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Purpose: Repetitive Transcranial Magnetic Stimulation (rTMS) is an important treatment option for medication resistant depression. It uses an electromagnetic coil that needs to be positioned accurately at a specific location and angle next to the head such that specific brain areas are stimulated. Existing image-guided neuronavigation systems allow accurate targeting but add cost, training and setup times, preventing their wide-spread use in the clinic. Mixed-reality neuronavigation can help mitigate these issues and thereby enable more widespread use of image-based neuronavigation by providing a much more intuitive and streamlined visualization of the target. A mixed-reality neuronavigation system requires two core functionalities: 1) tracking of the patient's head and 2) visualization of targeting-related information. Here we focus on the head tracking functionality and compare three different head tracking methods for a mixed-reality neuronavigation system. Methods: We integrated three head tracking methods into the mixed reality neuronavigation framework and measured their accuracy. Specifically, we experimented with (a) marker-based tracking with a mixed reality headset (optical see-through head-mounted display (OST-HMD)) camera, (b) marker-based tracking with a world-anchored camera and (c) markerless RGB-depth (RGB-D) tracking with a world-anchored camera. To measure the accuracy of each approach, we measured the distance between real-world and virtual target points on a mannequin head. Results: The mean tracking error for the initial head pose and the head rotated by 10° and 30° for the three methods respectively was: (a) 3.54±1.10 mm, 3.79±1.78 mm and 4.08±1.88 mm, (b) 3.97±1.41 mm, 6.01±2.51 mm and 6.84±3.48 mm, (c) 3.16±2.26 mm, 4.46±2.30 mm and 5.83±3.70 mm. Conclusion: For the initial head pose, all three methods achieved the required accuracy of < 5 mm for TMS treatment. For smaller head rotations of 10°, only the marker-based (a) and markerless method (c) delivered sufficient accuracy for TMS treatment. For larger head rotations of 30°, only the marker-based method (a) achieved sufficient accuracy. While the markerless method (c) did not provide sufficient accuracy for TMS at the larger head rotations, it offers significant advantages such as occlusion-handling and stability and could potentially meet the accuracy requirements with further methodological refinements.
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