Biomedical Engineering Reference
In-Depth Information
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Optimization procedure:
What algorithm will be used to search the
space of possible spatial mapping functions to determine which one yields
the maximum similarity?
These aspects will be discussed in the following sections.
14.2.1
The Reference Space
should minimize the positional variability of homol-
ogous anatomical (or functional) features after mapping data from different
subjects into the space. This frame of reference should arise directly from
a simple statistical analysis of the features. However, determination of this
space is dependent on the features selected in addition to the four aforemen-
tioned aspects of the spatial normalization procedure. This said, we will
describe a well known, practical reference space used for neuroscientific
research. Other reference spaces will be discussed in Section 14.2.5.
The ideal
reference space
14.2.1.1
Talairach Stereotaxic Space
Within the brain mapping community, the
standard reference frame is
based on the brain-based coordinate system first described by the French neuro-
surgeon Talairach.
de facto
2,30
This reference system provides a method for identifying
the location of a structure so that regions of interest can be compared between
brains using standard coordinates. Although originally developed to target
deep brain structures for stereotaxic neurosurgical procedures using pneu-
moencephalography, the Talairach stereotaxic system has become an interna-
tional standard for reporting the coordinates of brain locations obtained in
functional activation studies. This system has facilitated the development of
BrainMap, a database of spatially indexed functional brain data.
31
defined by Talairach is based on the identification of a
midplane line passing through the superior aspect of the anterior commis-
sure (AC) and the inferior aspect of the posterior commissure (PC), thus
defining the so-called AC-PC line. The origin of the space is defined by the
intersection of a vertical perpendicular line with the AC-PC line, passing
through the posterior aspect of the AC. This perpendicular is in the mid-
plane and is known as the VAC line. The coordinate system follows the con-
vention that the
The
stereotaxic space
x
-axis is in the left-right (LR) direction (positive towards the
right), the
-axis is in the posterior-anterior (PA) direction (positive anteri-
orly), and the
y
-axis is in the caudo-cranial (CC) direction (positive superiorly).
Individual data sets can be compared with the Talairach atlas by identify-
ing the AC-PC and VAC lines within the given volume so that they can be
aligned with the atlas (thus defining the
z
). In
order to account for different brain sizes, proportional scaling is used to par-
tition the brain into three piecewise linear components in the PA direction
(pre-AC, AC-PC, post-PC), two in the CC direction (above
Talairach spatial mapping function
below AC-PC),
and two in the LR direction (one for each hemisphere).
