Ultrasound Elastography for Biomedical Applications and Medicine

21 From Time Reversal to Natural Shear Wave Imaging

Stefan Catheline

LabTAU, INSERM‐University of Lyon, France

21.1 Introduction: Time Reversal Shear Wave in Soft Solids

The potential of time‐reversal methods [1] includes different areas such as telecommunication [2, 3], nondestructive testing [4], interactivity [5], medical imaging, and therapy [6]. It is an elegant and robust adaptive focusing method. In practical situations, a source or a strong reflector embedded in the medium is required in order to observe a forward wave scene. Then, a time‐reversal mirror is used to recreate the backward scene. The time‐reversed wave finally collapses on its source location. The problem of the source in a time‐reversal experiment is known in the medical field [7] as the “artificial ultrasonic stars”, in reference to analogous laser‐guided artificial stars used for optic adaptive focusing in astronomy [8]. In the time‐reversal experiments presented in Figure 21.1a, a reverberant solid cavity has been used. The great advantage of such a configuration is that a perfect time‐reversal cavity can be approached experimentally using a point source mounted on a vibrator: it is the so‐called one‐channel time‐reversal mirror [9]. The elastic field was measured noninvasively from the solid surface using a single transducer and ultrasonic techniques developed in the field of transient elastography [10]. In this medical field, propagation media are soft solids such as soft tissues, muscles, ...