A transcranial device and method for detecting cerebellar brain motion

Chiari Type I Malformation is a condition in which the cerebellar tonsils, small lobes on the undersurface of each half of the cerebellum, protrude through the base of the skull and press against the spinal cord. Assessing the motion of these herniated structures and their effects on cerebrospinal fluid dynamics is of significant clinical interest, particularly since the condition has been implicated in the formation of serious secondary disorders affecting the brain and spinal cord, such as hydrocephalus and syringomyelia. While MRI studies have shown no statistically significant differences in tonsillar motion of Chiari Type I patients compared to healthy individuals, surgeons have reported rapid tonsil motion as observed by intraoperative sonography during decompression surgery. However, it remains unclear whether this discrepancy is due to limitations of the MRI technique or decompression facilitating increased movement. Therefore, we aim to test the hypothesis that localized movement of cerebellar tonsils can be assessed non-invasively in the intact skull using ultrasound. Here, an investigation into the use of a novel methodology for transkull imaging in assessing cerebellar tonsil motion is presented. Two transducers (1MHz, 0.5 inches in diameter) were placed rostrocaudally on the frontal and suboccipital surfaces of a water-filled ex vivo human skull. A sinusoidal pulse was transmitted into the specimen from one transducer, and recorded by the receiving transducer at the opposite surface. Starting at the edge of the foramen magnum, the transducers were rotated at 8mm intervals in a counterclockwise direction. A tissue phantom was also used to mimic the cerebellar tonsils. Attenuation through the skull and motion detection in the tissue phantom was analyzed. It is shown that at a transducer frequency of 1MHz, our through transmission ultrasonic technique allows for a substantial energy transmission of up 8.6%. No signal was observed at the same points in ref- ection mode. To our knowledge, this level of energy transmission has been achievable only through a very limited temporal acoustic window, which fails in up to 29% of patients in a general population. Results demonstrate the feasibility of using this type of transducer system for a non-invasive pre-surgical assessment of cerebellar tonsil motion without the need for an acoustic window.