FLAIR images at 7 Tesla MRI highlight the ependyma and the outer layers of the cerebral cortex

- 7 T FLAIR images show a hyperintense rim at the outer layers of the cortex.
- We performed ex vivo experiments to clarify the anatomical correlates of the rim.
- We performed simulations to better understand potential underlying mechanisms.
- The rim corresponds to layer I of the cortex on high resolution ex vivo FLAIR.
- The rim can be explained by a long T2 in the outer layer, and is not an artifact.

ObjectivesFluid-attenuated inversion recovery (FLAIR) imaging is an important clinical 'work horse' for brain MRI and has proven to facilitate imaging of both intracortical lesions as well as cortical layers at 7 T MRI. A prominent observation on 7 T FLAIR images is a hyperintense rim at the cortical surface and around the ventricles. We aimed to clarify the anatomical correlates and underlying contrast mechanisms of this hyperintense rim.Materials and MethodsTwo experiments with post-mortem human brain tissue were performed. FLAIR and T2-weighted images were obtained at typical in vivo (0.8 mm isotropic) and high resolution (0.25 mm isotropic). At one location the cortical surface was partly removed, and scanned again. Imaging was followed by histological and immunohistochemical analysis. Additionally, several simulations were performed to evaluate the potential contribution from an artifact due to water diffusion.ResultsThe hyperintense rim corresponded to the outer - glia rich - layer of the cortex and disappeared upon removal of that layer. At the ventricles, the rim corresponded to the ependymal layer, and was not present at white matter/fluid borders at an artificial cut. The simulations supported the hypothesis that the hyperintense rim reflects the tissue properties in the outer cortical layers (or ependymal layer for the ventricles), and is not merely an artifact, although not all observations were explained by the simulated model of the contrast mechanism.Conclusions7 T FLAIR seems to amplify the signal from layers I-III of the cortex and the ependyma around the ventricles. Although diffusion of water from layer I into CSF does contribute to this effect, a long T2 relaxation time constant in layer I, and probably also layer II-III, is most likely the major contributor, since the rim disappears upon removal of that layer. This knowledge can help the interpretation of imaging results in cortical development and in patients with cortical pathology.