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After alignment and the modification of the headers, mapping from voxel to voxel is by A\B or B\A. Coregistration estimates a rigid-body matrix (R), which is used to update the voxel-to-world mapping of the moved image so that the A\B or B\A thing does as it should. Best regards. -John On Fri, 2010-05-21 at 15:55 +0200, Neggers, S.F.W. wrote: > Just to verify: > > The method I suggested was : voxel-to-world matrices A and B division, > eg A/B, or A*inv(B) which is the same mathematically. This in my > experience maps a coordinate in world space of image 1 to the world > space of image 2, assuming both images overlapped in voxel space. I just > guessed that is what Dan asked for, but I could easily be mistaken. I > use this to map a world space point p in an image to the same location > in that image after coregistration. > > John: what you suggest maps voxels in one image to voxels in another > image, right? This of course is something different. > > Don't want to cause any confusion, so just checking... > > So Dan, just pick what you need for the problem you are working on. > > Have a nice weekend, > > Bas > > -------------------------------------------------- > Dr. S.F.W. Neggers > Division of Brain Research > Rudolf Magnus Institute for Neuroscience > Utrecht University Medical Center > Visiting : Heidelberglaan 100, 3584 CX Utrecht > Room B.01.1.03 > Mail : Huispost B.01.206, P.O. Box 85500 > 3508 GA Utrecht, the Netherlands > Tel : +31 (0)88 7559609 > Fax : +31 (0)88 7555443 > E-mail : [log in to unmask] > Web : http://www.neuromri.nl/people/bas-neggers > http://www.neuralnavigator.com > -------------------------------------------------- > > > > > -----Oorspronkelijk bericht----- > > Van: SPM (Statistical Parametric Mapping) > > [mailto:[log in to unmask]] Namens John Ashburner > > Verzonden: vrijdag 21 mei 2010 15:24 > > Aan: [log in to unmask] > > Onderwerp: Re: [SPM] Finding affine transformation parameters > > > > To register two images of the same subject, the objective is > > to be able to map from voxels in one image to voxels in the > > other. Doing this requires the "voxel to world" matrices in > > the headers. So, to go from image 1 (whos voxel-to-world > > mapping is M1) to image 2 (whose mapping is M2), you would need to: > > > > 1) Map from voxels in image 1 to world space (multiply by M1) > > 2) Rotate and translate this world space into alignment with > > the world space of image 2 (multiply by R) > > 3) Map from this world space back to voxels in image 2 > > (multiply by the inverse of M2). > > > > So the overall matrix is: M2\R*M1 > > And the mapping from voxels in image 2 to those in image 1 is: > > inv(M1)*inv(R)*M2 > > > > > > The six parameters computed by spm_coreg are as described in > > spm_matrix.m . They paramaterise R (or its inverse). Going > > back from a matrix R to the parameters can be done by spm_imatrix. > > > > Best regards, > > -John > > > > > > On Fri, 2010-05-21 at 13:42 +0100, Dan Golding wrote: > > > Hi, > > > > > > I would like to find the 12 parameters for an affine > > transformation between two nifti images. I have been looking > > into using spm_coreg and spm_matrix but I couldn't quite > > understand how to use them form their help. spm_coreg outputs > > 6 parameters, I'm not sure what these 6 are? > > > > > > Also is there a function that does the reverse of > > spm_matrix, i.e. one that can take an affine transformation > > matrix and split it into 3 rotations, 3 scalings etc...? > > > > > > Thanks > > > Dan > > > > > > > -- > > John Ashburner <[log in to unmask]> > > > > ------------------------------------------------------------------------------ > > De informatie opgenomen in dit bericht kan vertrouwelijk zijn en is > uitsluitend bestemd voor de geadresseerde. Indien u dit bericht onterecht > ontvangt, wordt u verzocht de inhoud niet te gebruiken en de afzender direct > te informeren door het bericht te retourneren. Het Universitair Medisch > Centrum Utrecht is een publiekrechtelijke rechtspersoon in de zin van de W.H.W. > (Wet Hoger Onderwijs en Wetenschappelijk Onderzoek) en staat geregistreerd bij > de Kamer van Koophandel voor Midden-Nederland onder nr. 30244197. > > Denk s.v.p aan het milieu voor u deze e-mail afdrukt. > > ------------------------------------------------------------------------------ > > This message may contain confidential information and is intended exclusively > for the addressee. If you receive this message unintentionally, please do not > use the contents but notify the sender immediately by return e-mail. University > Medical Center Utrecht is a legal person by public law and is registered at > the Chamber of Commerce for Midden-Nederland under no. 30244197. > > Please consider the environment before printing this e-mail. > -- John Ashburner <[log in to unmask]>