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Chapter 3 Hippocampal segmentation using multiple atlases 47
3.1.1.2 Non-rigid registration
In the comparison by Yassa and Stark (2009), pairwise cross-participant registra-
tions were performed over 20 MR images of healthy subjects using the state-of-
the-art registration algorithms. The manual delineations of subcortical structures
including hippocampus, entorhinal cortex, perirhinal cortex, temporopolar cortex
and parahippocampal cortex of each subject were warped to the rest 19 images.
Methods such as Talairach alignment and 3dWarpDrive in AFNI (Cox, 1996),
normalization in SPM (Ashburner and Friston, 1999, 2005), LDDMM (Beg et al.,
2005; Miller et al., 2005), DARTEL (Ashburner, 2007), and Demons (Thirion,
1998; Vercauteren et al., 2009) were used for registration, and evaluated by the
overlap between the warped and the manual segmentation. The results showed
that the Demons algorithm aligns the hippocampus and other subcortical struc-
tures with the highest overlap.
The Demons algorithm (Thirion, 1996, 1998) is based on the notion of Maxwell’s
demons, which are fictitious agents on a semi-permeable membrane directing the
diffusion process toward the decrease of the total entropy, thus contradicting the
second law of thermodynamics. 1 In Demons algorithm, the transformation be-
tween the source and the target images is modeled by diffusion processes with the
control of demons driven by the force of optical flow (Horn and Schunck, 1981;
Barron et al., 1994). The diffeomorphic Demons is developed by Vercauteren et al.
(2007, 2009) in which the unconstrained update of displacement is projected back
to the Lie group of diffeomorphisms through the exponential map (Mahony and
Manton, 2002; Malis, 2004).
3.1.2 Similarity based atlas selection
In multi-atlas based segmentation, the warped atlases are selected according to
their registration accuracy such that the errors in the less well aligned atlases
1 Relevant literature on Maxwell’s demons is summarized by Leff and Rex (1990), and more
recent review of the paradox and the solutions by Maruyama et al. (2009).