Can we make a wave revive its past life? The development of ‘time-reversal mirrors’ for different types of waves — ultrasound, microwave and visible light — has made it possible to exploit time reversibility. Such devices bounce waves back and forth with a target like a game of tennis, with the waves becoming more focused over time. These instruments offer original solutions for imaging complex disordered environments.

There has recently been a surge of interest in applying ultrasound to the brain as a direct or adjunctive therapeutic agent. Ultrasound energy is already being used in clinical pilot studies to treat such neurological issues as anxiety, depression and even coma, and to facilitate the uptake of drugs through the blood brain barrier. However, there are still fundamental questions regarding the nature of ultrasound interaction with neural tissues.