1. A computer-implemented automatic tracing algorithm for a quantitative analysis of continuous structures, applied for analyzing a 3D image of the continuous structures, and each the continuous structure having at least one characteristic element, and the algorithm comprising the steps of: S 1 : capturing the 3D image of the continuous structure and digitizing the 3D image such that the 3D image is comprised of a plurality of voxels; S 2 : selecting a starting point voxel and assigning a starting point code N 1 to the starting point voxel, wherein the starting point voxel is selected from any voxel of the 3D image; S 3 : setting an intensity threshold of the 3D image, and starting with the starting point voxel to encode all the voxels of the 3D image that match with the intensity threshold, and the step S 3 further comprising the steps of: S 31 : assigning a voxel code to all the voxels adjacent to the starting point voxel N 1 and matched with the intensity threshold, and the voxel code being incremented from the starting point code N 1 by an integer to N 2 ; S 32 : using the voxel with a voxel code of N 2 as an adjacent starting point, and assigning a voxel code of N 3 to each the voxel adjacent to the voxel with the voxel code of N 2 and matched with the intensity threshold; and S 33 : repeating S 32 until all the voxels of the 3D image matched with the intensity threshold are encoded; and S 4 : using a standard code tracking to identify the characteristic element of the 3D image, and the standard code tracking comprising the steps of: S 41 : setting a tracing codelet (Np+t−1, Np+t−2,. .. , Nt), wherein the tracing codelet is comprised of the three or more connected voxels, the tracing codelet has a length of p voxels, a starting point at a voxel with a voxel code of Np+t−1, an end point at a voxel with a voxel code of Nt, and an integer t starts at 1 and increases as a tracing progress; S 42 : tracing by the tracing codelet from each direction of the starting point voxel N 1 to the adjacent voxel code by using a source field encoding methodology; S 43 : using the adjacent voxel code as a tracing codelet starting point and repeating the step S 42 until all the voxel codes of the 3D image are traced; and S 44 : determining each the characteristic element in the 3D image of the continuous structure, and recording the voxel code at the position of the characteristic element of the continuous structure.