A dual mode hemispherical sparse array for B-mode skull localization and passive acoustic mapping within a clinical MRI guided focused ultrasound platform

Previous work has demonstrated that passive acoustic imaging may be used alongside MRI for monitoring of focused ultrasound therapy. However, current implementations have generally made use of either linear arrays originally designed for diagnostic imaging or custom narrowband arrays specific to in-house therapeutic transducer designs, neither of which is fully compatible with clinical MR-guided focused ultrasound devices. Here we have designed an array which is suitable for use within an FDA-approved MR-guided transcranial focused ultrasound device, within the bore of a 3 Tesla clinical MRI scanner. The array is constructed from 5 × 0.4mm piezoceramic disc elements arranged in pseudorandom fashion on a low profile laser-cut acrylic frame designed to fit between the therapeutic elements of a 230 kHz InSightec ExAblate 4000 transducer. By exploiting thickness and radial resonance modes of the piezo discs the array is capable of both B-mode imaging at 5 MHz for skull localization, as well as passive reception at the second harmonic of the therapy array for mapping of acoustic sources such as emissions from cavitation. The strengths and limitations of the system for passive acoustic imaging during in vivo experiments will be discussed, utilizing robust and conventional time and frequency domain beamforming methods.