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

High resolution transcranial imaging of the brain requires adaptive focusing techniques in order to correct the defocusing effect of the skull bone (absorption, diffusion and refraction of ultrasounds). Here is presented a noninvasive brain imaging device made of two identical "twin" linear arrays located on each side of the skull. It is shown how to separate the respective influence of the two bone windows on the path of an ultrasonic wave going from one array to the other, and estimate at each frequency the attenuation and phase shift locally induced by each of the bone windows. This information is then used to correct, for a wide band signal, the wave fronts that have to be sent through the skull in order to obtain a good focusing inside 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 sidelobe levels are decreased up to 10 dB. 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.