I'm computing affine transforms for co-registering anatomical MRI scans. Sometimes I would like to re-use those transforms, so in a case similar to the example here https://dipy.org/documentation/1.2.0./examples_built/affine_registration_3d/... my final transform is the result of this command: rigid = affreg.optimize ( static, moving, transform, params0, static_affine, moving_affine, starting_affine = translation.affine ); final = rigid; after first having aligned the centres of gravity. After this command I can call resampled = final.transform ( moving ); and then I get the desired result. What I don't understand is how to store the transform and re-use it later. I have tried to just save the object 'final' with np.savez ( 'transformation.npz', final ); and then load it later with with np.load ( 'transformation.npz', allow_pickle = True ) as npzfile: final = npzfile [ 'arr_0' ]; and then re-run resampled = final.transform ( moving ); but that returns the following error: resampled = final.transform ( moving ); AttributeError: 'numpy.ndarray' object has no attribute 'transform' So what I understand from this is that the output 'rigid' from the affreg.optimize() command has an attribute 'transform', but when I save it and then load it again it does not any more. Is there a(nother) way to save and (re)load transforms? Many thanks Alle Meije