MRI assessment of different types of chronic demyelinating lesions in patients with multiple sclerosis using quantitative susceptibility mapping (QSM)

Histologically, chronic demyelinating lesions in multiple sclerosis (MS) have been shown to include inactive lesions that do not change over time and active or “smouldering" lesions that tend to enlarge over time and are surrounded by pro-inflammatory activated microglial cells that are loaded with iron. To identify “smouldering" foci of demyelination and assess the “latent” inflammatory process in the brain, MRI sequences sensitive to the detection of substances with paramagnetic properties, including iron, must be used. They include an innovative technique such as quantitative susceptibility mapping (QSM).Objective: to identify, using MRI different types of chronic demyelination foci in MS, based on iron distribution and the degree of damage (myelination) in their structure.Material and methods. The patterns of iron distribution in demyelinating lesions in 90 MS patients were investigated using QSM. In addition, two lesions with different iron distribution patterns were randomly selected on the QSM map for each patient, in which the magnetic transfer ratio (MTR), indirectly reflecting the degree of myelination, was calculated. The identified changes were also compared with visualization of lesions in standard MRI modes (T1 MPRAGE, T2 FLAIR).Results. Despite the predominantly identical visualization in T2 FLAIR mode, chronic foci of demyelination show different patterns on the QSM maps, which is due to the peculiarities of iron distribution: some foci are not detected on QSM, while others are visualized either in the form of a homogeneous or a ring-shaped pattern. When comparing QSM data with MTR, it was found that MTR indicators were highest in non-visualized lesions (demyelination is minimal), while damage was most pronounced in lesions with ring-shaped iron distribution.Conclusion. Different patterns of iron distribution in demyelination foci compared to the degree of myelination in these foci according to MTR were identified using QSM, which is of great importance for the evaluation of latent inflammation and the development of the neurodegenerative process in MS.