Docteur de l'université Automatic Segmentation and Shape Analysis ...

66 Chapter 3 Hippocampal segmentation using multiple atlases
of clinical trials. The initial goal of ADNI was to recruit 800 adults, ages 55 to
90, to participate in the research – approximately 200 cognitively normal older
individuals to be followed for 3 years, 400 people with MCI to be followed for 3
years, and 200 people with early AD to be followed for 2 years.
In the experiments, two separate atlas sets were used. One consisted of 138 normal
control (NC) subjects, and the other with 99 patients diagnosed of Alzheimer’s
disease (AD). The hippocampal volumes are semi-automated segmentations pro-
vided by ADNI, using high-dimensional brain mapping tool SNT, commercially
available from Medtronic Surgical Navigation Technologies (Louisville, CO). SNT
hippocampal volumetry has been previously validated on the normal aging, MCI
and AD subjects (Hsu et al., 2002). It first uses 22 control points manually placed
on the individual brain MRI as local landmarks. Fluid image transformation
is then used to match the individual brains to a template brain (Christensen
et al., 1997). The segmentations were manually edited by qualified reviewers if
the boundaries delineated by SNT were not accurate.
3.4.2.2 Experimental results
We perform a leave-one-out cross-validation on each atlas set. Each NC atlas was
registered to all other cases in NC set, and each AD atlas was registered to all the
others in the AD set. The registrations were performed by affine transformation
using a robust block matching approach (Ourselin et al., 2001) with 12 degrees of
freedom, which is followed by non-rigid registration using non-parametric diffeo-
morphic Demons algorithm (Vercauteren et al., 2007), transforming the atlases by
diffeomorphic displacement fields. In total 138 × 137 + 99 × 98 = 28 608 NRRs
were performed, in which 235 failed.
For a given atlas, the labels from other atlases were selected and combined using
LWV. NMI and correlation coefficients were used as similarity metrics in the atlas
selection. The similarity metrics were evaluated on a region of interest (ROI)
containing the hippocampus to be segmented. The ROI is defined by the labeling
of the atlas closest to the target with padding of 15-voxel width.