AC coordinates

[m.versteijlen]

Hi,

I was looking at the electrode coordinates from STN DBS in MNI space, and congruently with your paper on probabilistic conversion, the z-coordinates were at about -6. But the AC z-coordinate was stated to be at about +3 or 4, which is strange because that would place the electrode 9 mm below the AC, while when converted to AC/PC coordinates it was at 3 mm below MCP.
Accoding to Brett (2002) the origin of the MNI space was at about 4 mm above the AC.
Therefore my question is, how is the AC coordinate defined? Is the sign for the z-coordinate the same as for the other coordinates, up being positive and down negative?

Kind regards,

Monique

[andreashorn]

Hi Monique,

I assume you’re referring to the reco.acpc coordinates inside the ea_reconstruction.mat file?
Indeed, I just checked and am not entirely sure if these are correct. In fact, the option to write out ACPC coordinates has been disabled by default a while ago but if you have an older installation (with older prefs), it would still do it in your case.

There is an MNI to ACPC tool (inside the tools menu) in which you can map from MNI to ACPC based on selected patients. In there, it is unequivocal how coordinates are mapped and to which point they refer to. Please use this one instead.

The AC lead-dbs uses is at -0.5948, 2.1606, -3.2329 mm which I drew in per hand. However, it’s in 2009b NLIN ASYM space as everything in Lead-DBS, so not the 152 space Brett refers to (more on this here: http://www.lead-dbs.org/?p=1241).

Hope this helps!

Best, Andy

(will check and correct if necessary the acpc coord writeout in the next release).

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