Transmission-enhancing grated plate may allow for previously inaccessible ultrasonic imaging

Ultrasound is a widely used, non-invasive medical imaging technique but has limitations in certain biological tissue, where impedance mismatch can prevent ultrasonic energy from being transmitted. To address this, Zhou et al. propose an impedance-mismatched brass plate with single-sided periodic gratings to enhance ultrasonic transmission.

The researchers created a thin, brass plate with periodic circular gratings on a single side. This is in contrast with previous studies on the effects of periodic structures on transmission enhancement, which have had gratings on both sides rather than just one. By immersing the plate in a water tank, the researchers were able to observe the transmission of an excited ultrasonic field through the plate and understand the impact of the gratings. Lamb waves, which are couplings between longitudinal and transverse waves, were excited by the grating.

When the grating period is equal to the wavelength of the first Lamb mode, the researchers found the plate helps overcome the impedance mismatch. The resonance increases the transmission coefficient, leading to nearly full transmission of the ultrasonic waves through the plate. These results agree with simulations, and comparisons with uniform brass plates of various sizes indicate the improved transmission is a consequence of the gratings.

This improvement in ultrasonic transmission can ultimately make previously inaccessible ultrasonic imaging possible. Following additional work in developing a three-dimensional spherical shell model of the grating structure to properly fit over the skull, the impedance mismatch between the skull and intracranial tissue may be overcome, a first step towards ultrasonic cranial imaging. “Because the skull is curved and its thickness varies, it cannot be treated simply as a plate,” said author Zheng Xu.

Source: “Enhancement of ultrasonic transmission using a patch patterned with single-sided periodic gratings,” by Qinxin Zhou, Zheng Xu, Wei Xu, Lei Xue, and Liming Cheng, Journal of Applied Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5119734 .