). As shown in Figure 2.a, the 810 nm wavelength for tPBM and (Fig 2.e) 1064 nm wavelength for iPBM produced the highest energy accumulation. As shown in Fig. 2.b,e, optical power density is linearly correlated with energy. Moreover, in Fig. 2.c, we show that Caucasian (white) skin accumulates higher energy than other modelled skin colours. A maximum of 15% of the incidental energy for tPBM and 1% for iPBM reach the cortex (Fig 2.a). These correspond to a minimum of 100 and 40,000 minutes to reach the 3 J/cm3 target for tPBM (810nm and 100 mW/cm2) and iPBM (1064nm and 5 mW/cm2), respectively.

We found that the optimal wavelength depends on the penetrated tissue types. Thus, 810 nm and 1064 nm are optimal in tPBM and iPBM, respectively. The simulation also illustrated energy deposition being a linear function of power density. Moreover, melanin produces skin pigmentation and is the main variable in characterizing skin colour [8]. This study is the first to account for skin colour as a PBM consideration, demonstrating light skin being most conducive to light propagation. Moreover, we predict a maximum of 15% of the incidental energy is deposited into brain tissue, higher than previously reported using cadaver heads and skull fragments [9]. Even then, we show that at 300 mW/cm2 and 810 nm, at least 40 min of irradiation is required to reach the currently assumed optimal energy of 3J/cm3 (Fig 2.b). However, clinical research has shown increases in brain rhythms [10], and cognitive improvements with much lower energy dosages. This highlights the need to further understand the dynamic physiological processes impacting the PBM response in vivo.

Non-Invasive Stimulation Methods Other ¹

Classification and Predictive Modeling ²

Modeling

Structures

Other - Photobiomodulation (PBM); Monte Carlo Simulation