Focusing by spatiotemporal inverse filter: Application to heterogeneous and absorbing medium

Abstract

In most applications of ultrasound, a first step consists in focusing a broadband ultrasonic beam deeply inside the medium. Nevertheless, sound speed, density, or absorption heterogeneities may strongly degrade the focusing. Different techniques have been developed to correct such aberrations (time reversal, speckle brightness,...). Recently, we investigated a new broadband focusing technique, the spatiotemporal inverse filter, based on the inversion of the propagation operator relating a transducer array to a set of control points inside the medium. As this inversion is achieved both in space and time, it allows one to calculate the set of temporal signals to be emitted by each array element for an optimal focusing on a chosen control point. Potential applications have been experimentally investigated in heterogeneous and absorbing media where classical geometrical focusing is highly degraded. The strong improvement achieved by inverse filter compared to time reversal and classical focusing emphasizes the great potential of multichannel systems having the ability to apply completely different waveforms on each element. Particularly, intraplate echoes suppression in NDT, high quality focusing through a human skull, as well as hyper-resolution in reverberating media highlight its great interest in various fields such as medical imaging, nondestructive testing, room acoustics, and underwater acoustics.