## 2 Materials and Methods
### Clinical Workflow  
The initial step of biopsy procedure planning is selecting a trajectory along which the injection will be performed. The operator first selects a point at the target location, then a point on a skin at the injection site; afterwards, a straight line is drawn from the target spot through the injection site and above the patient's body. There are several rings around the designated line, which are intended to guide the operator by appropriately highlighting the needle's position in relation to the trajectory. After the trajectory is set, it can be revised by the operator in both the 3D and 2D views. This is a crucial point, at which the operator verifies the trajectory and ensures that the incision will not endanger any important organs. A sample selected trajectory is presented on Figure 4.  
The subsequent phase involves superimposing data within the holographic space directly onto the patient's body. To perform this process, the patient undergoes scanning with radiological markers strategically placed in specific locations of the body surface that serve as reference points and are clearly visible in the image data across multiple slices. The operator begins by selecting each marker on 2D slices, assigning them unique identifiers, and then selecting corresponding physical markers in the holographic space. Sample selected markers are presented in Figure 5. Afterwards, the hologram representing the patient's anatomy is transformed to minimize the distance between corresponding physical and virtual markers while preserving the real scale of the patient scan. The result of superimposing the data captured by the HMD lens is presented in Figure 6.  
Figure 4: Selected trajectory visualization. The **left-hand** image presents a volumetric rendering of the phantom’s CT scan with the chosen trajectory, while the **right-hand** image presents a 2D slice in the sagittal plane with the indicated injection site and target location. On the trajectory (red line) the first red cross symbol indicated injection point, the second landing point.  
The final step encompasses needle tracking to achieve precise alignment with the chosen trajectory. The rings and trajectory line dynamically change colors based on the needle's alignment accuracy. Additionally, the operator can perceive guiding lines extending from both needle ends to the trajectory line, providing a visual indication of the needle's proximity to the intended path. The HMD needle tracking interface is presented in Figure 7.  
When selecting a mixed reality headset for surgical environments, careful consideration of the operating room's unique demands is essential. The primary options are see-through and video pass-through headsets. See-through headsets allow for direct viewing of the surgical field through transparent glass, overlaying virtual elements onto this view. In contrast, video pass-through headsets relay external camera feeds onto internal screens, potentially introducing latency and visual disturbances.  
Figure 5: Selected markers in the data. The **left-hand** image presents a volumetric rendering of the phantom’s CT scan, while the **right-hand** image presents the 2D slice in the coronal plane with radiological marker.  
Figure 6: Data superimposition captured from HMD lens.  
See-through headsets offer the precise real-time visualization crucial for surgical accuracy while maintaining uninterrupted visual contact between surgeon and patient, facilitating seamless communication within the surgical team. Additionally, see-through headsets streamline the integration of augmented reality overlays, enhancing surgical precision. Therefore, in the context of surgical procedures, see-through headsets stand out as the preferred choice for optimal performance and compatibility. In this study, we have chosen the HoloLens 2 headset, as it is suitable for use by a clinicians in the operating room.