We developed and experimentally realized a novel type of focal transducer called a void-centric zone plate ultrasonic transducer (VZPUT). Although VZPUT is a planar transducer, it can focus ultrasound and control the focal length using a single-channel electric network. Further, unlike a classical Fresnel zone plate, the VZPUT has space for an imaging probe at the center, making it useful for therapeutic focused ultrasound (FUS). To realize VZPUT, we develop a novel void-centric zone plate (VZP) that allows design freedom for the radius of the space at the center. Then, we fabricate the piezoelectric VZP, an acoustic metamaterial, considering its structural vibration modes that appear within the operating frequency range. The VZPUT demonstrated applicability to therapeutic FUS in that it could control the focal point by the operating frequency with real-time monitoring images from the imaging probe mounted at the center.

The water immersion through-transmission method is widely used owing to its simplicity and high precision in measuring sound speed. However, when measuring the sound speed of a compound material whose host medium is water, such as hydrogel tissue-mimicking material (TMM), the reproducibility and reliability of the measurements obtained under the state of water immersion method were found to be low. To solve this problem, the application of contact transmission method instead of the water immersion method was examined in this study. Unlike the water immersion method, the direct-contact method requires stability in target stiffness, which is difficult in TMM hydrogels due to their low-stiffness properties. Therefore, it is important to find a material that acts as a propagation medium, maintains thickness, and prevents percentage change of the material component in TMM. As a propagation medium, it is necessary for the material to have a low attenuation coefficient value to increase the signal-to-noise ratio. Among the investigated candidate materials, polymethyl methacrylate (PMMA) was selected, owing to its lowest signal attenuation value compared to other candidate materials. A measuring jig was designed and manufactured with this material. The sound speeds of agar and gelatin TMMs were measured by the conventional water immersion through-transmission method and the proposed indirect contact-type method for comparison. In the case of the conventional water immersion method, as the measurement was repeated, the measured sound speeds showed a decreasing trend. By contrast, the indirect contact-type measurements using the PMMA jig showed relatively high reproducibility and reliability.

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