Susceptibility, diffusion and relaxation contrast in NMR microscopy at high resolution : a thesis presented in partial fulfilment of the requirement for the degree of Master of Science in physics at Massey University

Abstract

An integrated approach to the functional NMR imaging of plant tissue at moderately-high transverse resolution (23 µm) was undertaken. Attention was paid to all the possible commonly-known influences, such as sources of nuclear spin relaxation or of artefacts, relevant to the final image intensity of the different tissues. While it was not clear at the outset which influences might prove to be significant, two phenomena in particular, susceptibility inhomogeneity and correlated diffusion effects, were selected for detailed investigation using simple model systems constructed from small glass tubes and rods combined with aqueous solutions, before continuing on to more complex plant samples. Simulated images compared well with the experimental results in these studies. Preliminary images of a stem of an intact Stachys sylvatica L. plant showed that the apparent T₂ relaxation time is much less (an order of magnitude) than the T₁ relaxation time in all tissues. A range of diagnostic pulse sequences was then carried out on this and similar stems in order to reveal the signatures for different models of T₂ relaxation which might explain this fact (assuming that the water protons imaged fall within the extreme-narrowed region of Bloembergen, Purcell and Pound theory). It was found that measures were necessary to avoid the complicating factor of attenuation due to diffusion in the applied read gradient, specifically the use of Carr-Purcell-Meiboom-Gill (CPMG) refocusing pulses. Susceptibility inhomogeneity seemed important in sensitive gradient echo images, but further experiments at different B₀ strengths revealed that it (and chemical shift exchange) does not contribute significantly to the spin echo image contrast. The Brownstein-Tarr model of relaxation at boundaries and surfaces (without local field offsets) was also considered as a possibility, but was ruled out for at least some of the tissues (those which display a CPMG pulse-spacing dependence). Another alternative explanation is short-range dipole interactions between water protons and protons of more slowly-moving molecules, which should be abundant in the particular cells which escape the other hypotheses, but it is difficult to confirm this within the scope of the pulse sequences used here. More progress might be possible with proper multicomponent T₂ analysis and improved knowledge of subcellular structure of our particular tissues.