Pulsed EPR imaging: image reconstruction using selective acquisition sequences.

Pulsed nuclear magnetic resonance imaging (NMRI) apparatus has developed very quickly. On the other hand, the development of apparatus for pulsed electron paramagnetic resonance imaging (EPRI) has been very slow. This fact is due to the extremely reduced relaxation times of the paramagnetic probes. EPR linewidths are larger than typical NMR linewidths. These large linewidths are also responsible for a substantial worsening of spatial resolution. Due to the brevity of the electronic relaxation times, not all the acquisition/reconstruction techniques currently used in NMRI (such as spin-echo, gradient-echo, etc) can be applied in pulsed EPRI. In fact, the usable sequences in pulsed EPRI are only acquisitions from projections, where it is possible to use stationary magnetic field gradients. Moreover, the use of high fixed magnetic field gradients induces a short decay time constant T2*. The low T2* value can make it impossible for the analogue to digital conversion system (ADC) to reproduce signal variations during the whole acquisition interval and the resolution can worsen. A new pulsed EPRI acquisition sequence from projections, based on selective reception, is presented that is particularly useful in solving the problems of worsening of spatial resolution associated with the use of an ADC. In order to demonstrate the capabilities of our acquisition method, simulated numerical tests will also be reported.

Optimization of axial RF field distribution in low-frequency EPR loop-gap resonators.

A novel coupling method that optimizes the axial RF distribution of low-frequency EPR loop-gap resonators is presented. It consists of a resonant coupling loop positioned at the centre of a two-section loop-gap resonator. This arrangement ensures a symmetrical distribution of the radio frequency field along the axis of the resonator. The design of a central coupling system suitable for EPR resonators operating at about 220 MHz is described. Experimental results show that with the central coupling system the RF field is symmetrical and has a very good axial homogeneity (100% of the resonator length).

Two-dimensional 220 MHz Fourier transform EPR imaging.

In the last decade radiofrequency continuous-wave EPR spectrometers have been developed to detect and localize free radicals in vivo. Only recently, pulsed radiofrequency EPR spectrometers have been described for imaging applications with small samples. In the present work, we show the first two-dimensional image obtained at 220 MHz on a large phantom (40 ml) that simulates typical conditions of in vivo EPR imaging. This pulsed EPR apparatus has the potential to make the time required for three-dimensional imaging compatible with the biological half-life of normally used paramagnetic probes.