Nuclear magnetic resonance microscopy with 4-microns resolution: theoretical study and experimental results

Nuclear magnetic resonance (NMR) microscopy with 4-microns resolution, a step closer to the 1-micron resolution with which in vivo cellular imaging would be possible is described. An analysis of the ultimate resolution and voxel size dependent signal-to-noise ratio (SNR) in NMR microscopy is presented and experimentally verified. For microscopic scale objects (less than 1-mm diameter), the SNR based on the geometrical scale factor(s) is found to be proportional to sn where n less than 2, rather than n = 3 as previously supposed. This comes about because of a drastic reduction in sample noise coupled with a significant sensitivity gain realized in small diameter radiofrequency coils. A new pulse sequence which reduces both diffusion dependent resolution degradation and signal attenuation is presented. The selection of optimal bandwidth and acquisition time for maximal SNR is discussed. Experimental results obtained on both a 2.0-T whole-body system and a 7.0-T small bore system adapted for microscopy indicate the potentials of 4-microns resolution microscopy with the existing magnets.