Electron Paramagnetic Resonance Imaging

Electron Paramagnetic Resonance Imaging (EPRI) may become a useful tool for determining crucial oxygen levels in tumors and other biological tissue.

Oxygen is central to many diseases; for example, the absence of oxygen makes a cancer cell more resistant to radiation and chemotherapy. Taking advantage of the properties of electrons in certain biochemical compounds, Charles Pelizzari (c-pelizzari@uchicago.edu) and his colleagues use a novel technique to form images of the oxygen distribution in small animals with millimeter spatial resolution. In a talk at the AAPM meeting, Pelizzari's group will present EPR oxygen images superimposed on MRI anatomical images of animals. Developing these tools at the Center for In-Vivo EPR Imaging at the University of Chicago, the researchers create these important maps of oxygen levels by magnetically manipulating the unpaired electrons in certain oxygen-containing molecules, including free radicals.

Most electrons in atoms and molecules form pairs that mutually cancel out their internal magnetic properties, but unpaired electrons can give the atom/molecule “paramagnetic” properties that cause them to be weakly attracted to an external magnetic field. Electron paramagnetic resonance imaging (EPRI) obtains pictures of molecules with unpaired electrons in a way that is similar to the way MRI obtains images of atomic nuclei such as the hydrogen in water: an image is formed when paramagnetic molecules, lined up in a magnetic field, absorb and then re-emit electromagnetic waves in or near the microwave portion of the spectrum. Using a series of magnetic fields that vary in strength over a given region of space, these emissions can be reconstructed into a 3D image.

Where EPRI is advantageous over MRI is in providing quantitative images of the distribution of oxygen in living tissues. Pelizzari expects that one day this EPR methodology will obtain submillimeter-resolution maps and also be scaled up to human dimensions. A potential long-term benefit of EPR imaging should be in obtaining both maps of radiation-resistant tumor regions before treatment and in providing quick feedback on the results of cancer therapy in days or even hours, without the use of PET scans which require radioactivity. (Meeting talk WE-D-I-609-8)