-- From http://nifti.nimh.nih.gov/nifti-1/documentation/faq#Q14

The NIfTI format allows storage on disk to be in either a
left- or right-handed coordinate system. However, the format
includes an implicit spatial transformation into a RIGHT-HANDED
coordinate system. This transform maps from data coordinates
(e.g. column i, row j, slice k), into some real world (x,y,z)
positions in space. These positions could relate to
Talairach-Tournoux (T&T) space, MNI space, or patient-based
scanner coordinates.

***************************************************************

-- From BW (original) download page, linked from:
http://www.bic.mni.mcgill.ca/brainweb/selection_normal.html

image dimensions: zspace yspace xspace
    dimension name         length         step        start
    --------------         ------         ----        -----
    zspace                    181            1          -72
    yspace                    217            1         -126
    xspace                    181            1          -90

***************************************************************

My take on this, x is fastest changing, so in Matlab the order
is (x,y,z). First voxel -- voxel(1,1,1) in SPM convention --
maps to world(-90,-126,-72), voxel(91,127,73) goes to
world(0,0,0).
Suggested spm vol matrix
 M = [1 0 0 -91; 0 1 0 -127; 0 0 1 -73; 0 0 0 1];

***************************************************************

-- From BW 20 Models (subject 42), grey-level download page:
http://www.bic.mni.mcgill.ca/brainweb/anatomic_normal_20.html

image dimensions: zspace yspace xspace
    dimension name         length         step        start
    --------------         ------         ----        -----
    zspace                    181            1       -72.25
    yspace                    256            1      -145.75
    xspace                    256            1      -127.75

***************************************************************

My take on this, x is fastest changing, so in Matlab the order
is (x,y,z). First voxel -- voxel(1,1,1) in SPM convention --
maps to world(-127.75,-145.75,-72.25),
voxel(128.75,146.75,73.25) goes to world(0,0,0).
Suggested spm vol matrix
 M = [1 0 0 -128.75; 0 1 0 -146.75; 0 0 1 -73.25; 0 0 0 1];

***************************************************************

-- From BW 20 Models (subject 42), *discrete-model* download:
http://www.bic.mni.mcgill.ca/brainweb/anatomic_normal_20.html

image dimensions: zspace yspace xspace
    dimension name         length         step        start
    --------------         ------         ----        -----
    zspace                    362          0.5       -72.25
    yspace                    434          0.5      -126.25
    xspace                    362          0.5       -90.25

***************************************************************

My take on this, x is fastest changing, so in Matlab the order
is (x,y,z). First voxel -- voxel(1,1,1) in SPM convention --
maps to world(-90.25,-126.25,-72.25),
voxel(181.5,253.5,145.5) goes to world(0,0,0).
Suggested spm vol matrix
 M = [0.5 0 0 -90.75; 0 0.5 0 -126.75; 0 0 0.5 -72.75; 0 0 0 1];

***************************************************************

Test: download uncompressed raw byte (rawb) from BrainWeb site,
Matlab functions read these and write out NIfTI hdr+img pair.
blah.img is exactly the same as blah.rawb
spm_check_registration shows blah.img in register with
spm5/canonical/single_subj_T1.nii (including left/right!)

Note: world-origin appears not to be exactly on AC, but I think
this is because BW is registered to ICBM152 or 352 or similar,
rather than anatomically located.