Abstract
Objectives: Fluid-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 Methods: Two 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.
Results: The 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.
Conclusions: 7 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
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 Methods: Two 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.
Results: The 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.
Conclusions: 7 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
| Original language | English |
|---|---|
| Title of host publication | Organization for Human Brain Mapping |
| Publication status | Published - 2014 |