Accurate information of the microstructure properties of the corpus callosum (CC) is of upmost importance to understand its anatomical and functional connectivity to the cortical areas. Due to the complexity of the structure, it is extremely challenging for quantitatively assessing and identifying the distinct microstructures and functional subdivisions within CC in vivo using neuroimaging approaches. In this work, we hypothesized that parametric MR T₁ relaxation times could reflect the callosal microstructure properties such as the various fiber sizes with unique anatomical connectivity to the functionally relevant cortical areas. To test this hypothesis, we studied healthy subjects at ultrahigh field of 7T, which provides high-resolution parametric T₁ images with sufficient signal-to-noise ratio (SNR). In addition, DTI data were acquired to verify the anatomical connectivity between CC and projected cortical regions.

Eighteen subjects (12 M / 6 F, 32.4±13.1 years ) participated in this study. All MRI studies were performed at 7.0T/90cm MRI scanner (Siemens) with a 32-channel head coil (Nova).
T₁ mapping: The parametric T₁ image across the mid-sagittal CC slice was measured using the single-shot fast spin-echo sequence ^[1] with seven inversion recovery times. A high in-plane resolution (500x500 μm²) was applied to reduce the partial volume effect from surrounding tissues. The optimal RF power was applied with actual flip angle method ^[2]. Callosal Parcellation: Parcellation method was described in detail previously ^[3]. Briefly, after segmentation of mid-sagittal region of CC on the T₁-weighted images, equidistant surface points making up the surface boundaries were calculated and 7 regions were parcellated according to Witelson’s model ^[4]. T₁ Normalization and Spatial Registration: To account for the T₁ variations of individual subjects, all measured T₁ values were normalized by dividing their mean T₁ values . Each normalized T₁ maps were then spatially registered into the T₁ image of a representative subject using linear transformation ^[5]. Diffusion Tractography: For the confirmation of anatomical connectivity of CC to the cortical areas, DTI data were acquired (7s repetition time, b₀ = 2100 s/mm² with 110 diffusion gradient directions), and were processed according to HCP pipeline ^[6] with FDT diffusion toolbox including BEDPOSTX, which allows to model crossing fibers within each voxel.

Figure 1 shows the averaged T₁ map of the human CC displays a distinct distribution of T₁ values across different CC subdivision; highest T₁s in the inferior splenium, and lowest in rostrum. Furthermore, it is worthy to note that T₁s within the same subdivision shows highly inhomogeneous distribution (Fig. 2); for instance, the inferior part of splenium shows significantly higher T₁ compared to the posterior one, thus, high-resolution T₁ image is critical to identify such differentiation within a small subdivision. So far, post-mortem histology studies ^[7] have well defined the microstructural properties of callosal fibers such as size and density. The reported ex vivo regional differentiation of callosal microstructure are strikingly consistent with the T₁ distribution as imaged in this study (comparison result in Fig. 3). The high similarities between the MR T₁ relaxometry and histological analysis strongly support that callosal T₁ values may reflect the fiber diameters. Thus, two regions of interest (ROIs) was selected: one with high T₁ value (large fibers) and low T1 (small fibers), then identifying their projected cortices using tractography method. Figure 4 illustrates the connections between the selected CC ROIs and cortical lobes. Interestingly, the inferior part of splenium with large fiber size is connected to the visual sensory cortex, in contrast, the posterior part of splenium with small fiber size is connected to the parietal-temporal lobes with higher functionality. This observation is also consistent with the facts that the medium size fibers in the CC are connected to somatosensory and motor cortices, in contrast, the smallest size fibers in genu are connected to the front lobe with much complex functionality. These CC-cortical connections are in good agreement with previous diffusion tensor MRI study ^[8-9]. Our results suggest that the large CC fibers could facilitate fast communication of electrical signals between CC and sensory cortices, and in contrast, small CC fibers could facilitate complex signal but with relatively slow speed with higher functionality. Therefore, spatial distribution of T₁ relaxometry of the CC provides unique microstructural and functional specialization in the human brain.The results show the utility of complementary parametric T₁ approaches to quantitatively assess the fiber microstructure of the corpus callosum and unique functional connectivity to cortical regions. Therefore, this imaging approach could provide a robust and useful biomarker for delineation of the axonal fiber abnormality, and clinically potential for the white matter diseases.