However, I find the figure relative to the (left) preauricular point in that page somewhat confusing.
What I typically find in the sagittal orientation is the absence of a "hole" between the tragus and the helix, and rather a clear "hole" below the tragus, see figure attached.
I wonder whether I am systematically wrong in this identification, or my subjects have systematically different ears from yours.
Since I find it difficult to identify the right point between the tragus and the helix - as in many cases the "dip" is very shallow and imprecise - do you use any other tip from the other two orientations?
Hi Marco,
This is a difficult point to identify, especially on some MRI scans. The most important thing is that which ever point you chose on the subject is the same point you choose on the MRI. You might want to experiment with selecting the tragus or a point that you can more readily identify on the subject and then find that point on the MRI. We find that making a ‘best guess’ in combination with the automatic registration using head points really does a great job with the final registration.
Beth
In fact, as I was saying, I am using the same point that Brainstorm suggests to use, but the reference image (sagittal orientation) that you propose in http://neuroimage.usc.edu/brainstorm/CoordinateSystems
is puzzling to me because I usually do not find the same pattern. What I was asking for is: how do you visually select this point? Do you identify it easily on each subject or do you have less simple cases like the one I have posted? Otherwise, do you think that my identification is wrong? If my image is not clear enough, I can post others.
Hi Marco,
This point is not always easy to find. Depending on the quality of the scan and/or if the ears were cut off. Each subject is different. The point you selected looks like a good guess if the point you selected on the subject was in the small fold just above the tragus. If you move out just a few more slices, this fold can become more obvious, but not always. Again it depends on the subject. It is important to collect a nice even distribution of points over the head surface (~100) and then use the automatic registration using these headpoints. This will help reduce errors that are common for selecting these points.
Does that help?
Beth
Thanks a lot Beth for your feedback, it reassures me that our procedure is correct (we also digitize many points over the head surface to adjust co-registration).
