Abstract
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. Conventional and quadrisected annular arrays with three elements were implemented by loading the electrode patterns onto an 850 kHz 1-3 composite PZT disc, with a diameter of 31 mm, 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). 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. The MR compatibility was improved by the increased resistance of the electrodes in the quadrisected array maintaining dynamic focusing capabilities. The quadrisected annular design can be a fundamental structure for a larger MR-compatible segmented array transducer generating multiple acoustic foci.
Highlights
T HERAPEUTIC applications of focused ultrasound have expanded from treating tumors with high-intensity beams to modulating brain activity with low-intensity acoustic beams
While the precision in targeting specific brain regions is important in modulating brain functionalities, acoustic beam distortions caused by the different speeds of sound in the skull cannot be compensated for by a single element transducer with a fixed focal depth
The commercial therapeutic array transducer can successfully compensate for skull distortions and precisely focus acoustic beams at deep brain targets, the array structure composed of a group of small transducer patches on the inner surface of a spherical hemisphere can barely focus the acoustic beams in cortical regions [9]
Summary
T HERAPEUTIC applications of focused ultrasound have expanded from treating tumors with high-intensity beams to modulating brain activity with low-intensity acoustic beams. The commercial therapeutic array transducer can successfully compensate for skull distortions and precisely focus acoustic beams at deep brain targets, the array structure composed of a group of small transducer patches on the inner surface of a spherical hemisphere can barely focus the acoustic beams in cortical regions [9]. Another application of LIFU is to open the blood–brain barrier (BBB) for drug delivery into the brain parenchyma, which requires precise
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