(DWI)
Magnetic resonance imaging is sensitive to
diffusion, because the
diffusion of water molecules along a field
gradient reduces the MR signal. In areas of lower
diffusion the signal loss is less intense and the display from this areas is brighter. The use of a
bipolar gradient pulse and suitable
pulse sequences permits the acquisition of
diffusion weighted images (images in which areas of rapid
proton diffusion can be distinguished from areas with slow
diffusion).
Based on
echo planar imaging,
multislice
DWI is today a standard for
imaging brain infarction. With enhanced gradients, the whole
brain can be scanned within seconds. The degree of
diffusion weighting correlates with the strength of the
diffusion gradients, characterized by the
b-value, which is a function of the
gradient related parameters: strength, duration, and the period between
diffusion gradients.
Certain illnesses show restrictions of
diffusion, for example demyelinization and cytotoxic edema. Areas of cerebral infarction have decreased apparent
diffusion, which results in increased
signal intensity on
diffusion weighted
MRI scans.
DWI has been demonstrated to be more sensitive for the early detection of stroke than standard pulse
sequences and is closely related to temperature mapping.
DWIBS is a new
diffusion weighted
imaging technique for the whole body that produces PET-like images. The
DWIBS sequence has been developed with the aim to detect lymph nodes and to differentiate normal and hyperplastic from metastatic lymph nodes. This may be possible caused by alterations in microcirculation and water diffusivity within cancer metastases in lymph nodes.
See also
Diffusion Weighted Sequence,
Perfusion Imaging,
ADC Map,
Apparent Diffusion Coefficient, and
Diffusion Tensor Imaging.
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