Marieke van Rosmalen

General discussion 143 8 Interpretation of changes in fat fraction and T2 relaxation times in the peripheral nerves are mainly based on our (limited) knowledge of the histological structure of the peripheral nerves and MRI physics. Interpretation of fat fraction is based on the histological fact that the peripheral nerves are surrounded by myelin. Myelin contains a high content of lipids, and the formation of the myelin sheath requires high levels of fatty acid and lipid synthesis, together with the uptake of extracellular fatty acids. 65 A decrease of the fat fraction in peripheral nerves may therefore reflect demyelination or another process that could decrease fat fraction, such as the increase of water surrounding the nerves, for example due to oedema. An increased T2 relaxation time indicates, based on MRI physics, an increase of the amount of water, for example due to oedema caused by an inflammatory reaction. However, just as interpretation of changes in diffusion parameters, these correlations between histology and changes in fat fraction and T2 relaxation time should be carefully interpreted. Table 8.2 summarizes the (assumed) correlations between quantitative MRI parameters and histology. Table 8.2 Possible correlations of quantitative MRI parameters and histological changes Quantitative MRI parameter Definition 1 Assumed histological correlation FA Measures degree of anisotropy of water molecules Tissue injury, nonspecific MD Summary measure of AD and RD Tissue injury, nonspecific AD Degree of diffusion of water along the main axis When decreased: axonal loss RD Degree of diffusion of water perpendicular on tissue When increased: demyelination, breakdown or change in permeability of axolemma, loss of neurofilaments and microtubules, axonal swelling, increase extracellular space. T2 relaxation time Constant of the fitted exponential decay curve of signal intensity and different echo times When increased: oedema Fat fraction Percentage of fat of the total signal on an MR image When decreased: demyelination, oedema Abbreviations: MRI = magnetic resonance imaging; FA = fractional anisotropy; MD = mean diffusivity; RD = radial diffusivity; AD = axial diffusivity. 1 For an extensive explanation see chapter 1 of this thesis. Despite the limitations described in the previous paragraph, quantitative MRI techniques can be explored to study the condition of the peripheral nerves in vivo in patients with chronic inflammatory neuropathies. A few studies already showed differences in diffusion tensor parameters and T2 relaxation times in smaller cohorts of patients with CIDP, MMN and