Coordinate systems

Subject Coordinate System (SCS)

Definition

How to position the points

Nasion (NAS)

Left pre-auricular point (LPA)

Right pre-auricular point (RPA)

Talaraich coordinate system

The atlas that Jean Talairach established for human brain is base on two points, the posterior and anterior commissure, and the midsagittal plane. The third point needed here, the interhemispheric point, is used to define this plane. To learn more about the Talairach coordinate system, please visit this Wikipedia page.

Anterior commissure (AC)

Description of the anterior commissure at this Wikipedia page.

Technique to localize it:

• Use the axial orientation.
• Start from a slice just below the corpus callosum and move around it (slightly up or down)
• You are looking for a small fiber tract that connects the two hemispheres
• You should find two spots corresponding to this description. The more frontal one is the anterior commissure, the other one is the posterior commissure
• Once you have localized it: if it is visible on more than one slice, use the upper one (the one closest to the top of the head).

Talairach coordinates: (0, 0, 0).

Posterior commissure (PC)

Description of the posterior commissure at this Wikipedia page.

Technique to localize it:

• Use the axial orientation
• It is a fiber tract similar to the anterior commisure, but a few centimeters more posterior.
• If it is visible on more than one slice, pick the lowest one.

Talaraich coordinates: (0, -23, 0)

Interhemispheric point (IH)

Pick any point in the interhemispheric space, somewhere in the topmost part of the head. The further the point is from the commissures, the better.