Magnetic resonance imaging of brain iron

Abstract

The development of effective staining and other tissue analysis techniques has long provided pathologists with a means of studying [1–4] and quantifying [5] the regional distribution of the non-heme iron stored in the brain. However, the nature of these techniques precluded premortem studies. Soon after the introduction of magnetic resonance (MR) imaging in the 1980s, it was recognized that some of the MR contrast present in brain images correlated closely with the previously established patterns of brain iron deposition [6]. The initial studies were performed using 1.5 tesla (T) magnets but it was quickly noted that this contrast increased rapidly with the use of higher field strength magnets [7–9]. In the early 1990s, only a handful of whole-body magnets operating at field strengths above 1.5 T were in use, and these were confined to research applications. However, fully featured clinical scanners operating at 3 T are now coming into widespread use, and whole-body research systems operating at 7 and 8 T are available at some research centers. As a result, the study of iron-dependent contrast at high magnetic field strengths is now much more feasible than was previously the case. The MR signal arises from mobile protons in the tissues. In brain, these protons are almost entirely present in the solvent water molecules as H2O. The image contrast arises from variations among brain regions of the density of solvent water and the longitudinal (T1) and transverse (T2) relaxation times of the associated protons [10]. Generally, regions with short values of T1 appear bright on T1-weighted images and regions of short T2 appear dark on T2-weighted images. The presence of magnetic ions can alter T1 and T2 and thereby the tissue contrast [11]. Many metallic ions essential to brain function (sodium, potassium, calcium, magnesium, and zinc) are nonmagnetic and have no impact on MR images. However, ions of the transition group (e.g., iron, manganese, and copper), and of the rare earth group (e.g., gadolinium and dysprosium) exhibit nonzero magnetic moments in many compounds and do have the potential of affecting MR contrast.