Pulse echo imaging through a human skull: in vitro experiments

Brain imaging can be performed on newborns by focusing through the fontanel but at the present time, on the adults, echographic brain imaging remains limited to images obtained through the temple, using a small aperture. In order to perform high resolution imaging of adult brain, we proposed recently 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. Thanks to this technique, experimental transmit focusing through the skull is now comparable to the optimal focusing in homogeneous medium, i.e. down to -35 dB. Moreover spatial reciprocity teaches us that focusing either in transmit or in receive mode are equivalent.. We show that in the transmit-receive mode, focusing through the skull theoretically doubles the focusing pattern quality, leading to a -70 dB sidelobe level. Experimental images of phantoms placed behind a half human skull will be presented and compared to the one obtained with a classical transmit-receive focusing.