The transformation from an ultrasound (US) image plane to the coordinate system of a position sensor attached to the US transducer can be computed with US calibration algorithms. This knowledge can be used in many applications, including freehand 3D US and US guided surgical navigation. We present a software system assisting the surgeon to position a radio-frequency (RF) tumor ablation needle using augmented ultrasound, thus simplifying the treatment by (1) dividing it into two simple consecutive tasks, lesion finding and needle placement, and (2) relating the needle to the US plane at any time.