Biomedical Engineering Reference
In-Depth Information
impulse response” (HIR) function as a way to understand the
minimal evolution of vascular events so that responses to complex
neural activity can be modeled and predicted
(40-45)
.Forexam-
ple, in rats, the OIS response to a 2-s-long whisker or forepaw
stimulus begins 0.5-1 s after stimulus onset, peaks at 2.5-3 s,
and returns to baseline by 4-5 s
(40, 42-46)
. In humans, the
full-width-at-half-maximum (FWHM) is 5-7 s
(47-49)
, and can
be improved to 3-4 s if the contribution from large vessels is
removed
(50)
.
There is growing evidence indicating that the fundamental
spatial and temporal characteristics of the hemodynamic response
are fine enough to resolve subcortical activity and suggesting that
hemodynamic regulation occurs at a spatial scale that is much finer
than the resolution of typical current human or animal fMRI, and
implies that higher information content can be obtained with fur-
ther technological improvements. However, it is not clear to what
extent hemodynamic signals will be able to map elemental neu-
ronal populations, and thus continued research on understanding
the spatial and temporal evolution of the hemodynamic response
will be essential to increase the applicability of neuroimaging to
the study of functional brain organization. The ability to prop-
erly measure and quantify CBF with MRI is a major determi-
nant of progress into our understanding of brain function. In
the present work, we review the dynamic arterial spin labeling
(DASL) method to measure CBF and the CBF hemodynamic
response with high temporal resolution.
2. Dynamic
Arterial Spin
Labeling
Cerebral blood flow can be measured with MRI by using endoge-
nous arterial water as a perfusion tracer according to a number
of approaches collectively known as arterial spin labeling (ASL)
(51-54)
. The major advantages of ASL are that non-invasive
and quantitative measurements of CBF can be performed and
repeated indefinitely. Unlike other MRI methods for measuring
CBF that use exogenous contrast agents, in ASL, the tracer is
imaged against the brain tissue water background. Therefore, the
general principle of ASL is to differentiate the net magnetization
of arterial water flowing proximally to the brain from the net mag-
netization of brain tissue water. The labeled arterial water flows
through the brain, causing a net decrease in magnetization due to
its mixing (with or without exchange) with the brain tissue water,
which is proportional to the flow rate and therefore it may be used
to calculate CBF in the conventional units of
ml blood
100
g tissue
min
.
In addition, it is necessary to acquire two images, usually in an
interleaved manner, to determine CBF: one with spin labeling,
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