The invention relates to a calibration block for determining a transformation between an ultrasound (US) image and a tracking coordinate system, the calibration block (10) comprising a mounting part (20) for mounting an ultrasound probe (2) to the calibration block (10) in a locked position, and a guidance pathways defining part (30) for defining a plurality of coplanar guidance pathways (32), wherein the calibration block (10) is configured to receive the ultrasound probe (2) in the locked position so that the plurality of guidance pathways (32) is aligned with an imaging plane of the ultrasound probe (2). The invention further relates to a corresponding method and computer program. The invention provides provide an improved calibration apparatus, calibration system and corresponding calibration method for determining a transformation between a US image and a tracking coordinate system.
