A DWI file (DWI = "diffusion-weighted images") contains the diffusion-weighted data of a DMR project. A DMR project links to the DWI file with a "DWI Prefix". If the name of the DMR project is, for example, "S1_DTI.dmr", the DWI prefix will be "S1_DTI" after saving a created project accordingly. The "StorageDataFormat" entry in the DMR file describes which of three possible arrangements of the DWI data are used in a given project. Note that the content in the file(s) are not changed when different storage formats are used, the only thing what changes is the arrangement of the data within the file (different arrangements of the data are more useful for different purposes).
The DWI prefix changes when the data is preprocessed reflecting any applied preprocessing steps but it can also be changed to any name, if desired. When BrainVoyager accesses the data, it uses the DWI prefix and a volume/slice number to read the correct information from a DWI file. While there are three possible arrangements of the data within a single DWI file (specified by values 2, 3, and 4), DMR-DWI projects only use storage formats "3" and "4" (for formats "1" and "2", see STC files). Storage format "3" specifies that the data is stored as a sequence of volumes, i.e. all slices of the first scan are stored first, then all slices of the second volume and so on. The number of volumes is determined by the "NrOfVolumes" value and the number of slices within a volume is determined by the "NrOfSlices" value in the DMR text file. Each image is assumed to have the dimensions specified as "ResolutionX" (or NrOfColumns) and "ResolutionY" (or NrOfRows) in the corresponding DMR file. Each data element (intensity value) is represented either in 2 bytes (unsigned short) or in 4 bytes (float) as determined by the "DataFormat" entry in the FMR file. For storage format "3", the data is organized in four loops (most inner loop on the right):
TimePoints > Slices > Rows > Columns - Data
Since the three inner loops index all voxels of a volume, we can also express this storage arrangement as:
TimePoints > Voxels - Data
indicating that for each time point index (volume, data point), all voxel measurements are stored in contiguous positions on disk (at least virtually) and in contiguous addresses in memory.
When the "StorageDataFormat" value in the DMR file is "4", the data is expected to be organized in the same way as used for VTC files. In this format, the inner loop is the one across time points allowing to read the time series of a single voxel as a contiguous block. The data is, thus, organized as follows:
Slices > Rows > Columns > TimePoints
Since the first three loops index all voxels of a volume, we can express this storage arrangement also as:
Voxels > TimePoints - Data
indicating that for each voxel index, all repeated measurements are stored in contiguous positions on disk (at least virtually) and in contiguous addresses in memory.
Note that the DWI file does not contain any header information.