Biomedical Engineering Reference
In-Depth Information
98
1%. This was accompanied by an enhancement in the mag-
nitude of the power spectrum in the low frequency range. Two
of the rats studied showed a very minimal (2%) decrease in the
mean baseline BOLD signal intensity during exsanguination while
five rats showed no significant change. Frequencies centered at
0.02, 0.03, 0.07, 0.10 and 0.125 Hz were significantly enhanced.
Figure 12.1b,e
shows the spatially averaged power spectrum of
the BOLD signal and the low-pass filtered BOLD signal time
series respectively from the whole brain in a typical rat during
exsanguination.
Figure 12.1c,f
shows the average power spectrum and the
low-pass filtered BOLD signal time series from the whole brain
in the same rat after blood replacement. The total time duration
between blood withdrawal and replacement was around 30
minutes in all experiments. As indicated by the standard deviation
maps from a typical rat, the enhancement in BOLD signal
fluctuations was the maximum in cerebral cortex (
Fig. 12.1i
).
Withdrawn blood volume, when replaced, led to a partial recov-
ery of the BOLD signal fluctuations (
Fig. 12.1j
). An interesting
observation was the enhancement in the fluctuations in the very
low frequency range below 0.01 Hz after replacement of blood
(
Fig. 12.1c
).
The regional distribution of the low frequency BOLD fluc-
tuation was analyzed from specific anatomical regions namely
cerebral cortex, caudate putamen, hippocampus and thalamus
traced according to the rat stereotaxic atlas
(32)
. In any typ-
ical rat, the spatially averaged Fourier power of the low fre-
quency BOLD fluctuations in the different anatomical regions
were cerebral cortex
±
caudate putamen
(
Fig. 12.2a-d
). The same order in the Fourier power over
anatomical regions was observed over all rats. Exsanguination also
led to an increased power in most frequencies below 0.1 Hz over
all rats.
Temporal characteristics and spatial distribution of the low
frequency physiological fluctuations were analyzed using seed
voxels within the previously described anatomical regions of inter-
est (ROI). Randomly selected seed voxels within each ROI were
correlated with voxel time courses from the whole brain after
low-pass filtering (cutoff frequency of 0.1 Hz).
Figure 12.3a,c
shows typical activation maps obtained by correlating the time
course of a seed voxel from the sensorimotor cortex, hippocam-
pus and thalamus with all other regions of the brain. Though
small in area, highly correlated clusters were observed across the
sensorimotor cortex from either hemisphere with the seed voxel
chosen from the sensorimotor cortex (
Fig. 12.3a
). However,
correlated clusters were not detectable with seed voxels chosen
from the hippocampus or thalamus (
Fig. 12.3b,c)
. Exsanguina-
tion, which increased the amplitude of BOLD signal fluctuations
>
hippocampus
>
thalamus
>
