Similar to many other AR applications, the Freehand SPECT system overlays co-registered and tracked virtual and real objects on the video images thanks to real-time tracking. A pair of calibrated infrared cameras is used to track retro-reflective targets. The tracking accuracy strongly depends on the positioning of the Freehand SPECT device, which usually takes place during the preparation phase. The quality of the reconstructed model and the generated AR and VR images also depends on tracking accuracy. Moreover, due to the presence of the OR crew and other intra-operative equipment, repositioning the device is only possible in a limited manner, moving the camera head. This emphasizes the importance of the positioning of the device before the surgery starts, which should be done by an experienced and trained person. Unsuitable placement of the device may further lead to improper and unreliable AR visualization. An example for such a case is shown in Figure 11 where the right axilla is not visible in the camera view during the surgery due to inappropriate positioning of the system (Figure 11(b)), and the users had to switch to the VR mode (Figure 11(d)). This also explains the seventh column in (Figure 8(a)), which corresponds to this particular surgery, where the VR percentage is much higher than other surgeries of the same surgeon as well as the other two main surgeons, although the system is used less than average.