Three-dimensional whole body imaging of spin probes in mice by time-domain radiofrequency electron paramagnetic resonance

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Abstract
Imaging of stable paramagnetic spin probes in phantom objects and in vivo was evaluated using a RF time domain EPR spectrometer/imager operating at 300 MHz. Projections were collected using static magnetic field gradients and images were reconstructed using filtered back-projection techniques. Results from phantom objects containing approximately 1017 spins of stable paramagnetic probes with single narrow EPR spectra provide three-dimensional spatial images with resolution better than 2 mm. When the spin probe was administered to mice, the spin probe accumulation was temporally observed in the thoracic, abdominal, and pelvic regions. A three-dimensional image (from 144 projections) from a live mouse was collected in 5 min. Using fiducial markers, the spin probe accumulation in organs such as liver, kidney, and bladder could be observed. Differences in the oxygen status between liver and kidney were observed from the EPR images from mice administered with spin probe, by treating the time-domain responses with convolution difference approach, prior to image reconstruction. The results from these studies suggest that, with the use of stable paramagnetic spin probes and time-domain RF EPR, it is possible to perform in vivo imaging on animals and also obtain important spatially resolved physiologic information. Magn Reson Med 43:375–382, 2000.