Chemistry Reference
In-Depth Information
Although the specific biophysical mechanisms in the proposed hypotheses dif-
fer, they are all based on the assumption that the ADCs are different in the
intra- and extracellular spaces. This two-compartment (i.e., biexponential model
Section II.H) interpretation of DW-MRI experiments was earlier suggested by
Niendorf et al. [41]. Using very high diffusion sensitivities, a double-exponential
relationship of the signal- vers us-
b
curve was observed in rats, with one fast and
one slow diffusion component.
Recent work by Latt et al. [86] investigated the dependence on the diffusion time
of a plot of image signal with
b
-value, based on the assumption that the curves
originate from different compartments. This time-dependence should be due to
restricted diffusion within the compartments and/or water exchange between them
through permeable membranes. Their study showed that the signal versus
b
curves
in subacute ischemic lesions show a diffusion time (
T
D
) dependence, as measured
with a clinical MRI scanner. They conjecture that the observed signal curve split
up (a reduced normalized signal intensity at the longer
T
D
) is an effect of exchange
between different water pools with different ADCs. A similar result was observed
by Pfeuffer et al. [87] in the rat brain
in vivo
.
The DWI [88] has been extensively used to detect acute ischemic brain injury
much earlier after onset of the ischemic injury than conventional MRI is capable
of. It makes it possible to distinguish acute from chronic ischemic changes, which
may have an impact on treatment. In the acute phase, ADC are initially reduced
by 30-50% within 30 min of the onset of focal ischemia [89], more severely in the
WM than in the GM in acute and early subacute infarcts [90]. The ADC threshold
values are useful in predicting tissue viability and stroke outcome [91].
2. Diffusion Tensor Imaging
The characterization of tissue ADC depends both on experimental parameters
and tissue microstructure. In tissue, such as gray matter, the scalar magnitude
of the ADC is independent of the direction in which the diffusion sensitizing
gradient is applied [31]. Other tissue types, such as white matter or skeletal muscle,
demonstrate anisotropic water diffusion, and therefore a strong dependence on the
direction of the applied gradient [7]. In WM, the angle between the fiber tract axis
and the applied field gradient is the critical factor in determining the magnitude
of the ADC and the maximum ADC is measured when the diffusion gradient is
applied parallel to this axis.
Diffusion tensor imaging exploits this characteristic of white matter fibers, by
measuring the proton diffusion along many different directions [92]. Diffusion
weighted acquisitions along a minimum of six directions are required, and the
tensor estimate is improved as the number of directions increases. Gradients with
directions other than
x
,
y
, and
z
are achieved by combinations of these three
gradients. The diffusion tensor is a 3
×
3 matrix with six unique elements. The
