Ultrasound field measurement using a binary lens

TL;DR

This paper introduces a binary lens device for ultrasound field measurement that improves signal gain and reduces artifacts, enabling accurate characterization of focused ultrasound fields in air.

Contribution

A novel binary plate lens design focusing both planar and spherical waves is proposed, along with a prototype device for ultrasound field measurement in air.

Findings

Good correlation with nonlinear acoustic simulations

Mean pressure differences of 10-12% over 3dB FWHM

Mean pressure differences of 12-17% over 6dB FWHM

Abstract

Field characterization methods using a scattering target in the absence of a point-like receiver have been well described in which scattering is recorded by a relatively large receiver located outside the field of measurement. Unfortunately, such methods are prone to artifacts due to averaging across the receiver surface. To avoid this problem while simultaneously increasing the gain of a received signal, the present study introduces a binary plate lens designed to focus spherically-spreading waves onto a planar region having a nearly-uniform phase proportional to that of the target location. The lens is similar to a zone plate, but modified to produce a bi-convex-like behavior, such that it focuses both planar and spherically spreading waves. A measurement device suitable for characterizing narrowband ultrasound signals in air is designed around this lens by coupling it to a target and planar receiver. A prototype device is constructed and used to characterize the field of a highly-focused 400 kHz air transducer along 2 radial lines. Comparison of the measurements with numeric predictions formed from nonlinear acoustic simulation showed good relative pressure correlation, with mean differences of 10% and 12% over center 3dB FWHM drop and 12% and 17% over 6dB.