MRI Magnetic Susceptibility Mapping In Vivo*

The magnetic susceptibility of living tissues depends on their composition and microstructure. Therefore, the emerging, non-invasive, magnetic resonance imaging (MRI) technique of quantitative susceptibility mapping (QSM) is beginning to yield clinically useful information on pathophysiology-related changes in tissue composition and microstructure. I will outline the physical principles underpinning QSM and describe QSM applications we have developed in sickle cell anaemia, healthy brain ageing and head-and-neck cancer.

Weak tissue susceptibilities (Schenck, Med Phys 1996) cause small magnetic field perturbations seen in phase of the complex MRI signal. In QSM we calculate susceptibility maps from MRI phase images. MRI phase is a 0 to 2π angle in the complex plane and must be unwrapped. The unwrapped images are dominated by large-scale background phase variations caused by the relatively large air-tissue susceptibility difference. Once the background field variations are removed the result is the input for susceptibility calculation. This is an ill-posed inverse problem for which many regularisation methods have been proposed. There is a plethora of algorithms available for each stage in the QSM pipeline and the MRI QSM community is working to achieve consensus on the best methods.

QSM has important advantages over phase images and a widespread precursor known as susceptibility weighted imaging (SWI): it overcomes the non-local and orientation-dependent phase contrast (Shmueli, MRM 2009) to improve visualisation of tissue structure and composition.

Clinical applications are emerging based on QSM’s sensitivity to tissue calcifications, iron, myelin and deoxyhaemoglobin content. QSM highlights iron-rich brain structures in Parkinson’s disease, microbleeds and haemorrhages and distinguishes these from calcifications. QSM allows quantification of venous oxygenation with functional QSM now able to detect brain activity.