Elsevier

Clinical Radiology

Volume 64, Issue 1, January 2009, Pages 74-83
Clinical Radiology

Pictorial review
Susceptibility weighted imaging: a new tool in magnetic resonance imaging of stroke

https://doi.org/10.1016/j.crad.2008.04.022Get rights and content

Susceptibility weighted imaging (SWI) is a magnetic resonance (MR) technique that is exquisitely sensitive to paramagnetic substances, such as deoxygenated blood, blood products, iron, and calcium. This sequence allows detection of haemorrhage as early as 6 h and can reliably detect acute intracerebral parenchymal, as well as subarachnoid haemorrhage. It detects early haemorrhagic transformation within an infarct and provides insight into the cerebral haemodynamics following stroke. It helps in the diagnosis of cerebral venous thrombosis. It also has applications in the work-up of stroke patients. The sequence helps in detecting microbleeds in various conditions, such as vasculitis, cerebral autosomal dominant arteriopathy, subacute infarcts and leucoencephalopathy (CADASIL), amyloid angiopathy, and Binswanger's disease. The sequence also aids in the diagnosis of vascular malformations and perinatal cerebrovascular injuries. This review briefly illustrates the utility of this MR technique in various aspects of stroke diagnosis and management.

Introduction

Susceptibility weighted imaging (SWI) is an magnetic resonance (MR) technique that is exquisitely sensitive to paramagnetic substances, such as deoxygenated blood, blood products, iron, and calcium.1 This sequence is a high-resolution three-dimensional (3D), fully velocity-compensated, gradient echo (GRE) sequence, wherein the phase images are used to create a phase mask after unwrapping and high pass filtering, which is then multiplied with the magnitude images to enhance the conspicuity of small veins and other paramagnetic substances.2, 3 The clinical utility of this technique has been described in variety of neurological disorders, such as trauma, tumours, vascular malformations, multiple sclerosis, venous thrombosis, and stroke.4, 5, 6, 7 In this article we review the various applications of SWI in the diagnosis, evaluation, and management of stroke.

Section snippets

Principles of SWI

The local magnetic heterogeneity induced by paramagnetic, diamagnetic, and ferromagnetic substances can result in overall signal loss in GRE images. The susceptibility effect is highest in GRE techniques at long echo times and higher field strengths. The high spatial resolution 3D fast low angle shot (FLASH) technique is used, as this is extremely sensitive to susceptibility effects.1 All MR image data have magnitude and phase information, though the phase information is not routinely utilized

Clinical applications of SWI in stroke

Neuroimaging plays a crucial role in the evaluation of patients presenting with acute stroke symptoms. Although patient symptoms and clinical examinations may suggest the diagnosis, only brain imaging studies can confirm the diagnosis and differentiate haemorrhage from ischaemia with high accuracy, and this distinction is extremely important as the treatment decisions are dependent on this. SWI has been found to be a useful imaging sequence in the imaging of stroke patients. Broadly, the

Conclusion

This article briefly outlines the usefulness of SWI in the evaluation of acute and chronic stroke patients. This technique is extremely sensitive to even traces of haemorrhage and has potential diagnostic, as well as therapeutic, implications in acute stroke patients. It can also provide valuable diagnostic input in various stroke-related conditions and stroke mimics, and this technique should be included in the routine evaluation of stroke patients.

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      Recently, several reports have shown that T2*WI and/or SWI are more useful MR sequences than conventional MR sequences in detecting CVT.4,7,8 Moreover, T2*WI and/or SWI are more useful diagnostic imaging tools for direct clot detection in the acute phase of cerebral venous thrombosis, resulting from the susceptibility effects of deoxyhemoglobin within the blood clots.9,10 However, in this case, we could not detect the thrombus clearly in the cerebral venous sinus using T2*WI and SWI: the thrombus had not yet become large because it was in the hyperacute phase of CVT on the third day after symptom onset and the venous outflow had been maintained through the hypoplastic cerebral venous sinus on the contralateral side.

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    This paper was presented as a scientific exhibit at the RSNA 2007 meeting at Chicago and was awarded Certificate of Merit.

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