*2.1.1.1. Frame-based stereotaxis*

Today, Leksell G frame is still the most widely used, consisting of two parts—one fixed quadrangular and the other moving, the so-called arc. The fixed part consists of four graduated rods. On the right or left side of the patient, there is a diagonal bar in the form of the letter "N" that connects the corners.

Of the four basic geometric systems used in frame-based navigation systems, the "arc radius" system is the most commonly used, which is the base of the Leksell frame. The principle of the C arc is based on the ability to reach the center of the C arc from all directions by moving the probe along the radius of the frame. The frame itself is fixed on the patient's head, while the C arc is a movable part with the possibility of penetrating with a puncturing needle through any point on the convexity of the skull. The advantages of the Leksell frame design are the ability to reach any intracranial point with great precision and the ability to use it directly under the control of CT or MRI. One of the disadvantages is the bulkiness of the device itself, the obstruction of the operating field, and the "non-real time" procedure if there is no radiological supervision (CT or MRI).

By setting up the frame and the effect of the CT scanning, the center is first marked in anterior–posterior direction. The center is observed on axial CT scans in the form of the letter X by joining the frame corners. On the vertical plane, the center is marked by pulling the corner line through the mid-point of the letter "N." After defining the center, the target point is defined in relation to the geometric center of the frame and it can be accurately calculated, after which the C arc is moved and the probe is placed in the center of the lesion.

### *2.1.1.2. Frameless stereotaxis*

Next evolutionary step in the development of stereotaxis was frameless stereotaxis. Framebased stereotaxis is a clearly defined relationship between two coordinate systems (preoperative and intraoperative) and a precise position in relation to the planned procedure without further need for determining coordinates. Frameless stereotaxis, unlike frame-based, uses "point pair" registration to establish a relationship between preoperative images and surgical field or coordinate systems relative to preoperative images and patients during surgery. To connect these two coordinate systems, "registration" is necessary to establish a common relationship between two coordinate systems [9].

The frameless system consists of two parts, an infrared scanner and a dynamic frame, that is fixed to the Mayfield holder. The dynamic scanner transmits the coordination system data to the operating field and integrates them with software with preoperative images. The dynamic framework requires at least three fixed registration points (nasion, lateral cannula, frontal tuber, and tragus). In the further course, an additional multipoint registration on the curvature of the skull is used for the purpose of co-registration and refining the matching of the two coordinating systems. Registration is performed by marker or laser. Once the registration has been made, it is necessary to carry out a check for early identification of errors. The advantages of the frameless system are ease of use, as well as intraoperative possibility, altering the route or plan without the need for additional CT scanning, not taking up a large space, the possibility of absolute freedom of manipulation of the operating field, and no preoperative planning and determination of the coordination point on CT or MRI.

As the leading flaw of the frameless system, there is a decrease in precision compared to frame based, most likely because of less defined registration points in comparison to clearly defined points. However, these deviations from the target point in the brain do not exceed a value of 2–5 mm; they are expected and they are generally accepted when executing the procedure.
