Ultrasonic Inverse Scattering Transmission Tomography for Brain Imaging

使用常规脉冲回波B模式技术的超声经颅脑成像尚未实现。这是由于声波在通过颅骨传输时受到很大衰减的事实。此外，颅骨的强散射违反了图像形成过程中的理论假设。本文提出了一种利用逆散射CT技术定量测量人脑声速截面图像的方法。为了减少声波衰减，在头部表面上观察到100 kHz范围的低频传输波，并将其用作逆散射图像计算的可用输入数据。在这种情况下，一般的线性化逆散射技术假设弱散射的前提下，不能满足经颅脑成像的要求。为此，开发了一种排除颅骨影响的方法。例如，头部表面上的观察波曾经被转换为在利用颅骨的形状和声学特性的知识传输颅骨之前的波。随后，减去照明波分量以获得真正的前向散射波分量。最后对预处理后的数据进行线性化逆散射处理，得到声速图像。为了验证该方法的有效性，通过有限差分仿真计算进行了仿真检验。采用椭圆形人体颅骨体模，最大厚度为8mm。作为ROI，假设血肿病变的声速比周围组织大3%。试验结果表明，该方法能获得定量精度较好的声速图像，特别是能清楚地分辨出假定的血肿群。

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.