BrainVoyager version: 22.4.6
Dataset used: Getting Started Guide data

This document shows in a step by step example how to transform a VOI to a POI and how to transform a POI back to a VOI.

VOI to POI

To start, we open the MNI-normalized VMR of sub-01.

We link the VTC from our Getting Started Guide dataset: 
sub-01_ses-04_task-blocked_run-01_bold_SCCTBL_3DMCTS_THPGLMF3c_MNI.vtc and compute a single study GLM.

We define a set of regions of interest that show increased activation in response to stimuli presented in the right visual field (RVF) as compared to stimuli presented in the left visual field (LVF).

We load these functionally defined VOIs based on the contrast [Houses_RVF +1] + [Faces_RVF +1] ≠ [Faces_LVF -1] + [Houses_LVF -1] in the 'Volumes-of-Interest' dialog.

In a next step we need to load a cortical mesh that we would like to use to sample these regions in surface space. For the current example we use a slightly inflated cortical mesh of the left hemisphere.

If you decide to use an inflated or flattened mesh, please make sure that the correct folded mesh representation is linked to this mesh in the 'Mesh Morphing' dialog (via the menu 'Meshes -> Mesh Morphing'), otherwise the regions cannot be properly sampled.

Please open the 'Patches-Of-Interest' dialog via the menu 'Meshes -> Patches-Of-Interest..' Here we click on 'Options' to open the 'POI Analysis Options' and switch to the 'POI Functions' tab.

In the 'VOIs -> POIs' field we can choose to create a set of POIs from the currently loaded VOIs in Volume space by clicking on 'Create POIs'.

The options in the appearing dialog can be used to decide how to sample the VOIs from each vertex of the loaded cortical surface file. If the option 'Sample volume data exactly along mesh vertices' is selected, the mesh vertices are projected into the VMR and the vertices will sample the VOIs from the voxels in volume space that are closest to the location of the mesh vertices using trilinear interpolation. A detailed description of the different options can be found in the section 'Creation of Mesh Time Course Data - From Volume to Mesh Time Courses' in the BrainVoyager User's Guide. For now we keep the default settings, which will sample from voxels close to the mesh vertices but will also integrate data along the vertex normals.

After the POI creation is finished, all POIs will be listed in the Patches-Of-Interest list and can be saved to disk using the button 'Save'.

If you would like to see from which voxels the VOIs are sampled, you can project the mesh vertices into the VMR using the 'Mesh Spatial Transformations' dialog. This dialog can be invoked via the menu 'Meshes -> Spatial Transformations...'. Please click 'Mesh -> VMR' to project the mesh as a VOI into the current VMR.

The yellow outline in the screenshot below represents the cortical surface reconstruction with respect to the location of the VOIs in Volume space.

POI to VOI

We can perform the conversion also 'backwards' from Surface to Volume space, i.e. create VOIs based on POIs. This can be performed in the same tab of the 'POI Analysis Options' that was used to convert VOIs to POIs. In the current example we back-transform the newly created set of POIs back to VOIs by clicking on 'Create VOI' keeping the default sampling choices.

You will notice that the resulting VOIs are close, but not identical to the original set of VOIs due to the repeated interpolation.