Rapid Imaging of Free Radicals In Viva using Snapshot Field-Cycled PEDRI

Introduction Proton electron double resonance imaging (PEDRI) is a wellestablished method for imaging free radicals in biological samples and small animals [ 11. The EPR resonance of the free radical under study is irradiated during the acquisition of an MR image; the image intensity in parts of the sample containing free radical is altered by the Overhauser effect. In order to reduce the EPR irradiation power for in viva experiments, field-cycled PEDRI (FC-PEDRI) is often employed [2], in which the EPR irradiation is applied at low field (the evolution field, BOE 5 mT) hence low frequency and low power and the NMR signals are measured at higher field (the detection field, BsD 60 mT). “Conventional” FC-PEDRI requires a -3 x Tt period of EPR irradiation for every step of the phase encoding gradient, so that the total acquisition time can be several minutes. Furthermore, each period of EPR irradiation deposits energy into the sample, so that the SAR limit can easily be reached or exceeded. Snapshot FC-PEDRI was proposed a number of years ago in order to reduce the acquisition time and SAR [3], the idea being to apply a single, high-power EPR irradiation at field BoE, followed by rapid acquisition of the image data at BoD. A segmented acquisition scheme for Overhauser imaging was also proposed [4]. The feasibility of snapshot FC-PEDRI was demonstrated using longTi samples with detection at 10 mT, however implementation of snapshot FC-PEDRI with in vivo samples was found to be difficult, due to the rapid decay of Overhauser-enhanced magnetisation following the EPR irradiation.