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If these points are not marked correctly, the misalignment in Tailairach space will propagate through the workflow and result in erroneous tissue segmentation. 

Here we discuss some important points that will help you ensure you are marking these points correctly. 

First, the image below is at the early step in the MIP workflow where the user is prompted to to mark the AC, PC and MP.

Image 1: The "Original Image": Subject MRI as it appears in the MIP before being registered in Tailairach space.

After marking the AC, PC and MP and then clicking the "Update" button, the Transformed Image will show the results of the registration, in other words, how the orientation of the MRI changed compared to the original image as a result of being registered in Tailairach space. 

When the AC, PC and MP are are all marked correctly, the head in the Transformed Image will be tilted slightly forward compared to the Original Image. This is true in the image below of the Transformed Image where all landmarks were marked correctly.

 Image 2: Correct: The "Transformed Image": Same subject MRI as in Image 1, after being correctly registered in Tailairach space.  Note the head is tilted slightly forward compared to the Original Image. 

If one or more of the points is marked incorrectly, this can cause the head to be tilted backwards - towards the posterior - as show in Image 3, below.  This will result in erroneous tissue segmentation where cerebral tissue can be mistakenly marked as cerebellum, since a posterior head shift in Tailairach space will push the cerebrum downwards into the plane that is typically populated only by the cerebellum. This illustrates the importance of accurately marking these points.

Image 3: Incorrect: The "Transformed Image": Same subject MRI as in Image 1, after being incorrectly registered in Tailairach space.  Note the head is tilted slightly backwards compared to the Original Image.  

To avoid incorrect Tailairach registration, the three images below demonstrate the correct position of the AC, PC and MP on the same example subject's MRI.  The green crosshairs denote exactly the points marked for the AC, PC and MP.  Do note that these anatomical features may look different from patient to patient.  

Image 4: The Anterior Commissure (AC) is the lowest apex that sits below the lateral ventricle and corpus collosum, and is denoted here by the green crosshair. 

Image 5: The Posterior Commissure (PC) is the forward most tip of the cylindrical structure located on top of the pons and anterior to the hind brain, and is denoted by the green crosshair. 
 

Image 6: The Middle Point (MP) sits equi-distant between the AC and PC and occupies the hemispheric divide. It is denoted by the green crosshair.

Image 7: The three points exist together as a triangle.   

With the AC, PC and MP all marked correctly, the image will be correctly registered in Tailairach space, and ready for the next step.  

You can read more about the entire process of creating an IHM in the GeoSource 3.1 Research Manual.