Level-set methods have proven to be powerful and flexible for computer vision and medical imaging applications. Unfortunately, their drawback is the high computational effort required, which hampers the applicability to imaging tools for real clinical use. A new interactive 3D segmentation tool is presented, addressing operational speed issues by computing level-set surface models interactively. Real time segmentation is achieved by implementing a sparse field level set solver on the Graphics Processing Unit. The tool enables users - even non-expert - to produce good, reliable segmentations in a short time and with few training. A validation study is ongoing addressing wrist bone segmentations in magnetic resonance images of patients affected by Rheumatoid Arthritis. Preliminary positive results of qualitative and quantitative validation are presented and discussed at the end of the paper.