Diffusion-weighted magnetic resonance imaging

Medical imaging technique

Diffusion-weighted magnetic resonance imaging (DW-MRI or DWI) is a specialized form of magnetic resonance imaging (MRI) that is particularly sensitive to the random Brownian motion of water molecules within a voxel of tissue. This technique is widely used in the field of neuroimaging and has applications in the diagnosis and monitoring of various medical conditions, particularly in the detection of acute ischemic stroke.

Principles of Diffusion-weighted Imaging

Illustration of diffusion tensor imaging

Diffusion-weighted imaging exploits the diffusion properties of water molecules to generate contrast in MR images. In biological tissues, water diffusion is not free but is restricted by cellular structures such as membranes and macromolecules. DWI measures the apparent diffusion coefficient (ADC), which reflects the degree of water diffusion within tissue.

The basic principle involves applying a pair of diffusion-sensitizing gradients in the MRI sequence. These gradients dephase and then rephase the spins of water protons. If water molecules have moved between the application of the gradients, the rephasing is incomplete, resulting in signal attenuation. The degree of attenuation is proportional to the amount of diffusion.

Applications in Medicine

Cerebral infarction after 4 hours on ADC MRI

DWI is particularly useful in the early detection of stroke, as it can identify ischemic changes within minutes of onset. In acute ischemic stroke, the diffusion of water is restricted due to cytotoxic edema, leading to a high signal on DWI and a low ADC value.

Beyond stroke, DWI is used in the evaluation of brain tumors, abscesses, and traumatic brain injury. It is also valuable in assessing the integrity of white matter tracts through diffusion tensor imaging (DTI), a related technique that provides information about the directionality of water diffusion.

Technical Considerations

DTI axial ellipsoids

The quality of DWI can be affected by several factors, including the strength and timing of the diffusion gradients, the choice of b-value (a parameter that reflects the sensitivity of the sequence to diffusion), and the presence of artifacts such as eddy currents and susceptibility effects.

Advanced techniques such as echo-planar imaging (EPI) are often used to acquire DWI data rapidly, minimizing motion artifacts. However, EPI is sensitive to magnetic field inhomogeneities, which can lead to geometric distortions.

Diffusion Tensor Imaging

Diffusion tensor imaging (DTI) is an extension of DWI that models the diffusion process as a tensor, allowing for the characterization of anisotropic diffusion. This is particularly useful in mapping the orientation of white matter tracts in the brain, providing insights into the structural connectivity of the brain.

Related Pages

• Magnetic resonance imaging
• Neuroimaging
• Stroke
• Brain tumor
• Diffusion tensor imaging

Cerebral infarction after 4 hours on DWI MRI

MRI DWI sequence showing restricted diffusion in the mesial dorsal thalami