Wideband Image-Based Transskull Refocusing Using Dual-Mode Ultrasound Arrays

A new image-based wideband transcranial refocusing algorithm is presented together with ex vivo experimental verification results. A dual-mode ultrasound array (DMUA) prototype and supporting real-time imaging platform are used for the acquisition and processing of refocusing beams through ex vivo skull samples. The algorithm utilizes pre-beamforming DMUA echo data to perform optimal refocusing in multiple frequency bands within the transducer bandwidth based on user selection of the target and any critical points on the skull where the exposure is to be minimized. Hydrophone measurements were performed in water tank with and without the skull in the path of the beam. These measurements clearly demonstrated that the focusing gain has non-monotonic frequency dependence for any target point. Moreover, the focusing gain profiles vary considerably between different target points. In the former case, the measurements were performed with and without refocusing. Additional experimental verification was obtained using ex vivo skull samples embedded in tissue-mimicking phantoms. Transient temperature rises at the target location (thermocouple junction) were recorded with and without optimal refocusing. The results from both setups clearly demonstrate the increase in focusing gain at the target location and its frequency dependence. The results clearly demonstrate that real-time, site-specific refocusing allows for synthesizing refocused tFUS beams with the maximum operating bandwidth allowable by the transmission characteristics of the skull to the target point(s).