Radio frequency ablation is a minimally invasive intervention that introduces -- under 2D ultrasound guidance and via a needle-like probe -- high-frequency electrical current into non-resectable hepatic tumors. These recur mostly on the periphery, indicating errors in probe placement. Hypothesizing that a contextually correct 3D display will aid targeting and decrease recurrence, we have developed a prototype guidance system based on a head-tracked 3D display and motion-tracked instruments. We describe our reasoning and our experience in selecting components for, designing and constructing the 3D display. Initial candidates were an augmented reality see-through head-mounted display and a virtual reality "fish tank" system. We describe the system requirements and explain how we arrived at the final decision. We show the operational guidance system in use on phantoms and animals.