Manganese‐enhanced magnetic resonance imaging (MEMRI) of brain activity and applications to early detection of brain ischemia

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Abstract
Divalent manganese ion (Mn2+) has been reported to be a useful contrast agent for functional MRI, through a technique named activity‐induced manganese‐dependent MRI (AIM). In AIM, signal enhancement is related to functional increases in calcium influx, and therefore AIM is, thus far, the only MRI method able to map brain activation in vivo independently of the surrogate hemodynamic changes used in functional MRI. Because of its high signal‐to‐noise ratio (SNR) and high sensitivity, AIM allows the use of multi‐slice or three‐dimensional MRI techniques to map functional activity at high spatial resolution. In the present review, we define AIM as a functional MRI tool based on the administration of divalent ionized manganese through an open or disrupted blood–brain barrier (BBB). The adequacy and efficacy of AIM in detecting neural activation is described in light of supporting experiments on inhibition of calcium channels, FOS expression, and on direct comparison to BOLD‐ and perfusion‐based functional MRI. Two main applications of AIM, mapping brain activation in rat somatosensory cortex, as well stroke research based on the well‐established middle cerebral artery occlusion model, are described in detail. Methodological problems associated with a strong dependence on anesthetic conditions, potential corruption due to disruption of the BBB, and unspecific increase of the baseline signal due to acoustical noise are discussed. Finally, recommended preparation methods and experimental protocols for AIM are introduced. Copyright © 2004 John Wiley & Sons, Ltd.