New method of aberration correction for ultrasonic brain imaging

Ultrasonic imaging systems capabilities are strongly dependent on the focusing quality of the ultrasonic beam. In the case of brain imaging, the skull strongly degrades the ultrasonic focusing pattern by introducing strong phase and amplitude aberrations of the wavefront. In previous work, this degradation of the beam focus had been partially corrected by coupling the time reversal focusing process to an amplitude compensation of the emitted signals. In that case, the optimal focus was reproduced down to - 20 dB, but the sidelobe level remained at about -25 dB. We propose here a new focusing technique, called spatio-temporal inverse filter, based on the inversion of the propagation operator at each frequency within the bandwidth of our transducers. Simulations and experiments will be presented that show the effectiveness of the technique. Experimental focusing through the skull is now comparable to the optimal focusing in homogeneous medium. It will be explained how the set of emission signals can be used in order to achieve both transmit and receive focusing. In the transmit-receive mode, focusing through the skull reaches the optimal level obtained in water down to -70 dB (i.e. constrained only by experimental noise levels). This could lead to high quality real time brain imaging and Doppler flow mapping.