Development and characterization of a superresolution ultrasonic transducer

Highly sensitive ultrasound probes are needed to expand the capabilities of biomedical ultrasound and industrial non-destructive testing (NDT). Pursuing better imaging quality, while keeping fabrication costs low, is an important trend in the current development of ultrasound imaging systems. In this paper, we report the development and characterization of an ultrasonic transducer that (super)focuses ultrasonic waves beyond the so-called diffraction limit, i.e., the beamwaist is roughly narrower than one wavelength. The transducer comprises an additive manufactured case with a circular flat piezoelectric actuator fixed at the bottom and a core-shell lens (with a stainless steel core and a polymer shell) placed at the probe's conical tip. The core-shell lens is responsible to superfocusing effect of ultrasonic waves. Operating at approximately 3 MHz, the transverse and axial resolution for C- and B-scan images are, respectively, 0.65λ and 3λ/2, with the wavelength being λ = 0.5 mm. Whereas the system depth-of-field is 6.3λ. To demonstrate the transducer capability to resolve subwavelength structures, we successfully obtain images of a copper wire forming a Y-intersection, whose branches a diameter similar to human hair (0.15 mm ≈ 0.3λ). Our results represent a solid step toward the development of ultrasonic superresolution transducer applied for biomedical imaging and shallow NDT of materials.