Quantitative Susceptibility Mapping: Concepts and Applications

Purpose: To review the fundamental principles of susceptibility-weighted imaging (SWI) and quantitative susceptibility mapping (QSM), and to discuss recent clinical developments. Methods: SWI is a magnetic resonance imaging method that takes advantage of magnitude signal loss and phase information to reveal anatomic and physiologic information about tissue and venous vasculature. The method enhances image contrast qualitatively, relying on phase shifts due to differences in magnetic susceptibility between tissues. QSM, extending SWI in an elegant way, is a new sophisticated postprocessing technique that numerically solves the inverse source-effect problem to derive local tissue magnetic susceptibility (source) from the measured magnetic field distribution (effect) as it is reflected in the phase images of gradient-echo sequences. Results: SWI has meanwhile been established in numerous clinical as well as basic biomedical applications due to its ability to highlight tissue structures and compounds that are difficult to detect by conventional magnetic resonance imaging (MRI), including iron, calcifications, small veins, blood, and bones. The field of QSM has also progressed rapidly, both in terms of optimizing the post-processing strategies and algorithms as well as in gaining ground for new clinical applications that take advantage of its quantitative nature and improved specificity to identify the magnetic signature of lesions. Conclusions: Though magnetic susceptibility may be a major nuisance producing image artifacts in MRI, recent work has transformed it into a useful source of image contrast. Both SWI and QSM are gaining increasing acceptance in clinical practice. In particular, QSM provides new insights into tissue composition and organization due to its more direct relation to the actual physical tissue magnetic properties.