Currently, gadolinium bolus perfusion
imaging is the most widely used MR method for detecting perfusion of the
brain parenchyma.[30] This technique documents the relative perfusion
of different brain areas and has proven particularly sensitive to
changes in focal perfusion. Both hypoperfusion resulting from arterial
thromboembolism and hyperperfusion from revascularization can be
detected, introducing the interesting possibility of this technique
being used not only to detect ischemia but also to monitor treatment
outcome.
Technique. Perfusion imaging uses the intravenous contrast
agent gadolinium to assess tissue perfusion (Fig. 1). Contrast is
administered as a rapid bolus into a peripheral vein and arrives at the
cerebral capillaries about 20 seconds after injection, the exact time
depending both on the patient's cardiac output and cerebral blood flow
(CBF). A rapid MR sequence (T2*-weighted gradient-echo echo-planar
sequence) is repeated every few seconds as the bolus of contrast passes
through the cerebral vasculature, allowing the first-pass effects of the
contrast on the parenchyma to be observed.
Gadolinium is commonly used in other forms of MR imaging, but its use in
perfusion imaging is unique. Most contrast-enhanced imaging is
performed with T1-weighted imaging, using the T1-shortening effects of
gadolinium. Perfusion imaging, however, is performed with T2*-weighted
imaging. Unlike MR angiography, which detects intravascular contrast, or
T1-weighted parenchymal imaging, which documents leakage of contrast
from the blood-brain barrier (BBB), perfusion imaging detects the local
effects of intravascular contrast on the surrounding tissue. The
contrast remains confined to the capillaries, but its paramagnetic
effect causes inhomogeneity of the local magnetic field. As a result,
signal from tissue immediately surrounding the vessel decreases.
Integrity of the BBB is important because leakage of gadolinium into the
tissues can prolong this effect. The decrease in signal depends both on
the vascular concentration of contrast and the concentration of small
vessels within the imaged area.[3]
Perfusion Variables. From the drop in MR signal observed during
a contrast perfusion study, it is possible to calculate the changing
concentration of gadolinium within a voxel (proportional to 1/T2*). This
knowledge can be used to calculate a number of physi ological variables
of interest using tracer kinetic theory. The most commonly used
variable in gadolinium perfusion imaging is the relative mean cerebral
blood volume (rCBV). This value is calculated easily from a graph of
gadolinium concentration as a function of time, as the area under the
curve. Other variables that can be calculated include the time from
injection to peak signal drop (time to peak, TTP) and the time for
contrast to pass through a voxel (relative mean transit time, rMTT).[3]
These nonquantative variables cannot provide absolute measures of blood
flow, like xenon or perfusion CT. A separate measure of blood velocity
is required to obtain an accurate measurement of CBF. Several techniques
have been suggested,[19] but whether the information gained from CBF
will be superior to rCBV is uncertain.
Imaging Cerebral Ischemia. Focal hypoperfusion from arterial
thrombo embolism decreases both CBV and CBF and increases mean transit
time. It remains unclear which of these variables will prove most useful
in clinical practice. Early results have suggested that a low rCBV may
most accurately predict eventual infarct volume, while infarct growth
may be best demonstrated by the difference between low CBF on perfusion
imaging and diffusion restriction on diffusion-weighted imaging.[24]
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