In this presentation,we shall introduce the basic elements of susceptibility weighted imaging (SWI) and how one can use phase information to enhance image contrast.SWI is a 3D phase enhanced gradient echo imaging method that shows increased sensitivity in the detection of venous blood and blood products such as clot or hemorrhages.Phase images are directly related to local changes in the magnetic field and can be used to create quantitative susceptibility maps (QSM) which we refer to as SWI and mapping or SWIM from which one can deduce local iron content.We show that brain iron can be monitored in a variety of diseases and used to quantify progression of disease and also to measure local venous oxygen saturation.More specifically,we consider the application of SWI and QSM (SWIM) to trauma,stroke,tumors,Sturge-Webber disease (SWD) and multiple sclerosis (MS).1) Trauma: High resolution susceptibility weighted imaging (SWI) has been shown to be superior to conventional gradient echo imaging in detecting hemorrhages caused by traumatic brain injury (TBI).Small lesions are more visible on SWI compared to conventional GRE.2) Stroke: Both computed tomography (CT) and MRI have been used for initial radiologic evaluation of acute stroke and patient selection for thrombolytic therapy.MRI is more sensitive than CT for identifying acute infarcts.3) SWD: We have studied many cases of SWD and found that SWI shows abnormalities in the white matter consistent with abnormal transmedullary veins (these appear to drain from cortex which is hypometabolic but still not dead).Many patients also have prominent cortical SWI abnormalities as well.4) MS: SWI is revealing many new facets of the lesions in MS especially areas of macrophage activity.These include lesions with: increased iron content,central dark areas,peripheral dark areas but not veins; peripheral veins and connecting veins between lesions as well as: enhanced iron content tissue not yet associated with a lesion,increased iron content in the basal ganglia,lesions seen with SWI not seen with conventional methods and MS lesions seen with conventional imaging where SWI shows nothing.In summary,SWI and SWIM provide a powerful means to study neurovascular diseases.