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

26 Chapter 2 Literature Review
methods (Murgasova et al., 2006, 2007), and FreeSurfer whole brain segmentation
(Fischl et al., 2002).
The use of multiple atlases manually traced by experts improves the accuracy of the
segmentation based on a single atlas (Rohlfing et al., 2004a,b; Heckemann et al.,
2006), by reducing the bias towards each single atlas, and taking the advantage of
a priori knowledge encoded in the atlas set. Multiple atlases are transformed from
the coordinates in the space of atlas to the target image to be segmented by pair-
wise non-rigid registration (NRR). The labeling of each voxel in the target image
is then determined by fusing the warped segmentations of the candidate atlases,
usually by a vote rule. The errors made by individual atlas may be corrected by
other atlases in the label fusion. It has been shown to be a viable approach to the
delineation of subcortical structures in brain MR images (Babalola et al., 2008).
The inter-subject variability may exceed the transformation space searched by the
NRR algorithms, such that the error of mismatch between the image and the atlas
may be propagated to the segmentation result. Registration failure will lead to the
mislabeling. The errors in the result of multi-atlas segmentation thus arise either
from the atlases used, or due to the misregistration. In practice, the accuracy of
the method depends on the following design choices
• the initial atlas set
• the registration algorithms
• the atlas selection strategy after the registration
• and the label fusion method.
A representative but unbiased set of atlases is usually chosen to target the popu-
lation of interest. Cohort atlases from the population are found to perform better
than using a single standard atlas (Carmichael et al., 2005). Though it is not
always readily available, a larger atlas set to cope with the variability in a diverse
population (Wolz et al., 2010) or a specific sub-population (Shen et al., 2010). It
can be obtained by propagation from a smaller set of manually segmented atlases.