Biomedical Engineering Reference
In-Depth Information
While resting state connectivity maps were generated to show
the spatial location of regions that correlated significantly with a
representative time course in the brain, they do not necessarily
provide much information regarding the interaction between
the different subregions. Connectivity patterns between differ-
ent regions of interest (both for the visual and sensorimotor sys-
tems) were obtained using cross-correlation analysis. For each
subregion chosen for the sensorimotor or visual systems, prin-
cipal component analysis was computed
(36)
. Using PCA, a set
of orthogonal data sets based on their energy content was deter-
mined. Briefly, PCA is a multivariate technique that replaces the
measured variables by a new set of uncorrelated variables (prin-
cipal components), arranged in the order of decreasing standard
deviation (SD) or energy distribution. For this study, the first two
components accounted for more than 75% of the standard devi-
ation for each region of interest and were therefore used. The
resulting time series (consisting of the first PCA) for each subre-
gion was correlated with every other time component to obtain a
pair-wise correlation matrix.
While significant temporal correlation typically represents
similar time-series structure, it does not give any information
regarding the frequency components that may give rise to
high correlation coefficients. Therefore, in addition to cross-
correlation, coherence maps were generated from which the con-
tribution of specific frequencies could be estimated and compared
with the temporal correlation map.
3. Results
BOLD weighted MR images at 9.4 T were obtained in isoflurane-
anesthetized rats during various physiological conditions using a
gradient echo EPI sequence. During normal physiological condi-
tions, all animals imaged had MAP values ranging between 90 and
120 mmHg with baseline fluctuations in the fMRI-BOLD signal
intensity in the frequency range 0.01-0.125 Hz.
Figure 12.1a,d
shows the spatially averaged power spectrum and the low-pass fil-
tered BOLD signal time series from the whole brain during nor-
mal physiological condition for a typical rat.
After MR measurements during normal physiological condi-
tion, rats were exsanguinated where a small volume (8 ml/kg)
of blood was gradually withdrawn from each animal resulting
in hypovolemia and hypotension. MAP dropped from the nor-
mal control value of 110
7 mmHg during exsan-
guination in all rats and led to an enhancement in the amplitude
of the low frequency BOLD fluctuations. No significant change
was observed in S
a
O
2
during exsanguination which remained at
±
10
−
68
±
