Abstract: The present invention relates to a medical imaging apparatus (10). The apparatus comprises an ultrasound acquisition unit including an ultrasound probe (14) for acquiring ultrasound image data of a patient (12). An image data interface (18) is provided for receiving 3D medical image data of the patient and a position determining unit (28) for determining a position of the ultrasound probe. A calibration unit (24) determines a transfer function between the ultrasound image data and the 3D medical image data at a plurality of positions (C) of the ultrasound probe and provides a corresponding plurality of calibrated transfer functions and calibration positions. A computation unit (26) synchronizes the ultrasound image data and the 3D medical image data on the basis of a position of the ultrasound probe and the plurality of calibrated transfer functions, said computation unit is further adapted to weight the calibrated transfer functions on the basis of a position of the ultrasound probe.