The time reversal kaleidoscope: a new concept of smart transducers for 3D imaging

The design of two dimensional arrays for real time 3D imaging is a major challenge in medical ultrasound. Several thousand of transducers are typically needed to achieve the beam focusing and steering in a large 3D region of interest. Here we report a completely new approach for producing 3D images with a very small number of transducers using the combined concepts of time reversal mirrors and chaotic reverberating cavities. A small number of transducers glued on the surface of a solid cavity are used in a time reversal mode. If one face of the cavity is in contact with a fluid medium (like the body), ultrasonic waves transmitted by the transducers may take benefit of cavity reverberations to focus in any point of the fluid by using time reversal techniques. Due to the reverberations inside the cavity, waves emitted by each transducer are reflected hundred of times, creating at each reflection ´virtual´ transducers that can be observed by any observer located in the fluid. The result of such an operation is that a small number of transducers (typically 32) is multiplied by a number greater than hundred to create a kaleidoscopic transducer array presenting equivalent performances than conventional 2D matrices made of thousands of transducers. The time reversal technique allows us to use these ´virtual´ transducers to focus with a very weak side lobe level on each point of the fluid. First experimental 3D images obtained with a pioneer prototype will be presented. Beyond the scope of 3D medical imaging, this combination of time reversal processing with chaotic reverberating cavities leads to the concept of a ´smart´ transducer.