How aging affects cortical layers

What is it about?

The cortical mantle is not homogeneous but can be subdivided into up to six distinctive layers or laminae based on cytoarchitectural features. Neither in vivo nor ex vivo magnetic resonance (MR) imaging has currently the necessary resolution to capture the cytoarchitectural features needed to distinguish between different cytoarchitectural layers. Cortical myelin and iron bound to cortical myelin represent the main contributors to the cortical MR signal and histological research has not only shown considerable concordance between cyto- and myeloarchitecture but also that inner layers (layers V-VI) are more heavily myelinated than outer layers (layer i-IV) The aim of this study was to investigate the following questions: 1. Can high resolution images obtained with the MP2RAGE sequence at 7T in combination with k-means clustering be used to distinguish between heavily and sparsely myelinated layers in cortical gray matter? 2. Does this approach provide meaningful biological information? MP2RAGE images of 45 healthy controls (age: 19-75, f/m = 23/22) from the ATAG data repository were used to generate three different contrasts: 1. T1 weighted image (UNI). 2. T1 relaxation image (T1map). 3. INVC/T1map ratio (RATIO). K-means clustering identified 6 clusters/tissue maps (cerebrospinal fluid, cerebrospinal/gray matter transition, white matter, white matter/gray matter transition, heavily myelinated cortical GM (dGM), sparsely myelinated cortical GM (sGM)). The resulting maps showed good agreement with histological maps of cortical myelin in the literature. Volume-based analyses and surface-based analyses were used to compare dGM and sGM volume/thickness of young adults (n = 27, 19-27 years) with those of older adults (n = 18, 42-75 years) at p<0.001 uncorrected. Both approaches found age-related dGM loss/thinning in the mid-posterior cingulate and parahippocampal/entorhinal gyrus and age-related sGM losses in lateral, mesial and orbitofrontal frontal, insular cortex and superior temporal gyrus.

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Why is it important?

Based on this observation it can be concluded that MP2RAGE derived UNI, T1map and RATIO contrasts can be used to identify dGM and sGM. Considering the close relationship between cortical myelo- and cytoarchitecture, the findings reported here indicate that this new technique might provide new insights into the nature of cortical GM loss in different physiological and pathological conditions.

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