The Format of VTC Files

A VTC file contains the functional data (time series) of one experimental run (one functional scan) in a 3D space (native, AC-PC, Talairach, MNI) without slice gap. The binary file contains a variable-length header followed by the actual 4D data. The data is not stored as a sequence of volumes (time points), but contains for each voxel a continous stream of values representing the voxel's time course. Having time as the inner loop in this way speeds up reading time course data for voxels from a file.

VTC Header

BYTES	DATA TYPE	DEFAULT	DESCRIPTION
2	short int	2	version number
N	byte		name of FMR file whose STC data has been transformed (*1)
M	byte	<none>	name of the linked protocol (PRT) file (*1)
2	short int		NrOfVolumes (number of volumes, measurements, time points)
2	short int	3	VTC resolution, i.e. 3 -> one voxel = 3 x 3 x 3 mm (*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)
2	short int		Hemodynamic delay, simple shift value (*4)
4	float		TR [ms] (*4)
4	float		Hemodynamic function, delta parameter (*4)
4	float		Hemodynamic function, tau parameter (*4)
2	short int		Segment size, used for time course separation (*4)
2	short int		Segment offset, used for time course separation (*4)

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

(*2) The VTC resolution is defined with respect to the resolution of a VMR file. Typically, one VMR voxel has the resolution of 1 (Talairach) mm3. A VTC 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 2563 data set for Talairach VTC data, (see explanations below).

(*4) These values are included in the header only if the file version number is at least 2. You can ignore these values, they might be dropped in future versions.

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

Example. With a VTC resolution of 3 and the default dimension 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 VTC Files

The Format of VTC Files

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