Apparatus for calibrating an ultrasound transducer providing B-scans for two-dimensional (2D) images, includes: an ultrasound probe for providing B-scans; a position sensing device, the position sensing device being attached to the ultrasound probe and operating as part of a position sensing system in cooperation with a fixed sensing control unit, for labeling the B-scans with their respective relative positions and orientations (pose); a phantom marker for being imaged by the ultrasound probe for providing measurements which together with known physical properties of the phantom marker are used to derive calibration information for relating measurement data from the position sensing device to the poses of the B-scans to construct a 3D image; and the phantom marker comprising an encoded line object with distinctive calibration characteristics indicative of position along the line object, wherein the line object is disposed in a generally circumferential manner about a common axis with the probe.