| This thesis contains four studies, each of which involves measurement of water-NMR relaxation in peripheral nerve in some manner. The central basis of these studies is that peripheral nerve water exists in three broadly unique environments--myelinic, intra-axonal, and extra-axonal.; In the first study, multi-echo imaging identified three tranverse relaxation {dollar}(Tsb2){dollar} components in the frog sciatic nerve, including a long-lived component {dollar}(Tsb2 > 200{dollar} ms) which previously had only been identified in vitro. The existence of a long-lived {dollar}Tsb2{dollar} component indicated echo times of 200-300 ms may provide maximal contrast-to-noise (CNR) (nerve to muscle) in {dollar}Tsb2{dollar}-weighted images. Averaging selected images from the multi-echo image set, the CNR was increased by a factor of nearly three.; In the second study, multi-echo imaging and in-vitro measurements showed progressive changes in the {dollar}Tsb2{dollar}-spectra of frog sciatic nerve undergoing Wallerian degeneration. The two most apparent changes as degeneration progressed were a reduction from three well-resolved {dollar}Tsb2{dollar} components to one and a decline in the fraction of the spectra associated with short-lived {dollar}Tsb2.{dollar} The former change appears to reflect a collapse of myelinated fibres, while the latter a combination of interstitial oedema and myelin loss.; The third study found that each of the three {dollar}Tsb2{dollar} components of peripheral nerve water exhibited unique longitudinal relaxation {dollar}(Tsb1){dollar} and magnetisation transfer characteristics. Simulations demonstrated that mobile water exchange between axonal and myelinic components was not necessary to explain their similar steady-state magnetisation transfer contrast (MTC)s, and reasoning dictated that water exchange cannot be the primary mechanism for this similarity. Rather, the similar MTC of the two shorter-lived {dollar}Tsb2{dollar} components results from differing intrinsic {dollar}Tsb1{dollar}s. Therefore, interpreting MTC change to solely reflect a change in degree of myelination could lead to erroneous conclusions.; Finally, the fourth study used computer simulations and experimental data to demonstrate that when using sub-optimal spoiler gradients in a multi-echo imaging sequence, increasing the first spoiler gradient slightly reduces the fraction of unwanted signal by several times, resulting in {dollar}Tsb2{dollar} measurements within 1% of those obtained using optimal spoiler gradients. Use of this spoiler adjustment reduces the peak spoiler gradient requirement by a factor of 2-4. |
