> First I would like to say thanks to John Ashburner for answering my
> question. I have another which again shows how ignorant I am of spm! I
> was wondering if any of you could explain to me what the values when
> defining the parameters for spatial normalisation mean. When you use the
> define parameters only function , after it has spatially corrected for one
> volume you get a set of values , i just would like to know the units and
> what it is describing. I suspect its the value in millimeters to apply to
> the whole group so as to line it up with the template but i am not sure.
The 'Determine Parameters Only' option estimates how best to warp a source image so that
it matches a template. The results of this procedure encode a warp that is stored in a *_sn3d.mat file.
This warp can then be applied to any image that is rigidly registered to the source image.
Exactly what the stored parameters mean is a little bit tricky to explain. One of the elements is
the "Affine" matrix, which is an affine mapping between the source image and template such that
x' = a11*x + a12*y + a13*z + a14
y' = a21*x + a22*y + a23*z + a24
z' = a31*x + a32*y + a33*z + a34
Then there is a set of basis function coefficients that parameterise a nonlinear warp. For more information
about what these represent, then take a look at:
Ashburner J, Friston KJ (1999) "Nonlinear Spatial Normalization using Basis Functions"
Human Brain Mapping 7(4):254-266
which is available from:
http://www.fil.ion.ucl.ac.uk/spm/dox.html
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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