gms | German Medical Science

Objective: The process of glioma resection is characterized by the trade-off between a maximization of tumor resection extent and the minimization of the risk for functional disorders of the patient. Several intraoperative techniques like 5-ALA fluorescence are available to provide morphological information about the tumor extent during the surgical procedure, nevertheless specific functional testing of brain tissue can only be performed in awake cases. Here, we present the first results of a novel approach to assess the functional state of the brain tissue by evaluating the hemodynamic response of the tissue that can be observed with Intraoperative Optical Imaging (IOI) after Direct Cortical Stimulation (DCS) of the exposed cortical surface.

Methods: Measurements with the optical imaging technique were performed as a proof of concept on three patients that underwent resection of superficial gliomas. The exposed cortical surface of each patient was directly electrically stimulated (I_Stim = 6 mA, T_Stim = 5 s) with a bipolar electrode on multiple stimulation sites while simultaneously images of the cortical surface where acquired with a camera attached to the surgical microscope. Light wavelength filtering was performed within the optical path of the camera at λ = 568 nm (FWHM = 10 nm). Hemodynamic responses were extracted from all stimulation sites using a dynamic linear model. Classification for tumor or non-tumor response was performed based on neuro-navigation data. Various characteristic features (peak signal amplitude, signal duration etc.) of the hemodynamic response were extracted and compared for both response types.

Results: The group of hemodynamic response models extracted from tumor tissue sites are characterized by a statistical significant lowered maximal signal amplitude of relative reflectance change compared to the hemodynamic response models extracted from non-tumor sites (Wilcoxon rank sum test, p = 0.003). The other calculated model features showed no significant difference. Furthermore, the results are revealing that several anatomical influences, e.g. a thickened arachnoid membrane or the stimulation over sulci may influence the stimulation results.

Conclusion: The investigations demonstrate evidence that intraoperative tissue differentiation by optical imaging of direct cortical stimulation might be possible. Significant differences were especially observed in the peak signal amplitude that correlates with the amount of blood volume changes.