In vivo imaging of free radicals: applications from mouse to man.

Free radicals and other paramagnetic species, play an important role in cellular injury and pathophysiology. EPR spectroscopy and imaging has emerged as an important tool for non-invasive in vivo measurement and spatial mapping of free radicals in biological tissues. Extensive applications have been performed in small animals such as mice and recently applications in humans have been performed. Spatial EPR imaging enables 3D mapping of the distribution of a given free radical while spectral-spa-tial EPR imaging enables mapping of the spectral information at each spatial position, and, from the observed line width, the localized tissue oxygenation can be determined. A variety of spatial, and spectral-spatial EPR imaging applications have been performed. These techniques, along with the use of biocompatible paramagnetic probes including particulate suspensions and soluble nitroxide radicals, enable spatial imaging of the redox state and oxygenation in a variety of biomedical applications. With spectral-spatial EPR imaging, oxygenation was mapped within the gastrointestinal (GI) tract of living mice, enabling measurement of the oxygen gradient from the proximal to the distal GI tract. Using spatial EPR imaging, the distribution and metabolism of nitroxide radicals within the major organs of the body of living mice was visualized and anatomically co-registered by proton MRI enabling in vivo mapping of the redox state and radical clearance. EPR imaging techniques have also been applied to non-invasively measure the distribution and metabolism of topically applied nitroxide redox probes in humans, providing information regarding the penetration of the label through the skin and measurement of its redox clearance. Thus, EPR spectroscopy and imaging has provided important information in a variety of applications ranging from small animal models of disease to topical measurement of redox state in humans.