3.1 Principles of Stereotaxis

Stereotaxis tells us where a surgical instrument is in space. It allows one to limit surgical exposure because adjacent localization landmarks are not needed. Accuracy is the crucial element of functional neurosurgery. By improving accuracy, surgical risk is reduced. While target inaccuracy of 3–4 mm is of little consequence during exposure of a 4 cm tumor, greater precision is needed in functional neuromodulation. ¹

Originally, stereotactic neurosurgery simply involved registration and guidance of a probe to a specific point in space. Contemporary techniques of surgical navigation, or frameless sterotaxy, rely instead on registration of volumes, thereby increasing the method’s versatility. An instrument’s location is now known in real time and can be tracked relative to known points in the patient’s anatomy, either with a traditional frame, optical tracking devices or with magnetic fields. ²

3.2 Error

While stereotactic accuracy was at first thought to be only a characteristic of the frame used, Maciunas demonstrated that this was one of many factors. ³ Others include the accuracy of the imaging modality, the weight-bearing status of the frame, and patient position. ⁴ For instance, the prone position is less accurate. It results in a mean error of 2.8 mm in a traditional frame. ⁴

CT is generally more accurate than MRI since MRI is subject to field inhomogeneity. ⁵ As described in the previous chapter, this can be minimized with appropriate quality control and phantom testing. Often, commercial software is used to merge the images from MRI and CT. However, these software packages use proprietary algorithms that are not transparent to the user. They should be used with caution. ³ For lesion generation or placement of electrodes for deep brain stimulation, many have felt that microelectrode recording, macroelectrode recording or, at least intraoperative testing is needed to counteract the inherent inaccuracies of stereotactic surgery with a traditional frame. ^{6 , 7}

3.3 Frameless Stereotaxis

Because of its ease of use and patient comfort the field has been moving towards frameless stereotaxy. With frameless surgery, the burden of frame placement, stereotactic planning and imaging while the patient is still wearing the frame is obviated. The fixed head can be referenced to a previously obtained MRI or CT with surface contours, landmarks, adhesive scalp fiducials or bone screws that were inserted before surgery. Imaging can be done at varied times before surgery. A wand can then be tracked relative to a reference arc using either optical or magnetic guidance. ^{8 , 9 , 10} A bulky stereotactic frame is not in the way of the approach and there is greater airway access. Finally, the added time needed to input the coordinates into a stereotactic frame is avoided.

For biopsies or depth electrode placement for epilepsy monitoring, an articulated arm can be attached to the head holder in a fixed relationship to the skull. Software can help the surgeon line up a trajectory to use a twist drill and then a biopsy needle or electrode by progressive adjustment of the articulating arm.

While the accuracy of these techniques has been criticized as being inadequate for many functional procedures (see previous section), the accuracy can be improved with different registration techniques. Not surprisingly, less mobile fiducials yield greater registration precision. Mascott et al., showed that skull-based fiducials are more accurate than non-implanted methods (1.7 ± 0.7 mm vs 4.0 ± 1.7 mm). ¹¹ Thompson et al., also found that bone fiducials yield a lower registration error when compared to skin-based fiducials (1.35 mm versus 1.85 mm). ¹²

If greater accuracy or multiple trajectories are needed, robotic systems such as ROSA (Zimmer Biomet, Warsaw, IN) are an option. Planning can be done in advance. Instead of making multiple joint adjustments along an articulated arm, the robot is connected to the head and “drives” to each trajectory. The ROSA robot has an accuracy of 1.59 mm when using a 3 T MRI scan with frameless registration on a phantom. ¹³ With CT-based imaging this decreases to 0.3 mm. ¹³ The increased speed that the ROSA provides makes it most useful in stereotactic EEG (sEEG) in which 10–15 different trajectories are needed during depth electrode placements. ¹⁴ At our center, we can place a depth electrode every 6 minutes with this device.

Related posts:

4 MRI and CT Imaging with Stereotactic Neurosurgery 8 Microelectrode Recordings in Deep Brain Stimulation Surgery for Movement Disorders 22 Epilepsy: Extra-Temporal Surgery with Invasive Monitoring 29 Neurosurgery Treatment for Anorexia Nervosa 36 Ablative Procedures for Trigeminal Neuralgia 39 Neuroprosthetics

Stay updated, free articles. Join our Telegram channel

Join