Optimisation of FC-PEDRI for in vivo free radical imaging

A model for studying magnetisation during FC-PEDRI experiments has been developed and verified experimentally. The optimum EPR irradiation frequency, which maximises the Overhauser enhancement while minimising the RF power deposition in sample, was investigated using the FC-DNP technique. Experiments to verify the model were carried out in the EPR frequency range between 20 and 180 MHz and with different applied EPR RF power for free radicals with different EPR linewidths. The optimum EPR irradiation frequency was found to be dependent on the applied power level and on the EPR linewidths. Methods to obtain NMR images with T1-weighted contrast were also investigated. The most successful method was found to be a combination of Inversion Recovery and FC-PEDRI. This technique yields both high T1 contrast between samples and good contrast between images with different Tl values. The free radical distribution is recorded with the same SNR as those collected with the conventional FC-PEDRI technique. Rapid imaging of FC-PEDRI has also been studied. The number of EPR irradiation periods can be optimised to give a good free radical distribution image while reducing the power deposition in the sample. With 4 EPR irradiation periods, the difference image displays the free radical distribution comparable with the conventional technique (64 EPR irradiation periods). The power deposition in the sample is reduced by a factor of 16 and acquisition time is reduced by a factor of 4. Finally, inversion recovery spin preparation has been added to the hybrid fast technique to yield different anatomical information while reducing power deposition in the sample. It was found that an IR hybrid FISP FC-PEDRI pulse sequence could be used to generate T1 contrast in images which show similar detail and intensity of the free radical distribution as those obtained using hybrid FISP FC-PEDRI. (author)