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Hi again, probably, I was misunderstood due to a bad formulation of my question. I tried to ask: how many additional (transformation-) parameters could you include in the SPM-coregistration-algorithm, without losing in accuracy. Is the information incorporated in the mutal information (MI)-obtimisation function "sufficient enough" to calculate 12 (or even more) parameters correctly, instead of the usual 6 parameters, assuming a dataset with high resolution, and assuming that, each additional parameter changes the optimisation function less or in the same order of magnitude as the parameters before. Thanks, Siawoosh. John Ashburner wrote: > > as regards to the coregistration: Is the MI-obtimaization function "good > > enough" to yield reasonable results, if you use a 12-parametric > > transformation > > > > instead of the 6-parametric transformation? > > > > > Actually, SPM's coregistration only does a 6 parameter transform by > > > default, although it can be coaxed into using 12 - but not via the user > > > interface. > > This is a question that will differ from dataset to dataset. The validity > depends entirely upon the assumptions that can be made of your data. If you > use relatively new and well calibrated scanners, then the voxel sizes should > be set pretty accurately in the header. If this is the case, then a > 6-parameter rigid body model will be better. Perhaps the question should be > rephrased: > > "Is my data good enough to use a six-parameter rigid registration model > instead of a 12-parameter affine model?" > > Best regards, > > -John >