1. A computer-implemented method for estimating a diameter of an object of interest, comprising: providing a volume of interest comprising a marker identifying a location of the object of interest; extracting a sub-volume of voxels around the marker; initializing at least two volumes of the sub-volume, wherein each of the voxels has at least two values corresponding to the at least two volumes, respectively; performing, iteratively, a diffusion operation and a reaction operation on the voxels to update at least two values; comparing, for each voxel, the at least two values to a threshold to assign each voxel to one of the at least two volumes, wherein the assignment of the voxels is a segmentation result; performing a boundary check to determine whether the object of interest is solitary; estimating a diameter of the object of interest from the segmentation result upon determining the object of interest to be solitary; and extracting the object of interest by performing ellipsoid fit on the sub-volume and estimating a diameter of the object of interest upon determining the object of interest to be non-solitary.