The Format of VDW Files

The following table describes VDW file format 2 and also version 1 by indicating the entries which have been changed.

VDW Header

BYTES	DATA TYPE	DEFAULT	DESCRIPTION
2	short int	2	version number
S	byte		name of DMR file whose DWI data has been transformed (*1)
2	short int	1	number of protocols (NP) attached to VDW (new v2)
NP x Mi	byte	<none>	NP names, each specifying a linked protocol (PRT) file (*1)
2	short int	0	index specifying the "current" protocol (new v2)
2	short int		NrOfVolumes (number of volumes, measurements, time points)
2	short int	3	resolution relative to VMR, e.g. "3" - 1 voxel = 3 x 3 x 3 VMR voxels (*2)
2	short int	57	XStart (*3)
2	short int	231	XEnd (*3)
2	short int	52	YStart (*3)
2	short int	172	YEnd (*3)
2	short int	59	ZStart (*3)
2	short int	197	ZEnd (*3)
1	byte		left-right convention flag (*4) (new v2)
1	byte		reference space flag (*5) (new v2)
4	float		TR [ms]
4	int		TE [ms]
1	byte		GradientDirectionsVerified, boolean entry, values "1" or "0"
1	byte		GradientXDirInterpretation, values "1" - "6" (*6)
1	byte		GradientYDirInterpretation, values "1" - "6" (*6)
1	byte		GradientZDirInterpretation,values "1" - "6" (*6)
1	byte		GradientInformationAvailable
N x 4	float		IF table available, NrOfVolumes x 4 - gradient X/Y/Z plus B - values
1	byte		NrOfSpatialTransformations, used to update direction table
			IF (NrOfPastSpatialTransformations > 0) list of past spatial transformations (*7)

(*1) Variable length ("S" bytes), the end of the name is indicated by '0'.

(*2) The VDW resolution is defined with respect to the resolution of a VMR file. Typically, one VMR voxel has the resolution of 1 (Talairach) mm3.

A VDW resolution value of 3 means that one VTC voxel encompasses 3 x 3 x 3 = 27 VMR voxels. Other resolutions are possible, i.e. 1 and 2.

(*3) These values indicate the relative position within a VMR volume (must be a 256^3 data set for Talairach VDW data, (see explanations below).

(*4) The left-right convention flag may assume one of three values: "1" specifies radiological convention ("left-is-right"), "2" specifies neurological convention ("left-is-left") and "0" indicates that the convention is unknown.

(*5) The reference space flag may have the following values: "1" indicates that the functional data is aligned to a 3D data set in "native" space

(e.g. in the position scanned), "2" indicates ACPC space, "3" indicates "Talairach" space and "0" indicates that the reference space is not known.

(*6) ...

(*7) The structure of how spatial transformations are stored is described in the topic about VMR files.

VDW data

Probably similar to VTC data:

Each data element (intensity value) is represented in 2 bytes (unsigned short). The data is organized in four loops:

DimZ

DimY

DimX

DimT

The inner loop (DimT = NrOfVolumes) contains the time series for each VTC voxel. The spatial dimensions can be computed from the header info as follows:

DimX = (XEnd-XStart) / VTC-resolution
DimY = (YEnd-YStart) / VTC-resolution
DimZ = (ZEnd-ZStart) / VTC-resolution

With a VTC-Resolution of 3 and the default values provided above, this results in:

DimX = 58
DimY = 40
DimZ = 46

With these values and a typical NrOfVolumes of 200, the total number of bytes (for data without header) is therefore:

DimX * DimY * DimZ * NrOfVolumes * 2 = 42,688,000

or approximately 40MB.

Note that the axes terminology follows the internal BrainVoyager (BV) format. The mapping to Talairach axes is as follows:

BV X front -> back = Y in Tal space
BV Y top -> bottom = Z in Tal space
BV Z left -> right = X in Tal space

The Format of VDW Files

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