> I have a dataset with FDG images that came off the PET scanner in two
> different methods. Some are floats, others are signed 16-bit. SPM doesn't
> seem to mind...did a test analysis with mixed datatypes, and didn't
> complain. Could someone comment on how SPM does this, and whether I should
> convert all to one format or another? Also could someone remind me where I
> cam find the scale factor in Matlab, I would like to take a peek at it and
> see the differences there between the two types.
You can see the scale factor using the <Display> button. Alternatively,
you can get it from:
V=spm_vol(spm_get(1,'*.img'));
V.pinfo(1,:)
SPM is programmed to check the datatype from the .hdr file (stored in V.dim(4)).
Depending what it is, then SPM will use different routines for reading in the
data and converting it to double precision floating point.
You should be able to analyze your data if the datatypes are mixed, providing
there is enough dynamic range in the integer representations (and if the
data is scaled appropriately for each plane). Be careful of using 8 bit data
though, as only 256 different intensities can be represented. This may not
be enough accuracy for heavily smoothed data, or for fMRI.
Best regards,
-John
--
Dr John Ashburner.
Functional Imaging Lab., 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
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