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

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 7T MRI. A prominent observation on 7T 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 (0.8mm isotropic) and high resolution (0.25mm 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 7T 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.

[1]  H. C. Torrey.  Bloch Equations with Diffusion Terms , 1956 .

[2]  Jeroen van der Grond,et al.  Increased number of microinfarcts in Alzheimer disease at 7-T MR imaging. , 2013, Radiology.

[3]  Jaco J. M. Zwanenburg,et al.  Fluid attenuated inversion recovery (FLAIR) MRI at 7.0 Tesla: comparison with 1.5 and 3.0 Tesla , 2009, European Radiology.

[4]  K. Uğurbil,et al.  Magnetic field and tissue dependencies of human brain longitudinal 1H2O relaxation in vivo , 2007, Magnetic resonance in medicine.

[5]  Amandine Jullienne,et al.  Aquaporin and brain diseases. , 2014, Biochimica et biophysica acta.

[6]  P. Kalus,et al.  Cortical layer I changes in schizophrenia: a marker for impaired brain development? , 2005, Journal of Neural Transmission.

[7]  D. Le Bihan,et al.  Diffusion tensor imaging: Concepts and applications , 2001, Journal of magnetic resonance imaging : JMRI.

[8]  R. Turner,et al.  Layer-Specific Intracortical Connectivity Revealed with Diffusion MRI , 2012, Cerebral cortex.

[9]  Peter R Luijten,et al.  High‐resolution magnetization‐prepared 3D‐FLAIR imaging at 7.0 Tesla , 2010, Magnetic resonance in medicine.

[10]  Peter R Luijten,et al.  In Vivo Detection of Cerebral Cortical Microinfarcts with High-Resolution 7T MRI , 2013, Journal of cerebral blood flow and metabolism : official journal of the International Society of Cerebral Blood Flow and Metabolism.

[11]  Thomas Wisniewski,et al.  The neuropathology of autism: defects of neurogenesis and neuronal migration, and dysplastic changes , 2010, Acta Neuropathologica.

[12]  J R MacFall,et al.  Mechanisms of Contrast in NMR Imaging , 1984, Journal of computer assisted tomography.

[13]  Junjie Liu,et al.  Laminar profiles of functional activity in the human brain , 2007, NeuroImage.

[14]  Arzu Arslan,et al.  Perirolandic cortex of the normal brain: low signal intensity on turbo FLAIR MR images. , 2003, Radiology.

[15]  Pierre J Magistretti,et al.  Aquaporins in Brain: Distribution, Physiology, and Pathophysiology , 2002, Journal of cerebral blood flow and metabolism : official journal of the International Society of Cerebral Blood Flow and Metabolism.

[16]  Jeff H. Duyn,et al.  High-field MRI of brain cortical substructure based on signal phase , 2007, Proceedings of the National Academy of Sciences.

[17]  K. Uğurbil,et al.  Spin‐echo fMRI in humans using high spatial resolutions and high magnetic fields , 2003, Magnetic resonance in medicine.

[18]  J. Zwanenburg,et al.  High‐resolution MRI of the carotid arteries using a leaky waveguide transmitter and a high‐density receive array at 7 T , 2013, Magnetic resonance in medicine.

[19]  Mike P. Wattjes,et al.  Lesion detection at seven Tesla in multiple sclerosis using magnetisation prepared 3D-FLAIR and 3D-DIR , 2011, European Radiology.

[20]  P. Gowland,et al.  Cortical lesion load correlates with diffuse injury of multiple sclerosis normal appearing white matter , 2014, Multiple sclerosis.

[21]  K. Scheffler.  A pictorial description of steady-states in rapid magnetic resonance imaging , 1999 .

[22]  Peter R Luijten,et al.  Generalized multiple-layer appearance of the cerebral cortex with 3D FLAIR 7.0-T MR imaging. , 2012, Radiology.

[23]  M. Kleinnijenhuis,et al.  Layer-specific diffusion weighted imaging in human primary visual cortex in vitro , 2013, Cortex.

[24]  Eric E. Smith,et al.  Cerebral microinfarcts: the invisible lesions , 2012, Lancet Neurology.

[25]  M. D. Del Bigio,et al.  Ependymal cells: biology and pathology , 2009, Acta Neuropathologica.

[26]  P. Jakob,et al.  Diffusion generated T1 and T2 contrast. , 2008, Journal of magnetic resonance.

[27]  S. Shipp.  Structure and function of the cerebral cortex , 2007, Current Biology.

[28]  R. Turner,et al.  Microstructural Parcellation of the Human Cerebral Cortex – From Brodmann's Post-Mortem Map to in vivo Mapping with High-Field Magnetic Resonance Imaging , 2011, Front. Hum. Neurosci..

[29]  Milan Sonka,et al.  3D Slicer as an image computing platform for the Quantitative Imaging Network. , 2012, Magnetic resonance imaging.