Measurement of water-NMR relaxation in peripheral nerve

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 $(T\sb2)$ components in the frog sciatic nerve, including a long-lived component $(T\sb2 > 200$ ms) which previously had only been identified in vitro. The existence of a long-lived $T\sb2$ component indicated echo times of 200-300 ms may provide maximal contrast-to-noise (CNR) (nerve to muscle) in $T\sb2$-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 $T\sb2$-spectra of frog sciatic nerve undergoing Wallerian degeneration. The two most apparent changes as degeneration progressed were a reduction from three well-resolved $T\sb2$ components to one and a decline in the fraction of the spectra associated with short-lived $T\sb2.$ 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 $T\sb2$ components of peripheral nerve water exhibited unique longitudinal relaxation $(T\sb1)$ 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 $T\sb2$ components results from differing intrinsic $T\sb1$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 $T\sb2$ 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.