# Review of Microsoft HoloLens Applications over the Past Five Years
## 3 Results
### Applications in Medical and Healthcare Fields
#### 3.1.1 Medical and Surgical Aids and Systems
The sorted information was merged and displayed using the HoloLens and the surgeon could view the patient's medical images and head simultaneously. In this study, the accuracy of the alignment algorithm was measured using spatial reference points with known positions; the improved algorithm was determined to be extremely accurate, exhibiting errors within 3 mm. [PERSON] et al. [33] quantified the accuracy of virtual object placement methods for an AR device in neurosurgery. The accuracy was quantified for three different methods of virtual object placement (i.e., tap to place, 3-point correspondence matching and keyboard control). Consequently, it was found that using a keyboard that moves in 5 mm increments was the most accurate; moreover, the tap to place method was the most efficient, which was followed by the keyboard and 3-point methods. [PERSON] et al. [34] verified the reproducibility and accuracy of preoperative planning in maxilla repositioning surgery that was performed using computer-aided design/manufacturing technologies and MR surgical navigation. To achieve this, they used new registration markers and the HoloLens headset. The reproducibility and accuracy were evaluated by comparing the preoperative virtual operation 3D image with the one-month postoperative computed tomography (CT) image. Subsequently, it was confirmed that no  
Figure 2: Scene performing ureteroscopy in full immersion simulation environment. (**A**) View of the user to a nonsuser when the HoloLens is worn. (**B**) Simulated view of what the user of the HoloLens sees [3]. statistically significant difference existed between any of the points on any of the axes and that the method could reproduce the maxillary position with high accuracy. [PERSON] et al. [35] proposed a prototype of a holographic interface (HI) that can visualize 3D MRI data for planning neurosurgical procedures. The proposed HI immersed the user in an MR scene, which included MRI data and virtual renderings, and was implemented to interact with objects in MR. Additionally, the user can immediately control the scanner connected to the MRI scanner. They performed a preliminary qualitative evaluation of the proposed HI and found that holographic visualization of high-resolution 3D MRI data can provide an intuitive and interactive perspective of complex brain vasculature and anatomical structures. [PERSON] et al. [36] developed a real-time navigation framework, which provides a 3D holographic view of the vascular system, to reduce the X-ray exposure that may occur during endovascular interventions using X-ray imaging and convert the two-dimensional (2D) X-ray images into 3D images. Because the presented framework uses calibrations based on extrinsic landmarks, the virtual objects are precisely aligned with the real world. They extracted the patient's surface and vascular tree from preoperative CT data and registered this information to the patient using a magnetic tracking system. The developed system was evaluated by experienced vascular surgeons, radiologists and thoracic surgeons; it was confirmed that their approach reduced X-ray exposure in endovascular procedures, while allowing novices to acquire the required skills faster. [PERSON] et al. [37] used AR to obtain information contained in preoperative computed tomography angiography (CTA) images. They depicted the separated volumes of osseous, vascular, skin, soft tissue structures and relevant vascular perforators on the CTA scans, to generate 3D images that were then converted to polygonal models and rendered using a custom application within the HoloLens stereo HMD. Figure 3 presents the original CTA image, segmentation and corresponding polygonal models. Moreover, during the surgery, a combination of tracked hand gestures and voice commands was used to allow the surgeon to register the model directly to their respective subjects.