use mri_realigned from the mat file

tutorial/coregistration_opm.md

@@ -546,6 +546,8 @@ After reading in the MRI, you can check the coordinate system with `ft_determine
 
     % read in the anatomical MRI
     mri = ft_read_mri('example3_anatomical.nii');
+
+    % check the coordinate system
     ft_determine_coordsys(mri, 'interactive', 'no');
 
 {% include image src="/assets/img/tutorial/coregistration_opm/mri_notaligned.png" width="400" %}
@@ -556,22 +558,30 @@ As the above figure shows, the axes are labeled as 'unknown', but it seems that
 We will explicitly align the MRI to an anatomical landmark-based coordinate system next, which requires interactive identification of the relevant landmarks (Nasion, Left, and Right pre-auricular points).
 
     % define a head coordinate system linked to the anatomical landmarks
-    cfg          = [];
-    cfg.coordsys = 'neuromag';
-    mri          = ft_volumerealign(cfg, mri);
-    ft_determine_coordsys(mri, 'interactive', 'no');
+    cfg           = [];
+    cfg.coordsys  = 'neuromag';
+    mri_realigned = ft_volumerealign(cfg, mri);
+
+    % check the coordinate system after realignment
+    ft_determine_coordsys(mri_realigned, 'interactive', 'no');
 
 {% include image src="/assets/img/tutorial/coregistration_opm/mri_aligned.png" width="400" %}
 _Figure: anatomical MRI image with a 'neuromag' head coordinate system._
 
+{% include markup/info %}
+During MRI coregistration you have to click on the nasion, left and right ear. The landmark points at the ears are [ambiguous](/faq/how_are_the_lpa_and_rpa_points_defined) and if you click on different points, your alignment could be off by a centimeter or so. In the subsequent code we will use some rotation and translation parameters. To make sure that your subsequent results match what is presented here, you should download [example3_face_mri_realigned.mat](https://download.fieldtriptoolbox.org/tutorial/coregistration_opm/example3_mri_realigned.mat) and load it in MATLAB.
+
+    load example3_mri_realigned.mat % this contains the realigned mri
+{% include markup/end %}
+
 ### Alignment of the 3D scan face with the MRI
 
 We can extract the scalp surface from the anatomical MRI and interactively align this with the face from the 3D scan. In theory, an automatic algorithm, such as the iterative closest point (ICP) algorithm could be used, but the results of that highly depends on the number of points, and the quality of the initial alignment. Hence, in this example we stick to a manual alignment.
 
     % segment the scalp
     cfg          = [];
     cfg.output   = 'scalp';
-    seg          = ft_volumesegment(cfg, mri);
+    seg          = ft_volumesegment(cfg, mri_realigned);
 
     % create a mesh for the scalp
     cfg             = [];