Diffusion-weighted imaging

Diffusion weighted imaging (DWI) is a form of MR imaging based upon measuring the random Brownian motion of water molecules within a voxel of tissue. The relationship between histology and diffusion is complex, however generally densely cellular tissues or those with cellular swelling exhibit lower diffusion coefficients, and thus diffusion is particularly useful in tumour characterisation and cerebral ischaemia. 

Terminology

A great deal of confusion exists in the way the clinicians and radiologists refer to diffusion restriction, with both groups often appearing to not actually understand what they are referring to.

The first problem is that the term "diffusion weighted imaging" is used to denote a number of different things: 

1. isotropic diffusion map (what most radiologists will refer to as DWI)
2. sequence which results in generation of DWI, b=0 and ADC maps
3. a more general term to encompass all diffusion techniques including diffusion tensor imaging

Additionally confusion also exists in how to refer to abnormal restricted diffusion. This largely stems from the initial popularisation of DWI in stroke, which presented infarcted tissue as high signal on isotropic maps and described it merely as "restricted diffusion", implying that the rest of the brain did not demonstrateddemonstrate restricted diffusion, which is clearly not true. Unfortunately this short-hand is appealing and widespread rather than using the more accurate but clumsier "diffusion demonstrates greater restriction than one would expect for this tissue".

To make matters worse many are not aware with the idea of T2 shine-through, another cause of high signal on DWI.

A much safer and more accurate way of referring to diffusion restriction is to remember that we are referring to actual ADC values, and to use wording such as "the region demonstrates abnormally low ADC values (abnormal diffusion restriction)" or even "high signal on isotropic images (DWI) is confirmed to represent abnormal restricted diffusion on ADC maps". 

Physics

As opposed to free diffusion of water kept inside a container, diffusion of water inside a voxel of brain tissue, for example, is hindered primarily by cell membrane boundaries, and thus represents the combined water diffusion in a number of compartments: 

• diffusion within the intracellular fluid
  • within the cytoplasm generally
  • within organelles
• diffusion within extracellular fluid
  • interstitial fluid
  • intravascular
  • lymphatic
  • various biological cavities e.g. ventricles of the brain
• diffusion between intra and extracellular compartments

The contribution of each one of these will depend on the tissue and pathology. For example, in acute cerebral infarction it is believed that the decrease in ADC values is the result of a combination of water moving into the intracellular compartment (where it's diffusion is more impeded by organelles than it is in the extracellular space) and the resulting cellular swelling narrowing the the extracellular space ⁶. Similar mechanisms result in low ADC values in highly cellular tumours (e.g. small round blue cell tumours (e.g. lymphoma / PNET) and high grade gliomas (GBM)).  

Clinical application

DW imaging has a major role in the following clinical situations ^3-5: 

• early identification of ischemic stroke
• differentiation of acute from chronic stroke
• differentiation of acute stroke from other stroke mimics
• differentiation of epidermoid cyst from arachnoid cyst
• differentiation of abscess from necrotic tumors
• assessment of cortical lesions in CJD
• differentiation of herpes encephalitis from diffuse temporal gliomas
• assessment of the extent of diffuse axonal injury
• grading of gliomas and meningiomas (need further study)
• assessment of active demyelination 

MRI sequence

Figure 1 depicts a spin echo sequence with diffusion gradients added. The gradient coil used to produce the diffusion need not be a separate gradient or gradients from those used for spatial encoding. The degree of diffusion weighting is dependent primarily on the area under the diffusion gradients and on the interval between the gradients. Other factors include the effect of the spatial localization gradients and the size of the voxels. 

• stationary water molecule - unaffected by the diffusion gradients and hence retain their signal.
• moving water molecule - acquire phase information by the first gradient but are not rephased by the second, hence losing their signal.

See also

• b values
• apparent diffusion coefficient
• diffusion tensor imaging
• diffusion weighted MRI in acute stroke
• restricted diffusion in the basal ganglia
• intracranial lesions which show restricted diffusion