> Could anybody explain exact difference between old analyze(7.5) format
> and SPM analyze file format?
The SPM version of Analyze image format is based on the old Analyze format
but with a few small differences. A full description of the SPM version is
found in spm_format.man.
Scalefactors are done differently: in SPM99 the glmax, glmin, cal_max and
cal_min fields are not used for mapping between integer values in the .img
files and some calibrated value. Instead, the images are multiplied by a
scalefactor stored in dime. roi_scale (or dime.funused1 in dbh.h files where
roi_scale does not exist).
An "origin" field is included in the SPM99 version where hist.originator is
cast as three short integers that contain voxel co-ordinates somewhere near
the AC. If the images have a .mat file, then this information is ignored.
Origin values of [0 0 0] will default to mean the centre of the volume.
hk.sizeof_hdr is used by SPM99 to ascertain if the .hdr file needs to have
its byte orders changed, along with the byte ordering of the .img file.
vox_offset is partially used by SPM99. Positive integer values indicate how
far to skip into the .img file before reading the data. This value must be
equivalent to a whole number of voxel elements (i.e. a multiple of 2 for
short ints, multiple of 4 for integer and float and a multiple of 8 for
doubles).
Proper Analyze images use a left-handed co-ordinate system, whereas Talairach
& Tournoux use a right handed system. This is why spatially normalised
images are left-right flipped. (See page 22 of
http://www.fil.ion.ucl.ac.uk/~john/thesis/chapter2.pdf). Images are assumed
to be transverse - unless a .mat file specifies otherwise (hist.orient is
ignored).
SPM99 completely ignores the presence of fields in an Analyze .hdr other than
those read by spm_hread.m.
Best regards,
-John
--
Dr John Ashburner.
Wellcome Department of Cognitive Neurology.
12 Queen Square, London WC1N 3BG, UK.
tel: +44 (0)20 78337491 or +44 (0)20 78373611 x4381
fax: +44 (0)20 78131420
http://www.fil.ion.ucl.ac.uk/~john
mail: [log in to unmask]
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