Radiology: 7T MRI opens new windows into MS lesions
Jeff H. Duyn, PhD, of the National Institutes of Health in Bethesda, Md., and colleagues wanted to test the effectiveness of recently available ultra-high-field-strength MRI systems in characterizing MS. Specifically, they wanted to see if the improved contrast of susceptibility-weighted 7T MRI images, which is highly variable within and among MS lesions, is correlated to levels of iron and myelin.
The researchers obtained high-spatial-resolution susceptibility-weighted 7T MR images from 21 MS patients and investigated contrast patterns in quantitative phase and R2* images in areas defined as chronic MS lesions. Positive or negative phase shifts were assessed in each lesion. Duyn et al also performed postmortem MR imaging at 7T and 11.7T and the results were correlated with immunohistochemical staining for myelin, iron and ferritin.
Results showed that 60.5 percent of the lesions had a normal phase and a reduced R2*, while 38.2 percent had negative phase shift and a variety of appearances on R2* maps. When compared with histologic findings, R2* reduction corresponded to severe loss of iron and myelin, while negative phase shift corresponded to focal iron deposits with myelin loss.
“Changes in these parameters are interpreted as indicators of myelin and axon loss, which may follow the initial inflammatory process in MS-induced [white matter] lesions,” wrote the authors.
MS is a demyelinating disease, though prior to the current study, it was not known the extent to which myelin and iron contributed to contrast variations in susceptibility-weighted MR images.
“The observed R2* decreases suggest profound myelin loss, whereas negative phase shifts suggest a focal iron accumulation,” concluded the authors. “The tissue concentration of nonheme iron and myelin are particularly relevant to MS as they may be reflective of the underlying disease process.”
The researchers noted that one of the limitations of the study was that only one postmortem brain was available for investigation. Not all lesion types seen in vivo were available from those tissue samples, so the generalizability of the results is limited.
Other factors besides iron and myelin, such as fiber orientation and tissue microstructure, might play a role in image contrast and could be considered in future research.