Multi-component T1 relaxation and magnetisation transfer in peripheral nerve

We report here a study of longitudinal relaxation (T1) and magnetisation transfer (MT) in peripheral nerve. Amphibian sciatic nerve was maintained in vitro and studied at a magnetic field strength of 3 T. A CPMG pulse sequence was modified to include either a saturation pulse to measure T1 relaxation or an off-resonance RF irradiation pulse to measure MT. The resulting transverse relaxation (T2) spectra yielded four components corresponding to three nerve compartments, taken to result from myelinic, axonal, and inter-axonal water, and a fourth corresponding to the buffer solution water in which the nerve sample was bathed. Each nerve component was analysed for T1 relaxation and MT. All three nerve T2 components exhibited unique T1 relaxation and MT characteristics, providing further support for the assignment of the components to unique physical compartments of water. Numerical investigation of T1sat measurements of each of the three nerve T2 components indicates that while the two shorter-lived exhibit similar steady-state magnetisation transfer ratios (MTRs), their respective MT properties are quite different. Simulations demonstrate that mobile water exchange between these two components is not necessary to explain their similar steady-state MTR. In the context of the assignment of these two components to signal from myelinic and axonal water, this is to say that these two microanatomical regions of nerve may exhibit similar steady-state MTR characteristics despite possessing widely different MT exchange rates. Therefore, interpreting changes in MTR solely to reflect a change in degree of myelination could lead to erroneous conclusions.