Towards ultrasonic brain imaging

Ultrasonic imaging system capabilities are strongly dependent on the focusing quality of the ultrasonic beam. The beam width and sidelobe level constrain respectively the resolution and contrast of the final image. In the case of brain imaging, it is well-known that the skull strongly degrades the ultrasonic focusing pattern by introducing substantial 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 based on the calculation of the spatio-temporal inverse filter of the propagation. Experimental focusing through the skull is now comparable to the focusing in a homogeneous medium. In the transmit-receive mode, focusing through the skull could reach the optimal level obtained in water down to -70 dB (i.e. constrained only by experimental noise levels.)