DWI = Diffusion-weighted imaging
ADC = apparent diffusion coefficient
"DWI is sensitive to the microscopic random motion of the water
molecule protons, a value known as the apparent diffusion coefficient
(ADC), which is measured and captured by this type of imaging" Water
molecules in the brain move in the direction of the gradient of the
magnetic field that is used for this scan. "They accumulate a phase
shift in their transverse magnetization relative to that of a
stationary one, and this phase shift is related directly to the signal
attenuation of the image." ADC drops by 30-50% within 30 minutes of
the start of ischemia. DWI is very sensitive and reliable in detecting
acute ischemic stroke, even in its early stages.
PWI = Perfusion-weighted imaging
This gives information about the perfusion status of the brain, ie how
the blood is able to flow through the brain tissue.
"The most commonly used technique is bolus-contrast tracking (other
techniques include blood oxygen level and arterial spin tagging). The
imaging is based on the monitoring of a nondiffusible contrast
material (gadolinium) passing through brain tissue"
A combination of DWI and PWI has been shown to be better than ordinary
MRI in detecting the early stages of stroke and up to 48 hours after
it occurs. The two methods used together give information about where
and how large the infarct is in the brain and how it is developing.
This gives valuable help to physicians when they decide on treatment,
and also helps them to predict the likely outcome for the patient.
GRE = gradient-recalled echo imaging. DWI and PWI are not so useful in
detecting hemorrhages within the brain. GRE is a particularly
sensitive technique to detect acute intraparenchymal hemorrhage.
FLAIR = fluid attenuated inversion recovery imaging. This is another
technique that is useful to detect hemorrhage, especially acute,
subacute, and chronic subarachnoid hemorrhage, although it does show
up intraparenchymal and subdural hemorrhage as well. Areas of
hemorrhage appear as areas of high signal intensity. When there is
high signal intensity in blood vessels in the brain, this shows slow
blood flow or thrombosis. FLAIR also detects areas of recent ischemia,
which show up as "fluffy" areas of increased signal intensity.
All quoted text is from:
Magnetic Resonance Imaging (MRI) in Acute Stroke by Souvik Sen, MD,
Director, Assistant Professor of Neurology, Department of Neurology,
Division of Cerebrovascular Disease, Neuroscience Institute, JFK
Medical Center, Seton Hall University
http://www.emedicine.com/neuro/topic431.htm (This article has lots
more information and also pictures of the techniques)
Additional information from:
"Diagnostic Evaluation of Stroke" by Robert J. Wityyk, MD and Norman
J. Beauchamp, Jr., MD, MHS, Neurologic clinics, Volume 19. Number 2.
"FLAIR MR imaging in acute stroke" by S. Makkat, J.E. Vandevenne,
P.M. Parizel, A.M.A. De Schepper from Antwerp, Belgium
Search strategy on Google: 1. MRI, stroke, DWI 2. stroke, FLAIR
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