QSM

Quantitative Susceptibility Mapping (QSM) is an MRI-based imaging technique that aims to quantify the magnetic susceptibility of tissues. Magnetic susceptibility is a property that governs how a material becomes magnetized in an external magnetic field, and in biology it is influenced by substances such as iron, calcium, and myelin. QSM produces maps that reflect spatial variations in this susceptibility, providing information complementary to conventional MRI contrasts.

QSM derives susceptibility from the phase information of gradient-echo MRI data. The typical processing workflow includes phase unwrapping to remove artificial discontinuities, removal of background fields not arising from the tissue of interest, and solving an ill-posed dipole inversion to recover the underlying susceptibility distribution. Because the inverse problem is mathematically unstable, regularization or prior information is often used to obtain stable, physiologically plausible maps. The resulting images are usually expressed in units of parts per million (ppm) relative to a reference material such as water.

Applications of QSM are broad in neuroimaging and beyond. In the brain, QSM can help quantify iron deposition in regions like the basal ganglia, detect calcifications, and assess myelin-related changes. It is used in research on neurodegenerative diseases (e.g., Parkinson’s disease, multiple sclerosis, Alzheimer’s disease), neurovascular disorders, and functional brain studies that require mineral and tissue property information. Clinically, QSM is increasingly integrated as a supplementary biomarker alongside conventional MRI sequences, though its adoption varies by institution and scanner capabilities.

Limitations include the inherent non-uniqueness of the dipole inversion, sensitivity to noise and artifacts near air-tissue interfaces, and dependence on accurate background field removal and brain masking. Interpretation can be complicated by mixed tissue contributions and calibration differences, so standardization across platforms remains an ongoing effort.