ABSTRACT
The concept of susceptibility weighted imaging (SWI) began with the study of the role of veins in blood oxygen level–dependent (BOLD) imaging and the ability to use phase instead of T2* to examine oxygen consumption in the brain. This gave rise to studying susceptibility using MR phase images in the early 1990s and was subsequently used to detect oxygenation in the venous blood. Phase signal induced by the magnetic susceptibility differences (Δχ) between tissues can be utilised as a new type of contrast in MRI, different than the relaxation times or spin density. Clinically relevant tissues such as venous blood, clot (paramagnetic), calcium (diamagnetic) and iron laden tissue have a unique Δχ relative to surrounding tissues. After removing the low-frequency global background fields, the phase itself can be a superb source of image contrast. Hence, the filtered phase was then used as a means to alter magnitude contrast, and the resultant composite magnitude was named susceptibility weighted imaging or SWI. The filtered phase was further utilised to generate a map of different susceptibility sources using the inverse process. This method is known as quantitative susceptibility mapping (QSM), which provides quantification of biomarkers such as iron, calcium and superparamagnetic iron oxide particles and can be used to assess cerebral activity in normal and diseased areas. Using short TEs, it is also possible to image structures with high susceptibilities, such as air and bone, with QSM. This chapter discusses the potential of T2*, SWI and QSM in clinical applications focused on the brain and technical considerations associated with these techniques
