Electron paramagnetic resonance imaging

(EPRI) can determine the oxygen content of tumors and other biological tissue. Low levels of oxygen make a cancerous tumor more resistant to radiation and chemotherapy; higher doses could be administered to such areas—provided those areas could be found. With the goal of mapping biological oxygen in three dimensions, researchers at the University of Chicago are developing a new technique that lets them magnetically manipulate unpaired electrons in certain oxygen-containing molecules. With an appropriate contrast agent present, each little volume element of a sample produces an absorption line whose width gives the oxygen content at that location. The team has thus been able to quantify and image, with millimeter spatial resolution, the oxygen distribution in small animals. By superimposing those images on anatomically superior MRI images, regions rich or poor in oxygen can be located, as reported by Charles Pelizzari at the July meeting in Seattle of the American Association of Physicists in Medicine. For example, in these 7-mm panels showing a mouse tumor, the upper images were obtained just prior to a treatment and the lower ones four days later; the left images were made with MRI (with the tumor outlined); the right ones, showing a noticeable change in oxygen content (bright areas), were made with EPRI. Pelizzari thinks that images like these may have potential for biologically based planning and assessment of radiation therapy. (C. Haney et al. , AAPM Meeting talk WE-D-I-609-8. )