A closer look into DESIRE for NMR microscopy

The major challenge of nuclear magnetic resonance (NMR) microscopy at a spatial resolution of a few micrometers is to obtain a sufficiently high signal-to-noise-ratio (SNR) within a reasonable measurement time. As a particular difficulty, molecular self-diffusion poses a serious limitation to true spatial resolution and SNR if conventional Fourier encoding techniques are used. Opposed to that, the alternative DESIRE (Diffusion Enhancement of SIgnal and REsolution) approach to NMR microscopy utilises diffusion to increase the SNR. Being a real-space imaging method, spatial localisation is accomplished by saturation pulses while diffusion continuously replaces the saturated by unsaturated spins. For this technique a signal enhancement of up to three orders of magnitude has been predicted and initial experimental data have provided a proof of principle. In the present work, a detailed investigation of one-dimensional (1D) DESIRE is presented including simulations of a real implementation of the method, a quantitative experimental analysis, and basic 1D imaging. The simulations reveal the importance and provide the means of ensuring the true spatial resolution for this particular way of localisation, enable the selection of useful experimental parameters, and predict the specific image contrast to be expected around barriers restricting diffusion. Experimental data are presented with resolutions down to 3 microm and DESIRE enhancement up to 25 that are in good agreement with the simulation results. In particular, 1D DESIRE imaging in a phantom confirms the expected signal drop close to barriers due to spatially restricted diffusion.