Single-shot T1 mapping of the corpus callosum: a rapid characterization of fiber bundle anatomy.

Sabine Hofer,Jens Frahm,Xiaoqing Wang,Volkert Roeloffs

doi:10.3389/fnana.2015.00057

Abstract

Using diffusion-tensor magnetic resonance imaging and fiber tractography the topographic organization of the human corpus callosum (CC) has been described to comprise five segments with fibers projecting into prefrontal (I), premotor and supplementary motor (II), primary motor (III), and primary sensory areas (IV), as well as into parietal, temporal, and occipital cortical areas (V). In order to more rapidly characterize the underlying anatomy of these segments, this study used a novel single-shot T1 mapping method to quantitatively determine T1 relaxation times in the human CC. A region-of-interest analysis revealed a tendency for the lowest T1 relaxation times in the genu and the highest T1 relaxation times in the somatomotor region of the CC. This observation separates regions dominated by myelinated fibers with large diameters (somatomotor area) from densely packed smaller axonal bundles (genu) with less myelin. The results indicate that characteristic T1 relaxation times in callosal profiles provide an additional means to monitor differences in fiber anatomy, fiber density, and gray matter in respective neocortical areas. In conclusion, rapid T1 mapping allows for a characterization of the axonal architecture in an individual CC in less than 10 s. The approach emerges as a valuable means for studying neocortical brain anatomy with possible implications for the diagnosis of neurodegenerative processes.

Highlights

The corpus callosum (CC) is a unique structure in placental mammals and by far the largest fiber bundle in the human brain interconnecting the two cerebral hemispheres with more than 300 million fibers (de Lacoste et al, 1985; Clarke and Zaidel, 1994; Aboitiz and Montiel, 2003)
The T1 relaxation times in the somatomotor region were in most subjects significantly higher compared to all other CC regions; significance only failed in subject VIII for the anterior midbody
We found variations of T1 relaxation times in four different regions of the human CC, which are in line with the known fiber anatomy in respective topographic locations

Summary

Introduction

The corpus callosum (CC) is a unique structure in placental mammals and by far the largest fiber bundle in the human brain interconnecting the two cerebral hemispheres with more than 300 million fibers (de Lacoste et al, 1985; Clarke and Zaidel, 1994; Aboitiz and Montiel, 2003). In humans and other primates the CC features a rough topographical representation of different cortical areas, in which anterior cortical areas are connected through the anterior CC and posterior areas through posterior regions (Aboitiz et al, 1992; Hofer and Frahm, 2006; Hofer et al, 2008; van der Knaap and van der Ham, 2011). The CC can be identified by conventional magnetic resonance imaging (MRI), there are no in vivo anatomic landmarks that clearly delimit distinct callosal areas in a midsagittal cross-section (van der Knaap and van der Ham, 2011). The density of thin fibers is most apparent in the anterior CC (genu, region I), with fiber diameters between 0.4 and 1 μm. Unmyelinated fibers were found to be scarce, except in the genu where they comprised about 16% of total fibers (Aboitiz et al, 1992)

Methods

Results

Conclusion