Diffusion tensor imaging (DTI) was demonstrated to successfully trace three-dimensional trajectory of neuronal fiber tracts in vivo and has been widely utilized in many clinical applications. Recently, many studies further utilized fiber tractography to construct brain structural connectivity networks that were demonstrated capable of detecting the disruptions of structural networks caused by different disorders^1-4. However, most structural connectivity studies were conducted using conventional single-shot DTI. As conventional DTI has problems of intra-voxel fiber crossings and susceptibility distortions and may result in false fiber tracts and detrimentally affect the analysis of connectivity networks. Therefore, the purpose of this study was to utilize PROPELLER echo-planar DTI technique with probabilistic tractography to construct brain connectivity networks and statistically compare the results of connectivity networks between male and female subjects.Forty healthy subjects (Male/Female=20/20, age=18-22 y/o) who had no history of neurological disease participated in this study. All imaging data were acquired from a 3.0T MR scanner (General Electric, Milwaukee, WI, USA). After acquiring high-resolution three-dimensional T1-weighted images, this study performed both single-shot and PROPELLER DTI acquisitions by applying diffusion-sensitizing gradient in 30 non-collinear directions with b-value=1000 s/mm² plus one b0 image, and other imaging parameters were kept identical. The scan time for single-shot and PROPELLER DTI was 5 min 29 sec and 11 min 12 sec respectively. Moreover, distortion-free turbo-spin-echo T2-weighted images were also acquired for comparisons of susceptibility distortions between single-shot and PROPELLER DTI. All imaging data were transferred to a standalone workstation running FSL (FMRIB Software Library, Oxford) and MATALB (Mathworks, Natick, MA, USA) programs. After the pre-processing of DTI datasets, the BET (Brain Extraction Tool, FSL) was then performed on both single-shot and PROPELLER DTI datasets to remove non-brain background signals, and the DTIFTI tool was used to calculate the fractional anisotropy and mean diffusivity maps. For connectivity analysis, a Bayesian estimation of crossing fibers (BEDPOSTX, FMRIB, Oxford, UK) was performed on both datasets to estimate multiple fiber orientations of each voxel in the brain. Afterwards, the template T1 images, which define the 116 AAL (Automatic Anatomical Labeling) cortical regions, were spatially transformed to match the individual T1 images using linear affine and non-linear demon registrations, from which the entire cerebral cortex of individual brain was divided into 116 AAL cortical regions and were used as seeding regions to trace neuronal fibers using a probabilistic tractography (PROBTRACKX, FMRIB, Oxford, UK) with 5000 seeds per voxel. Finally, a connectivity matrix that contains the information of normalized number of fiber tracts between 116 cortical regions was obtained. A two-sample t test was performed to show the difference of connectome between single-shot and PROPELLER DTI and between male and female subjects in single-shot and PROPELLER DTI. The difference was considered significant as P < 0.005.The statistical comparisons showed that the structural connectivity between most cortical regions was significantly higher in PROPELLER DTI than single-shot DTI; however, PROPELLER DTI exhibited significantly lower structural connectivity than single-shot DTI in regions near frontal lobe and brain stem areas, as shown in Fig. 1. The comparisons of structural connectivity between male and female subjects further revealed that male subjects had significantly higher intra-hemispheric connectivity than female, and female subjects exhibited significantly higher inter-hemispheric connectivity than male subjects in both single-shot and PROPELLER DTI, as shown in Figs. 2 and 3, respectively, consistent with the previous findings³. Notably, the conventional single-shot DTI exhibited more sex difference of structural connectivity in regions near the frontal lobe and brain stem areas than that of PROPELLER DTI. As there were susceptibility distortions in frontal and brain stem areas in single-shot DTI, the sex difference revealed in those regions might be erroneously enhanced by susceptibility distortions.

Although PROPELLER echo-planar DTI took longer scan time than single-shot DTI, it was helpful to reduce susceptibility distortions and was more suitable for analyzing structural connectivity networks than those using single-shot DTI technique in regions with air-tissue and bone-tissue interfaces.