1.1. Depth perception in volume renderingSpatial information (or depth) is a main benefit of using 3Ddatasets in medical procedures. The 2D/3D overlay allows the inter-ventionist to understand the 3D structure more quickly, especiallyin complex cases. The transfer functions (TFs) defined to create thisoverlay are crucial and are used to classify data in volume render-ing based on intensity, gradient magnitude [3], curvature [4] and/orlocal structures [5]. The visualization of the overlay must be mean-ingful for the interventionist, and in this sense depth perceptionhas been an important issue for volume rendering. Recently, Zhenget al. proposed a method for the improvement of perceptually basedordering in volume rendering [6]. They set up an energy functionbased on quantitative perception models to measure the qualityof the images in terms of the effectiveness of depth-ordering andtransparency perception. Guided by the function, they use a con-jugate gradient method to iteratively and judiciously enhance theresults. Alternatively, Kniss et al. proposed an interactive shadingmodel based on volumetric light attenuation effects to incorpo-rate volumetric shadows in volume rendering [7]. Solteszova et al.proposed a new method called chromatic shadowing based on ashadow transfer function to handle the over-darkening problem,thus allowing for better perception of details in the shadowedareas [8]. Bruckner et al. introduced volumetric halos in the vol-ume rendering pipeline to facilitate depth perception. The halosare known as the darkened or brightened regions surrounding theedges of a targeted structure. The added halos help judge the spatial