Biomedical Engineering Reference
In-Depth Information
circulating compounds may have influenced the flow fluctuations
even after replacement of withdrawn blood. Further, it is also pos-
sible that the return to normal levels may be beyond the measure-
ment window of the present study.
4.2. Dependence
of BOLD Signal
Fluctuations on CBF
and CBV
LDF studies indicate that a decrease in MAP to threshold of
autoregulatory limits does not decrease mean blood flow val-
ues but increases amplitude of CBF fluctuations, which fluctuates
across the same mean value prior to drop in MAP
(5, 43)
.Fur-
thermore, spontaneous fluctuations in BOLD and CBF signals
and their dependence on MAP have been observed under differ-
ent anesthesia
(43, 44)
. The hypocapnia dependent modulation
in the amplitude of the low frequency BOLD signal fluctuations,
suggests that they are strongly connected to spontaneous CBF
fluctuations that have a similar dependence on MAP
(5)
.
BOLD signal response when acquired with a gradient-echo
sequence is sensitive to vascular caliber and density. Differences
in metabolic regulation and vascular density in different regions
affect the BOLD contrast to noise ratio (
Fig. 12.1h)
. The rat
cortex has a larger blood volume than the thalamus and other
deeper structures of the brain
(45)
. The Fourier power of the
low frequency physiological fluctuations in the different anatom-
ical regions and their enhancement in response to exsanguination
were cerebral cortex
caudate putamen
(
Fig. 12.2
), following a vascular density or CBV weighted
dependence of the BOLD signal fluctuations. The enhancement
in the amplitude of the low frequency fluctuations in BOLD sig-
nal with a decrease in intravascular pressure and tone similar to
LDF fluctuations during exsanguination
(5)
indicates a strong
link between fluctuations in brain oxygenation and CBF fluctu-
ations in the microvascular network. While this suggests a myo-
genic component in the generation of the observed low frequency
fluctuations in BOLD signal, some of the results also support
underlying neural activity. Though simultaneous measurements of
neuronal activity were not carried out in the present study, direct
measurement of neuronal signals (spike rate, LFP or EEG) from
both hemispheres of the sensorimotor cortex during exsanguina-
tion would help clarify the myogenic and neuronal contribution
to the observed BOLD signal fluctuations.
>
hippocampus
>
thalamus
>
4.3. Implications
for Resting State
Connectivity
Connectivity maps in humans have been generated by cross-
correlating the signal time course of every voxel in the brain with
a seed voxel chosen from a region where brain activation from
the respective sensory or motor stimuli is expected
(26, 46, 47)
.
Using a similar analysis, during normal resting conditions in the
anesthetized rat, cross-correlating a seed voxel from a region
of interest encompassing the sensorimotor cortex, hippocampus
