Rapid imaging of free radicals in vivo using field cycled PEDRI

Imaging of free radicals in vivo using an interleaved field-cycled proton-e lectron double-resonance imaging (FC-PEDRI) pulse sequence has recently bee n investigated. In this work, in order to reduce the EPR (electron paramagn etic resonance) irradiation power required and the imaging time, a centric reordered snapshot FC-PEDRI pulse sequence has been implemented. This is ba sed on the FLASH pulse sequence with a very short repetition time and the u se of centric reordering of the phase-encoding gradient, allowing the most significant free induction decay (FID) signals to be collected before the s ignal enhancement decays significantly. A new technique of signal phaseshif t correction was required to eliminate ghost artefacts caused by the instab ility of the main magnetic held after field cycling. An FID amplitude corre ction scheme has also been implemented to reduce edge enhancement artefacts caused by the rapid change of magnetization population before reaching the steady state. Using the rapid pulse sequence, the time required for acquis ition of a 64 x 64 pixel FC-PEDRI image was reduced to 6 s per image compar ed with about 2.5 min with the conventional pulse sequence. The EPR irradia tion power applied to the sample was reduced by a factor of approximately 6 4. Although the resulting images obtained by the rapid pulse sequence have a lower signal to noise than those obtained by a normal interleaved FC-PEDR I pulse sequence, the results show that rapid imaging of free radicals in v ivo using snapshot FC-PEDRI is possible.