High resolution ultrasonic brain imaging: adaptive focusing based on twin-arrays

Transcranial imaging of the brain is currently limited by the defocusing effect of the skull bone (absorption, diffusion and refraction of ultrasounds). A brief review of the various techniques developed in the last decades to correct the aberrations induced by the skull bone is first presented. A noninvasive brain imaging device is presented that takes into account the defocusing effect of the skull. This device is made of two identical "twin" linear arrays located on each side of the head. It is shown how to differentiate the respective influence of the two bone windows on the path of an ultrasonic wave going from one array to the other, and how to estimate at each frequency the attenuation and phase shift locally induced by each of the bone windows. This information is then used to perform adaptive focusing through the skull. Compared to uncorrected wave fronts, the spatial shift of the focal point is cancelled, the width of the focal spot is reduced, and sidelobes level is decreased up to 10dB. Simulated structural transcranial images of a brain model are presented to exhibit the improvement in image quality provided by this new noninvasive adaptive focusing method.