@@ -721,37 +721,46 @@ within the `[*_]dwi.bval` and `[*_]dwi.bvec` files) MAY change across DWI runs.
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** Gradient orientation file formats** .
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The ` [*_]dwi.bval ` and ` [*_]dwi.bvec ` files MUST follow the
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- [ FSL format] ( https://fsl.fmrib.ox.ac.uk/fsl/docs/#/diffusion/index?id=diffusion-data-in-fsl ) :
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- The ` [*_]dwi.bvec ` file contains 3 rows with * N* space-delimited floating-point numbers
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- (corresponding to the * N* volumes in the corresponding NIfTI file.)
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- The first row contains the * x* elements, the second row contains the * y* elements and
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- the third row contains the * z* elements of a unit vector in the direction of the applied
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- diffusion gradient, where the * i* -th elements in each row correspond together to
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- the * i* -th volume, with ` [0,0,0] ` for * non-diffusion-weighted* (also called * b* =0 or * low-b* )
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- volumes.
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- Following the FSL format for the ` [*_]dwi.bvec ` specification, the coordinate system of
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- the * b* vectors MUST be defined with respect to the coordinate system defined by
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- the header of the corresponding ` _dwi ` NIfTI file and not the scanner's device
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- coordinate system (see [ Coordinate systems] ( ../appendices/coordinate-systems.md ) ).
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- The most relevant limitation imposed by this choice is that the gradient information cannot
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- be directly stored in this format if the scanner generates * b* -vectors in * scanner coordinates* .
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-
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- Example of ` [*_]dwi.bvec ` file, with * N* =6, with two * b* =0 volumes in the beginning:
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-
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- ``` Text
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- 0 0 0.021828 -0.015425 -0.70918 -0.2465
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- 0 0 0.80242 0.22098 -0.00063106 0.1043
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- 0 0 -0.59636 0.97516 -0.70503 -0.96351
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- ```
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-
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- The ` [*_]dwi.bval ` file contains the * b* -values (in s/mm<sup >2</sup >) corresponding to the
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- volumes in the relevant NIfTI file), with 0 designating * b* =0 volumes, space-delimited.
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-
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- Example of ` [*_]dwi.bval ` file, corresponding to the previous ` [*_]dwi.bvec ` example:
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-
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- ``` Text
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- 0 0 2000 2000 1000 1000
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- ```
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+ [ FSL format] ( https://fsl.fmrib.ox.ac.uk/fsl/docs/#/diffusion/index?id=diffusion-data-in-fsl ) .
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+
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+ The ` [*_]dwi.bvec ` file contains 3 rows with * N* space-delimited floating-point numbers,
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+ corresponding to the * N* volumes in the corresponding NIfTI file.
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+ Across these three rows,
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+ each column encodes three elements of a 3-vector for the corresponding image volume;
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+ each vector MUST be either of unit norm,
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+ or optionally the vector ` [0.0,0.0,0.0] `
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+ for * non-diffusion-weighted* (also called * b* =0 or * low-b* ) volumes.
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+ These values are to be interpreted as cosine values with respect to the image axis orientations
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+ as defined by the corresponding NIfTI image header transformation;
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+ * unless* the image axes defined in the corresponding NIfTI image header
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+ form a right-handed coordinate system
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+ (that is, the 3x3 matrix of direction cosines has a positive determinant),
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+ in which case the sign of the first element of each 3-vector must be inverted
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+ for this interpretation to be valid.
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+ Note that this definition of orientations with respect to the NIfTI image axes
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+ is * not* equivalent to the DICOM convention,
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+ where orientations are instead defined with respect to the scanner device's coordinate system
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+ (see [ Coordinate systems] ( ../appendices/coordinate-systems.md ) ).
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+
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+ The ` [*_]dwi.bval ` file contains the * b* -values (in s/mm<sup >2</sup >)
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+ corresponding to the volumes in the relevant NIfTI file,
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+ with 0 designating * b* =0 volumes; space-delimited.
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+
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+ Examples of ` [*_]dwi.bvec ` and ` [*_]dwi.bval ` files,
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+ corresponding to a NIfTI image with 6 volumes
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+ with the first two volumes having no diffusion sensitization:
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+
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+ - ` [*_]dwi.bvec ` :
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+ ``` Text
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+ 0 0 0.021828 -0.015425 -0.70918 -0.2465
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+ 0 0 0.80242 0.22098 -0.00063106 0.1043
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+ 0 0 -0.59636 0.97516 -0.70503 -0.96351
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+ ```
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+
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+ - `[_]dwi.bval`:
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+ ```Text
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+ 0 0 2000 2000 1000 1000
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+ ```
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### Multipart (split) DWI schemes
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