Spatial and spectral-spatial EPR imaging of free radicals and oxygen in the heart

It has been hypothesized that free radical metabolism and oxygenation in living organs, such as the heart, may vary over particular anatomic locations within these tissues. When there is a lack of tissue perfusion, as occurs during myocardial ischemia or infarction, radical metabolism and oxygenation are greatly changed, however, it was not previously possible to spatially map these alterations within the heart. Therefore, we developed EPR imaging instrumentation to enable three-dimensional spatial as well as spectral-spatial EPR imaging of 2, 3, or 4 dimensions. Utilizing this instrumentation, at L-band, high quality images of the distribution of nitroxide free radical labels and suspensions of oxygen sensitive chars have been performed on lossy samples of up to 25 mm in size. In the isolated rat heart, imaging experiments were performed to determine the kinetics of radical clearance at different locations within the heart during myocardial ischemia. This kinetic data showed the existence of regional and transmural differences in myocardial radical clearance. In terms of image quality and resolution, we observed with nitroxides that resolutions as high as 1 mm could be obtained, enabling visualization of the of the overall shape of the heart and location of the right and left ventricles. With char suspensions resolutions as high as 0.1 mm were possible, enabling visualization of the ventricles, aortic root, and proximal coronary arteries. Overall our results demonstrate that spatial and spectral-spatial EPR imaging are powerful techniques which enable the visualization of localized differences in free radical metabolism and oxygenation within the heart.