PEDRI (Proton-Electron Double-Resonance Imaging) is a technique for imaging the distribution of free radicals within the body. PEDRI is able to detect low concentrations of free radicals and can also produce very high quality images, showing the distribution of free radicals in large samples. PEDRI makes use of a double resonance magnetic resonance technique called the Overhauser Effect.
The Overhauser Effect is essentially a combination of nuclear magnetic resonance (NMR) and electron spin resonance (ESR). The NMR signal is obtained in the usual way, by applying pulses of radiowaves at the NMR frequency. At the same time, the free radical's unpaired electrons are irradiated by applying an irradiation (radiowave or microwave) at the ESR frequency. If there is good magnetic coupling between the unpaired electrons and the water hydrogen nuclei, the ESR irradiation can cause a transfer of polarisation from the electrons to the nuclear spins, resulting in an amplification or "enhancement" of the measured NMR signal. In theory, the enhancement can be up to 330-fold, though in practice enhancement factors of 10- to 50-fold are more commonly observed.PEDRI is the imaging version of the Overhauser experiment. The ESR resonance of the free radicals of interest is irradiated during the collection of an MR image. The Overhauser effect causes an enhancement of the NMR signal in parts of the sample containing the free radical, and these regions "light up" in the resulting image, showing the distribution of the free radical.