Are functional deficits in concussed individuals consistent with white matter structural alterations: combined FMRI & DTI study

Concussion: Beyond the Cascade

Sport concussion affects millions of athletes each year at all levels of sport. Increasing evidence demonstrates clinical and physiological recovery are becoming more divergent definitions, as evidenced by several studies examining blood-based biomarkers of inflammation and imaging studies of the central nervous system (CNS). Recent studies have shown elevated microglial activation in the CNS in active and retired American football players, as well as in active collegiate athletes who were diagnosed with a concussion and returned to sport. These data are supportive of discordance in clinical symptomology and the inflammatory response in the CNS upon symptom resolution. In this review, we will summarize recent advances in the understanding of the inflammatory response associated with sport concussion and broader mild traumatic brain injury, as well as provide an outlook for important research questions to better align clinical and physiological recovery.

Assessing blood oxygen level-dependent signal variability as a biomarker of brain injury in sport-related concussion

Mild traumatic brain injury is a complex neurological disorder of significant concern among athletes who play contact sports. Athletes who sustain sport-related concussion typically undergo physical examination and neurocognitive evaluation to determine injury severity and return-to-play status. However, traumatic disruption to neurometabolic processes can occur with minimal detectable anatomic pathology or neurocognitive alteration, increasing the risk that athletes may be cleared for return-to-play during a vulnerable period and receive a repetitive injury. This underscores the need for sensitive functional neuroimaging methods to detect altered cerebral physiology in concussed athletes. The present study compared the efficacy of Immediate Post-concussion Assessment and Cognitive Testing composite scores and whole-brain measures of blood oxygen level-dependent signal variability for classifying concussion status and predicting concussion symptomatology in healthy, concussed and repetitively concussed athletes, assessing blood oxygen level-dependent signal variability as a potential diagnostic tool for characterizing functional alterations to cerebral physiology and assisting in the detection of sport-related concussion. We observed significant differences in regional blood oxygen level-dependent signal variability measures for concussed athletes but did not observe significant differences in Immediate Post-concussion Assessment and Cognitive Testing scores of concussed athletes. We further demonstrate that incorporating measures of functional brain alteration alongside Immediate Post-concussion Assessment and Cognitive Testing scores enhances the sensitivity and specificity of supervised random forest machine learning methods when classifying and predicting concussion status and post-concussion symptoms, suggesting that alterations to cerebrovascular status characterize unique variance that may aid in the detection of sport-related concussion and repetitive mild traumatic brain injury. These results indicate that altered blood oxygen level-dependent variability holds promise as a novel neurobiological marker for detecting alterations in cerebral perfusion and neuronal functioning in sport-related concussion, motivating future research to establish and validate clinical assessment protocols that can incorporate advanced neuroimaging methods to characterize altered cerebral physiology following mild traumatic brain injury.

Diffusion-Weighted Imaging in Mild Traumatic Brain Injury: A Systematic Review of the Literature

There is evidence that diffusion-weighted imaging (DWI) is able to detect tissue alterations following mild traumatic brain injury (mTBI) that may not be observed on conventional neuroimaging; however, findings are often inconsistent between studies. This systematic review assesses patterns of differences in DWI metrics between those with and without a history of mTBI. A PubMed literature search was performed using relevant indexing terms for articles published prior to May 14, 2020. Findings were limited to human studies using DWI in mTBI. Articles were excluded if they were not full-length, did not contain original data, if they were case studies, pertained to military populations, had inadequate injury severity classification, or did not report post-injury interval. Findings were reported independently for four subgroups: acute/subacute pediatric mTBI, acute/subacute adult mTBI, chronic adult mTBI, and sport-related concussion, and all DWI acquisition and analysis methods used were included. Patterns of findings between studies were reported, along with strengths and weaknesses of the current state of the literature. Although heterogeneity of sample characteristics and study methods limited the consistency of findings, alterations in DWI metrics were most commonly reported in the corpus callosum, corona radiata, internal capsule, and long association pathways. Many acute/subacute pediatric studies reported higher FA and lower ADC or MD in various regions. In contrast, acute/subacute adult studies most commonly indicate lower FA within the context of higher MD and RD. In the chronic phase of recovery, FA may remain low, possibly indicating overall demyelination or Wallerian degeneration over time. Longitudinal studies, though limited, generally indicate at least a partial normalization of DWI metrics over time, which is often associated with functional improvement. We conclude that DWI is able to detect structural mTBI-related abnormalities that may persist over time, although future DWI research will benefit from larger samples, improved data analysis methods, standardized reporting, and increasing transparency.

Memory retrieval brain-behavior disconnection in mild traumatic brain injury: A magnetoencephalography and diffusion tensor imaging study

Mild traumatic brain (mTBI) injury is often associated with long-term cognitive and behavioral complications, including an increased risk of memory impairment. Current research challenges include a lack of cross-modal convergence regarding the underlying neural-behavioral mechanisms of mTBI, which hinders therapeutics and outcome management for this frequently under-treated and vulnerable population. We used multi-modality imaging methods including magnetoencephalography (MEG) and diffusion tensor imaging (DTI) to investigate brain-behavior impairment in mTBI related to working memory. A total of 41 participants were recruited, including 23 patients with a first-time mTBI imaged within 3 months of injury (all male, age = 29.9, SD = 6.9), and 18 control participants (all male, age = 27.3, SD = 5.3). Whole-brain statistics revealed spatially concomitant functional-structural disruptions in brain-behavior interactions in working memory in the mTBI group compared with the control group. These disruptions are located in the hippocampal-prefrontal region and, additionally, in the amygdala (measured by MEG neural activation and DTI measures of fractional anisotropy in relation to working memory performance; p < .05, two-way ANCOVA, nonparametric permutations, corrected). Impaired brain-behavior connections found in the hippocampal-prefrontal and amygdala circuits indicate brain dysregulation of memory, which may leave mTBI patients vulnerable to increased environmental demands exerting memory resources, leading to related cognitive and emotional psychopathologies. The findings yield clinical implications and highlight a need for early rehabilitation after mTBI, including attention- and sensory-based behavioral exercises.

Brain predictors of fatigue in rheumatoid arthritis: A machine learning study

Background

Fatigue is a common and burdensome symptom in Rheumatoid Arthritis (RA), yet is poorly understood. Currently, clinicians rely solely on fatigue questionnaires, which are inherently subjective measures. For the effective development of future therapies and stratification, it is of vital importance to identify biomarkers of fatigue. In this study, we identify brain differences between RA patients who improved and did not improve their levels of fatigue based on Chalder Fatigue Scale variation (ΔCFS≥ 2), and we compared the performance of different classifiers to distinguish between these samples at baseline.

Methods

Fifty-four fatigued RA patients underwent a magnetic resonance (MR) scan at baseline and 6 months later. At 6 months we identified those whose fatigue levels improved and those for whom it did not. More than 900 brain features across three data sets were assessed as potential predictors of fatigue improvement. These data sets included clinical, structural MRI (sMRI) and diffusion tensor imaging (DTI) data. A genetic algorithm was used for feature selection. Three classifiers were employed in the discrimination of improvers and non-improvers of fatigue: a Least Square Linear Discriminant (LSLD), a linear Support Vector Machine (SVM) and a SVM with Radial Basis Function kernel.

Results

The highest accuracy (67.9%) was achieved with the sMRI set, followed by the DTI set (63.8%), whereas classification performance using clinical features was at the chance level. The mean curvature of the left superior temporal sulcus was most strongly selected during the feature selection step, followed by the surface are of the right frontal pole and the surface area of the left banks of the superior temporal sulcus.

Conclusions

The results presented evidence a superiority of brain metrics over clinical metrics in predicting fatigue changes. Further exploration of these methods may support clinicians to triage patients towards the most appropriate fatigue alleviating therapies.

Altered white matter diffusivity and subtle motor function in a pilot cohort of adolescents with sports-related concussion

Background and objective:

Adolescents with sports-related concussion (SRC) demonstrate acute and persistent deficits in subtle motor function. However, there is limited research examining related neurological underpinnings. This pilot study examined changes in motor-associated white matter pathways using diffusion tensor imaging (DTI) and their relationship with subtle motor function.

Methods:

Twelve adolescents with SRC (12–17 years) within two-weeks post-injury and 13 never-injured neurotypical peers completed DTI scanning. A subset of 6 adolescents with SRC returned for a follow-up visit post-medical clearance from concussion. Subtle motor function was evaluated using the Physical and Neurological Examination of Subtle Signs (PANESS).

Results:

Adolescents with SRC showed higher mean diffusivity (MD) of the superior corona radiata and greater subtle motor deficits compared to controls. Across all participants, greater subtle motor deficits were associated with higher (more atypical) MD of the superior corona radiata. Preliminary longitudinal analysis indicated reduction in fractional anisotropy of the corpus callosum but no change in the MD of the superior corona radiata from the initial visit to the follow-up visit post-medical clearance.

Conclusions:

These findings support preliminary evidence for a brain–behavior relationship between superior corona radiata microstructure and subtle motor deficits in adolescents with SRC that merits further investigation.

White matter abnormalities in active elite adult rugby players

The recognition, diagnosis and management of mild traumatic brain injuries are difficult and confusing. It is unclear how the severity and number of injuries sustained relate to brain injuries, such as diffuse axonal injury, diffuse vascular injury and progressive neurodegeneration. Advances in neuroimaging techniques enable the investigation of neuropathologies associated with acute and long-term effects of injury. Head injuries are the most commonly reported injury seen during professional rugby. There is increased vigilance for the immediate effects of these injuries in matches, but there has been surprisingly little research investigating the longer-term effects of rugby participation. Here, we present a longitudinal observational study investigating the relationship of exposure to rugby participation and sub-acute head injuries in professional adult male and female rugby union and league players using advanced MRI. Diffusion tensor imaging and susceptibility weighted imaging was used to assess white matter structure and evidence of axonal and diffuse vascular injury. We also studied changes in brain structure over time using Jacobian Determinant statistics extracted from serial volumetric imaging. We tested 41 male and 3 female adult elite rugby players, of whom 21 attended study visits after a head injury, alongside 32 non-sporting controls, 15 non-collision-sport athletic controls and 16 longitudinally assessed controls. Eighteen rugby players participated in the longitudinal arm of the study, with a second visit at least 6 months after their first scan. Neuroimaging evidence of either axonal injury or diffuse vascular injury was present in 23% (10/44) of players. In the non-acutely injured group of rugby players, abnormalities of fractional anisotropy and other diffusion measures were seen. In contrast, non-collision-sport athletic controls were not classified as showing abnormalities. A group level contrast also showed evidence of sub-acute injury using diffusion tensor imaging in rugby players. Examination of longitudinal imaging revealed unexpected reductions in white matter volume in the elite rugby players studied. These changes were not related to self-reported head injury history or neuropsychological test scores and might indicate excess neurodegeneration in white matter tracts affected by injury. Taken together, our findings suggest an association of participation in elite adult rugby with changes in brain structure. Further well-designed large-scale studies are needed to understand the impact of both repeated sports-related head impacts and head injuries on brain structure, and to clarify whether the abnormalities we have observed are related to an increased risk of neurodegenerative disease and impaired neurocognitive function following elite rugby participation.

Diffusion Tensor Imaging in Sport-Related Concussion: A Systematic Review Using an a priori Quality Rating System

Diffusion tensor imaging (DTI) of brain white matter (WM) may be useful for characterizing the nature and degree of brain injury after sport-related concussion (SRC) and assist in establishing objective diagnostic and prognostic biomarkers. This study aimed to conduct a systematic review using an a priori quality rating strategy to determine the most consistent DTI-WM changes post-SRC. Articles published in English (until June 2020) were retrieved by standard research engine and gray literature searches (N = 4932), using PRISMA guidelines. Eligible studies were non-interventional naturalistic original studies that conducted DTI within 6 months of SRC in current athletes from all levels of play, types of sports, and sex. A total of 29 articles were included in the review, and after quality appraisal by two raters, data from 10 studies were extracted after being identified as high quality. High-quality studies showed widespread moderate-to-large WM differences when SRC samples were compared to controls during the acute to early chronic stage (days to weeks) post-SRC, including both increased and decreased fractional anisotropy and axial diffusivity and decreased mean diffusivity and radial diffusivity. WM differences remained stable in the chronic stage (2-6 months post-SRC). DTI metrics were commonly associated with SRC symptom severity, although standardized SRC diagnostics would improve future research. This indicates that microstructural recovery is often incomplete at return to play and may lag behind clinically assessed recovery measures. Future work should explore interindividual trajectories to improve understanding of the heterogeneous and dynamic WM patterns post-SRC.

Brain function associated with reaction time after sport-related concussion

Concussion is associated with significant functional disturbances in the first week post-injury. Computerized neurocognitive testing tools have become widely adopted in concussion management, to identify specific domains of impairment and obtain more objective measures of recovery. Reaction time (RT) slowing is a common sequela of concussion, however, the functional brain networks that underlie RT performance remain under-studied in both healthy and concussed athletic cohorts. This study used blood-oxygenation-level-dependent function magnetic resonance imaging (BOLD fMRI) to evaluate resting brain function of 45 university-level athletes with concussion in the first week post-injury, along with a control cohort of 102 athletes without recent concussion. We evaluated the main effects of concussion and RT on functional connectivity, along with concussion × RT interactions, using multivariate analysis techniques. Concussion was associated with reduced connectivity throughout the brain, whereas RT slowing was associated with elevated connectivity in parietal and temporal regions, for both control and concussed groups. For the concussed group, RT slowing was also associated with disrupted connectivity between fronto-insular and default mode networks. For concussed athletes, the brain networks associated with slower post-injury RT also showed similar but non-significant associations with longitudinal changes in RT performance relative to pre-injury baseline. These study findings provide new insights into the effects of concussion on neurocognitive function and suggest the presence of functional brain networks that are specific to concussion-related RT slowing.

Targeting the Cerebrovascular System: Next-Generation Biomarkers and Treatment for Mild Traumatic Brain Injury

The diagnosis, prognosis, and treatment of mild traumatic brain injuries (mTBIs), such as concussions, are significant unmet medical issues. The kinetic forces that occur in mTBI adversely affect the cerebral vasculature, making cerebrovascular injury (CVI) a pathophysiological hallmark of mTBI. Given the importance of a healthy cerebrovascular system in overall brain function, CVI is likely to contribute to neurological dysfunction after mTBI. As such, CVI and related pathomechanisms may provide objective biomarkers and therapeutic targets to improve the clinical management and outcomes of mTBI. Despite this potential, until recently, few studies have focused on the cerebral vasculature in this context. This article will begin by providing a brief overview of the cerebrovascular system followed by a review of the literature regarding how mTBI can affect the integrity and function of the cerebrovascular system, and how this may ultimately contribute to neurological dysfunction and neurodegenerative conditions. We then discuss promising avenues of research related to mTBI biomarkers and interventions that target CVI, and conclude that a clinical approach that takes CVI into account could result in substantial improvements in the care and outcomes of patients with mTBI.

The role of diffusion tensor imaging in characterizing injury patterns on athletes with concussion and subconcussive injury: a systematic review

Traumatic brain injury (TBI) is a major public health problem. The majority of TBIs are in the form of mild TBI (also known as concussion) with sports-related concussion (SRC) receiving public attention in recent years.Here we have performed a systematic review of the literature on the use of Diffusion Tensor Imaging (DTI) on sports-related concussion and subconcussive injuries. Our review found different patterns of change in DTI parameters between concussed and subconcussed groups. The Fractional Anisotropy (FA) was either unchanged or increased for the concussion group, while the subconcussed group generally experienced a decrease in FA. A reverse pattern was observed for Mean Diffusivity (MD) - where the concussed group experienced a decrease in MD while the subconcussed group showed an increase in MD. However, in general, discrepancies were observed in the results reported in the literature - likely due to the huge variations in DTI acquisition parameters, and image processing and analysis methods used in these studies. This calls for more comprehensive and well-controlled studies in this field, including those that combine the advanced brain imaging with biomechancial modeling and kinematic sensors - to shed light on the underlying mechanisms behind the structural changes observed from the imaging studies.

MRI detection of impairment of glymphatic function in rat after mild traumatic brain injury

We investigated the effect of mild traumatic brain injury (mTBI) on the glymphatic pathway using contrast-enhanced magnetic resonance imaging (CE-MRI) and quantified with kinetic parameters obtained from an advanced two-compartment model. mTBI was induced in male Wistar rats using a closed head impact. Animals with and without mTBI (n = 7/group) underwent the identical MRI protocol 10-weeks post-injury, including T2-weighted imaging and 3D T1-weighted imaging with intra-cisterna magna injection of contrast agent (Gd-DTPA). The parameters of infusion rate, clearance rate and clearance time constant, characterizing the kinetic features of glymphatic tracer transport in a living brain, were quantified in multiple brain tissue regions. In the majority of examined regions, our quantification demonstrated significantly reduced infusion and clearance rates, and significantly increased clearance time constant in the mTBI animals compared to the healthy controls. These data indicate that mTBI induces chronic changes in influx and efflux of contrast agent and glymphatic pathway dysfunction. While the reduced efficiency of glymphatic function after mTBI was apparent in brain, regional evaluation revealed heterogeneous glymphatic effects of the mTBI in different anatomical regions. The suppression of glymphatic function, rather than the presence of focal lesions, indicates a persistent injury of the brain after mTBI. Thus, dynamic CE-MRI in conjunction with advanced kinetic analysis may offer a useful methodology for an objective assessment and confirmatory diagnosis of mTBI.

Comparison of season-long diffusivity measures in a cohort of non-concussed contact and non-contact athletes

INTRODUCTION

Concern surrounding short- and long-term consequences of participation in contact sports has become a significant public health topic. Previous literature utilizing diffusion tensor imaging in sports-related concussion has exhibited notable variety of analysis methods and analyzed regions of interest, and largely focuses on acute effects of concussion. The current study aimed to compare diffusivity metrics across a single season within athlete cohorts with no history of concussion.

METHODS

A prospective cohort of 75 contact and 79 non-contact division I athletes were compared across diffusion tensor imaging metrics (i.e. TRACULA); examinations were also performed assessing the relationship between neuroimaging metrics, head impact exposure metrics (in-helmet accelerometer), and neurocognitive variables. Assessment occurred at pre-and post-season time points.

RESULTS

Seasonal changes in fractional anisotropy and mean diffusivity values did not differ between athlete cohorts, nor did they differ within cohort groups, across pre- and post-season scans. Specific to contact athletes, positive associations were found between uncinate fasciculus mean diffusivity values and season linear acceleration (p =.018), season rotational acceleration (p =.017), and season hit severity (p =.021).

CONCLUSIONS

Results suggest an influence of impact frequency, type, and severity on white matter integrity in select brain regions in contact athletes. Current findings expand our knowledge of anatomical changes over the course of a single season, and underscore the importance of considering methodology when interpreting findings in this population, as differing image analysis techniques may lead to different conclusions regarding significant effects.

An Image Registration-Based Morphing Technique for Generating Subject-Specific Brain Finite Element Models

Finite element (FE) models of the brain are crucial for investigating the mechanisms of traumatic brain injury (TBI). However, FE brain models are often limited to a single neuroanatomy because the manual development of subject-specific models is time consuming. The objective of this study was to develop a pipeline to automatically generate subject-specific FE brain models using previously developed nonlinear image registration techniques, preserving both external and internal neuroanatomical characteristics. To verify the morphing-induced mesh distortions did not influence the brain deformation response, strain distributions predicted using the morphed model were compared to those from manually created voxel models of the same subject. Morphed and voxel models were generated for 44 subjects ranging in age, and simulated using head kinematics from a football concussion case. For each subject, brain strain distributions predicted by each model type were consistent, and differences in strain prediction was less than 4% between model type. This automated technique, taking approximately 2 h to generate a subject-specific model, will facilitate interdisciplinary research between the biomechanics and neuroimaging fields and could enable future use of biomechanical models in the clinical setting as a tool for improving diagnosis.

Microstructure of the Corpus Callosum Long after Pediatric Concussion

OBJECTIVE

The long-term effects of pediatric concussion on white matter microstructure are poorly understood. This study investigated long-term changes in white matter diffusion properties of the corpus callosum in youth several years after concussion.

METHODS

Participants were 8-19 years old with a history of concussion (n = 36) or orthopedic injury (OI) (n = 21). Mean time since injury for the sample was 2.6 years (SD = 1.6). Participants underwent diffusion magnetic resonance imaging, completed cognitive testing, and rated their post-concussion symptoms. Measures of diffusivity (fractional anisotropy, mean, axial, and radial diffusivity) were extracted from white matter tracts in the genu, body, and splenium regions of the corpus callosum. The genu and splenium tracts were further subdivided into 21 equally spaced regions along the tract and diffusion values were extracted from each of these smaller regions.

RESULTS

White matter tracts in the genu, body, and splenium did not differ in diffusivity properties between youth with a history of concussion and those with a history of OI. No significant group differences were found in subdivisions of the genu and splenium after correcting for multiple comparisons. Diffusion metrics did not significantly correlate with symptom reports or cognitive performance.

CONCLUSIONS

These findings suggest that at approximately 2.5 years post-injury, youth with prior concussion do not have differences in their corpus callosum microstructure compared to youth with OI. Although these results are promising from the perspective of long-term recovery, further research utilizing longitudinal study designs is needed to confirm the long-term effects of pediatric concussion on white matter microstructure.

Baseline vs. cross-sectional MRI of concussion: distinct brain patterns in white matter and cerebral blood flow

Neuroimaging has been used to describe the pathophysiology of sport-related concussion during early injury, with effects that may persist beyond medical clearance to return-to-play (RTP). However, studies are typically cross-sectional, comparing groups of concussed and uninjured athletes. It is important to determine whether these findings are consistent with longitudinal change at the individual level, relative to their own pre-injury baseline. A cohort of N = 123 university-level athletes were scanned with magnetic resonance imaging (MRI). Of this group, N = 12 acquired a concussion and were re-scanned at early symptomatic injury and at RTP. A sub-group of N = 44 uninjured athletes were also re-imaged, providing a normative reference group. Among concussed athletes, abnormalities were identified for white matter fractional anisotropy and mean diffusivity, along with grey matter cerebral blood flow, using both cross-sectional (CS) and longitudinal (LNG) approaches. The spatial patterns of abnormality for CS and LNG were distinct, with median fractional overlap below 0.10 and significant differences in the percentage of abnormal voxels. However, the analysis methods did not differ in the amount of change from symptomatic injury to RTP and in the direction of observed abnormalities. These results highlight the impact of using pre-injury baseline data when evaluating concussion-related brain abnormalities at the individual level.

Mining multi-site clinical data to develop machine learning MRI biomarkers: application to neonatal hypoxic ischemic encephalopathy

BACKGROUND

Secondary and retrospective use of hospital-hosted clinical data provides a time- and cost-efficient alternative to prospective clinical trials for biomarker development. This study aims to create a retrospective clinical dataset of Magnetic Resonance Images (MRI) and clinical records of neonatal hypoxic ischemic encephalopathy (HIE), from which clinically-relevant analytic algorithms can be developed for MRI-based HIE lesion detection and outcome prediction.

METHODS

This retrospective study will use clinical registries and big data informatics tools to build a multi-site dataset that contains structural and diffusion MRI, clinical information including hospital course, short-term outcomes (during infancy), and long-term outcomes (~ 2 years of age) for at least 300 patients from multiple hospitals.

DISCUSSION

Within machine learning frameworks, we will test whether the quantified deviation from our recently-developed normative brain atlases can detect abnormal regions and predict outcomes for individual patients as accurately as, or even more accurately, than human experts. Trial Registration Not applicable. This study protocol mines existing clinical data thus does not meet the ICMJE definition of a clinical trial that requires registration.

Pathophysiology of Concussion

Although concussion has been a subject of interest for centuries, this condition remains poorly understood. The mechanistic underpinnings and accepted definition of concussion remain elusive. To make sense of these issues, this article presents a brief history of concussion studies, detailing the evolution of motivations and experimental conclusions over time. Interest in concussion as a subject of scientific inquiry has increased with growing concern about the long-term consequences of mild traumatic brain injury (TBI). Although concussion is often associated with mild TBI, these conditions-the former a neurological syndrome, the latter a neurological event-are distinct, both mechanistically and pathobiologically. Modern research primarily focuses on the study of the biomechanics, pathophysiology, potential biomarkers and neuroimaging to distinguish concussion from mild TBI. In addition, mild TBI and concussion outcomes are influenced by age, sex, and genetic differences in people. With converging experimental objectives and methodologies, future concussion research has the potential to improve clinical assessment, treatment, and preventative measures.

Diffusion tensor imaging changes following mild, moderate and severe adult traumatic brain injury: a meta-analysis

Diffusion tensor imaging quantifies the asymmetry (fractional anisotropy; FA) and amount of water diffusion (mean diffusivity/apparent diffusion coefficient; MD/ADC) and has been used to assess white matter damage following traumatic brain injury (TBI). In healthy brains, diffusion is constrained by the organization of axons, resulting in high FA and low MD/ADC. Following a TBI, diffusion may be altered; however the exact nature of these changes has yet to be determined. A meta-analysis was therefore conducted to determine the location and extent of changes in DTI following adult TBI. The data from 44 studies that compared the FA and/or MD/ADC data from TBI and Control participants in different regions of interest (ROIs) were analyzed. The impact of injury severity, post-injury interval (acute: ≤ 1 week, subacute: 1 week-3 months, chronic: > 3 months), scanner details and acquisition parameters were investigated in subgroup analyses, with the findings indicating that mild TBI should be examined separately to that of moderate to severe injuries. Lower FA values were found in 88% of brain regions following mild TBI and 92% following moderate-severe TBI, compared to Controls. MD/ADC was higher in 95% and 100% of brain regions following mild and moderate-severe TBI, respectively. Moderate to severe TBI resulted in larger changes in FA and MD/ADC than mild TBI. Overall, changes to FA and MD/ADC were widespread, reflecting more symmetric and a higher amount of diffusion, indicative of white matter damage.

Neurobiological effect of selective brain cooling after concussive injury

The search for effective treatment facilitating recovery from concussive injury, as well as reducing risk for recurrent concussion is an ongoing challenge. This study aimed to determine: a) feasibility of selective brain cooling to facilitate clinical symptoms resolution, and b) biological functions of the brain within athletes in acute phase of sports-related concussion. Selective brain cooling for 30 minutes using WElkins sideline cooling system was administered to student-athletes suffering concussive injury (n=12; tested within 5±3 days) and those without history of concussion (n=12). fMRI and ASL sequences were obtained before and immediately after cooling to better understanding the mechanism by which cooling affects neurovascular coupling. Concussed subjects self-reported temporary relief from physical symptoms after cooling. There were no differences in the number or strength of functional connections within Default Mode Network (DMN) between groups prior to cooling. However, we observed a reduction in the strength and number of connections of the DMN with other ROIs in both groups after cooling. Unexpectedly, we observed a significant increase in cerebral blood flow (CBF) assessed by ASL after selective cooling in the concussed subjects compared to the normal controls. We suggest that compromised neurovascular coupling in acute phase of injury may be temporarily restored by cooling to match CBF with surges in the metabolic demands of the brain. Upon further validation, selective brain cooling could be a potential clinical tool in the minimization of symptoms and pathological changes after concussion.

Comparing Region of Interest versus Voxel-Wise Diffusion Tensor Imaging Analytic Methods in Mild and Moderate Traumatic Brain Injury: A Systematic Review and Meta-Analysis

Diffusion tensor imaging is a magnetic resonance imaging technique that is uniquely capable of detecting microstructural tissue damage in mild and moderate traumatic brain injuries (TBIs). To date, it remains unknown if two common analytic techniques, region of interest (ROI) versus voxel-wise (VW) analyses, detect injury in similar locations. The purpose of this systematic review and meta-analysis was to directly compare the regions of abnormality elucidated by each method. Twenty-seven ROI and 11 VW studies met our inclusion criteria. Our ROI meta-analysis identified 11 regions, including the splenium of the corpus callosum, where fractional anisotropy (FA) was significantly decreased in TBI patients, compared with controls. Likewise, we identified higher mean diffusivity/apparent diffusivity constant in the genu, body, and splenium of the corpus callosum. Alternatively, our VW analysis identified one region of high FA in the right superior longitudinal fasciculus and seven regions of low FA, with the two largest located in the corpus callosum. High mean diffusivity and high radial diffusivity, both in the right inferior longitudinal fasciculus, also was revealed by our VW analysis. Moreover, we have shown that the magnitude of damage in the corpus callosum revealed by ROI analysis (z = -3.15) is greater than that demonstrated by VW analysis (z = -1.41). Overall, this study indicates that both ROI and VW analytic methods are sensitive to low FA in the corpus callosum; however, the ROI method has more power to detect the full extent of tissue abnormality in the corpus callosum. More research utilizing standardized methods and reporting is essential to fully characterize the extent to which ROI and VW analyses can concordantly detect other locations of pathology in mild and moderate TBI patients.

Altered brain microstructure in association with repetitive subconcussive head impacts and the potential protective effect of jugular vein compression: a longitudinal study of female soccer athletes

PURPOSE

To (1) quantify white matter (WM) alterations in female high school athletes during a soccer season and characterise the potential for normalisation during the off-season rest period, (2) determine the association between WM alterations and exposure to repetitive subconcussive head impacts, and (3) evaluate the efficacy of a jugular vein compression collar to prevent WM alterations associated with head impact exposure.

METHODS

Diffusion tensor imaging (DTI) data were prospectively collected from high school female soccer participants (14-18 years) at up to three time points over 9 months. Head impacts were monitored using accelerometers during all practices and games. Participants were assigned to a collar (n=24) or non-collar group (n=22). The Tract-Based Spatial Statistics approach was used in the analysis of within-group longitudinal change and between-group comparisons.

RESULTS

DTI analyses revealed significant pre-season to post-season WM changes in the non-collar group in mean diffusivity (2.83%±2.46%), axial diffusivity (2.58%±2.34%) and radial diffusivity (3.52%±2.60%), but there was no significant change in the collar group despite similar head impact exposure. Significant correlation was found between head impact exposure and pre-season to post-season DTI changes in the non-collar group. WM changes in the non-collar group partially resolved at 3 months off-season follow-up.

DISCUSSION

Microstructural changes in WM occurred during a season of female high school soccer among athletes who did not wear the collar device. In comparison, there were no changes in players who wore the collar, suggesting a potential prophylactic effect of the collar device in preventing changes associated with repetitive head impacts. In those without collar use, the microstructural changes showed a reversal towards normal over time in the off-season follow-up period.

A review of the effects of physical activity and sports concussion on brain function and anatomy

Physical activity has been associated with widespread anatomical and functional brain changes that occur following acute exercise or, in the case of athletes, throughout life. High levels of physical activity through the practice of sports also lead to better general health and increased cognitive function. Athletes are at risk, however, of suffering a concussion, the effects of which have been extensively described for brain function and anatomy. The level to which these effects are modulated by increased levels of fitness is not known. Here, we review literature describing the effects of physical activity and sports concussions on white matter, grey matter, neurochemistry and cortical excitability. We suggest that the effects of sports concussion can be coufounded by the effects of exercise. Indeed, available data show that the brain of athletes is different from that of healthy individuals with a non-active lifestyle. As a result, sports concussions take place in a context where structural/functional plasticity has occurred prior to the concussive event. The sports concussion literature does not permit, at present, to separate the effects of intense and repeated physical activity, and the abrupt removal from such activities, from those of concussion on brain structure and function.

Longitudinal Changes in Resting State Connectivity and White Matter Integrity in Adolescents With Sports-Related Concussion

OBJECTIVES

The aim of this study was to investigate alterations in functional connectivity, white matter integrity, and cognitive abilities due to sports-related concussion (SRC) in adolescents using a prospective longitudinal design.

METHODS

We assessed male high school football players (ages 14-18) with (n=16) and without (n=12) SRC using complementary resting state functional MRI (rs-fMRI) and diffusion tensor imaging (DTI) along with cognitive performance using the Immediate Post-Concussive Assessment and Cognitive Testing (ImPACT). We assessed both changes at the acute phase (<7 days post-SRC) and at 21 days later, as well as, differences between athletes with SRC and age- and team-matched control athletes.

RESULTS

The results revealed rs-fMRI hyperconnectivity within posterior brain regions (e.g., precuneus and cerebellum), and hypoconnectivity in more anterior areas (e.g., inferior and middle frontal gyri) when comparing SRC group to control group acutely. Performance on the ImPACT (visual/verbal memory composites) was correlated with resting state network connectivity at both time points. DTI results revealed altered diffusion in the SRC group along a segment of the corticospinal tract and the superior longitudinal fasciculus in the acute phase of SRC. No differences between the SRC group and control group were seen at follow-up imaging.

CONCLUSIONS

Acute effects of SRC are associated with both hyperconnectivity and hypoconnectivity, with disruption of white matter integrity. In addition, acute memory performance was most sensitive to these changes. After 21 days, adolescents with SRC returned to baseline performance, although chronic hyperconnectivity of these regions could place these adolescents at greater risk for secondary neuropathological changes, necessitating future follow-up. (JINS, 2018, 24, 781-792).

Functional Magnetic Resonance Imaging and Oculomotor Dysfunction in Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) is a significant cause of disability, especially when symptoms become chronic. This chronicity is often linked to oculomotor dysfunction (OMD). To our knowledge, this is the first prospective study to localize aberrations in brain function between mTBI cohorts, by comparing patients with mTBI with OMD with an mTBI control group without OMD, using task and resting-state functional magnetic resonance imaging (fMRI). Ten subjects with mTBI who had OMD (OMD group) were compared with nine subjects with mTBI who had no findings of OMD (control group). These groups were determined by a developmental optometrist using objective testing for OMD. The (convergence) task fMRI data demonstrated significantly decreased brain activity, measured as decreases in the blood oxygen level dependent (BOLD) signal, in the OMD group compared with the control group in three brain regions: the left posterior lingual gyrus, the bilateral anterior lingual gyrus and cuneus, and the parahippocampal gyrus. When doing a seed-based resting state fMRI analysis in the lingual/parahippocampal region, a large cluster covering the left middle frontal gyrus and the dorsolateral pre-frontal cortex (Brodmann areas 9 and 10), with decreased functional correlation in the OMD group, was identified. Together these observations provide evidence for neural networks of interactions involving the control of eye movement for visual processing, reading comprehension, spatial localization and navigation, and spatial working memory that appear to be decreased in mTBI patients with OMD compared with mTBI patients without OMD. The clinical symptomatology associated with post-traumatic OMD correlates well with these MRI findings.

Hybrid Diffusion Imaging in Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) is an important public health problem. Although conventional medical imaging techniques can detect moderate-to-severe injuries, they are relatively insensitive to mTBI. In this study, we used hybrid diffusion imaging (HYDI) to detect white matter alterations in 19 patients with mTBI and 23 other trauma control patients. Within 15 days (standard deviation = 10) of brain injury, all subjects underwent magnetic resonance HYDI and were assessed with a battery of neuropsychological tests of sustained attention, memory, and executive function. Tract-based spatial statistics (TBSS) was used for voxel-wise statistical analyses within the white matter skeleton to study between-group differences in diffusion metrics, within-group correlations between diffusion metrics and clinical outcomes, and between-group interaction effects. The advanced diffusion imaging techniques, including neurite orientation dispersion and density imaging (NODDI) and q-space analyses, appeared to be more sensitive then classic diffusion tensor imaging. Only NODDI-derived intra-axonal volume fraction (V_ic) demonstrated significant group differences (i.e., 5–9% lower in the injured brain). Within the mTBI group, V_ic and a q-space measure, P₀, correlated with 6 of 10 neuropsychological tests, including measures of attention, memory, and executive function. In addition, the direction of correlations differed significantly between groups (R² > 0.71 and p_interation < 0.03). Specifically, in the control group, higher V_ic and P₀ were associated with better performances on clinical assessments, whereas in the mTBI group, higher V_ic and P₀ were associated with worse performances with correlation coefficients >0.83. In summary, the NODDI-derived axonal density index and q-space measure for tissue restriction demonstrated superior sensitivity to white matter changes shortly after mTBI. These techniques hold promise as a neuroimaging biomarker for mTBI.

Altered cortical activation and connectivity patterns for visual attention processing in young adults post-traumatic brain injury: A functional near infrared spectroscopy study

AIMS

This study aimed at understanding the neurobiological mechanisms associated with inattention induced by traumatic brain injury (TBI). To eliminate the potential confounding caused by the heterogeneity of TBI, we focused on young adults postsports-related concussion (SRC).

METHODS

Functional near-infrared spectroscopy (fNIRS) data were collected from 27 young adults post-SRC and 27 group-matched normal controls (NCs), while performing a visual sustained attention task. Task responsive cortical activation maps and pairwise functional connectivity among six regions of interest were constructed for each subject. Correlations among the brain imaging measures and clinical measures of attention were calculated in each group.

RESULTS

Compared to the NCs, the SRC group showed significantly increased brain activation in left middle frontal gyrus (MFG) and increased functional connectivity between right inferior occipital cortex (IOC) bilateral calcarine gyri (CG). The left MFG activation magnitude was significantly negatively correlated with the hyperactive/impulsive symptom severity measure in the NCs, but not in the patients. The right hemisphere CG-IOC functional connectivity showed a significant positive correlation with the hyperactive/impulsive symptom severity measure in patients, but not in NCs.

CONCLUSION

The current data suggest that abnormal left MFG activation and hyper-communications between right IOC and bilateral CG during visual attention processing may significantly contribute to behavioral manifestations of attention deficits in patients with TBI.

Mild Traumatic Brain Injury Induces Structural and Functional Disconnection of Local Neocortical Inhibitory Networks via Parvalbumin Interneuron Diffuse Axonal Injury

Diffuse axonal injury (DAI) plays a major role in cortical network dysfunction posited to cause excitatory/inhibitory imbalance after mild traumatic brain injury (mTBI). Current thought holds that white matter (WM) is uniquely vulnerable to DAI. However, clinically diagnosed mTBI is not always associated with WM DAI. This suggests an undetected neocortical pathophysiology, implicating GABAergic interneurons. To evaluate this possibility, we used mild central fluid percussion injury to generate DAI in mice with Cre-driven tdTomato labeling of parvalbumin (PV) interneurons. We followed tdTomato+ profiles using confocal and electron microscopy, together with patch-clamp analysis to probe for DAI-mediated neocortical GABAergic interneuron disruption. Within 3 h post-mTBI tdTomato+ perisomatic axonal injury (PSAI) was found across somatosensory layers 2-6. The DAI marker amyloid precursor protein colocalized with GAD67 immunoreactivity within tdTomato+ PSAI, representing the majority of GABAergic interneuron DAI. At 24 h post-mTBI, we used phospho-c-Jun, a surrogate DAI marker, for retrograde assessments of sustaining somas. Via this approach, we estimated DAI occurs in ~9% of total tdTomato+ interneurons, representing ~14% of pan-neuronal DAI. Patch-clamp recordings of tdTomato+ interneurons revealed decreased inhibitory transmission. Overall, these data show that PV interneuron DAI is a consistent and significant feature of experimental mTBI with important implications for cortical network dysfunction.

Multimodal Assessment of Recurrent MTBI across the Lifespan

Recurrent mild traumatic brain injuries (mTBI) and its neurological sequelae have been the focus of a large number of studies, indicating cognitive, structural, and functional brain alterations. However, studies often focused on single outcome measures in small cohorts of specific populations only. We conducted a multimodal evaluation of the impact of recurrent mTBI on a broad range of cognitive functions, regional brain volume, white matter integrity, and resting state functional connectivity (RSFC) in young and older adults in the chronic stage (>6 months after the last mTBI). Seventeen young participants with mTBI (age: 24.2 ± 2.8 (mean ± SD)) and 21 group-wise matched healthy controls (age: 25.8 ± 5.4 (mean ± SD)), as well as 17 older participants with mTBI (age: 62.7 ± 7.7 (mean ± SD)) and 16 group-wise matched healthy controls (age: 61.7 ± 5.9 (mean ± SD)) were evaluated. We found significant differences in the verbal fluency between young participants with mTBI and young healthy controls. Furthermore, differences in the regional volume of precuneus and medial orbitofrontal gyrus between participants with mTBI and controls for both age groups were seen. A significant age by group interaction for the right hippocampal volume was noted, indicating an accelerated hippocampal volume loss in older participants with mTBI. Other cognitive parameters, white matter integrity, and RSFC showed no significant differences. We confirmed some of the previously reported detrimental effects of recurrent mTBI, but also demonstrated inconspicuous findings for the majority of parameters.

A systematic review of neuroimaging findings in children and adolescents with sports-related concussion

BACKGROUND

Sport-related concussion (SRC) generally does not result in structural anomalies revealed through clinical imaging techniques such as MRI and CT. While advanced neuroimaging techniques offer another avenue to investigate the subtle alterations following SRC, the current pediatric literature in this area has yet to be reviewed. The aim of this review is to systematically explore the literature on magnetic resonance spectroscopy (MRS), diffusion tensor imaging (DTI), functional magnetic resonance imaging (fMRI), and cortical thickness following SRC in children and adolescents.

METHODS

A systematic Pubmed search using the preferred reporting items for systematic reviews and meta-analysis guidelines was conducted independently for each neuroimaging method. Studies were screened for inclusion based on pre-determined criteria.

RESULTS

A total of 26 studies were included (MRS = 4, DTI = 10, fMRI = 11, cortical thickness = 1). A total of 16 studies were conducted solely with male athletes, while 10 studies recruited an unequal number of male and female athletes.

CONCLUSIONS

While MRI and CT are generally unrevealing, advanced neuroimaging techniques demonstrated neurometabolic, microstructural, and functional alterations following SRC in athletes younger than 19 years of age in the acute, subacute, and chronic phases of recovery. However, more studies are needed to fully understand the impact of SRC on the developing brain in children and adolescents.

Blood biomarkers are associated with brain function and blood flow following sport concussion

BACKGROUND

Secondary injury pathophysiology after sport-related concussion (SRC) is poorly understood. Blood biomarkers may be a useful tool for characterizing these processes, yet there are limitations in their application as a single modality. Combining blood biomarker analysis with advanced neuroimaging may help validate their continued utility in brain injury research by elucidating important secondary injury mechanisms. Hence, the purpose of this study was to evaluate co-modulation between peripheral blood biomarkers and advanced functional brain imaging after SRC.

METHODS

Forty-three university level athletes from 7 sports were recruited (16 recently concussed athletes; 15 healthy athletes with no prior history of concussion; 12 healthy athletes with a history of concussion). Seven blood biomarkers were evaluated: s100B, total tau (T-tau), von Willebrand factor (vWF), brain derived neurotrophic factor (BDNF), peroxiredoxin (PRDX)-6, monocyte chemoattractant protein (MCP)-1 and -4. Resting-state functional MRI was employed to assess global neural connectivity (Gconn), and arterial spin labelling was used to evaluate cerebral blood flow (CBF). We tested for concurrent alterations in blood biomarkers and MRI measures of brain function between athlete groups using a non-parametric, bootstrapped resampling framework.

RESULTS

Compared to healthy athletes, recently concussed athletes showed greater concurrent alterations in several peripheral blood biomarker and MRI measures: a decrease in T-Tau and Gconn, a decrease in T-Tau and CBF, a decrease in Gconn with elevated PRDX-6, a decrease in CBF with elevated PRDX-6, and a decrease in Gconn with elevated MCP-4. In addition, compared to healthy athletes with no concussion history, healthy athletes with a history of concussion displayed greater concurrent alterations in blood biomarkers and Gconn; lower GConn covaried with higher blood levels of s100B and MCP-4.

CONCLUSION

We identified robust relationships between peripheral blood biomarkers and MRI measures in both recently concussed athletes and healthy athletes with a history of concussion. The results from this combinatorial approach further support that human concussion is associated with inflammation, oxidative stress, and cellular damage, and that physiological perturbations may extend chronically beyond recovery. Finally, our results support the continued implementation of blood biomarkers as a tool to investigate brain injury, particularly in a multimodal framework.

Structural imaging of mild traumatic brain injury may not be enough: overview of functional and metabolic imaging of mild traumatic brain injury

A majority of patients with traumatic brain injury (TBI) present as mild injury with no findings on conventional clinical imaging methods. Due to this difficulty of imaging assessment on mild TBI patients, there has been much emphasis on the development of diffusion imaging modalities such as diffusion tensor imaging (DTI). However, basic science research in TBI shows that many of the functional and metabolic abnormalities in TBI may be present even in the absence of structural damage. Moreover, structural damage may be present at a microscopic and molecular level that is not detectable by structural imaging modality. The use of functional and metabolic imaging modalities can provide information on pathological changes in mild TBI patients that may not be detected by structural imaging. Although there are various differences in protocols of positron emission tomography (PET), single photon emission computed tomography (SPECT), functional magnetic resonance imaging (fMRI), electroencephalography (EEG), and magnetoencephalography (MEG) methods, these may be important modalities to be used in conjunction with structural imaging in the future in order to detect and understand the pathophysiology of mild TBI. In this review, studies of mild TBI patients using these modalities that detect functional and metabolic state of the brain are discussed. Each modality's advantages and disadvantages are compared, and potential future applications of using combined modalities are explored.

Connectomic markers of symptom severity in sport-related concussion: Whole-brain analysis of resting-state fMRI

Concussion is associated with significant adverse effects within the first week post-injury, including physical complaints and altered cognition, sleep and mood. It is currently unknown whether these subjective disturbances have reliable functional brain correlates. Resting-state functional magnetic resonance imaging (rs-fMRI) has been used to measure functional connectivity of individuals after traumatic brain injury, but less is known about the relationship between functional connectivity and symptom assessments after a sport concussion. In this study, rs-fMRI was used to evaluate whole-brain functional connectivity for seventy (70) university-level athletes, including 35 with acute concussion and 35 healthy matched controls. Univariate analyses showed that greater symptom severity was mainly associated with lower pairwise connectivity in frontal, temporal and insular regions, along with higher connectivity in a sparser set of cerebellar regions. A novel multivariate approach also extracted two components that showed reliable covariation with symptom severity: (1) a network of frontal, temporal and insular regions where connectivity was negatively correlated with symptom severity (replicating the univariate findings); and (2) a network with anti-correlated elements of the default-mode network and sensorimotor system, where connectivity was positively correlated with symptom severity. These findings support the presence of connectomic signatures of symptom complaints following a sport-related concussion, including both increased and decreased functional connectivity within distinct functional brain networks.

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Analyzed relationship between resting brain function and symptoms of concussion

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Whole-brain analysis, using both univariate and multivariate methods

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Symptoms associated with lower connectivity for frontal/temporal/insular network

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Symptoms associated with higher connectivity for default-mode/sensorimotor network

Acute white matter changes following sport-related concussion: A serial diffusion tensor and diffusion kurtosis tensor imaging study

Recent neuroimaging studies have suggested that following sport-related concussion (SRC) physiological brain alterations may persist after an athlete has shown full symptom recovery. Diffusion MRI is a versatile technique to study white matter injury following SRC, yet serial follow-up studies in the very acute stages following SRC utilizing a comprehensive set of diffusion metrics are lacking. The aim of the current study was to characterize white matter changes within 24 hours of concussion in a group of high school and collegiate athletes, using Diffusion Tensor and Diffusion Kurtosis Tensor metrics. Participants were reassessed a week later. At 24 hours post-injury, the concussed group reported significantly more concussion symptoms than a well-matched control group and demonstrated poorer performance on a cognitive screening measure, yet these differences were nonsignificant at the 8-day follow-up. Similarly, within 24-hours after injury, the concussed group exhibited a widespread decrease in mean diffusivity, increased axial kurtosis and, to a lesser extent, decreased axial and radial diffusivities compared with control subjects. At 8 days post injury, the differences in these diffusion metrics were even more widespread in the injured athletes, despite improvement of symptoms and cognitive performance. These MRI findings suggest that the athletes might not have reached full physiological recovery a week after the injury. These findings have significant implications for the management of SRC because allowing an athlete to return to play before the brain has fully recovered from injury may have negative consequences. Hum Brain Mapp 37:3821-3834, 2016. © 2016 Wiley Periodicals, Inc.

White matter alterations over the course of two consecutive high-school football seasons and the effect of a jugular compression collar: A preliminary longitudinal diffusion tensor imaging study

The cumulative effects of repetitive subclinical head impacts during sports may result in chronic white matter (WM) changes and possibly, neurodegenerative sequelae. In this pilot study, we investigated the longitudinal WM changes over the course of two consecutive high-school football seasons and explored the long-term effects of a jugular vein compression collar on these WM alterations. Diffusion tensor imaging data were prospectively collected both pre- and postseason in the two consecutive seasons. Participants were assigned into either collar or noncollar groups. Tract-based spatial statistics (TBSS) approach and region of interest-based approach were used to quantify changes in WM diffusion properties. Despite comparable exposure to repetitive head impacts, significant reductions in mean, axial, and/or radial diffusivity were identified in Season 1 in multiple WM regions in the noncollar group but not in the collar group. After an 8- to 9-month long off-season, these changes observed in the noncollar group partially and significantly reversed but also remained significantly different from the baseline. In Season 2, trend level WM alterations in the noncollar group were found but located in spatially different regions than Season 1. Last, the WM integrity in the collar group remained unchanged throughout the four time points. In conclusion, we quantitatively assessed the WM structural changes and partial reversal over the course of two consecutive high-school football seasons. In addition, the mitigated WM alterations in athletes in the collar group might indicate potential effect of the collar in ameliorating the changes against repetitive head impacts. Hum Brain Mapp 39:491-508, 2018. © 2017 Wiley Periodicals, Inc.

Injury prediction and vulnerability assessment using strain and susceptibility measures of the deep white matter

Reliable prediction and diagnosis of concussion is important for its effective clinical management. Previous model-based studies largely employ peak responses from a single element in a pre-selected anatomical region of interest (ROI) and utilize a single training dataset for injury prediction. A more systematic and rigorous approach is necessary to scrutinize the entire white matter (WM) ROIs as well as ROI-constrained neural tracts. To this end, we evaluated injury prediction performances of the 50 deep WM regions using predictor variables based on strains obtained from simulating the 58 reconstructed American National Football League head impacts. To objectively evaluate performance, repeated random subsampling was employed to split the impacts into independent training and testing datasets (39 and 19 cases, respectively, with 100 trials). Univariate logistic regressions were conducted based on training datasets to compute the area under the receiver operating characteristic curve (AUC), while accuracy, sensitivity, and specificity were reported based on testing datasets. Two tract-wise injury susceptibilities were identified as the best overall via pair-wise permutation test. They had comparable AUC, accuracy, and sensitivity, with the highest values occurring in superior longitudinal fasciculus (SLF; 0.867-0.879, 84.4-85.2, and 84.1-84.6%, respectively). Using metrics based on WM fiber strain, the most vulnerable ROIs included genu of corpus callosum, cerebral peduncle, and uncinate fasciculus, while genu and main body of corpus callosum, and SLF were among the most vulnerable tracts. Even for one un-concussed athlete, injury susceptibility of the cingulum (hippocampus) right was elevated. These findings highlight the unique injury discriminatory potentials of computational models and may provide important insight into how best to incorporate WM structural anisotropy for investigation of brain injury.

Mild Traumatic Brain Injury Evokes Pyramidal Neuron Axon Initial Segment Plasticity and Diffuse Presynaptic Inhibitory Terminal Loss

The axon initial segment (AIS) is the site of action potential (AP) initiation, thus a crucial regulator of neuronal activity. In excitatory pyramidal neurons, the high density of voltage-gated sodium channels (NaV1.6) at the distal AIS regulates AP initiation. A surrogate AIS marker, ankyrin-G (ankG) is a structural protein regulating neuronal functional via clustering voltage-gated ion channels. In neuronal circuits, changes in presynaptic input can alter postsynaptic output via AIS structural-functional plasticity. Recently, we showed experimental mild traumatic brain injury (mTBI) evokes neocortical circuit disruption via diffuse axonal injury (DAI) of excitatory and inhibitory neuronal systems. A key finding was that mTBI-induced neocortical electrophysiological changes involved non-DAI/ intact excitatory pyramidal neurons consistent with AIS-specific alterations. In the current study we employed Thy1-yellow fluorescent protein (YFP)-H mice to test if mTBI induces AIS structural and/or functional plasticity within intact pyramidal neurons 2 days after mTBI. We used confocal microscopy to assess intact YFP+ pyramidal neurons in layer 5 of primary somatosensory barrel field (S1BF), whose axons were continuous from the soma of origin to the subcortical white matter (SCWM). YFP+ axonal traces were superimposed on ankG and NaV1.6 immunofluorescent profiles to determine AIS position and length. We found that while mTBI had no effect on ankG start position, the length significantly decreased from the distal end, consistent with the site of AP initiation at the AIS. However, NaV1.6 structure did not change after mTBI, suggesting uncoupling from ankG. Parallel quantitative analysis of presynaptic inhibitory terminals along the postsynaptic perisomatic domain of these same intact YFP+ excitatory pyramidal neurons revealed a significant decrease in GABAergic bouton density. Also within this non-DAI population, patch-clamp recordings of intact YFP+ pyramidal neurons showed AP acceleration decreased 2 days post-mTBI, consistent with AIS functional plasticity. Simulations of realistic pyramidal neuron computational models using experimentally determined AIS lengths showed a subtle decrease is NaV1.6 density is sufficient to attenuate AP acceleration. Collectively, these findings highlight the complexity of mTBI-induced neocortical circuit disruption, involving changes in extrinsic/presynaptic inhibitory perisomatic input interfaced with intrinsic/postsynaptic intact excitatory neuron AIS output.

A prospective microstructure imaging study in mixed-martial artists using geometric measures and diffusion tensor imaging: methods and findings

Although diffusion magnetic resonance imaging (dMRI) has been widely used to characterize the effects of repetitive mild traumatic brain injury (rmTBI), to date no studies have investigated how novel geometric models of microstructure relate to more typical diffusion tensor imaging (DTI) sequences. Moreover, few studies have evaluated the sensitivity of different registration pipelines (non-linear, linear and tract-based spatial statistics) for detecting dMRI abnormalities in clinical populations. Results from single-subject analyses in healthy controls (HC) indicated a strong negative relationship between fractional anisotropy (FA) and orientation dispersion index (ODI) in both white and gray matter. Equally important, only moderate relationships existed between all other estimates of free/intracellular water volume fractions and more traditional DTI metrics (FA, mean, axial and radial diffusivity). These findings suggest that geometric measures provide differential information about the cellular microstructure relative to traditional DTI measures. Results also suggest greater sensitivity for non-linear registration pipelines that maximize the anatomical information available in T1-weighted images. Clinically, rmTBI resulted in a pattern of decreased FA and increased ODI, largely overlapping in space, in conjunction with increased intracellular and free water fractions, highlighting the potential role of edema following repeated head trauma. In summary, current results suggest that geometric models of diffusion can provide relatively unique information regarding potential mechanisms of pathology that contribute to long-term neurological damage.

White matter microstructure in athletes with a history of concussion: Comparing diffusion tensor imaging (DTI) and neurite orientation dispersion and density imaging (NODDI)

Sport concussion is associated with disturbances in brain function in the absence of gross anatomical lesions, and may have long-term health consequences. Diffusion-weighted magnetic resonance imaging (MRI) methods provide a powerful tool for investigating alterations in white matter microstructure reflecting the long-term effects of concussion. In a previous study, diffusion tensor imaging (DTI) showed that athletes with a history of concussion had elevated fractional anisotropy (FA) and reduced mean diffusivity (MD) parameters. To better understand these effects, this study compared DTI results to neurite orientation dispersion and density imaging (NODDI), which was used to estimate the intracellular volume fraction (V_IC ) and orientation dispersion index (ODI). Sixty-eight (68) varsity athletes were recruited, including 37 without a history of concussion and 31 with concussion >6 months prior to imaging. Univariate analyses showed elevated FA and decreased MD for concussed athletes, along with increased V_IC and reduced ODI, indicating greater neurite density and coherence of neurite orientation within white matter. Multivariate analyses also showed that for athletes with a history of concussion, white matter regions with increased FA had increased V_IC and decreased ODI, with greater effects among athletes who were imaged a longer time since their last concussion. These findings enhance our understanding of the relationship between the biophysics of water diffusion and concussion neurobiology for young, healthy adults. Hum Brain Mapp 38:4201-4211, 2017. © 2017 Wiley Periodicals, Inc.