1. Medical imaging apparatus (10) comprising:
- an ultrasound acquisition unit including an ultrasound probe (14) for acquiring ultrasound image data of a patient (12),
- an image data interface (18) for receiving 3D medical image data of the patient,
- a position determining unit (28) for determining a position of the ultrasound probe,
- a calibration unit (24) adapted to determine 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 adapted to provide a corresponding plurality of calibrated transfer functions (TCaio, TQ.II) and calibration positions, and
- a computation unit (26) adapted to synchronize 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.

2. Medical imaging apparatus as claimed in claim 1, wherein the computation unit is adapted to weight the calibrated transfer functions on the basis of a distance between the ultrasound probe and the plurality of calibration positions.
3. Medical imaging apparatus as claimed in claim 2, wherein the computation unit is adapted to weight the calibrated transfer functions on the basis of a distance between a predefined position (34) at a tip of the ultrasound probe and a plurality of the calibration positions.
4. Medical imaging apparatus as claimed in claim 2, wherein the computation unit is adapted to weight the calibrated transfer functions on the basis of a distance (d) between a predefined position within the ultrasound image data and a plurality of calibration positions.

5. Medical imaging apparatus as claimed in claim 4, wherein the predefined position within the ultrasound image data is a centre of the ultrasound image data.
6. Medical imaging apparatus as claimed in claim 1, wherein the computation unit is further adapted to weight the calibrated transfer functions by means of relative weight factors (a).
7. Medical imaging apparatus as claimed in claim 1, wherein the computation unit is further adapted to weight the calibrated transfer functions on the basis of absolute weight factors (a).
8. Medical imaging apparatus as claimed in claim 1, wherein the calibration unit is further adapted to determine a 3D rigid transformation (Q) for each calibration position.
9. Medical imaging apparatus as claimed in claim 1, wherein the computation unit is further adapted to determine a general transfer function (T) for a current position of the ultrasound probe on the basis of the calibrated transfer functions weighted by weight factors.
10. Medical imaging apparatus as claimed in claim 1, further comprising an image processing unit (22) for providing fused image data and on the basis of the synchronized ultrasound image data and 3D medical image data and an image data interface for providing the fused image data to a display unit (28).
11. Medical imaging method for combining different medical image data, comprising the steps of:
- acquiring ultrasound data of a patient (12) by means of an ultrasound probe
(14),
- receiving 3D medical image data of the patient,
- determining a position of the ultrasound probe by means of a position determining unit (28),
- determining a transfer function between the ultrasound image data and the medical image data at a plurality of positions of the ultrasound probe and providing a

corresponding plurality of calibrated transfer functions and calibration positions, and
- synchronizing the ultrasound image data and the 3D medical image data on the basis of a position of the ultrasound probe and the plurality of transfer functions.