A reactive-diffusion method for estimating a diameter of an object of interest includes providing a volume of interest including a plurality of voxels, initializing at least two volumes of the volume of interest, wherein each of the voxels has at least two values corresponding to the at least two volumes of the volume of interest, respectively, performing a diffusion operation and a reaction operation on the voxels to adjust the 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, and estimating a diameter of the object of interest from the segmentation result, wherein the object of interest is represented by at least one of the at least two volumes but less than all the volumes.