Correlation of ultrasound phase with physical skull properties.

Noninvasive treatment of brain disorders using focused ultrasound (US) requires a reliable model for predicting the distortion of the field due to the skull using physical parameters obtained in vivo. Previous studies indicate that control of US phase alone is sufficient for producing a focus through the skull using a phased US array. The present study concentrates on identifying methods to estimate phase distortion. This will be critical for the future clinical use of noninvasive brain therapy. Ten ex vivo human calvaria were examined. Each sample was imaged in water using computerized tomography (CT). The information was used to determine the inner and outer skull surfaces, thickness as a function of position, and internal structure. Phase measurement over a series of points was obtained by placing a skull fragment between a transducer and a receiver with the skull normal to the transducer. Correlation was found between the skull thickness and the US phase shift. A linear fit of the data follows that predicted by a homogeneous skull when average speed of sound 2650 m/s was used. Large variance (SD = 60 degrees, mean = 50 degrees ) indicates the additional role of internal bone speed and density fluctuations. In an attempt to reduce the variance, the skull was first studied as a three-layer structure. Next, density-dependent bone speed fluctuation was introduced to both the single-layer and three-layer models. It was determined that adjustment of the mean propagation speeds using density improves the overall phase prediction. Results demonstrate that it is possible to use thickness and density information from CT images to predict the US phase distortion induced by the skull accurately enough for therapeutic aberration correction. In addition, the measurements provide coefficients for phase dependence on skull thickness and density that can be used in clinical treatments.