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Studying disease-related brain alterations in bipolar disorder with combined analysis of DKI and VBM1Department of Radiology, the University of Tokyo, Tokyo, Japan, 2Department of Radiology, Juntendo University, Tokyo, Japan, 3Department of Neuropsychiatry, the University of Tokyo, Tokyo, Japan, 4Department of Radiology, the University of Tokyo Hospital, Tokyo, Japan
Synopsis
Brain abnormalities in bipolar disorder were investigated with diffusion kurtosis imaging and voxel-based morphometry, using a framework for data-driven feature extraction from multivariate data. The result showed two components capturing effect of diagnosis, and these were driven by diffusion kurtosis measures in the white matter including the prefrontal-striatal-thalamic pathways, cerebellum, and medial temporal lobes. Our results indicate diffusion kurtosis imaging can provide unique information that is sensitive to the abnormalities in bipolar disorder, and that interrelationship among different measures is a promising avenue to study neuronal circuits relevant to the disease.
Introduction
Bipolar disorder (BD) is a common affective disorder that has a large burden on society. Though the exact etiologies of BD remain unknown, data from post-mortem, genetic, and imaging studies have provided evidence for substantial brain abnormalities.1-4 MRI studies using functional MRI,5 morphemetry,6,7 diffusion,8 and other modes of microstructural imaging9 have shown intriguing findings, that seem to relate one another. While most previous studies focused on each measure separately, studying the cross-information among different measures can bring us even more.10 Also, application of diffusion kurtosis imaging (DKI) in BD has been scarce, whereas its strength over diffusion tensor imaging (DTI) has been repeatedly reported in other psychological disorders like schizophrenia.11 In this study, we investigated brain abnormalities in patients with BD with DTI/DKI and voxel-based morphometry (VBM), using multivariate fusion analysis.12Methods
Participants: Data from 27 patients with BD (34.7 ± 10.7 years-old, 17 male) and 43 healthy controls (39.2 ± 8.0 years-old, 16 male) were analyzed. All participants were right-handed, and had no history of neuropsychological disease other than BD, alcohol or drug abuse, head trauma, or any abnormalities visible on conventional MRI.
Image acquisition: MRI data were acquired using a 3-T unit (Discovery MR750w, GE Healthcare). A single-shot EPI sequence was used with three diffusion weightings (b = 1000, 1500, and 2000 s/mm2) along 30 non-collinear directions, and 5 b= 0 s/mm2 volumes (voxel size = 1.88 × 1.88 × 2.50 mm3; δ/Δ = 35.1/44.7 ms). Structural 3D T1-weighted images were acquired using SPGR sequence (voxel size = 1.00 × 1.00 × 1.20 mm3).
Diffusion processing: DTI/DKI metrics were computed using DKE software13 after denoising,14 correction for Gibbs ringing,15 and correction for motion and eddy current distortions.16 The diffusion parameter maps (mean diffusivity (MD), fractional anisotropy (FA), mean kurtosis (MK), and kurtosis fractional anisotropy (KFA)) were projected onto mean FA skeleton created by FSL’s TBSS routine.
VBM processing: T1-weighted images were processed with FSL-VBM. For smoothing, we used an isotropic Gaussian kernel with a sigma of 4 mm.
Linked independent component analysis (LICA): The skeletonized diffusion maps and smoothed gray matter density maps were analyzed with FMRIB’s implementation of LICA (http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FLICA) (Figure 1). LICA was initialized with principal component analysis, where we determined model order (number of components) based on Bayesian model selection.17
Statistical test: We tested diagnostic group differences in all LICA components accounting for age and sex, using general linear model as implemented in Permutation Analysis of Linear Models.18 Significance threshold was set at p < 0.05 (two-tailed), corrected for multiple comparisons across components.
Results
Nine independent components were identified. Figure 2 shows the relative weights of the examined measures in each component, as well as the effect size of diagnosis. We found significant effect of diagnosis on two components (Component #3 and #6, p = 0.004 and 0.01, respectively) (Figures 3&4). We had another component indicating smaller FA in the corpus callosum of the patients, though this did not survive after correction for multiple comparisons (Component #4, Figure 5).Discussion and Conclusion
Data-driven feature extraction identified components that showed linked behavior in the multivariate space. Two components were sensitive to disease-related abnormalities, and these were driven by the diffusion metrics of white matter regions including the prefrontal-striatal-thalamic pathways, cerebellum, and medial temporal lobes (Figures 3&4). The present observation is consistent with the concept that BD is caused by disruption of the emotion control circuit formed by these anatomical structures,3,4 as well as with the previous DKI study that analyzed the cerebellum of patients with BD.19
The components capturing effect of diagnosis were dominated by the DKI measures, which suggests studying the white matter with DKI possibly enhances our sensitivity to brain abnormalities in BD, compared to DTI. The role of white matter in BD is now attracting increasing attentions,1,2 and such gain in sensitivity is expected to aid, for example, machine-learning approaches20,21 and investigation of heterogeneity under the same diagnostic category.22 Though not specific, smaller MK values observed in BD are in agreement with the histological studies that showed loss of oligodendrocytes and reduction of myelin.1,2 Also noteworthy is the opposite trend seen in a few regions, like anterior limb of the internal capsule and subcortical white matter of the medial orbito-frontal cortices (Figure 3), since these structures are also involved in the emotion control circuit3,4,23. Indeed, some studies suggested compensatory re-myelination occurring in the frontal cortical/subcortical region of patients with BD.2 Studying such (anti-)correlation among different measures as well as different anatomical locations may lead us to a more comprehensive understanding of the disease mechanism.
Acknowledgements
This study was partly supported by Japan Society for the Promotion of Science (JSPS) KAKENHI [Grant Number 16H06395, 16H06399, 16K21720, 17H04244, 18K07729, and Advanced Bioimaging Support [Grant Number 16H06280]], the Brain Mapping by Integrated Neurotechnologies for Disease Studies (Brain/MINDS), Integrated Research on Depression, Dementia and Development disorders by the Strategic Research Program for Brain Sciences from the Japan Agency for Medical Research and Development, AMED, UTokyo Center for Integrative Science of Human Behavior (CiSHuB), and International Research Center for NeuroIntelligence (IRCN).References
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