How the heterogeneity of the severely injured brain affects hybrid 2 diffuse optical signals: case examples and guidelines

Significance: A shortcoming for the routine clinical use of diffuse optics (DO) in the injured head has been that the results from commercial near-infrared spectroscopy-based devices are not reproducible, often give phys iologically invalid values and differ between systems. Besides the limitations due to the physics of continuous23 wave light sources, one culprit is the head heterogeneity and the underlying morphological and functional abnor malities of the probed tissue. Aim: To investigate the effect that different tissue alterations in the damaged head has on DO signals and provide guidelines to avoid data misinterpretation. Approach: DO measurements and computed tomography scans were acquired on brain-injured patients. The relationship between the signals and the underlying tissue types was classified on a case-by-case basis. Results: Examples and suggestions to establish quality control routines were provided. The findings suggested guidelines for carrying out DO measurements and speculations towards improved devices. Conclusions: This study advocates for the standardization of the DO measurements to secure a role for DO in neurocritical care. We suggest that blind measurements are unacceptably problematic due to confounding effects and care using a priori and a posteriori quality control routines that go beyond an assessment of the signal-to-noise ratio that is typically utilized.
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