Noninvasive Monitoring of Intracranial Pulse Waves

Abstract

The clinical management of several neurological disorders benefits from the assessment of intracranial pressure and craniospinal compliance. However, the associated procedures are invasive in nature. Here, we aimed to assess whether naturally occurring periodic changes in the dielectric properties of the head could serve as the basis for deriving surrogates of craniospinal compliance noninvasively. Methods: We designed a device and electrodes for noninvasive measurement of periodic changes of the dielectric properties of the human head. We characterized the properties of the device-electrode-head system by measurements on healthy volunteers, by computational modeling, and by electromechanical modeling. We then performed hyperventilation testing to assess whether the measured signal is of intracranial origin. Results: Signals obtained with the device on volunteers showed characteristic cardiac and respiratory modulations. Signal oscillations can be attributed primarily to changes in resistive properties of the head during cardiac and respiratory cycles. Reduction of end-tidal CO2, through hyperventilation, resulted in a decrease in the signal amplitude associated with cardiovascular action. Conclusion: Given the higher CO2 reactivity of intracranial vessels compared to extracranial ones, the results of hyperventilation testing suggest that the acquired signal is, in part, of intracranial origin. Significance: If confirmed in larger cohorts, our observations suggest that noninvasive capacitive acquisition of changes in the dielectric properties of the head could be used to derive surrogates of craniospinal compliance.

Theory for a non-invasive diagnostic biomarker for craniospinal diseases

“…Other signal sources, e.g., related to the pulsation of intra- and extra-cranial vasculature, or scalp tissues, have not been considered (see Andreas Spiegelberg et al, 2022 , Boraschi et al, 2022 regarding experimental evidence for the intra-cranial origin of the signal). For blood vessels with small diameters that are fully embedded in a homogeneous tissue, the signal contributions from diametrically opposite vessel sides are expected to compensate, such that only large vessels on the brain surface or within the scalp/face are relevant.…”

“…In a forthcoming study, computational model predictions will be experimentally validated in healthy and diseased subjects using the recently developed device from Andreas Spiegelberg et al (2022) .…”

“…This is a quasi-static approximation of Maxwell’s equations which neglects the displacement current. It is suitable in the frequency range of interest, i.e., around 1 MHz ( Andreas Spiegelberg et al, 2022 ), because the head dimensions are much smaller than the EM wavelength (multiple meters). For this study, Dirichlet boundary conditions (constant voltage) were used on the electrodes and insulating Neumann boundary conditions at the remaining surfaces.…”

“…Dirichlet boundary conditions were applied to the rectangular electrodes. The working frequency was chosen to be 1 MHz based on the recently developed device by Andreas Spiegelberg et al (2022) . Then, Eq.…”

“…This complicates both computations and experimental measurements. To overcome the measurement problems, Andreas Spiegelberg et al (2022) developed a new non-invasive impedance measurement device. This work aims at overcoming the computational issues.…”