The clinical prospect of treating spinal cord injury by photobiomodulation therapy (PBMT) is unclear as the spinal dosimetry of transcutaneous laser application is unknown, yet essential for clinical recommendations. Light irradiances at 9 sites over an 8cm length along the thoracic to lumbar segment of the spinal canal were measured in 6 cadaver dogs using a flexible intra-spinal probe, under a surface irradiance of 3.14W/cm2. The skin transmits 4-12% of 980nm PBMT light. On-contact technique is ~5 times more efficient than off-contact technique. Transcutaneous transmission of photobiomodulatory irradiance to the spinal canal will benefit from contact-probe technique and skin-clearing approaches.
Transcutaneous photobiomodulation of the spinal cord in rodent models has shown therapeutic effects to spinal cord injury. For translation to human medicine; however, it remains unknown if a therapeutic dose can reach the spinal cord with a non-invasive treatment protocol that does not cause collateral tissue damage. Our previous pilot test on a cadaver dog using a single photo-diode-sensor demonstrated that the transcutaneous transmission of 980nm light to the spinal canal through the shallowest section of the vertebrae was comparable to the transcranial transmission. The use of a single sensor to measure at multiple positions along the spinal canal challenged correlations of the results, due to variations of sensor-tissue geometry after repositioning the sensor within the spinal canal and closing the tissue for light transmission. To identify a potential transmission window for delivering an effective photobiomodulation therapy dose to the spinal cord, and to evaluate inter-subject variations, it is essential to maintain consistency in intra-spinal measurements at multiple positions for any one subject and across multiple subjects. We developed a flexible multi-channel photo-detector-array specifically for measuring the transmission of light to the spinal canal at multiple sites under fixed surface irradiation. The probe, with a 1/4" stem, contains 9 photo-diodes spaced 1cm apart allowing measurement over an 8cm length. Multi-site dosimetry at 980nm was tested in one cadaver dog at a fixed surface (shaved skin) irradiance of 3.18 W/cm2 by deploying the applicator-probe in the spinal canal after hemilaminectomy, and will be repeated in a total of six dogs.
Noninvasive photobiomodulation therapy (PBMT) of spinal cord disease remains speculative due to the lack of evidence for whether photobiomodulatory irradiances can be transcutaneously delivered to the spinal cord under a clinically acceptable PBMT surface irradiation protocol. We developed a flexible nine-channel photodetection probe for deployment within the spinal canal of a cadaver dog after hemilaminectomy to measure transcutaneously transmitted PBMT irradiance at nine sites over an eight-cm spinal canal length. The probe was built upon a 6.325-mm tubular stem, to the surface of which nine photodiodes were epoxied at approximately 1 cm apart. The photodiode has a form factor of 4.80 mm×2.10 mm×1.15 mm (length×width×height). Each photodiode was individually calibrated to deliver 1 V per 7.58 μW/cm2 continuous irradiance at 850 nm. The outputs of eight photodiodes were logged concurrently using a data acquisition module interfacing eight channels of differential analog signals, while the output of the ninth photodiode was measured by a precision multimeter. This flexible probe rendered simultaneous intraspinal (nine-site) measurements of transcutaneous PBMT irradiations at 980 nm in a pilot cadaver dog model. At a surface continuous irradiance of 3.14 W/cm2 applied off-contact between L1 and L2, intraspinal irradiances picked up by nine photodiodes had a maximum of 327.48 μW/cm2 without the skin and 5.68 μW/cm2 with the skin.
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