Directional local mean difference level set method for locating a urinary bladder lumen in brachytherapy

In this paper, we propose the directional local mean difference level set method (DLMD-LS). Our work is focused on the segmentation for a urinary bladder lumen in a T2 weighted image taken during brachytherapy. The boundary is detected as the region where the intensity means of the areas inside and outside the zero-level contour (edge) are high. The contour is controlled such that it stops evolving only at the boundary of the bright object inside the dark surrounding. Since the intensity mean is used, the DLMD-LS is more tolerant to noise than the conventional level set methods which detect the boundary as the part with large intensity gradient. Furthermore, the mean is calculated locally; therefore, the function is less affected to the different intensity distribution inside the lumen. The experiment demonstrates the superior noise tolerance and better capability to capture the blurred boundary of our proposed DLMD-LS to the directional level set method and the level set which detects the boundary according to the intensity distribution of the entire lumen.