Biomedical Engineering Reference
In-Depth Information
NIRS optode array was placed such that the optical signals were
sampled from the frontal and temporal lobes which are respec-
tively supplied by the anterior (ACA) and medial (MCA) cerebral
arteries (
Fig. 2.7
, left panel). This arrangement allowed for map-
ping across the borderline zones in between adjacent lobes known
to be susceptible to hypotensive provocation
(30)
, and ischemia,
both having severe clinical consequences
(31, 32)
. As seen in the
right panel of
Fig. 2.7
,
β
is significantly lower within the bor-
derline MCA territory than in the surrounding areas of the brain
cortex
(32)
. This regional difference in temporal complexity is
interpreted as a manifestation of an altered vasomotion activity
of the small arteries within the borderline MCA territory which
is less synchronized than in the neighboring regions. A less syn-
chronized vasomotion results in a less efficient “arterial pump”,
which otherwise could aid local perfusion when perfusion pres-
sure drops to critically low levels and this can lead to susceptibil-
ity of these borderline zones to ischemic insults - a well known
clinical scenario
(30, 31)
.
8. Towards
4 Dimensional
(Spatiotemporal)
Description of
Brain
Hemodynamics
Our optical approaches to assess dynamic complexity utilizing
surface measurements by LDF, LSI, and NIRS imaging did not
allow resolving the cerebrovascular dynamics in deeper regions of
the brain. For this purpose, fMRI was pursued (
Fig. 2.8
)
(17)
.
Out of the various MRI modalities, the gradient echo or BOLD
weighted EPI maps were analyzed. Coronal scanning across the
rat brain was carried out under normoxic, resting conditions when
the main determinant of the signal is local perfusion or fluctua-
tions in tissue oxygenation due to perfusion. As observed by the
Hurst exponent parametric map (
Fig. 2.8
, right panel), similar
to the optical modalities, the BOLD signal also shows sponta-
neous fluctuations at rest across the brain.
H
is around 0.5 indica-
tive of an uncorrelated pattern (random) within the white matter
where blood flow is lower than in gray matter, while
H
is much
higher in the cerebral cortex and in deep nuclear regions where
blood flow is much higher. It is worth noting, that the ranges
of
H
are comparable within the cortical microareas of the brain,
using either the optical imaging or magnetic resonance modali-
ties. Both of these signal modalities are of a correlated fGn tem-
poral fractal with
H
may have infer-
ence to regionally different metabolic activities, via differences in
regional neuronal firing rates mapped into the regional spectra of
the metabolically driven BOLD fluctuations.
Optical imaging and magnetic resonance are both suitable to
collect high definition spatiotemporal data sets (volumes) from
∼
0.7. Spatial differences in
β
