After the rebuilt, the ILM boundary was obtained, and each A-line of two OCT and OCTA volumes was translated based on the displacement between and corresponding ILM boundary Display Formula
(5)where and are the displacement of each A-line in reference and moving volumes, respectively, and is the coordinate of each A-line. Each A-line was then translated based on the corresponding displacements. To reduce the registration error caused by ILM segmentation, another registration between these two A-lines was applied to fine-tune the axial final positions. Before the registration, the reflectance of two A-lines [Figs. 4(a) and 4(b)] was first normalized to [0, 1] and filtered by a pixel Gaussian filter with 5 pixel standard deviation [Fig. 4(c)]. The registration was optimized by a cost function Display Formula
(6)where is the cost function of the registration, is the normalized reflectance of the reference A-line, is the normalized reflectance of the moving A-line after a displacement , and is the number of pixel in each A-line. The optimized displacement was selected when the cost function achieved the smallest value. In the final axial registration step, and were translated to and, respectively, [Fig. 4(d)].