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. In general simplified terms, highly cellular tissues or those with cellular swelling exhibit lower diffusion coefficients. Diffusion is particularly useful in tumor characterization and cerebral ischemia. The term “diffusion-weighted imaging” is used to denote a number of different things:

• Isotropic diffusion map (what most radiologists and clinicians will refer to as DWI)
• The pulse sequence that results in the generation of the various images
• a more general term to encompass all diffusion techniques including diffusion tensor imaging

Diffusion-weighted imaging has a major role in the following clinical situations:

• early identification of ischemic stroke
• differentiation of acute from chronic stroke
• differentiation of acute stroke from other stroke mimics
• differentiation of epidermoid cyst from an arachnoid cyst
• differentiation of abscess from necrotic tumors
• assessment of cortical lesions in Creutzfeldt-Jakob disease (CJD)
• differentiation of herpes encephalitis from diffuse temporal gliomas
• assessment of the extent of diffuse axonal injury
• grading of diffuse gliomas and meningiomas
• assessment of active demyelination
• grading of prostate lesions (see PIRADS)
• differentiation between cholesteatoma and otitis media

The fundamental idea behind diffusion-weighted imaging is the attenuation of T2* signal based on how easily water molecules are able to diffuse in that region. The more easily water can diffuse (i.e. the further a water molecule can move around during the sequence) the less initial T2* signal will remain. For example, water within cerebrospinal fluid (CSF) can diffuse very easily, so very little signal remains and the ventricles appear black. In contrast, water within brain parenchyma cannot move as easily due to cell membranes getting in the way and therefore the initial T2* signal of the brain is only somewhat attenuated. An important consequence of this is that if a region of the brain has zero T2* signal it cannot, regardless of the diffusion characteristics of that tissue, show signal on isotropic diffusion-weighted images.

Diffusion-weighted imaging has got immense potential in the field of onco-imaging. It is easy to implement and adds very little time to a standard MR examination. Malignant lesions have lower ADC values compared to surrounding normal tissue, edema and benign tumors in brain, head and neck malignancies, prostate and liver cancer.In DWI, the signal intensity varies inversely with the freedom of water proton diffusion. Imaging of liver fibrosis is based on the postulate that the molecular structure of collagen restricts diffusion of water protons. Several studies have demonstrated the ability of DWI to detect cirrhosis; however, studies evaluating the ability of DWI to stage fibrosis have shown mixed results.Like other modalities, using diffusion as an indirect marker of fibrosis makes the technique susceptible to confounding factors such as necroinflammatory activity, steatosis, and significant iron deposits in liver tissue. DWI provides useful information, increasing the sensitivity of MRI as a diagnostic tool, narrowing the differential diagnosis, providing prognostic information, aiding the treatment planning and evaluating response to treatment.

When diffusion measurements are being performed, the direction of water diffusion along the three orthogonal directions of the magnet (phase select, frequency select, and slice select) can be assessed independently by applying diffusion gradients in each of these directions. DW images that are the sum of the directionally acquired DW images are known as trace or index DW images. Another phenomenon that can be encountered is that of diffusion anisotropy. Diffusion anisotropy refers to unequal directional diffusion, which occurs as a result of tissue or structural organization. A good example of diffusion anisotropy is seen along the white matter tracts of the internal capsule in the brain. The diffusion motion appears relatively free in the head–foot direction along the long axis of the white matter tracts, but appears restricted in the anteroposterior and right–left directions across the neuronal fibers.

In the implementation of DWI in the body, two main strategies can be pursued: breath-hold imaging and non-breath-hold imaging. Breath-hold imaging allows a target volume (e.g., liver, kidney, and elsewhere in the abdomen) to be rapidly assessed. The images retain good anatomic detail and are usually not degraded by respiratory motion or volume averaging. Small lesions may be better perceived and the quantification of ADC is theoretically more accurate than with a non-breath-hold technique. One example of such a technique is breath-hold single-shot spin-echo EPI combined with parallel imaging (e.g., sensitivity encoding) and fat suppression.

Tumors are frequently more cellular than the tissue from which they originate and thus appear to be of relatively high signal intensity (restricted diffusion) at DWI.

DWI is being applied for the detection of liver metastases. In the liver, low value images that suppress the high-signal flow from the hepatic vessels, resulting in black blood images, have been found to be useful for lesion detection

DWI has become an indispensable imaging tool. It has well established roles in the fields of stroke imaging, white matter diseases and oncology. While conventional imaging provided only anatomical information, DWI has opened a new paradigm with information about molecular activity and cellular function. Recent advances in this field have touched new horizons with the arrival of functional diffusion MRI. Researchers believe that DWI has still not achieved to its full potential and it is expected that in future DWI may be able to solve the most complex puzzles of brain functioning.