Susceptibility weighted imaging (SWI), originally called BOLD venographic imaging is an imaging technique developed by Haacke et al in 2004. SWI is a fully velocity compensated high-resolution 3D gradient-echo sequence that uses magnitude and filtered-phase information to create a different form of contrast. 

This technique exploits the susceptibility differences in tissues such that signal from substances with different susceptibilities than their neighbouring tissues (such as venous blood or haemorrhage) will become out of phase. This special data acquisition and image processing produces an enhanced contrast magnitude image which is exquisitely sensitive to venous blood, haemorrhage and iron storage.  SWI offers the potential for:

• better diagnosis of disease and effect on patient management and prognosis.

• better follow up for longitudinal studies

• to be involved in clinical research

SWI has been found to provide additional clinically useful information that is often complementary to conventional MR imaging sequences.

Various applications of SWI:

1. Small hemorrhagic shearing lesions in DAI are only visible by using SWI.
2. More sensitive in detecting hemorrhage inside acute infarct and identifying the microangiopathic changes in Cerebral amyloid angiopathy (CAA), CADASIL and cerebral fat embolism.
3. Early detection of Sturge Weber Syndrome by depiciting abnormal transmedullary veins.
4. Distinguishing hemorrhage from tumoral veins if SWI is used both before and after administration of a contrast agent.
5. Venous connectivity and evidence of iron deposirion in Multiple sclerosis lesions.
6. Differentiating calcification from hemorrhage.
7. Increased iron deposition as in Parkinson disease, Multiple sclerosis lesions and hallervorden-spatz syndrome.
8. Depicting the small veins of the brain, phase Images of dentate Nucleus and the midbrain giving images analogus to the anatomical sections.