Images for neuronavigation are usually collected through CT and MRI. These images must contain fiducials for registration and have enough accurate axial images with a slice spacing and thickness of 1.0 mm. Possible magnetic resonance sequence images include those captured by means of functional MRI, diffusion tensor imaging, and magnetic resonance spectroscopy. Positron emission tomography images also can be used [4]. These images can also be fused onto a reference image to better provide surgical information intraoperatively. These image data are digitalized through the navigation system.

Before scanning preoperative image, the surgeon decides which fiducials to use for registration. There are two types of fiducials with respect to registration. The first is the surface marker used for point-pair registration. There are bone fiducials, anatomical landmarks, and adhesive markers. Bone fiducials are bone screws that are screwed into patient’s skull following scalp incision. They confer high accuracy but require an additional scalp incision for their placement. The method of registration through anatomical landmarks uses prominent anatomical points, such as the canthus, tragus, and bridge of the nose. But it is relatively less accurate because of the difficulty in precisely matching each point [5]. The other method involves attaching adhesive markers onto the patient’s scalp. It is commonly performed, has reliable accuracy, and is easy to use. However, movement and detachment of the markers cause errors during registration. As different types of adhesive markers are used between CT and MRI, correct markers should be selected carefully. If there are at least four makers adhered to the head, registration can be performed. However, more than 10 markers are recommended because more markers result in higher accuracy and some markers may not be available accidentally. Markers should be widely spaced and distributed evenly on a relatively immovable scalp (Fig. 3). The occipital scalp is more movable than the frontal scalp, so it is not a good location for marker placement because of the possibility for error.

Surface contour is also used for registration. Instead of matching corresponding points from surface markers to the preoperative image, the surface contour method involves scanning the curved surface of the face or head and matching the scanned surface to the preoperative images through multiple random points of the scanned surface [6]. The accuracy of this method is not universal, but this method is easy to use and fast to register [7,8].

Registration is an essential step in the frameless system. Unlike frame-based systems, frameless systems indirectly recognize the patient’s head and lesions through a tracker on the reference frame after imaging. So a spatial relationship between reference frame and the patient’s actual head must be established. This step is called ‘registration’ and is usually executed using the aforementioned fiducials.

There are two methods of registration: point-pair registration using surface markers and surface contour registration. During point-pair registration using adhesive markers, surgeons manually match the markers in the preoperative images and the corresponding adhesive marker on the patient’s head one by one. They must be careful not to move and detach the marker, especially during image scanning, the patient moving into the operating room, head positioning, and induction of general anesthesia. If the border or center of the markers is drawn by pen on the scalp immediately after their attachment, moved markers can be easily identified and excluded from the registration step. During head fixation, markers can also move as the scalp is pushed out by head clamping.

Surface contour registration involves drawing along the curved surface of the patient’s face and head with a pointer or laser beam. The system performs the registration step by matching the drawn curved surface to the surface of the head in preoperative images. Including a nose with a prominent curve of the face, a wide head surface area should be used to reduce error (Fig. 4). Areas of movable occipital and compressed temporal scalp that occur during image scanning should not be included for registration.

After registration, the head clamp and reference frame mounted on the patient’s head should not be moved because spatial relationships would have already been established between the patient’s head and the reference frame. Minimizing errors in the registration step is the most effective way to increase the quality of the frameless system. As navigation systems calculate the distance between corresponding points and display the root mean square error automatically, the extent of error could be checked after registration. About 2 mm of error is generally acceptable. Identifying the anatomical landmarks or surfaces between patient head and preoperative image is one of the manual methods of verifying accuracy.

Points of interest and trajectories are displayed on the monitor using the pointer. Optical and electromagnetic systems are commonly used tracking methods. In optical systems, the camera emits infrared light to the trackers and recognizes them by reflected light. The pointer is recognized by the system through its mounted trackers and the reference frame. Trackers can be attached to surgical instruments and microscopes, so they can be used as a pointer. This tracking method provides good accuracy, but a free line of sight between the camera and the trackers is needed to keep. If infrared light is blocked by an obstacle and is not reflected by foreign objects on the tracker, such as blood and water, the system does not work.

Electromagnetic systems are based on a generation of magnetic field and the presence of electromagnetic coils in tracking device [9]. The camera and reference frame in optical systems have a similar role to the magnetic field generator and patient tracker. Because this system uses electromagnetic fields, it is not necessary to maintain a free line of sight between the magnetic field generator and patient tracker. Unlike using the patient’s head and a fixed reference frame in the optical method, head fixation is not required because the patient tracker is attached on the patient’s head and is used instead of a reference frame. As head fixation is not required, this system allows the head to move freely during surgery. The pointer (or stylet) of the electromagnetic system is different than that of the optical system. In optical systems, the pointer is recognized indirectly by a tracker mounted on the opposite side of the pointer tip. However, the pointer in electromagnetic systems is flexible and thin, and recognized directly by the tip of the pointer inserted coil, so it is suitable for catheter insertion because the pointer can be inserted into a catheter. Electromagnetic systems also have some disadvantages. Their accuracy is decreased when the patient tracker moves and detaches. So, patient tracker should be attached at the malar eminence or a relatively immovable area of the scalp and not be moved during head rotation. Large magnetic field generator should also be close enough to the tracker and pointer because the magnetic field is not wide enough, so the surgeon may feel uncomfortable due to generator during surgery. And magnetic substances, including surgical instruments and head clamps, should be kept away from the field of surgery because of the interference caused by ferromagnetic materials [10].

The position and trajectory indicated by the pointer tip are visible on the image within the monitor. Basically, it is possible to see coronal, axial, sagittal, and other planes, such as inline view (along the axis of the pointer) and probe view (perpendicular to the probe axis). The probe view is useful for tumor biopsy and catheter insertion into a lesion. Once the position of interest is specified, the trajectory to that point can be previewed and constructed. The lesion can then be easily and safely accessed while following the trajectory represented by the pointer on the monitor.