The fact that Nifti1Image cannot contain boolean arrays is **really** annoying. Several calls to astype() can't be avoided for this reason, which incurs a copy each time. One can argue that this is usually not a bottleneck, but I think we should do something about it. I created in df08e063a0ae9fe466b61ca4149a1c90a0f2ec12 (see also PR #47) a NislImage class, that just contains two methods get_data() and get_affine() for this reason. It is a valid niimg according to is_a_niimg(), but cannot be saved as a Nifti file easily.