Time reversal of ultrasonic fields. I. Basic principles

TLDR

Time‑reversal of ultrasonic fields enables focusing through inhomogeneous media, building on classical adaptive time‑delay techniques for pulsed wave focusing. A time‑reversal mirror composed of a linear transmit‑receive transducer array samples the incident pressure, time‑reverses it, and re‑emits it to focus on a reflective target that acts as an acoustic source. Pulsed‑wave time‑reversal focusing achieves the optimal spatial‑temporal matched filter for inhomogeneous media, and has inspired concepts such as a time‑reversal cavity and iterative processing for automatic target sorting.

Abstract

Time reversal of ultrasonic fields represents a way to focus through an inhomogeneous medium. This may be accomplished by a time-reversal mirror (TRM) made from an array of transmit-receive transducers that respond linearly and allow the incident acoustic pressure to be sampled. The pressure field is then time-reversed and re-emitted. This process can be used to focus through inhomogeneous media on a reflective target that behaves as an acoustic source after being insonified. The time-reversal approach is introduced in a discussion of the classical techniques used for focusing pulsed waves through inhomogeneous media (adaptive time-delay techniques). Pulsed wave time-reversal focusing is shown using reciprocity valid in inhomogeneous medium to be optimal in the sense that it realizes the spatial-temporal matched filter to the inhomogeneous propagation transfer function between the array and the target. The research on time-reversed wave fields has also led to the development of new concepts that are described: time-reversal cavity that extends the concept of the TRM, and iterative time-reversal processing for automatic sorting of targets according to their reflectivity and resonating of extended targets.