MRI has been widely used as a non-invasive technique to probe neural tissue pathologies in variety of nervous system diseases. Observed changes in the MR parameters are thought to be associated with structural changes in neural system such as demyelination, inflammation and cellular loss. However, these degenerative processes may occur concurrently in neural system pathologies, and standard MR parameters are often incapable of distinguishing them. The goal of this study was to investigate the changes in MR parameters such as T1 and T1 relaxation and Magnetization Transfer during experimentally induced inflammation.

To achieve this goal, we have used an animal model of neural tissue inflammation by injecting tumour necrosis factor alpha(TNFa) into rat sciatic nerves. Two days after TNFa injection, the nerves were dissected and MR measurements were performed in vitro at 1.5 T. Additionally, immnohistologic and morphometric analysis was also performed to assess presence of inflammatory cells and the structural changes in the neural tissue. A total of 15 nerves were injected with TNFa and 4 nerves were injected with control PBS.

Histopathology revealed significant inflammation, as seen by significantly increased extracellular volume fraction (p < 0.05) and the presence of a large number of inflammatory cells two days post-TNFa injection. The processes of axonal loss or demyelination were negligible when compared to BSA control injected tissues (p > 0.05). All measured MR parameters changed with inflammation (p< 0.05). Both relaxation times T1 and T2 significantly increased with inflammation (50 – 70%), whereas magnetization transfer ratio (MTR) significantly decreased with inflammation (by approximately 25%). The measured MR parameters were also quantitatively correlated with the histopathological measure of extracellular volume fraction. The observed, multicomponent T2 relaxation was demonstrated to be the best technique in order to distinguish between the process of demyelination and inflammation.