# Intraoperative Use of Mixed Reality Technology in Median Neck and Branchial Cyst Excision
## 1 Introduction  
The existing methods of instrumental diagnostics in the preoperative phase (ultrasound, MSCT, and MRI) allow a rather accurate detection of topological peculiarities of the median neck and branchial cysts [1, 2]. However, the intraoperative image, with the change in the patient's positioning on the operating table, can considerably differ from the examination results. This leads to difficulties when compared to the images in the surgeon's mind. Therefore, there is a need for a solution that is based on augmented reality technology and overlays MRI or MSCT data on top of the patient during surgery procedures. This would provide surgeons a visualization of the interrelation between the cystic mass and the great vessels, as well as the interconnection of all neck organs; this would also allow the surgeons to rule out any abnormal changes in them. However, the classical approach of augmented reality is not enough because the final image is formed on a flat screen, which does not allow the surgeon to accurately determine the position of anatomical structures in 3D space. Thus, it is necessary to use wearable glasses that include a stereo display and the use of mixed reality technology [3]. The appearance of the Microsoft HoloLens augmented reality glasses on the market has revolutionized many areas, including medicine [4, 5]. This has opened up opportunities for more effective diagnostics and intraoperative treatment through the visualization of various types of data. Now, Microsoft HoloLens is widely used in research planning [6], training [7, 8], and in performing surgical operations [9] using mixed reality technology. If we consider the application of this technology directly in the operating room, the existing solutions can be divided into two categories:Medical Data Visualization Systems;
* Navigation Technologies through the Combination of Medical Data with the Patient during the Operation. In the first case, the main idea is to visualize CT/MRI data as well as information about the patient's condition through a series of virtual screens, which eliminates the need to use bulky racks with multiple monitors, therefore optimizing the space in the operating room [10]. In the second case, mixed reality glasses are used to compare a virtual model of the patient's anatomy, which was built based on MRI and CT data, with the real condition of the patient. This is achieved by superimposing these data onto the patient's body and creating the effect of an \"X-ray vision\" or a visualization at a separate point in space, where the patient's data is compared with the current situation during surgery in the form of a freestanding holographic model [11; 12]. In the case of using mixed reality technology as a navigation system, one of the main tasks is the patient's registration (a comparison of virtual MRI data/CT scan with the patient's position in the operating room). To do this, it is necessary to use spatial positioning systems, which can be marker-based (reference markers) [13] or marker-free (markerless); these systems are based on optical tracking technologies [14], magnetic induction sensors, or 3D scanning [15]. Systems using optical tracking, where infrared cameras are used as tracking devices, have proven themselves well and are actively used in brain surgeries (Medtronics, Brainlab), while providing a positioning accuracy of 1-2 mm. However, these systems have a significant drawback--the markers must always be in the field of view of the recording cameras, which complicates the operation process when several doctors working with the patient block the field of view of the device. Technologies using magnetic induction sensors do not have this disadvantage and offer even higher positioning accuracy, but they are difficult to implement and have a limited scope of work.