Ultrasonic Inverse Scattering Transmission Tomography for Brain Imaging

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Ultrasonic transcranial brain imaging using the conventional pulse echo B-mode technique has not been realized. This is due to the fact that sound wave suffers great attenuation when transmitting through the cranial bone. Furthermore, strong scattering of the cranial bone violates the theoretical assumption made in the image formation procedure. In this paper, inverse scattering computed tomographic technique was proposed as a solution to the quantitative sound speed cross-sectional image of a human brain. For the reduction of the sound wave attenuation, 100 kHz range low frequency transmission waves were observed on the head surface and used as available input data of the inverse scattering image calculation. In that case, ordinaly linearized inverse scattering technique assuming weak scattering precondition cannot meet the requirement of the transcranial brain imaging. To this end, a method for excluding the effect of cranial bone was developed. For instance, the observation waves on the head surface were once converted to the waves before transmitting the cranial bone with the knowledge of the shape and acoustic property of the cranial bone. Subsequently, the illumination wave components were subtracted to get the genuine forward scattering wave components. Lastly, applying the ordinary linearized inverse scattering procedures to the preprocessed data, sound speed image can be obtained. To verify the effectiveness of the proposed method, simulation examinations were made through the finite difference simulation calculations. Human cranium phantom with elliptical shape and maximum thickness 8mm was assumed. As a ROI, lesion of hematoma was assumed having sound speed 3% larger than surrounding tissue. As a results of the test examination, it was shown the sound speed image with relatively good quantitative precision could be obtained, in particular, the assumed hematoma legion could be clearly discriminated.