Dual-arrays brain imaging prototype: experimental in vitro results

Abstract

Transcranial imaging of the brain is currently limited by the defocusing effect of the skull bone (absorption, diffusion and refraction of ultrasound. 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 shifts locally induced by each of the bone windows. This information is then used to perform non invasive adaptive focusing through the skull. Compared to non corrected wavefronts used in the beamforming process of commercial scanners, the spatial shift of the focal point is cancelled, the width of the focal spot is reduced, and the sidelobes level is decreased up to 15dB. The technique has been used to acquire in vitro images of tissue phantoms behind a skull wall, exhibiting the image quality enhancement with respect to images obtained with cylindrical focusing. Further refinement should lead to an improvement of the image contrast greater than 20 dB compared to conventionnal scanners.