Noninvasive measurement of head dielectric properties as a novel method for monitoring intracranial volume variations

Abstract

Introduction: Intracranial pressure (ICP) monitoring and craniospinal compliance (CC) assessment can provide information for the diagnosis and management of neurological disorders. However, these tests are invasive, requiring probe placement within the cranial or spinal space. We present a method that may allow for the noninvasive derivation of CC surrogate metrics. It is based on the concept that cardiac and respiratory action modulate the dielectric properties of the head by periodic displacement of blood and cerebrospinal fluid (CSF), and that such modulations can be assessed noninvasively using capacitive measurements1. Through acquisitions on healthy volunteers, we show that the time course of the signal obtained in this fashion has characteristics reminiscent of typical ICP waveforms. We further show by physiological testing that the measured signal is, in part, of intracranial origin. Methods: A custom device for capacitive measurements was coupled to the volunteers’ heads through electrically isolated electrodes and hyperventilation testing was performed. As intracranial arteries have markedly higher CO2 reactivity than extracranial ones, changes in the measured signal produced by hypocapnia are expected to be primarily of intracranial origin. We focused on the peak-to-peak amplitude of signal oscillations produced by cardiac action (AMP) as the metric to compare normocapnic and hypocapnic conditions. Results: The acquired signal showed cardiac and respiratory oscillations. Hyperventilation reduced the end-tidal CO2 from 5.7% ± 0.2 to 4.2% ± 0.3 (P = 0.028). AMP was reduced in all investigated subjects during hyperventilation and returned to its baseline value during a control period. Discussion: We have developed a method for probing changes in the dielectric properties of the head induced by natural oscillations of the composition of the intracranial space due to cardiac and respiratory action. Hyperventilation testing suggests that signals obtained are, in part, of intracranial origin. CC surrogates could potentially be derived using this noninvasive approach.