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 -weighted images and quantitative 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 -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.
Highlights
Significant challenges arise when trying to relate high resolution Functional magnetic resonance imaging (fMRI) images to a standard brain using a standard three-dimensional coordinate system (Morosan et al, 2001; Maldjian et al, 2003)
Even the border of layer VI with the white matter is indistinct, because the density of neurons drops there are some neurons in the white matter and the junction is difficult to identify at high power magnification, as there are many small glial cells in all cortical areas that stain for Nissl substance and obscure the neuronal patterns
The longitudinal relaxation time, T1, and transverse relaxation time, T∗2, of the MR signal in brain tissue are predominantly influenced by the iron content (Fukunaga et al, 2010), myelination (Lee et al, 2012), and degree of binding of water to macromolecules within each imaging voxel (Bock et al, 2009)
Summary
Significant challenges arise when trying to relate high resolution fMRI images to a standard brain using a standard three-dimensional coordinate system (Morosan et al, 2001; Maldjian et al, 2003). It has become more common to try and relate coordinates for functional activity to a corresponding high-resolution structural image of the same brain (Fischl et al, 1999; Walters et al, 2003; Sigalovsky et al, 2006). This works well for many subcortical regions especially if they are large or have a distinctive structure; it is much more challenging when trying to identify functional areas of the neocortex. MRI cannot provide much useful information about the size, form, or even density of neurons that are the features used to classify cytoarchitectonic areas in the neocortex; it can detect the more distinctive areas of the hippocampal region because of their unique arrangement of cell bodies and fibers (Augustinack et al, 2014)
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