Traumatic brain injury, major depression, and diffusion tensor imaging: making connections

Neuroimaging correlates of traumatic brain injury and suicidal behavior

INTRODUCTION

There is an urgent need to define the neurobiological and cognitive underpinnings of suicidal ideation and behavior in veterans with traumatic brain injury (TBI). Separate studies implicate frontal white matter systems in the pathophysiology of TBI, suicidality, and impulsivity. We examined the relationship between the integrity of major frontal white matter (WM) systems on measures of impulsivity and suicidality in veterans with TBI.

METHODS

Fifteen male veterans with TBI and 17 matched healthy controls (HC) received clinical ratings, measures of impulsivity and MRI scans on a 3T magnet. Diffusion tensor imaging (DTI) data for the genu and cingulum were analyzed using Freesurfer and FSL. Correlations were performed for fractional anisotropy (FA) (DTI) values and measures of suicidality and impulsivity for veterans with TBI.

RESULTS

Significantly decreased in FA values in the left cingulum (P = 0.02), and left (P = 0.02) and total genu (P = 0.01) were observed in the TBI group relative to controls. Measures of impulsivity were significantly greater for the TBI group and total and right cingulum FA positively correlated with current suicidal ideation and measures of impulsivity (P <0.03).

CONCLUSION

These data demonstrate a significant reduction in FA in frontal WM tracts in veterans with mild TBI that was associated with both impulsivity and suicidality. These findings may reflect a neurobiological vulnerability to suicidal risk related to white matter microstructure.

Identification of neural targets for the treatment of psychiatric disorders: the role of functional neuroimaging

Neurosurgical treatment of psychiatric disorders has been influenced by evolving neurobiological models of symptom generation. The advent of functional neuroimaging and advances in the neurosciences have revolutionized understanding of the functional neuroanatomy of psychiatric disorders. This article reviews neuroimaging studies of depression from the last 3 decades and describes an emerging neurocircuitry model of mood disorders, focusing on critical circuits of cognition and emotion, particularly those networks involved in the regulation of evaluative, expressive and experiential aspects of emotion. The relevance of this model for neurotherapeutics is discussed, as well as the role of functional neuroimaging of psychiatric disorders.

Sex-dependent influences of obesity on cerebral white matter investigated by diffusion-tensor imaging

Several studies have shown that obesity is associated with changes in human brain function and structure. Since women are more susceptible to obesity than men, it seems plausible that neural correlates may also be different. However, this has not been demonstrated so far. To address this issue, we systematically investigated the brain's white matter (WM) structure in 23 lean to obese women (mean age 25.5 y, std 5.1 y; mean body mass index (BMI) 29.5 kg/m², std 7.3 kg/m²) and 26 lean to obese men (mean age 27.1 y, std 5.0 y; mean BMI 28.8 kg/m², std 6.8 kg/m²) with diffusion-weighted magnetic resonance imaging (MRI). There was no significant age (p>0.2) or BMI (p>0.7) difference between female and male participants. Using tract-based spatial statistics, we correlated several diffusion parameters including the apparent diffusion coefficient, fractional anisotropy (FA), as well as axial (λ_∥) and radial diffusivity (λ_⊥) with BMI and serum leptin levels. In female and male subjects, the putative axon marker λ_∥ was consistently reduced throughout the corpus callosum, particularly in the splenium (r = −0.62, p<0.005). This suggests that obesity may be associated with axonal degeneration. Only in women, the putative myelin marker λ_⊥ significantly increased with increasing BMI (r = 0.57, p<0.005) and serum leptin levels (r = 0.62, p<0.005) predominantly in the genu of the corpus callosum, suggesting additional myelin degeneration. Comparable structural changes were reported for the aging brain, which may point to accelerated aging of WM structure in obese subjects. In conclusion, we demonstrate structural WM changes related to an elevated body weight, but with differences between men and women. Future studies on obesity-related functional and structural brain changes should therefore account for sex-related differences.

An introduction to diffusion tensor image analysis

Diffusion tensor magnetic resonance imaging (DTI) is a relatively new technology that is popular for imaging the white matter of the brain. This article provides a basic and broad overview of DTI to enable the reader to develop an intuitive understanding of these types of data, and an awareness of their strengths and weaknesses.

Diffusion weighted imaging and neuropsychological correlates in adults with mild traumatic brain injury

Diffusion Tensor Imaging (DTI) is increasingly being used as a research tool in mild Traumatic Brain Injury (mTBI). This article reviews the concepts of diffusion tensor imaging, neuropsychological testing and results to date when applied to mTBI in adults. DTI is being used in conjunction with neuropsychological and electrophysiological measures to provide improved structural/functional correlations of mTBI. Future directions and applications of DTI in mTBI research are suggested.

White matter changes in healthy adolescents at familial risk for unipolar depression: a diffusion tensor imaging study

Alterations in white matter integrity of several cortical and subcortical circuits have been reported in relation to unipolar major depressive disorder. It is not clear whether these white matter changes precede the onset of illness. In all, 13 adolescent volunteers with no personal or family history of a psychiatric disorder (controls) and 18 adolescent volunteers with no personal history of a psychiatric illness including depression, but who were at high risk for developing unipolar depression by virtue of parental depression (high-risk youth), underwent diffusion tensor imaging studies. An automated tract-based spatial statistics method, a whole-brain voxel-by-voxel analysis, was used to analyze the scans. Population average diffusion parameter values were also calculated for each tract. Adolescents at high risk for unipolar depression had lower fractional anisotropy (FA) values in the left cingulum, splenium of the corpus callosum, superior longitudinal fasciculi, uncinate, and inferior fronto-occipital fasciculi than did controls. Altered white matter integrity in healthy adolescents at familial risk for unipolar depression suggests that it might serve as a vulnerability marker for the illness.

Prognostic prediction of therapeutic response in depression using high-field MR imaging

Despite significant advances in the treatment of major depression, there is a high degree of variability in how patients respond to treatment. Approximately 70% of patients show some improvement following standard antidepressant treatment and are classified as having non-refractory depressive disorder (NDD), while the remaining 30% of patients do not respond to treatment and are classified as having refractory depressive disorder (RDD). At present, there are no objective, neurological markers which can be used to identify individuals with depression and predict clinical outcome. We therefore examined the diagnostic and prognostic potential of pre-treatment structural neuroanatomy using support vector machine (SVM). Sixty-one drug-naïve adults suffering from depression and 42 healthy volunteers were scanned using structural magnetic resonance imaging (sMRI). Patients then received standard antidepressant medication (either tricyclic, typical serotonin-norepinephrine reuptake inhibitor or typical selective serotonin reuptake inhibitor). Based on clinical outcome, we selected two groups of RDD (n=23) and NDD (n=23) patients matched for age, sex and pre-treatment severity of depression. Diagnostic accuracy of gray matter was 67.39% for RDD (p=0.01) and 76.09% for NDD (p<0.001), while diagnostic accuracy of white matter was 58.70% for RDD (p=0.13) and 84.65% for NDD (p<0.001). SVM applied to gray matter correctly distinguished between RDD and NDD patients with an accuracy of 69.57% (p=0.006); in contrast, SVM applied to white matter predicted clinical outcome with an accuracy of 65.22% (p=0.02). These results indicate that both gray and white matter have diagnostic and prognostic potential in major depression and may provide an initial step towards the use of biological markers to inform clinical treatment. Future studies will benefit from the integration of structural neuroimaging with other imaging modalities as well as genetic, clinical and cognitive information.

In vivo mapping of microstructural somatotopies in the human corticospinal pathways

The human corticospinal pathway is organized in a body-centric (i.e., somatotopic) manner that begins in cortical cell bodies and is maintained in the axons as they project through the midbrain on their way to spinal motor neurons. The subcortical segment of this somatotopy has been described using histological methods on non-human primates but only coarsely validated from lesion studies in human patient populations. Using high definition fiber tracking (HDFT) techniques, we set out to provide the first in vivo quantitative description of the midbrain somatotopy of corticospinal fibers in humans. Multi-shell diffusion imaging and deterministic fiber tracking were used to map white matter bundles that originate in the neocortex, navigate complex fiber crossings, and project through the midbrain. These fiber bundles were segmented into premotor (dorsal premotor, ventral premotor, and supplementary motor area) and primary motor sections based on the cortical origin of each fiber streamline. With HDFT, we were able to reveal several unique corticospinal patterns, including the cortical origins of ventral premotor fibers and small (∼ 1-2 mm) shifts in the midbrain location of premotor versus primary motor cortex fibers. More importantly, within the relatively small diameter of the pyramidal tracts (∼ 5 mm), we were able to map and quantify the direction of the corticospinal somatotopy. These results show how an HDFT approach to white matter mapping provides the first in vivo, quantitative mapping of subcortical corticospinal topographies at resolutions previously only available with postmortem histological techniques.