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faq:how_are_the_lpa_and_rpa_points_defined [2012/08/17 10:43]
robert explain consequences of inconsistent use of LPA and RPA
faq:how_are_the_lpa_and_rpa_points_defined [2016/11/15 10:08]
robert
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-{{tag>​mri ctf source coordinate}}+{{tag>faq fiducial ​mri ctf source coordinate}}
  
 ====== How are the Left and Right Pre-Auricular (LPA and RPA) points defined? ======== ====== How are the Left and Right Pre-Auricular (LPA and RPA) points defined? ========
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 The problem with using the LPA and RPA according to this definition is that it can be really difficult to localize them precisely on the anatomical MRI. Mislocalization of these points can lead to severe misregistration between the MRI and MEG/EEG, and consequently affect the accuracy of source localization. The problem with using the LPA and RPA according to this definition is that it can be really difficult to localize them precisely on the anatomical MRI. Mislocalization of these points can lead to severe misregistration between the MRI and MEG/EEG, and consequently affect the accuracy of source localization.
  
-Because ​of the difficulty of localizing ​the LPA and RPA points in both EEG/MEG and in the anatomical MRIvarious labs have adopted slightly different conventions for the fiducial points for the ears.+<note warning>​ 
 +Regardless ​of which convention you use for the points representing ​the left and right ear landmarksyou have to be aware of it and use it consistently throughout. 
 +</​note>​
  
-Some labs use the junction between the tragus and the helix, marked with the red dot in the figure of the ear. It can be located ​much more precisely both anatomically and on the MRI slices.+ 
 +Because of the difficulty of localizing the LPA and RPA points in both EEG/MEG and in the anatomical MRI, various labs have adopted slightly different conventions for the fiducial points for the ears. Some labs use the junction between the tragus and the helix, marked with the red dot in the figure of the ear above, which can be located more precisely both anatomically and on the MRI slices. Other possibilities that are less frequently uses are the tip of the tragus, the ear lobe or the [[http://​en.wikipedia.org/​wiki/​Mastoid|mastoid]].
  
 At the Donders Centre in Nijmegen we use ear molds with a hole in them (see below) to attach the markers: In the MEG scanner we insert a small tube into the hole (the tube is also used for auditory stimulation) and attach the MEG localizer coil to the tube. In the MRI scanner we use the same ear molds, but rather insert a custom-made marker with a small drop of vitamine E into the hole. The position thereby obtained with the MEG localizer coils is as precisely as possible reproduced in the MRI, given the movement that is allowed by the ear molds. We have various sizes of ear molds, both at the MEG and MRI scanner, and subjects should use the same size in both scanners. At the Donders Centre in Nijmegen we use ear molds with a hole in them (see below) to attach the markers: In the MEG scanner we insert a small tube into the hole (the tube is also used for auditory stimulation) and attach the MEG localizer coil to the tube. In the MRI scanner we use the same ear molds, but rather insert a custom-made marker with a small drop of vitamine E into the hole. The position thereby obtained with the MEG localizer coils is as precisely as possible reproduced in the MRI, given the movement that is allowed by the ear molds. We have various sizes of ear molds, both at the MEG and MRI scanner, and subjects should use the same size in both scanners.
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 The consequence of different fiducial locations in different labs is that the terms "​LPA"​ and "​RPA",​ although used in software such as FieldTrip and other EEG/MEG tools, do not always refer to the same anatomical landmarks. In your analysis you have to take care that the positions consistently refer to the same landmarks, whether they are in front of the ear, on the tragus or in line with the ear canal. So whenever the software uses LPA and RPA, you have to be aware of your lab convention. The consequence of different fiducial locations in different labs is that the terms "​LPA"​ and "​RPA",​ although used in software such as FieldTrip and other EEG/MEG tools, do not always refer to the same anatomical landmarks. In your analysis you have to take care that the positions consistently refer to the same landmarks, whether they are in front of the ear, on the tragus or in line with the ear canal. So whenever the software uses LPA and RPA, you have to be aware of your lab convention.
  
-Note that for the [[http://​en.wikipedia.org/​wiki/​Nasion|nasion]],​ where we also place one of the MEG localizer coils, we do not use a MRI marker. The nasion is easy to identify in the anatomical MRI images.+Note that for the [[http://​en.wikipedia.org/​wiki/​Nasion|nasion]],​ where at the Donders Centre ​we also place one of the MEG localizer coils, we do not use a MRI marker. The nasion is easy to identify in the anatomical MRI images.
  
 ===== See also ===== ===== See also =====
   * BrainStorm documentation on [[http://​neuroimage.usc.edu/​brainstorm/​CoordinateSystems|coordinate systems]]   * BrainStorm documentation on [[http://​neuroimage.usc.edu/​brainstorm/​CoordinateSystems|coordinate systems]]
 +  * http://​www.proplugs.com for the ear molds we use at the Donders Centre