1. A method of detecting a structure in a three-dimensional (3D) computed tomography (“CT”) knee image, comprising: (a) creating an intensity prior mask based on a two-dimensional (2D) slice of the CT image; (b) receiving or estimating a seed point in the 2D slice, the seed point being in a Hoffa's pad region; (c) creating a geometric prior mask based on the seed point; (d) combining the intensity prior mask and the geometric mask to form a region mask; and (e) segmenting a level-set contour from the 2D slice, wherein the step of segmenting is constrained by the region mask.