Aganj, ImanIglesias, Juan EugenioReuter, MartinSabuncu, Mert RoryFischl, Bruce2022-07-122017-05-15Aganj, Iman, Juan Eugenio Iglesias, Martin Reuter, Mert Rory Sabuncu, Bruce Fischl. "Mid-Space-Independent Deformable Image Registration." NeuroImage 152 (2017): 158-170. DOI: 10.1016/j.neuroimage.2017.02.0551053-8119https://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37372577Aligning images in a mid-space is a common approach to ensuring that deformable image registration is symmetric – that it does not depend on the arbitrary ordering of the input images. The results are, however, generally dependent on the mathematical definition of the mid-space. In particular, the set of possible solutions is typically restricted by the constraints that are enforced on the transformations to prevent the mid-space from drifting too far from the native image spaces. The use of an implicit atlas has been proposed as an approach to mid-space image registration. In this work, we show that when the atlas is aligned to each image in the native image space, the data term of implicit-atlas-based deformable registration is inherently independent of the mid-space. In addition, we show that the regularization term can be reformulated independently of the mid-space as well. We derive a new symmetric cost function that only depends on the transformation morphing the images to each other, rather than to the atlas. This eliminates the need for anti-drift constraints, thereby expanding the space of allowable deformations. We provide an implementation scheme for the proposed framework, and validate it through diffeomorphic registration experiments on brain magnetic resonance images.en-USResearch Subject Categories::MEDICINE::Morphology, cell biology, pathology::Cell biology::NeuroscienceJournal Article2017-08-2120172022-07-1210.1016/j.neuroimage.2017.02.055