The term "SWI" stands for "susceptibility weighted imaging", an extremely sensitive MRI technique that detects hemosiderin deposited in brain tissue after any sort of hemorrhagic event. Hemosiderin, the end-product of blood degradation in brain, is basically a tiny deposit of iron that will be present forever. SWI is best performed on a 3.0 Tesla MRI scanner and is at least 4 - 6 times more sensitive at detecting micro-bleeds than any other susceptibility sequences available on any other equipment; typically gradient echo T2* sequences.
Micro-bleeds can be due to amyloid angiopathy, seen often in Alzheimer Disease (AD), or axonal shear injuries, seen in traumatic brain injury (TBI). They can also be noted in other disease states such as Binswanger (hypertensive) vasculitis. At UDI, we routinely include these sequences in Brain MRI scans performed to evaluate these clinical diagnoses.
In the SWI image below, the black dots are deposits of hemosiderin scattered throughout the brain of a 20 year old male who experienced several severe concussions. The linear black structures are intra-cerebral veins which are included because they contain paramagnetic de-oxygenated blood.