This thesis explores the effectiveness of a novel interaction model for visualizing 3D image data. The interaction model is based on user-sketched line segments known as sketch-lines. This thesis shows that sketch-lines provide simple, fast and precise interactive image slice positioning and 3D region of interest (ROI) delineation in volume images. These two user interactions form the basis of many image visualization tasks, including image slice-based exploration and inspection, cutaway, surgical planning, and model-based segmentation. The sketch-line model is combined with the use of a subdivision surface as well as a new image slice rotation model to support the implementation of the user interactions. To evaluate the effectiveness of the sketch-line model, this thesis measures its performance and compares it to other interaction models with the aid of two user studies. Several experiments applying the model to various 3D medical images are also performed to demonstrate its functionality and consistency.