sary to define a physical coordinate system with respect to the patient. In frame-based methods, these coordinates are defined by the arc system on the frame itself, whereas in image-guided neurosurgery a coordinate measuring device is used. Various technologies have been proposed for this purpose, including mechanical arms, radiofrequency transmitter/receiver coils, ultrasound spark gaps and optical trackers. We will consider the relative merits of these devices in the next section.

Having defined a physical coordinate system in theater, the problem is now to align the preoperative images to the patient in order to present the surgeon with image data that correspond to the patient's anatomy. This is achieved by identifying corresponding features in the pre-operative images and on the surface of the patient. These features will generally be landmarks, but may also include the skin surface or, as the operation proceeds, bone surface. The process of establishing correspondence between the image and the patient is termed "registration".

Defining the Patient Coordinates

Stereotactic Frames

Frame-based stereotactic neurosurgery has been an established clinical routine since the 1950s. Here, the patient coordinate system is defined by an arc device that attaches to the frame. The frame carries high-contrast imaging markers and is rigidly bolted to the patient's skull prior to imaging. An entry and target point is defined in the images, and the arc angles are calculated to achieve this trajectory according to the manufacturer's instructions. The bulky and somewhat invasive nature of such frames has limited their application. Since only a target and trajectory can be defined, frames are generally used only for biopsies or placement of electrodes or cannulae. They are widely regarded as highly accurate, though some studies have suggested that the accuracy may be overstated [1].

Mechanical Arms

The first frameless neuronavigation device to be widely used was the "Faro Arm", a mechanical device that attaches to the side of the table. Encoders on each of the axes of the arm enable calculation of the tip position. Problems with such a device are that the range of movement is

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