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"Key: BV = brain volume (WM + GM), CSF = cerebrospinal fluid, ICV = intracranial volume (WM + GM + CSF).

b., , ^ , Estimated compartment vol - True compartment vol Absolute volumetric error =-^-----—--x 100%.

"Key: BV = brain volume (WM + GM), CSF = cerebrospinal fluid, ICV = intracranial volume (WM + GM + CSF).

b., , ^ , Estimated compartment vol - True compartment vol Absolute volumetric error =-^-----—--x 100%.

with image registration in order to segment specific brain structures, including the segmentation of subcortical nuclei. Registration can be accomplished by spatially normalizing image data to a brain template that resides in a common reference space, namely the stereotaxic space. A commonly employed stereotaxic space in MR imaging is the Talairach space [35].

In MR brain imaging, the concept of segmenting image data via registration is often called atlas-guided segmentation. These methods seek to accurately register brain image data to a presegmented and prelabeled brain atlas (a template). If registration is successful, then both structural information and labels are transferred from the atlas to the image. The process of warping an atlas to a target image can be accomplished by linear [35-37] or nonlinear [38-41] transformations; however, because of large anatomical variability, precise segmentation of intricate brain structures, such as the cerebral cortex and specific thalamic nuclei, still remains a difficult problem.

A recently reported atlas-guided approach [12] showed high accuracy and reproducibility for segmentation of brain compartments such as the frontal, parietal, temporal, and occipital regions, as well as subcortical brain structures such as the caudate and lenticular nuclei, the hippocampus, and other specific ROIs. This method works as follows. Global volumetric distributions for WM, GM, and CSF are obtained with the adaptive Bayesian segmentation technique (refer to Section 3) and then individually normalized into the Talairach space [35], preserving the tissue volumetric units, with a 3D elastic warping model [39]. A coregistered (in Talairach space) digital atlas of neuroanatomy [42] is then used to transfer the labels and guide the volumetric segmentation of the normalized data. The accuracy of this segmentation approach lies in the precision of the warping transformation between the brain data and the prelabeled atlas. Figure 4 illustrates how the method works.

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