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视神经纤维自动识别与分析关键技术研究
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摘要
目前,在有关人和动物的神经科学研究中,神经纤维形态分析被广泛应用于正常或病理状态下的神经组织研究或诊断中。神经纤维及轴突的形态分析,对于对正确理解神经发育、损伤及退化程度,掌握神经纤维形态特征规律及分布具有重要意义。
     神经纤维形态分析能否客观反映神经科学现象,关键取决于兴趣区域(如轴突或髓鞘)的选择提取是否科学、准确。在神经纤维形态分析研究之初,这项工作依赖手动方式提取兴趣区域,即由有经验的人员从显微图像中勾画出兴趣区域的轮廓及边界。随着研究的深入,手动方式的弊端明显体现:第一,手动方式费时费力,分割结果的优劣很大程度上取决于操作者的经验知识,客观性和可重复性较差;第二,对于数量庞大的神经纤维,少量采样可以缓解科研人员的繁重劳动,但是获取数据的科学性和可靠性却受影响。
     因而,神经纤维形态的自动识别与分析已成为图像处理和神经科学领域科研工作者共同关注的研究热点。从上世纪80年代,相关的神经纤维识别算法和商业化软件不断被推出和改进,使神经纤维的识别效果不断提高。
     精确的神经纤维形态识别和分析是建立在准确的图像分割基础之上。由于神经系统组织成分复杂多变,给神经纤维识别带来诸多困难,例如细小神经纤维的识别、复杂背景下特征的提取、不规则神经纤维的提取等。这些困难在神经纤维分布密集的神经(如视神经)中表现尤为突出。
     本课题研究的首要目标是开发出适合视神经纤维分割的有效方法。在纤维识别中,我们充分利用神经纤维的解剖结构特点,先获取纤维髓鞘的中心线,即粗略锁定神经纤维;在此基础上通过中心线的内移、外移及曲线演化,进而获得轴突和髓鞘的精细轮廓。这一技术思想也是本研究的创新主体。
     在上文所述创新思想的驱动之下,本研究采用改进后的Watersnake进行神经纤维分割。在实验中分两步进行:第一步,由Watershed生成连续且拓扑保持的神经纤维的初始骨架,并剔除假性纤维和边界位置的不完整纤维;第二步,由Snake将初始骨架引导到轴突和髓鞘的精细位置上。本算法提取的神经纤维保持了位置的准确和外形的光滑。
     在算法的性能评价实验中,随机抽取纤维图像,将自动分割结果与采用PHOTOSHOP软件手动分割结果进行对照分析。在纤维识别率分析中,自动识别正确率95.7%,假阳性率3.3%。在轴突直径频数分布对照分析中,自动识别与手动识别方式下的轴突直径频数分布直方图相吻合,波峰位置未有偏移。
     从目前的实验结果来看,本研究的识别率稍高于Romero等人的研究(94.8%),而假阳性率方面不及前者(1.0%)。研究中发现,假阳性纤维主要来源于拥挤纤维簇之间形成的闭合区域。在Romero等人的研究中,其研究对象是坐骨神经纤维,而坐骨神经纤维的分布比视神经纤维的分布稀疏,这也许是其假阳性率低于本研究的原因之一。
     本研究中的图像来源于分辨率较高的透射电镜底片,Watersnake技术对分辨率低的图像(如光镜图像)是否同样适用,在后期研究中我们将继续探索和改进,拓宽该技术的应用范围。
Presently, in the neurological research related to human and animal, histological examination of nerve fibers is often complemented by morphometric analysis in both clinical and research settings. Nerve morphometric analysis could provide researcher some valuable parameters.
     The accurate isolation of the ROI (Region of Interesting) is the key to the nerve morphometric studies. In the early investigation of nerve morphometry, the ROI is achieved manually i. e. the experienced neuroscientists draw the outline of the ROI in the microscopy image. Gradually, the disadvantage of manual morphometry is noticeable. Manual nerve morphometry is extremely tedious, labour intensive and time-consuming, and segmentation of ROI depend on the subjective decision and experience of operator. To overcome this there have been efforts to devise representative sampling schemes to reduce the amount of data analysis required. However, the accuracy of results is dubious.
     As a result, automatic identification of nerve fibers has draws much attention from image processing and neuroscience communities. Since the eighties of the last century, algorithms and commercial softwares related to nerve fibers detection were reported and promoted to improve the efficiency of fiber detection.
     Nevertheless, precise segmentation is the basis of the fiber analysis. In the process of fiber detection, the failure always results from the diversity of nerve fiber, for instance, lack of prominent features and irregular in shape. In the nerve with dense population, for example the optic nerve, the detection becomes more difficult.
     At present investigation, an original scheme for automatic segmentation of optic nerve fiber is proposed. Most importantly, the anatomical structure of fiber is taken into account. First, the center line of myelin sheath is picked up; therefore, the location of fiber is determined. Next, the contour is moved in, moved out, and finally, the fine contour is achieved by evolution of contour.
     Motivated by the above original scheme, we introduced watersnake to segment nerve fibers. Initially, watershed extracts continuous and topological reserving sketch of the nerve fibers and the false fibers are get rid of. In the following step, the initial contours are steered to be good fit to the fine location of axon and myelin sheath. The final contours are smooth and accurate.
     The proposed method has been tested using nerve image by random sampling; the results from automation were compared with those of manual method with the help of PHOTOSHOP software. The overall detection rate and false alarm rate were found to be 95. 7% and 3. 3%, respectively. The axon diameter frequency distribution was comparable to that done manually, and the peak has no offset.
     The current results indicate that the detection rate is slightly superior to the research of Romero (94.8%) , but the false alarm rate is inferior to that (1.0%) . It was found that the false fibers primarily originate from the close region among the crowded nerve fibers. Probably the sciatic nerve fibers adopted in Romero's research are sparse, which result in the false alarm rate is lower than ours.
     In our investigation, we adopted electron microscopy photographs with high resolution. In the following research, we would continue our investigation and made the technique of watersnake to be generally applied, so that it could be adaptable to the images with low resolution (light microscopy).
引文
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