Susceptibility weighted imaging

Magnetic resonance imaging technique

Susceptibility weighted imaging (SWI) is a magnetic resonance imaging (MRI) technique that exploits the magnetic susceptibility differences of various tissues, such as blood, iron, and calcification, to generate high-contrast images. This technique is particularly useful in detecting microbleeds, venous structures, and other pathologies that may not be visible with conventional MRI sequences.

Principles

SWI is based on the principles of magnetic susceptibility, which refers to the degree to which a material can be magnetized in an external magnetic field. In SWI, the phase information from a gradient echo sequence is used to enhance the contrast of tissues with different susceptibilities. The technique involves the acquisition of both magnitude and phase images, which are then combined to produce a susceptibility-weighted image.

The phase images are particularly sensitive to the presence of paramagnetic substances such as deoxygenated blood and iron deposits. By applying a high-pass filter to the phase images, SWI can highlight these substances, providing a unique contrast that is not available in conventional MRI.

Applications

SWI is used in a variety of clinical and research settings. Some of the key applications include:

Neuroimaging

SWI is highly effective in detecting cerebral microbleeds, which are small chronic brain hemorrhages often associated with cerebral amyloid angiopathy and traumatic brain injury. It is also used to visualize venous structures and cerebral veins, which can be important in the assessment of venous thrombosis and other vascular disorders.

SWI image at 4 Tesla

Trauma

In cases of traumatic brain injury, SWI can detect small hemorrhages and diffuse axonal injury that may not be visible on conventional MRI sequences. This makes it a valuable tool in the assessment of head trauma.

Comparison of SWI and GRE in trauma

Further comparison of SWI and GRE in trauma

Neurodegenerative Diseases

SWI is used in the study of neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease, where it can detect iron deposition in the brain, which is thought to play a role in the pathophysiology of these conditions.

Vascular Malformations

SWI is useful in the evaluation of vascular malformations such as cavernous hemangiomas and arteriovenous malformations. It can provide detailed information about the venous architecture and the presence of associated hemorrhages.

SWI of Sturge-Weber syndrome

Other Applications

SWI is also used in the assessment of brain tumors, stroke, and multiple sclerosis. In stroke, it can help identify areas of hemorrhagic transformation. In multiple sclerosis, it can detect iron deposition in the basal ganglia and other regions.

Technique

The SWI technique involves the use of a high-resolution, three-dimensional, fully flow-compensated gradient echo sequence. The images are acquired with a long echo time to enhance the susceptibility effects. The phase images are processed to create a phase mask, which is then multiplied with the magnitude images to produce the final SWI.

Negative phase mask in SWI

Advantages and Limitations

SWI offers several advantages over conventional MRI techniques, including its ability to detect small hemorrhages and venous structures with high sensitivity. However, it also has limitations, such as susceptibility to artifacts from air-tissue interfaces and the need for specialized post-processing techniques.

Related pages

• Magnetic resonance imaging
• Gradient echo
• Cerebral microbleeds
• Traumatic brain injury
• Cerebral amyloid angiopathy