ReviewThe future of susceptibility contrast for assessment of anatomy and function
Introduction
Magnetic susceptibility is a physical quantity that describes the relationship between the magnetization of a material and the applied external magnetic field. Despite being of central importance in the field of magnetic resonance imaging, its perception resembles a rollercoaster as a blessing and a curse. Often considered as a source of unwanted image contrast leading to signal loss, distortions and image artifacts, it represents, on the other hand, the biophysical origin of functional MRI (fMRI), where the tiny difference in hemoglobin's magnetic susceptibility between the oxygenated and deoxygenated states is responsible for the induced signal changes (Ogawa et al., 1990). Without magnetic susceptibility and nature's involvement in creating bulk magnetic susceptibility changes upon activation there wouldn't be most likely this special issue!
With recent advances in scanner hardware, the development of ultrahigh field MRI (7 T and 9.4 T), multi-channel coil arrays, and sophisticated signal processing techniques, an up to ten fold improvement in sensitivity compared to typical clinical field strengths of 1.5 T to 3 T may be achieved, which enables visualization of hitherto unseen details of the architecture of the cerebral cortex of living human beings that are at least partially caused by local variations in the magnetic susceptibility of the brain (Budde et al., 2011, Deistung et al., 2008, Duyn et al., 2007, Koopmans et al., 2008, Marques et al., 2010, Prudent et al., 2010). Most recently, the field has moved forward to quantitative susceptibility mapping (QSM), a technique that uses phase information of gradient echo (GRE) data to produce maps of magnetic properties of biological tissues in vivo (de Rochefort et al., 2010, Schweser et al., 2011a, Shmueli et al., 2009). Since this article is concerned with current and future aspects of susceptibility contrast, it may nevertheless be worthwhile to start with a brief look into the past.
Section snippets
How it began
My first encounter with magnetic resonance imaging and magnetic susceptibility began in the mid-nineties, when I came to Mark Haacke's lab in St. Louis to start my work on understanding the BOLD phenomena and the role that venous blood played. Among the many efforts for improving our early understanding of the BOLD phenomenon, including spatial resolution and long echo time issues (Barth et al., 1997), we already began to explore new ways to create images highlighting susceptibility-affected
How it stands
With phase information of T2*-weighted gradient echo scans being recognized to provide impressive tissue contrast, the potential of delineating anatomical structures using MR phase imaging with high spatial resolution was quickly adopted, even at field strengths as low as 1.5 T (Rauscher et al., 2005a). Brain structures, such as deep brain nuclei and white matter fiber bundles, which are barely or not at all visible on the corresponding gradient echo magnitude images, were imaged with a wealth
What the future holds
Looking into the crystal ball is always dodgy, but in this instance some developments are clear. Beyond doubt quantitative susceptibility mapping will unfold its enormous scientific and clinical potentials in the future. In particular, since phase contrast can exceed the magnitude contrast by up to an order of magnitude at high field with gradient-echo imaging, transforming the GRE phase information into quantitative susceptibility maps will become increasingly more attractive with increasing
Acknowledgments
There are many individuals I have to acknowledge and thank for their invaluable contributions to my work over many years, too many to mention all of them here. Special gratitude, however, is dedicated to the current and previous members of my research group, with whom I had the privilege to work together over the years on this specific topic. These talented and gifted bright young individuals are Andreas Deistung, Enrico Dittrich, Berengar Lehr, Alexander Rauscher, Ferdinand Schweser, Jan
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