Ultrasonic Inverse Scattering Transmission Tomography for Brain Imaging

传统的脉冲回波b型超声经颅脑成像技术尚未实现。这是由于声波在通过颅骨时受到很大的衰减。此外，颅骨的强散射违背了图像形成过程中所做的理论假设。本文提出了逆散射计算机层析成像技术作为定量声速人脑横切面图像的解决方案。为了减小声波衰减，在头部表面观测到100 kHz范围的低频透射波，并将其作为逆散射图像计算的可用输入数据。在这种情况下，以弱散射为前提的普通线性化逆散射技术不能满足经颅脑成像的要求。为此，提出了一种排除颅骨影响的方法。例如，在了解颅骨的形状和声学特性后，将头表面的观察波转换为传输颅骨之前的波。然后，减去光照波分量，得到真实的前向散射波分量。最后，对预处理数据进行普通线性化逆散射处理，得到声速图像。为了验证所提方法的有效性，通过有限差分仿真计算进行了仿真检验。假设人颅骨幻像为椭圆形，最大厚度为8mm。作为ROI，假设血肿病变声速比周围组织大3%。试验检验结果表明，声速图像具有较好的定量精度，特别是能够清晰地分辨出假定的血肿军团。

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.