Types of Atlases

3.1 MRI

Beyond the anatomic atlases based upon postmortem and histologic material mentioned previously, the application of magnetic resonance to acquire detailed descriptions of anatomy in vivo is a driving force in brain mapping research. MRI data have the advantage of intrinsic three-axis registration and spatial coordinates [22], but have relatively low resolution and lack anatomic contrast in important subregions. Even high-resolution MR atlases, with up to 100-150 slices, a section thickness of 2 mm, and 2562 pixel imaging planes [30, 58], still result in resolutions lower than the complexity of many neuroanatomic structures. However, advances in the technology continue to push improvements in spatial and contrast resolution. A recent innovation in the collection of atlas-quality MRI involves the averaging of multiple coregistered scans (N = 27) from a single subject to overcome the lack of contrast and relatively poor signal-to-noise [51].

3.2 Multimodality Atlases

Characterizing a single subject with multiple imaging devices clearly combines the strengths of each imaging modality. In the Visible Human Project [96], two (male and female) cadavers were cryoplaned and imaged at 1.0 mm intervals, and the entire bodies were also reconstructed via 5000 postmortem CT and MRI images. The resulting digital datasets consist of over 15 gigabytes of image data. While not an atlas per se, the Visible Human imagery has sufficient quality and accessibility to make it a test platform for developing methods and standards [96]. The data has served as the foundation for developing related

FIGURE 1 Multimodality brain atlases. These atlases combine data from multiple imaging devices in a common coordinate space, providing a more comprehensive description of brain structure and function than can be obtained with a single modality. Brain structure can be mapped in vivo with computed tomography (CT) and magnetic resonance imaging (MRI). Full-color digital images of cryosectioned head specimens (Cryo) can be reconstructed in 3D, allowing anatomy to be delineated at an even finer scale [118]. Tissue sections can be stained histologically to reveal molecular content and regional biochemistry. Optical intrinsic signal imaging (OIS) monitors reflectance changes in electrically active cortex. Because of its high spatial and temporal resolution, OIS may complement assessments of brain function based on positron emission tomography (PET) or functional MRI. Comparison of data from multiple sources requires specialized registration approaches, which may invoke statistical dependencies between the imaging signals from different sensors [127, 130, 132]. See also Plate 104.

FIGURE 1 Multimodality brain atlases. These atlases combine data from multiple imaging devices in a common coordinate space, providing a more comprehensive description of brain structure and function than can be obtained with a single modality. Brain structure can be mapped in vivo with computed tomography (CT) and magnetic resonance imaging (MRI). Full-color digital images of cryosectioned head specimens (Cryo) can be reconstructed in 3D, allowing anatomy to be delineated at an even finer scale [118]. Tissue sections can be stained histologically to reveal molecular content and regional biochemistry. Optical intrinsic signal imaging (OIS) monitors reflectance changes in electrically active cortex. Because of its high spatial and temporal resolution, OIS may complement assessments of brain function based on positron emission tomography (PET) or functional MRI. Comparison of data from multiple sources requires specialized registration approaches, which may invoke statistical dependencies between the imaging signals from different sensors [127, 130, 132]. See also Plate 104.

atlases of regions of the cerebral cortex [27] and high-quality brain models and visualizations [91,100]. Using multimodality data from a patient with a localized pathology, and more recently the Visible Human data, Hohne and co-workers developed a commercially available brain atlas designed for teaching neuroanatomy (VOXEL-MAN; [49,50,80,117]). Data from single subjects, premortem and postmortem, provides a unique view into the relationship between in vivo imaging and histologic assessment. Mega et al. [64] scanned Alzheimer's patients in the terminal stages of their disease using both MRI and PET. These data were combined with 3D histologic images from the same subject postmortem, showing the gross anatomy [118] and a Gallyas stain of neurofibrillary tangles. This multimodal, but single subject, atlas of Alzheimer's disease relates the anatomic and histopathologic underpinnings to in vivo metabolic and perfusion maps of this disease (Fig. 2).

3.3 3D Anatomical Models

Modeling strategies currently used to represent brain data have been motivated by the need to extract and analyze the complex shape of anatomical structures, for high-resolution visualization and quantitative comparisons. Using standard 3D modeling approaches to examine often-studied structures such as the ventricles can provide a framework for mapping variation within and between different populations. Figure 3 shows models of the ventricles used to study the differences between a population diagnosed with a degenerative dementing disease and age-matched controls. Ray-tracing and surface rendering techniques can then be applied to parameterized or triangulated structure models [78,119] to visualize complex anatomic systems. An underlying 3D coordinate system is central to all atlas systems, since it supports the linkage of structure models and associated image data with spatially indexed neuroanatomic labels, preserving spatial information and adding anatomical knowledge.

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