Biomedical Engineering Reference
In-Depth Information
anatomical connections. The most highly acclaimed neurobiology textbooks con-
sider the vertical organization of the cortex to be a basic concept of neurobiology,
commonly referring to it as the columnar hypothesis [3]. Minicolumns are defined
as vertical organizations of neurons arranged in the neocortex. They grow in the
gray matter and communicate through the white matter. Minicolumns, rather than
the neurons, are believed to be the basic units of the brain in the sense that the en-
largement of the neocortex, in the phase of the brain development, is characterized
by an increase in the number of minicolumns rather than that of single neurons. In
the aging phase, the neocortex loses whole minicolumns, not randomly scattered
neurons. This is clearly illustrated in Figure 1, where the minicolumnar structure
for a 9-year-old child and a 67-year-old adult are depicted. It is quite obvious that
the neurons are lost within columns, not randomly. Any disturbance in the number,
structure, or organization of minicolumns will affect the development of the brain
and, as a result, its functionality. According to this, we hypothesize that the brain
developmental disorders and aging disorders can be diagnosed and analyzed in
terms of an analysis of the number as well as organization of the minicolumns in
the neocortex. These changes are expected to be captured
at a different scale
by the
Magnetic Resonance Images or the Diffusion Tensor Images. Consequently, level
sets and deformable models play a major role in analyzing the structural changes
of minicolumns, which allows one to draw conclusions and performmeasurements
regarding disease analysis and diagnosis. On one hand, there have been several
MRI studies in this field [5, 18, 19]. For example, Herbert et al. [18] conducted
a study to analyze autism and developmental language disorder through the vol-
umes of the white matter in several brain regions: the frontal lobe, parietal lobe,
temporal lobe, and occipital lobe (for the superficial layer), the corpus callosum,
cingulum, basal forebrain, and internal capsule (for the deep structures). Eliez et
al. [5] studied morphological alteration of temporal lobe gray matter in dyslexia,
and they also made some volumetric analysis for the whole brain tissues as well as
for different lobes. Chung et al. [19] recently presented analysis of cortical thick-
ness in autism studies. On the other hand, Diffusion Tensor Magnetic Resonance
Imaging (DT-MRI) has recently emerged as a noninvasive imaging modality that
can yield rich information about the white matter structure in the human brain and
aid in inferring the architecture of the underlying structure [20, 21]. The DT-MRI
technique was first introduced in the mid1980s [22, 23]. As of today it is the unique
noninvasive technique capable of probing and measuring the anisotropic diffusion
properties of water molecules in living tissues like brain or muscles [24]. The
Diffusion Tensor (DT) provides information about the intensity of water diffusion
in any direction at a certain point. In the specific case of the brain, the cellular
organization, in particular axonal cell membrane and myelin sheath, highly affects
water mobility. Hence, the measured DT becomes highly anisotropic and oriented
in areas of compact nerve fiber organization, providing an indirect method for
fiber tract identification. Motivated by this advantage of DT-MRI over traditional
MRI, numerous works have addressed the study of white matter diseases (such as
