Overview
This lecture reviews the principles of stereotactic (stereotaxy) neurosurgery, focusing on navigation, targeting methods, history, and error management in the discipline.
Direct vs. Stereotactic Neurosurgery
- Traditional neurosurgery targeted visible brain areas directly, often for Parkinson's disease or psychiatric procedures.
- Stereotactic surgery enables minimally invasive targeting of deep or hard-to-see brain structures.
Etymology and Historical Development
- "Stereotactic" comes from Greek (stereo = 3D, taxis = arrangement) and Latin (actus = touch).
- Early stereotactic frames included Zernov’s frame (1880s), Horsley-Clarke’s primate frame (1900s), and Spiegel-Wycis’ human frame (1940s).
- Lars Leksell developed the arc-quadrant system, allowing vector-based targeting.
Maps, Atlases, and Fiducials
- Stereotactic navigation relies on brain atlases, CT/MRI scans, and anatomical landmarks (fiducials) to define targets.
- Common landmarks include the nasion, external auditory meatus, and commissures (anterior/posterior).
Targeting Methods
- Indirect targeting uses landmarks to infer the location of invisible structures (e.g., ventral intermediate nucleus of thalamus).
- Direct targeting uses imaging where the target is visible (e.g., subthalamic nucleus on T2 MRI).
- AC-PC line (anterior-posterior commissure) is a standard reference for indirect targeting.
Registration and Localization
- Image registration aligns pre-op MRI and CT with fiducials or frame for precise navigation.
- Localizer boxes use geometry and known distances to determine spatial position in stereotactic space.
- Frameless systems use cameras and fiducials (bone, skin stickers, or surface features) or electromagnetic fields for instrument tracking.
Sources and Management of Error
- Errors arise from imaging distortions, frame placement, fiducial misalignment, and human measurement.
- MRI field inhomogeneity and voxel resolution limit precision.
- Best practices: spread fiducial markers, avoid linear placement, mark fiducial positions, and check measurements carefully.
Accuracy Comparisons
- Bone fiducials offer ~1 mm accuracy; skin fiducials and surface matching are less precise.
- Frames provide 1–3 mm accuracy; intraoperative MRI is most accurate (<1 mm).
- Human error is significant due to multiple required measurements per procedure.
Key Terms & Definitions
- Stereotaxy/Stereotactic surgery — Navigation and targeting within the brain using 3D coordinates.
- Fiducial — An external marker used for spatial reference in imaging and surgery.
- Atlas — A map of brain structures used for navigation.
- Direct targeting — Approach where the target is directly visible on imaging.
- Indirect targeting — Approach using landmarks to estimate the location of an unseen target.
- AC-PC line — Line connecting anterior and posterior commissures, used as a coordinate reference.
- Registration — Alignment of images/maps to the patient’s real anatomy.
Action Items / Next Steps
- Review notes on major stereotactic frames and targeting strategies.
- Study sources of error and best practices for fiducial placement.
- Optional: Consult recommended textbooks for deeper reading (as mentioned by the lecturer).