Therapeutic Quadrisected Annular Array for Improving Magnetic Resonance Compatibility.

Objective: Focused ultrasound has been applied in brain therapeutics. Although focusing ultrasonic beams on multiple arbitrary regions under the guidance of magnetic resonance imaging(MRI) is needed for precise treatments, current therapeutic transducers with large pitch sizes have been optimized to focus on deep brain regions. While annular arrays can adjust the beam foci from cortical to deep regions, their circular shape may generate eddy current-induced magnetic flux during MRI. In this study, a quadrisected annular array is proposed to address these limitations. Methods: Conventional and quadrisected annular arrays with three elements were implemented by loading the electrode patterns onto an 850kHz 1-3 composite PZT disc, with a diameter of 31mm, including three rings. MR compatibilities were demonstrated by imaging an MRI phantom with pulse sequences for B0 and B1 mapping and spin-echo imaging. Acoustic beam profiles, with and without a macaque monkey skull, were measured. A quadrisected transducer was also used to open the blood-brain barrier(BBB). Results: The flip angle distortion improved by 20% in spin-echo MR imaging. The acoustic beam distortions shifting the focal point from 36 to 41mm and elongating the focal zone from 10 to 15 mm could be recovered to nearly the original values. BBB openings in the hippocampus and basal region were also demonstrated. Conclusion: The MR compatibility was improved by the increased resistance of the electrodes in the quadrisected array maintaining dynamic focusing capabilities. Significance: The quadrisected annular design can be a fundamental structure for a larger MR-compatible segmented array transducer generating multiple acoustic foci.