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