Testing the safety of a custom NIRS sensor designed to monitor spinal cord hemodynamics

Background: A customized sensor, based on near-infrared spectroscopy (NIRS), was developed to non-invasively monitor spinal cord hemodynamics after acute spinal cord injury (SCI). However, the effect of direct contact and emission of the NIRS signals on the spinal cord tissue structure was not clear. This information is essential because even minimal heating or contact pressure from the NIRS sensor placed over the injured spinal cord may lead to further damage to the spinal cord tissue. Here, we evaluate the safety of the custom-made NIRS sensor as it is essential prior to its clinical translation. Methods: A custom-made multi-wavelength miniaturized NIRS sensor was placed extradurally on the spinal cord of six Yucatan mini-pigs who received a T10 SCI. After seven days of continuous data collection at 100 Hz, the sensor was removed and the spinal cord tissue was examined. Histological assessment of the spinal cord revealed evidence of cellular damage at the NIRS sensor placement in two animals. An in-vitro experiment was performed to evaluate the possibility of heat damage caused by the NIRS sensor. Using a digital thermometer with two probes, one directly touching the NIRS emitter and the other one at a control site one centimeter away from the emitter. The amount of heat generation of the NIRS emitter after seven days of continuous operation at 100 Hz was measured and compared with the control site. Results: In-vitro heat tests showed no heat generation by the NIRS sensor. The temperature measured from the emitter site of the NIRS sensor and control temperature probe was identical during the seven days. Conclusion: The custom-made NIRS sensor does not generate heat at the emitter site and physical damage observed with regards to histology is due to the compression applied from the sensor. By refining the sensor to be smaller and more flexible with an even surface, we will improve the safety of the NIRS sensor before translating this technology to human patients. The new sensor will be further examined.