Yoon D C
UCLA School of Dentistry, Section of Oral Radiology, PO Box 951668, Los Angeles, California 90095-1668, USA.
Dentomaxillofac Radiol. 2000 Jan;29(1):11-9. doi: 10.1038/sj/dmfr/4600487.
To provide a robust and convenient method by which radiographic images can be spatially aligned for digital radiographic subtraction without the use of manually selected reference points.
An automated method for image alignment is described which begins with the extraction of a large number of edge features (1500 or more pixels) from each of two radiographic images taken of the same anatomical region of a given patient. The features in the first radiograph are paired, pixel by pixel, with those in the second radiograph using a nearest neighbor criterion. The edge features in the first radiograph are aligned with those in the second radiograph by performing an affine transformation that is consistent with the projection geometry for a plantar parallel X-ray beam. Transformation parameters are determined which provide the smallest spatial alignment error between the two sets of equivalent features. These parameters are found by a closed-form analytic solution, thus enabling a computationally efficient implementation. The final transformation is then applied to the entire first image resulting in a close spatial match to the second image. The performance of three dentists using the automatic method was compared with their performance using a manual method of alignment for eight pairs of images.
The root mean squared error in image alignment for the automatic method was 14% lower than that with manual alignment. The variability for the automatic method was half that of the maximal method as measured by the residual error. The automatic method was also three times faster than the manual method.
This method could make digital subtraction more accessible to researchers and practising dentists. Batch mode implementation could enable the processing of large volumes of data. Restriction to a region of interest and improved feature extraction could further improve performance.
提供一种强大且便捷的方法，通过该方法可对射线图像进行空间对齐，以实现数字射线减影，而无需使用手动选择的参考点。
描述了一种图像对齐的自动化方法，该方法首先从给定患者同一解剖区域的两张射线图像中提取大量边缘特征（1500个或更多像素）。使用最近邻准则将第一张射线照片中的特征与第二张射线照片中的特征逐像素配对。通过执行与足底平行X射线束的投影几何一致的仿射变换，使第一张射线照片中的边缘特征与第二张射线照片中的边缘特征对齐。确定变换参数，这些参数可在两组等效特征之间提供最小的空间对齐误差。这些参数通过闭式解析解找到，从而实现计算效率高的实现。然后将最终变换应用于整个第一张图像，使其在空间上与第二张图像紧密匹配。比较了三位牙医使用自动方法和手动对齐方法对八对图像的性能。
自动方法在图像对齐中的均方根误差比手动对齐低14%。通过残差误差测量，自动方法的变异性是手动方法的一半。自动方法也比手动方法快三倍。
该方法可使研究人员和执业牙医更易于进行数字减影。批处理模式的实现可以处理大量数据。限制在感兴趣区域并改进特征提取可进一步提高性能。
