Susceptibility weighted imaging (SWI)

Susceptibility weighted imaging (SWI), originally called BOLD venographic imaging, uses a type of contrast in magnetic resonance imaging (MRI) different from traditional spin density, T1, or T2 imaging. SWI uses a fully flow compensated, long echo, gradient recalled echo (GRE) pulse sequence to acquire images. This method exploits the susceptibility differences between tissues and uses the phase image to detect these differences. The magnitude and phase data are combined to produce an enhanced contrast magnitude image which is exquisitely sensitive to venous blood, hemorrhage and iron storage. The imaging of venous blood with SWI is a blood-oxygen-level dependent (BOLD) technique which is why it was (and is sometimes still) referred to as BOLD venography. Due to its sensitivity to venous blood SWI is commonly used in traumatic brain injury (TBI) and for high resolution brain venographies but has many other clinical applications. SWI is offered as a clinical package by Philips and Siemens but can be run on any manufacturer’s machine at field strengths of 1.0 T, 1.5 T, 3.0 T and higher.

There have been major advances in studying susceptibility weighted imaging (SWI), filtered SWI phase images and quantitative susceptibility mapping (QSM).

The implementation of QSM, referred to as SWIM (susceptibility weighted imaging and mapping), is fast, robust and can be used with very high resolution matrices. SWIM also makes it possible to remove artifacts from SWI thereby creating a “true” SWI data set or tSWI.

Gradient echo T2 weighted image MRI is the 3–4 × more sensitive test than FLAIR for demonstrating intraparenchymal blood (which appears dark) due to high sensitivity to paramagnetic artifact. It is not as sensitive as SWI.

Susceptibility-weighted imaging (SWI) is a magnetic resonance imaging (MRI) technique where image contrast represents 'magnetic susceptibility effects'-a natural property of tissues. The applications of SWI are rapidly increasing, with much work being carried out to determine the usefulness of the technique in multiple disease states. Current clinical applications of the technique include detection of microbleeds, subarachnoid hemorrhage (SAH), ferromagnetic deposition in neurodegenerative disease, and characterization of cerebral tumors 1).

Susceptibility Weighted Imaging (SWI) is an MRI sequence with improved visualization of susceptibility differences between tissues, particularly sensitive for brain veins.

Automatically detect small hypointensities that may be subtle signs of chronic and acute damage due to both subconcussive and concussive injury 2).

The alterations suggest decreased extracellular space and decreased diffusivities in white matter tissue. This finding might be explained by swelling and/or by increased cellularity of glia cells. Even though these findings in and of themselves cannot determine whether the observed microstructural alterations are related to long-term pathology or persistent symptoms, they are important nonetheless because they establish a clearer picture of how the brain responds to concussion 3).

The susceptibility weighted imaging with 3 Tesla MRI-based subthalamic nucleus localization shows the best accuracy compared with T2-weighted and fluid-attenuated inversion recovery 3-T MRI. Therefore, the susceptibility-weighted 3-T MRI should be preferred for surgical planning when the operation procedure is performed under general anesthesia without microelectrode recordings in subthalamic nucleus deep brain stimulation 4).

Krishnan AS, Lansley JA, Jäger HR, Mankad K. New vistas in clinical practice: susceptibility-weighted imaging. Quant Imaging Med Surg. 2015 Jun;5(3):448-52. doi: 10.3978/j.issn.2223-4292.2015.03.03. PubMed PMID: 26029647; PubMed Central PMCID: PMC4426120.
Helmer KG, Pasternak O, Fredman E, Preciado RI, Koerte IK, Sasaki T, Mayinger M, Johnson AM, Holmes JD, Forwell LA, Skopelja EN, Shenton ME, Echlin PS. Hockey Concussion Education Project, Part 1. Susceptibility-weighted imaging study in male and female ice hockey players over a single season. J Neurosurg. 2014 Feb 4.[Epub ahead of print] PubMed PMID: 24490839.
Pasternak O, Koerte IK, Bouix S, Fredman E, Sasaki T, Mayinger M, Helmer KG, Johnson AM, Holmes JD, Forwell LA, Skopelja EN, Shenton ME, Echlin PS. Hockey Concussion Education Project, Part 2. Microstructural white matter alterations in acutely concussed ice hockey players: a longitudinal free-water MRI study. J Neurosurg. 2014 Feb 4. [Epub ahead of print] PubMed PMID: 24490785.
Polanski WH, Martin KD, Engellandt K, von Kummer R, Klingelhoefer L, Fauser M, Storch A, Schackert G, Sobottka SB. Accuracy of subthalamic nucleus targeting by T2, FLAIR and SWI-3-Tesla MRI confirmed by microelectrode recordings. Acta Neurochir (Wien). 2015 Mar;157(3):479-86. doi: 10.1007/s00701-014-2328-x. Epub 2015 Jan 18. PubMed PMID: 25596640.
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