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

46 Chapter 3 Hippocampal segmentation using multiple atlases
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.
. .
transformations
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{Tk}
.
.
.atlas images {Ik}
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.target image I
.NRR
. Atlas
Selection
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.
.
transformed atlas
images {Ik ◦ Tk}
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selected
subset A
.Interpolation
.atlas label maps {Lk}
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. Classifier
fusion
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.transformed label maps {Lk ◦ Tk}
.fused label map ˆ L for I
Figure 3.1: Diagram demonstrating the process segmentation using multiple
atlases.
3.1.1.1 Affine registration
The affine registration is performed using a robust block matching approach (Ourselin
et al., 2001). In the block matching step, the image blocks in the source are
matched to the target maximizing the image similarity metric. The optimal trans-
formation is then estimated by a least square regression matching the displacement
between the blocks in the source and the target. A multi-resolution implementa-
tion varying the sampling rate from coarse to fine is used. In practice, the regis-
tration is initially performed with a rigid transformation of 6 degrees of freedom
(SO(3) ⋉ R 3 ): 3D translation and 3D rotation. Based on the rigid initialization,
an affine registration is used to estimate the affine transformation T A of 12 degrees
of freedom (GL(3, R) ⋉ R 3 ): 3D translation, 3D rotation, 3D anisotropic scaling,
and 3D skew.