SWI Basics, Applications and Pitfalls

Susceptibility-weighted imaging (SWI) is a high spatial resolution, three-dimensional (3D), gradientecho (GRE) magnetic resonance (MR) technique [1]. It uses both the T2*-weighted magnitude and the phase information to emphasize the magnetic susceptibility differences of various tissues or substances, such as blood products, iron, and calcification, by post-processing the magnitude images with a phase mask [2]. Minimal intensity projection (minIP) images can further demonstrate tortuous vasculature and the continuity of vessels or abnormalities across slices [1-3]. Although most diagnostic MR imaging ignores phase information, phase images, however, contain a wealth of information about local susceptibility changes between tissues [1-10], which can be useful in measuring iron content [4], for instance, and other substances that change the local field.