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

18 Chapter 2 Literature Review
Depending on the application and imaging modalities used in acquisition, a wide
range of segmentation methods has been developed in order to deal with images
of different parts of body. With respect to the level of human involvement, these
methods can be classified into three categories:
• manual segmentation
• semi-automatic segmentation
• automatic segmentation.
Manual segmentation is usually performed by trained experts following certain
protocols defining regions of interest (ROIs) on the image. The anatomical knowl-
edge of the expert contributes to the segmentation when the boundaries of the
structures are difficult to detect solely based on image features. Although expert-
delineated segmentations are regarded as the ground truth or the golden standard,
in practice, manual approaches are subjected to both intra- and inter-rater vari-
ability induced by the the human factor. Considerable variability may result in
low reproducibility of the manual segmentation. In addition, manually tracing the
structures of interest by experts is a laborious task, which can be time consuming
and expensive. It might take more than one hour per structure for the expert to
manually define the boundary of the structure of interest in the volume, which is
impractical and inefficient, especially when large data set is involved. Computer
softwares are usually used to assist manual tracing and the edition of the bound-
aries, such as the publicly available MRIcron (Rorden and Brett, 2000), imageJ
(Abramoff et al., 2004), 3D Slicer (Pieper et al., 2006) and ITK-SNAP (Yushkevich
et al., 2006).
Computerized automatic and semi-automatic methods are introduced to reduce
the subjectivity in the manual segmentation, and to increase the efficiency and
reproducibility. Semi-automatic segmentation still requires manual operations to
introduce prior knowledge, provide guidance to the process, or correct the segmen-
tation results. The correction of boundaries sometimes can take as long as manual