Direct volume rendering (DVR) is a visualisation technique allowing users to create 2-D renditions from 3-D spatial datasets. This technique can assist medical users in both diagnosis and therapy planning. Currently users of such visualisation systems have limited means of selecting visualisation parameters to enhance important regions of interest (ROI). We propose a modification to 3-D texture-based volume rendering allowing users to visually enhance important regions, while retaining contextual information. Using a series of interleaved region slices, the algorithm assigns a different transfer function to the ROI and context. Knowledge about the human visual system is used to modify the two transfer functions creating "pop-out" effects. This approach is demonstrated using the perceptual characteristics of luminance and hue. The output of this research is the new ability for users to precisely control the highlighting of regions of interest and hence improve the visualisation process.