Health & Medical Neurological Conditions

Ultra-High-Field MR Imaging in Multiple Sclerosis

´╗┐Ultra-High-Field MR Imaging in Multiple Sclerosis

Imaging WM Lesions

Ultra-high-field MRI allows better definition of lesions located in the WM and GM, their morphology and their association with the vasculature at a resolution which is similar to that of pathological assessment. Several studies have shown that 7 T and 8 T systems detect a higher number of lesions within the brain WM in patients with established MS in comparison with 1.5 and 3 T scanners. This suggests that abnormalities detected using quantitative MR techniques in the normal-appearing WM (NAWM) are, at least in part, due to the presence of focal lesions which go undetected when using low-field magnets. Whether the assessment of lesion burden and distribution using ultra-high-field MRI scanners assists in making an earlier diagnosis of patients presenting with a clinically isolated syndrome suggestive of MS has not yet been evaluated. However, thanks to the morphological detail that can be seen with these scanners, several studies have contributed to the identification of some interesting lesion characteristics, which can aid the differential diagnosis between MS and other neurological conditions that can mimic the disease. Specifically, due to the better definition of the relationship between demyelinating lesions and the deep venous system, several studies have shown that MS plaques form around the microvasculature. This feature can help to distinguish WM lesions in MS patients from incidental WM lesions, and has been reinforced by an initial investigation of blood-brain barrier abnormalities in MS at 7 T, which showed that the majority of enhancing lesions are perivenular, and that the smallest lesions have a concentric pattern of enhancement, suggesting that they grow outward from a central vein. The presence of a central small vein and a rim of hypointensity on 7 T T2*-weighted magnitude images can also assist in the differentiation of WM lesions found in MS patients from those of patients with neuromyelitis optica spectrum disorders or Susac syndrome. In this latter condition, T1-hypointense lesions within the central part of the corpus callosum (CC), that are not commonly seen in MS, have also been detected.

The use of iron-sensitive MRI sequences has provided additional insights into MS lesion characteristics by showing, for example, the presence of a peripheral ring of iron deposition around some acute and chronic MS lesions. A correlative MR/pathology study has shown that iron deposition occurs predominantly within oligodendrocytes in the NAWM, while in active MS lesions iron accumulates inside macrophages at the periphery of the plaques. Perivascular iron deposits do also occur and are most likely a reflection of impairment of vascular permeability. Another combined MR/histology study has suggested that the pooled analysis of R2* and phase images may contribute to a better characterisation of the pathological features of MS lesions, since R2* abnormalities were found to correspond to severe loss of iron and myelin, whereas negative phase shift abnormalities were associated with focal iron accumulation. A recent longitudinal study has shown that ring-phase lesions remained unchanged over a 2.5-year period in five relapsing-remitting MS patients, thus challenging the notion that such lesions reflect the presence of acute activated iron-rich macrophages.

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