Histological Basis of Laminar MRI Patterns in High Resolution Images of Fixed Human Auditory Cortex

Abstract

Functional magnetic resonance imaging (fMRI) studies of the auditory region of the temporal lobe would benefit from the availability of image contrast that allowed direct identification of the primary auditory cortex, as this region cannot be accurately located using gyral landmarks alone. Previous work has suggested that the primary area can be identified in magnetic resonance (MR) images because of its relatively high myelin content. However, MR images are also affected by the iron content of the tissue and in this study we sought to confirm that different MR image contrasts did correlate with the myelin content in the gray matter and were not primarily affected by iron content as is the case in the primary visual and somatosensory areas. By imaging blocks of fixed post-mortem cortex in a 7 T scanner and then sectioning them for histological staining we sought to assess the relative contribution of myelin and iron to the gray matter contrast in the auditory region. Evaluating the image contrast in T2*-weighted images and quantitative R2* maps showed a reasonably high correlation between the myelin density of the gray matter and the intensity of the MR images. The correlation with T1-weighted phase sensitive inversion recovery (PSIR) images was better than with the previous two image types, and there were clearly differentiated borders between adjacent cortical areas in these images. A significant amount of iron was present in the auditory region, but did not seem to contribute to the laminar pattern of the cortical gray matter in MR images. Similar levels of iron were present in the gray and white matter and although iron was present in fibers within the gray matter, these fibers were fairly uniformly distributed across the cortex. Thus, we conclude that T1- and T2*-weighted imaging sequences do demonstrate the relatively high myelin levels that are characteristic of the deep layers in primary auditory cortex and allow it and some of the surrounding areas to be reliably distinguished.

Gray matter T1‐w/T2‐w ratios are higher in Alzheimer's disease

“…Previous studies have suggested that myelin is the predominant source of MR contrast in T1‐weighted and T2‐weighted images (Bock, Kocharyan, Liu, & Silva, ; Eickhoff et al, ; Laule et al, ; Wallace et al, ), and by using the ratio of these images the contrast sensitivity to detect myelin content is increased further due to attenuation of the shared intensity biases (Glasser & Van Essen, ). Although, other studies have questioned the sensitivity of the T1‐w/T2‐w ratio for myelin content, and argued that it is likely not the only factor contributing to the T1‐w/T2‐w image (Arshad et al, ; Righart et al, ; Uddin et al, ).…”

“…These changes tended to be most pronounced in anatomical areas known to be affected in AD such as the interior parietal lobule and precuneus, and were associated with higher levels of the neuronal injury marker tau and worse cognition. Wallace et al, 2016), and by using the ratio of these images the contrast sensitivity to detect myelin content is increased further due to attenuation of the shared intensity biases (Glasser & Van Essen, 2011). Although, other studies have questioned the sensitivity of the T1-w/T2-w ratio for myelin content, and argued that it is likely not the only factor contributing to the T1-w/T2-w image (Arshad et al, 2017;Righart et al, 2017;Uddin et al, 2018).…”

Extensive tonotopic mapping across auditory cortex is recapitulated by spectrally-directed attention, and systematically related to cortical myeloarchitecture

“…Finally, is there spatial correspondence between auditory cortical anatomy, as measured by the local change in R1-estimated myelination, and the strength of the fMRI-assessed strength of relative frequency selectivity? Post-mortem Gallyas staining to establish human myeloarchitecture reveals considerable variability in auditory cortical myelination that is associated with MRI signal change in the same brain (Wallace et al, 2016). Likewise, variation in cortical myelination estimated using T1/T2 ratio approaches also appears to correspond spatially with some functional variation in the superior temporal lobe (Glasser et al, 2016).…”

In-vivo magnetic resonance imaging (MRI) of laminae in the human cortex

“…This results in a more direct relationship with specific microstructural components, although not a trivial one-to-one relationship. Studies on the contribution of myelin and iron to different relaxation time constants showed that maps of R1 (¼1/T1) values are mainly dominated by myelin concentration whereas R2* (¼1/T2*) or quantitative susceptibility maps are particularly sensitive to iron (Wharton and Bowtell, 2012) in deep gray matter structures but sensitive to both myelin and iron in cortical gray matter (Stüber et al, 2014;Wallace et al, 2016). Proton density maps, however, can be used to visualize macromolecular tissue volume (Mezer et al, 2013).…”