Stereotaxis

Frameless	Frame based
Point-pair registration or surface contour registration can be used to establish the relationship between the preoperative images and that for the surgical field.	By using the same frame for both preoperative image acquisition and performing the surgery, the relationship between the two coordinate systems is known and no further transformation is required.

Patient’s head be immobilized with a head clamp and the operating table not be moved during surgery. To overcome the error that may be induced with patient or operating table movement, tracking units attached to the patient’s head or a surgical head clamp can be used.

Electromagnetic (rather than optical) tracking is the latest development in frameless stereotaxy and is used in pediatric neurosurgery as it avoids the need for head clamping (not possible in those without fused skull sutures).

Requires a set of at least three non-colinear points to be defined in the coordinates of the images.
These ‘fiducial points’ can consist of either natural anatomic landmarks (i.e. nasion, lateral canthus, or tragus), skin-applied markers, or bone-implanted markers.
These same fiducial points are defined again within the coordinate system of the surgical field in the operating room.
Software is then used to establish the relationship between the coordinates of the fiducial points in the image space and their counterpart in the surgical space.

Mapping a radiographic surface can be achieved by touching a number of multiple random points (termed cloud of points) or by scanning a surface with laser registration.
These surface-based algorithms allow the use of imaging obtained before the intent to operate, but are less accurate compared to the combination of natural landmarks and skin-applied fiducial markers.
Once the spatial relationship is established with either point-pair registration or surface contour registration, it is used throughout the remainder of the operation to map anatomic targets of the patient to that of the preoperative images.

Is the underpinning for all frame types, and describes the location of a point in space with three coordinates (x, y, z).

The polar coordinate system is based on the necessity that guidance of the probe be described with regard to the angle relative to a skull entry point.

Usually, a minimum of two angles (in planes orthogonal to each other) is required to characterize a unique trajectory;

In addition to these angles, the distance of the probe to the target lesion needs to be calculated (e.g. Spiegel-Wycis, Horsley-Clarke).

The arc-radius system (e.g. Leksell frame) is based on the concept that a probe equivalent to the radius of a semicircular arc will reach the center of the arc when introduced perpendicular to any point along the arc.
Adjustment of the arc (vertically and anterior-posterior) permits targeting of different points; the base of the arc is attached to a rigid head frame on the patient’s head which defines the corners of a cube visible on CT.
After the geometric center of the cube is defined, the relative location of the target can be calculated and the semicircular arc manipulated accordingly to place that lesion at the center of the arc.

Arc-phantom systems (e.g. CRW frame) allow mounting of the arc onto a phantom base frame to check the accuracy of the trajectory immediately before transferring it to the patient.

A burr hole-mounted system provides a limited range of possible intracranial target points with a fixed entry point (usually over non-eloquent brain): provided two angular degrees of freedom (horizontal plane, vertical plane) and a depth adjustment.