Susceptibility Weighted Imaging (SWI)

By Dianna Bardo M.D.  and Jeffrey M. Pollock M.D.

How it works: SWI or susceptibility weighted images are useful to visualize small blood vessels, micro-hemorrhages, blood clots, subtle differences in tissue signal, and tissue iron. Susceptibility weighted imaging is based on the phase image of a gradient echo pulse sequence.

Benefits: SWI sequences are a useful addition to numerous imaging protocols:

Trauma: Subtle injury, especially when associated with hemorrhage such as diffuse axonal injury and cerebral contusion.

Brain tumors: Detection of established and developing tumor neo-vascularity.

Alzheimer’s disease (AD): Alterations in brain iron over time and perhaps to distinguish AD from vascular dementia.

Multiple sclerosis (MS): Local brain iron increases in MS so SWI may be able to reveal plaques that are not detected on standard imaging sequences.

Vascular imaging: Arterial thrombosis – blood clots appear very hypointense

Vascular malformations – particularly sensitive to very small caliber vessels and areas with slow blood flow

Experimental – atherosclerosis and hemorrhage in a vessel wall, which may also be useful in cases of dissection

Stroke imaging: Shows changes of oxygen saturation – deoxyhemoglobin content in small vessels is increased in areas of infarction/ischemia

Micro-hemorrhage – such as is associated with basal ganglia infarction, patients undergoing thrombolysis, and in amyloidosis

Equipment:  Our MRI suite uses the latest Phillips 3.0 Tesla magnets coupled with a state of the art Phillips MR SWI post-processing package. Processing is performed by the technologist as soon as the scan is completed and images are available for immediate interpretation.

Exam Preparation: The technologist will interview you prior to scan to make sure you have no contraindications to being in the MRI scanner. Patients with braces or other metal near the head or neck may not be suitable for SWI because of the metal artifacts.

What to expect: SWI alone takes approximately 3-5 minutes of scanner time. It is very important to remain very still during this portion of the examination. The study is done in conjunction with routine anatomic imaging. No intravenous contrast is used during this portion of the examination.

References:
Hermier M, et. al.,Contribution of susceptibility-weighted imaging to  acute stroke assessment. Stroke 2004; 35:1989.
Sehgal V et. al., Clinical applications of neuroimaging with susceptibility-weighted imaging. J MRI 2005; 22(4): 439-450.