Experimental Study on CW Diffuse Reflective Optical CT

本项目是利用体表二维连续波源探测器阵列进行具有三维空间分辨率的漫射光学层析成像的可行性研究，旨在对受皮肤循环波动影响最小的大脑活动进行实时成像。在光子迁移的扩散方程近似中，我们基于Rytov近似对吸收系数的小扰动进行了正演求解，并利用简单正则化Moore-Penrose伪逆构造了三维层析重建。分析了重构误差和空间分辨率对正则化参数、目标深度和重构体素大小的依赖关系。研究表明，重建误差与空间分辨率之间的权衡不依赖于体素大小。然而，发现简单正则化逆重构灵敏度具有较强的深度依赖性，使得深度范围内更多的目标脑信号容易被浅层范围内不需要的皮肤循环信号所掩盖。我们设计了深度自适应体素大小方法和深度自适应正则化方法，这两种方法都能有效地克服这一严重问题。基于这些广泛的模拟研究，我们明确了空间分辨率、重建噪声、重建感兴趣区域和重建灵敏度均匀性等之间的权衡。为了评估实验可行性，我们修改了现有的商用光学地形仪，实现了重建的数字数据采集，并通过模拟研究检查了性能。最后，利用现有地形仪的源探测器系统，设计了一个实用的成像系统。该系统将在距离皮肤表面1cm深度的大脑皮层4cm × 4cm区域内以1cm的空间分辨率显示大脑活动，并抑制20 dB的有害皮肤循环信号。少

This project is a feasibility study on diffuse optical tomography which has 3-D spatial resolution using 2-dimensional CW source-detector arrays on the body surface, aiming at real time imaging of the brain activation minimally contaminated by skin circulation fluctuation. We formulated the forward problem based on the Rytov approximation for the small perturbation of the absorption coefficients in the diffusion equation approximation of the photon migration, and formulated the 3-D tomographic reconstruction using simple regularized Moore-Penrose pseudo inverse. We analyzed the dependencies of the reconstruction error and the spatial resolution on the regularization parameter, the target depth and the reconstruction voxel size. The study showed that the trade-off between the reconstruction error and the spatial resolution does not depend on the voxel size. The simple regularized inverse, however, was found to have strong depth dependencies of the reconstruction sensitivity so that the … More target brain signal in the depth would be easily masked by the unwanted skin circulation signal in the shallow range. We devised the depth-adaptive voxel size method and the depth-adaptive regularization, both of which were shown to be effective to overcome this serious problem. Based on these extensive simulation studies, we clarified trade-offs between spatial resolution, reconstruction noise, reconstruction region of interest and the uniformity of the reconstruction sensitivity etc. To assess the feasibility experimentally, we modified a existing commercial optical topograph, enabling digital data acquisition for the reconstruction, and checked the performance by phantom study. Finally, we designed a practical imaging system with the source-detector system of the existing topograph. The system would visualize the brain activity with the spatial resolution of 1cm in the 4cm X 4cm area of the cerebral cortex assumed at the 1cm depth from the skin surface with the 20 dB suppression of the unwanted skin circulation signal. Less