Biomedical Engineering Reference
In-Depth Information
Table 13.1
Onset time and time-to-peak for CBF and BOLD hemodynamic responses to
somatosensory stimulation in rats
CBF
∗
BOLD
∗
BOLD
†
Contrast
Cortical region
Deep
Surface
Deep
Surface
Layers I-III
Layers IV-V
Layer VI
Onset Time (s)
0.6
±
0.4 1.6
±
1.1 1.1
±
0.2 1.5
±
0.8 1.27
±
0.43 0.59
±
0.17 1.11
±
0.45
Time-to-Peak (s) 4.4
±
3.7 4.4
±
3.5 5.8
±
3.9 6.0
±
3.8 4.37
±
0.48 4.13
±
0.62 4.79
±
0.51
∗
m
3
. Temporal resolution 108 ms.
Data from Ref.
(27, 67)
. Spatial resolution: 470
×
470
×
2000
μ
†
Data from Ref.
(28)
. Spatial resolution: 200
m
3
. Temporal resolution 40 ms.
×
200
×
2000
μ
heterogeneity was maintained throughout the entire rising por-
tion of the BOLD hemodynamic response, both in amplitude
as well as in temporal aspect. Our data showed that layers IV-
V in the rat somatosensory cortex have the strongest temporal
resemblance to the stimulation paradigm, and that the onset times
from those cortical laminae are shorter than the respective laten-
cies from layers I-III and VI, suggesting that the hemodynamic
response originates in the central region of the cortex and prop-
agates up and down the cortical depth toward the supragranular
and infragranular layers
(28)
. This finding is consistent with the
hypothesis that changes in CBF start at the site of neural activity
and propagate upstream toward the feeding arterioles and arter-
ies
(69)
. Curiously, the shorter CBF and BOLD hemodynamic
responses onset times in cortical layer IV is consistent with the
expected order of neuronal current flow in the cortex
(70)
, thus
suggesting that the hemodynamic response could be preserving
the temporal order of cortical neuronal events. While CBF cannot
respond as fast to increased neuronal activity as electrical events,
still the cerebral vasculature could be constructed to follow elec-
trical activity as much as possible
(12, 16)
, making fMRI signals
useful for probing cortical activity with short latencies, such as
laminar communication.
4. Future
Applications
of DASL
The property of DASL to measure the three basic parameters
related to quantification of CBF make the technique attractive
for the measurement of hemodynamic parameters both in scien-
tific studies of human brain function as well as in clinical studies
of cerebrovascular diseases. The periodic repetition of the labeling
function gives DASL superior advantage in achieving CBF mea-
surements with high SNR due to the multiple sampling of the
