VOI files are simple text files describing a set of volumes-of-interest (VOIs), which can be linked to a VMR with compatible dimensions. As the 3D version of regions-of-interest (ROIs), the main information in a VOI file are lists of coordinates (X, Y, Z values) specifying the voxels belonging to a specific VOI. The most recent file version (4) is described below together with a specification to what has been changed since previous versions.
| ENTRY | DESCRIPTION |
| FileVersion: | The version of the VOI file: 4 |
| ReferenceSpace: | "BV" or "NATIVE", "ACPC", "TAL" (*1) |
| OriginalVMRResolutionX: | Voxel x resolution [mm] of original VMR (*2) |
| OriginalVMRResolutionY: | Voxel y resolution [mm] of original VMR (*2) |
| OriginalVMRResolutionZ: | Voxel z resolution [mm] of original VMR (*2) |
| OriginalVMROffsetX: | Offset in x dimension of original VMR (*2) |
| OriginalVMROffsetY: | Offset in y dimension of original VMR (*2) |
| OriginalVMROffsetZ: | Offset in z dimension of original VMR (*2) |
| OriginalVMRFramingCubeDim: | Framing cube of original VMR (*2) |
| LeftRightConvention: | Left-right convention of original VMR (*2) |
| SubjectVOINamingConvention: | "<VOI>_<SUBJ>" or "<SUBJ>_<VOI>" (*3) |
| NrOfVOIs: | Number N of VOIs defined subsequently |
| Loop over N VOIs | |
| NameOfVOI: | Name of VOI |
| ColorOfVOI: | Three values with R, G, B color components (0-255 range) |
| NrOfVoxels: | Number V of voxels defining current VOI |
| Loop over V voxels | |
| [x y z] | Three integer values per voxel in one row separated by blanks |
| End of loop over V voxels | |
| End of loop over N VOIs | |
| NrOfVOIVTCs: | Number T of VTC files (*4) |
| Loop over T VTC file names | |
| [VTC filename] | A string containing the name of one of the listed VTC files |
| End of loop o ver T VTC file names | |
(*1) This entry was named "CoordsType:" in previous versions. The reference space determines the space in which the VMR file resides linking this VOI file. Note that the three voxel coordinates are interpreted differently in the "TAL" space than in the "BV" (or "NATIVE") space.
(*2) These values are copied from the VMR file used when saving/creating the VOIs. They can be used to check whether other VMR files attempting to link a VOI file are compatible. The knowledge of these values also allows to make VOIs compatible to VMR files with different dimensions or left-right convention by adjusting the coordinate values accordingly.
(*3) This entry is used when VOI names refer to a nesting of VOIs under subjects, i.e. when several VOIs refer to the same homologue brain area in multiple subjects. This entry specifies whether in a compound "VOI subject" name the "subject ID" or the homologue VOI substring appears first. The two substrings must be separated by the underline ("_") character.
(*4) The specified list of VTC files is stored in the VOI file to quickly re-establish access to VTCs, which were previously used in the context of this VOI file. This is merely a convenience function, i.e. this information is strictly speaking not related to the definition of VOIs.