Comparison of deuterium NMR imaging methods and application to plants

Abstract

FID, gradient-echo, and spin-echo imaging sequences have been compared id order to determine the imaging sequence optimally suited for deuterium MRI. Each sequence was investigated for the polar and Cartesian spatial encoding mode. Analytical expressions for the SNR per unit time were derived. The most favorable imaging parameters, in particular the optimum acquisition time and repetition time for each sequence, were deduced. The sensitivities per unit time of the selected sequences at individually optimized parameters were compared. While the FID sequence (in polar mode) provides the optimum SNR, and is thus favorable for imaging at very low deuterium concentrations, this method is also very susceptible to artifacts and yields poor spatial resolution. In standard situations one will therefore choose the gradient-echo sequence in the Cartesian encoding mode, with a simple spoiling gradient interval. The 20% lower sensitivity of this sequence is compensated for by its significantly better overall image quality. Optimal parameters of this sequence with respect to SNR per unit time include an acquisition time of 0.7T 2 ∗ and a repetition time of 0.7 T 1. When deuterium MRI was applied to small plants, it was possible to demonstrate that (i) spatial structures of 0.3 mm can be resolved at high D 20 concentrations, (ii) concentrations as low as 20 m M can be imaged at spatial resolution of 5 mm, and (iii) time resolution can be chosen as low as 3 min in projectional images at a spatial resolution of 0.5 mm, if tracer concentrations in the range of 50% D 20 are applied. In kinetic image series the distribution patterns, the transport pathways, and the transport velocity of water in plants could be determined.