Ultrasound waves used in conventional biomedical imaging applications carry momentum and impart small forces to tissue during the imaging process. This nonlinear acoustic radiation force acts in the direction of the ultrasound beam and grows in magnitude with the square of the incident ultrasound pressure. In most biological tissues the momentum transfer arises from absorption of the ultrasound energy by the tissue, but if there is a large discontinuity in the tissue impedance additional force arises from reflection. In preliminary studies we examined physiological effects of ultrasound acoustic radiation force in the inner ear. When low intensity ultrasound was focused on the inner ear otolith organs, a relatively large acoustic radiation force was generated, pushing the otolith in the direction of the ultrasound beam, deflecting hair bundles in the direction of the beam, and modulating action potential discharge rate of afferent neurons. The force was generated primarily because of the acoustic impedance mismatch between endolymph and the otoconial mass, a mismatch that is not present in the semicircular canals or the cochlea. This property can be used to selectively activate individual otolith organs in prescribed directions using a remote noninvasive ultrasound transducer. The present R21 application is designed to develop ultrasound as controlled stimulus for otolith organs, and to examine physiological responses of vestibular organs and the cochlea. The technology has potential as a stimulus for testing the function of otolith organs and compensatory neural circuits, as a highly controlled stimulus for modulating individual otolith inputs to the brain, and as a noninvasive method for vestibular neuromodulation.
Currently available methods to test the function of vestibular otolith organs and the compensatory neural circuits they drive rely on intense air conducted sound, bone conducted vibration, or head acceleration as stimuli. Recent evidence suggests 1-5MHz low intensity focused ultrasound is a much more controllable stimulus for activation of individual organs and frequency-specific neural pathways. Safety and efficacy of this new approach will be examined.
| Iversen, M M; Zhu, H; Zhou, W et al. (2018) Sound abnormally stimulates the vestibular system in canal dehiscence syndrome by generating pathological fluid-mechanical waves. Sci Rep 8:10257 |
