Biomedical Engineering Reference
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and thalamus with the rest of the brain indicated sparse acti-
vation with coherent voxels in both hemispheres of the rat
cortex (
Fig. 12.3a-c)
. Exsanguination led to an increase in
the number of correlated voxels across both hemispheres of the
brain (
Fig. 12.3d-f)
. The spatial extent of the correlation maps
was dependent on the anatomical region of choice of the seed
voxels that were used for the cross-correlation indicating distinct
temporal characteristics of the fluctuations. The correlation maps
indicate mostly similar phase characteristics of the low frequency
BOLD signal fluctuations in the cortex and thalamus but a
different phase in the hippocampus (compare
Fig. 12.3d,e,f
).
Most of the cortical regions were negatively correlated with the
hippocampal seed voxel (
Fig. 12.3e)
. Within the sensitivity of
the present experimental protocol, the distinct phase synchrony
observed in the hippocampus may represent contribution from
specific extravascular factors, which may be neurometabolic
and/or neural signaling in origin. Thus, hypotension near the
autoregulatory limits can sufficiently enhance the MR-sensitivity
and, to a certain extent, may reflect the 'resting state' functional
networks in the anesthetized rat model.
Aliasing of frequencies greater than the critically sampled ones
can be a concern in the low frequency region. The frequencies
of respiration and heart rate in the isoflurane-anesthetized rats
were in the region of 1.2 Hz and over 5 Hz respectively and
the TR has to be less than 100 ms to theoretically avoid aliasing
(48)
. Recently, using very low and high sampling rates (TR
=
125
ms and TR
3 s), DeLuca et al.
(49)
have studied the aliasing
effects of physiological processes such as cardiac and respiratory
fluctuations on the resting state fluctuations in the BOLD sig-
nal. The cardiac and respiration frequencies were spatially distinct
from the resting state functional network and do not significantly
affect the low frequency BOLD fluctuations. In order to mini-
mize any aliasing effects, low-pass filtering (0.1 Hz cutoff) was
applied to all data before the correlation analysis. Further, spatial
distribution of prominent frequencies in the range between 0.01
and 0.1 Hz was ubiquitous across various anatomical regions with
a strong presence in the cortex. No distinct presence of specific
frequencies was observed in any one anatomical region either dur-
ing normal or exsanguinated conditions (
Fig. 12.4
). As much
of the fluctuation signal is located over multiple frequencies in
the superficial part of the cortex, a strong fluctuation signal is
also consistent on the base of the brain, midline and third ven-
tricle (
Fig. 12.4
). Ventricular spaces are susceptible to artifacts
from CSF pulsations from high frequency sources such as respira-
tory and cardiac cycles that may be aliased into the low frequency
region in the power spectrum.
It is interesting to note, however, that a similar high corre-
lation of the low frequency BOLD signal fluctuations in both
=
