E. Mark Haacke
The development of susceptibility-weighted imaging (SWI) began in the mid-1990s, just following the advent of blood oxygen saturation imaging. Our group had been involved in the early brain/vein debate, that the veins played the critical role in visualizing information on brain function,1 not diffusion. We set out to demonstrate this and in the process found ourselves creating very high-resolution susceptibility maps of the brain and veins in the days when people were using fairly low-resolution echo-planar scanning approaches. This led us to introducing “phase” as a means to map susceptibility,2 and this first paper preceded the initial concept of susceptibility mapping.1 When we realized that the phase could be high-pass-filtered to reveal regions of high levels of deoxyhemoglobin and high iron content, we also sought to use that information to enhance the usual magnitude images but without insisting that the radiologist had to understand phase per se, but rather incorporate the information into a new type of magnitude image.3 Thus, the enhanced-contrast SWI method was born.
Initially, SWI was thought best to be used to study cerebral venous anomalies such as telangiectasias or developmental venous anomalies, as well as imaging microbleeds. Both of these are imaged exquisitely well with SWI. However, since the original 1997 paper, more than 1000 citations exist for the use of SWI, and most of these are clinical citations. Surprisingly, and happily, once this technique was put into the hands of the clinicians (thanks to Siemens Medical Solutions adopting this as a clinical sequence), many new applications of SWI developed.4,5 Thanks to these clinical efforts, SWI has found applications in brain function (see also the paper by Ge et al in this issue6) and imaging cirrhosis of the liver.7 For this reason, we felt it appropriate to write both a technical review followed by a clinical review, back in 2009.4 Future applications are likely to include imaging the heart and atherosclerosis.8 Recently, the SWI concept received an award as one of the top 30 papers in Magnetic Resonance in Medicine as a technical paper.9
However, the most rewarding aspect of developing a method like this is to see it applied in the clinical world as a tool that helps not only to diagnose disease better but also to treat it. Two such examples of using SWI as a means to help determine treatment appeared recently for stroke10 and traumatic brain injury.11,12 Today we are expanding the use of SWI into a multiecho methodology for better determination of thrombosis and also as a means to better quantify susceptibility mapping13 in a new approach we refer to as susceptibility weighted imaging and mapping (SWIM). Finally, from a futuristic perspective, without a contrast agent, SWI cannot image arteries. However, with the use of an iron-based contrast agent, everything that has been done with veins prior to this now has the potential to be done with arteries, by changing their susceptibility and then using SWI.14 This offers the potential at high field to image both arteries and veins with SWI. Once put into the hands of clinicians, this could lead to many new discoveries in the presence and role of microvascular disease in neuroradiology. Several of these futuristic concepts will be presented at the upcoming meeting of the International Society of Magnetic Resonance in Medicine, in Toronto, Canada from May 30 to June 5, 2015.