In vivo high‐resolution 3D Overhauser‐enhanced MRI in mice at 0.2 T

Overhauser‐enhanced MRI (OMRI) offers the potentiality of detecting low‐concentrated species generated by specific biological processes. However molecular imaging applications of OMRI need significant improvement in spatial localization. Here it is shown that 3D‐OMRI of a free radical injected in tumor‐bearing mice can be performed at high anatomical resolution at a constant field. A 30 mm cavity operating at 5.43 GHz was inserted in a C‐shaped magnet for proton MRI at 0.194 T. Nude mice with or without brain‐implanted C6 rat glioma were positioned in the cavity and injected with TOPCA (1‐oxyl‐2,2,5,5‐tetramethyl‐2,5‐dihydro‐1H‐pyrrole‐3‐carboxylic acid). OMRI was performed in 3D within several minutes in the brain region without high overheating of the animals. Voxel size was 0.5 × 0.5 × 1 mm³, providing good delineation of brain regions. Signal amplifications ranged from 2 in tumors to 10 in vessels several minutes after TOPCA injection. Time‐course of signal enhancement could be measured by 2D OMRI at 15 s time intervals in a localized thin slice. The method opens the way for molecular imaging of biological activities able to generate OMRI‐visible free radicals. Copyright © 2012 John Wiley & Sons, Ltd.