Conceived and designed the experiments: VN DM GW. Performed the experiments: VN DC JO AM MC. Analyzed the data: VN DM GW DS J. Cross SV. Contributed reagents/materials/analysis tools: GW TAC DM JP MC . Wrote the paper: VN DC JO SV AM J. Cross DS MC PH J. Coles TAC JP GW DM.
The authors have declared that no competing interests exist.
Traumatic brain injury is a major cause of morbidity and mortality worldwide. Ameliorating the neurocognitive and physical deficits that accompany traumatic brain injury would be of substantial benefit, but the mechanisms that underlie them are poorly characterized. This study aimed to use diffusion tensor imaging to relate clinical outcome to the burden of white matter injury.
Sixty-eight patients, categorized by the Glasgow Outcome Score, underwent magnetic resonance imaging at a median of 11.8 months (range 6.6 months to 3.7 years) years post injury. Control data were obtained from 36 age-matched healthy volunteers. Mean fractional anisotropy, apparent diffusion coefficient (ADC), and eigenvalues were obtained for regions of interest commonly affected in traumatic brain injury. In a subset of patients where conventional magnetic resonance imaging was completely normal, diffusion tensor imaging was able to detect clear abnormalities. Significant trends of increasing ADC with worse outcome were noted in all regions of interest. In the white matter regions of interest worse clinical outcome corresponded with significant trends of decreasing fractional anisotropy.
This study found that clinical outcome was related to the burden of white matter injury, quantified by diffusivity parameters late after traumatic brain injury. These differences were seen even in patients with the best outcomes and patients in whom conventional magnetic resonance imaging was normal, suggesting that diffusion tensor imaging can detect subtle injury missed by other techniques. An improved
Traumatic brain injury (TBI) is a major cause of morbidity and mortality worldwide. The extent and severity of traumatic brain injury is greatly underestimated by X-ray computed tomography (CT) and conventional magnetic resonance imaging (MRI), which often correlate poorly with functional outcome
There is an increasing belief that many of the cognitive deficits following TBI may be the consequence of traumatic axonal injury (TAI), which may be subtle and is poorly quantified with conventional imaging techniques. MRI with diffusion tensor imaging (DTI) characterizes the diffusion of water molecules in tissue environments, which is influenced by the microstructural organization of tissues and their constituent cells, and can provide unique insights into pathophysiology, particularly in white matter. The diffusion tensor can be used to represent the magnitude of water diffusion (quantified as the apparent diffusion coefficient, ADC), whether such diffusion is directionally non-uniform (anisotropy), and the orientation of that direction (eigenvectors/eigenvalues). Indeed, previous studies have used the technique in TBI, and typically found consistent reductions in fractional anisotropy (FA) in classical areas affected by TAI, even when conventional MRI showed no lesion. These regions include the subcortical white matter in the frontal and temporal regions, splenium of the corpus callosum, posterior limb of the internal capsule, and the cerebral peduncles
Despite these accumulating data on DTI in TBI, previous studies have reported on small numbers of patients and/or addressed a limited range of outcome categories. We wished to examine how clinical outcomes related to the burden of white matter injury, with outcomes ranging from the vegetative state to patients with no or minimal sequelae.
Ethical approval was obtained from the Cambridgeshire 2 Research Ethics Committee, and written informed consent, or written assent from next-of-kin where appropriate, were obtained in all cases in accordance with the Declaration of Helsinki.
Sixty-eight patients who had sustained TBI underwent MR imaging using a 3 Tesla Siemens Magnetom Total Imaging Matrix (TIM) Trio. Thirty-six controls (healthy volunteers) underwent an identical imaging protocol. This included DTI, 3D T1 weighted structural imaging (magnetization prepared rapid gradient echo; MPRAGE), a Fluid Attenuated Inversion Recovery (FLAIR) sequence, a gradient echo (GE) sequence, and a dual echo (proton density/T2) sequence. The DTI parameters were as follows; 12 non-collinear directions, 5 b values ranging from 338 to 1588 s/mm2, 5 b = 0 images, acquisition matrix size 96×96, field of view 192 mm×192 mm, 63 axial slices, 2 mm slice thickness, TR = 8300 ms, TE = 98 ms. All scans were visually inspected and four patients with translational head movement greater than 5 mm during the diffusion sequence were removed prior to data analysis. This left a dataset of 64 patients and 36 controls. All conventional images were inspected by two neuroradiologists (JC and DS), blinded to whether the images were from control subjects or patients with TBI, and to the outcome category of individual patients. The presence and location of lesions were noted. Subsequent creation of regions of interest (ROIs) took account of this information, and ensured that they did not include lesioned tissue, since blood products may cause signal dropout in DTI.
The DTI data underwent eddy current correction and FA, ADC and eigenvalue maps were created using the Oxford Centre for fMRI of the Brain's (FMRIB's) Diffusion Toolbox (
ROIs, chosen due to their predilection for damage post TBI, were manually drawn using Analyze 7.0 (
Top from left to right; whole brain grey matter (blue), whole brain white mater (white), the supratentorial white matter (red), right and left cerebellar peduncles (green and blue) and the cerebellar cortex (yellow). Bottom from left to right; right and left pons (light blue and yellow), dorsal (yellow) and ventral (red) midbrain, thalamus (green and blue), anterior corpus callosum (red) and posterior corpus callous (light blue).
Patients were categorized into groups using the Glasgow Outcome Scale (GOS), which uses six simple questions in the domains of physical, neuropsychological and social disability, and is the most widely used outcome measure post TBI
Statistical analyses were conducted using SPSS14.0 (
The patient demographic details are shown in
Controls | TBI patients | ||||
(n = 36) | GOS 5(n = 21) | GOS 4(n = 20) | GOS 3(n = 16) | GOS 2(n = 7) | |
|
38 (24 to 70) | 32 (18 to 59) | 38 (20 to 60) | 38.8 (17 to 63) | 39 (21 to 67) |
|
306 (172 to 1252) | 387 (174 to 1341) | 373 (192 to 1130) | 198 (105 to 681) | |
|
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Male | 27 (75) | 14 (66.7) | 14 (70) | 8 (50) | 5 (71) |
Female | 9 (25) | 7 (33.3) | 6 (30) | 8 (50) | 2 (29) |
|
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Motor Vehicle Collision | 17 (81) | 14 (70) | 10 (62.5) | 3 (42.9) | |
Assault | 1 (4.8) | 2 (10) | 1 (6.3) | 2 (28.6) | |
Fall | 3 (14.3) | 4 (20) | 5 (31.3) | 2 (28.6) |
GOS = Glasgow Outcome Score at time of scan
The TBI patients were divided into groups based on Glasgow Outcome Score (GOS) at the time of imaging.
Clinical outcome showed an inverse trend with ADC in all ROIs (
The central lines in the boxes denote the median values, the upper and lower edges the 75th and 25th percentiles, the error bars the 90th and 10th percentiles and the closed circles the data outside these percentiles. *** p<0.0001; NS, non-significant. SWM: supratentorial white matter, ACC: anterior corpus callosum, PCC: posterior corpus callosum. C = controls, GR = good recovery, MD = moderate disability, SD = severe disability, VS = vegetative state.
The central lines in the boxes denote the median values, the upper and lower edges the 75th and 25th percentiles, the error bars the 90th and 10th percentiles and the closed circles the data outside these percentiles. *** p<0.0001; NS, non-significant. SWM: supratentorial white matter, ACC: anterior corpus callosum, PCC: posterior corpus callosum. C = controls, GR = good recovery, MD = moderate disability, SD = severe disability, VS = vegetative state.
C = controls, GR = good recovery, MD = moderate disability, SD = severe disability, VS = vegetative state. * p<0·0013; **<0.001; *** p<0.0001; NS, non-significant.
C = controls, GR = good recovery, MD = moderate disability, SD = severe disability, VS = vegetative state. * p<0·0013; ** <0.001; *** p<0.0001; NS, non-significant.
FA | ADC | |||||||||||
Regions of Interest | GOS 5 Vs GOS 4 | GOS 4 Vs GOS 3 | GOS 3 Vs GOS 2 | Favourable Vs Unfavourable(P value) | Area under the ROC curve(95% CI) | P value | GOS 5 Vs GOS 4 | GOS 4 Vs GOS 3 | GOS 3 Vs GOS 2 | Favourable Vs Unfavourable(P value) | Area under the ROC curve(95% CI) | P value |
Supratentorial white matter | 0.565 | 0.002 | 0.010 | 0.000 | 0.877(0.773 to 0.980) | 0.000 | 0.433 | 0.004 | 0.014 | 0.000 | 0.896(0.798 to 0.994) | 0.000 |
Anterior corpus callosum | 0.261 | 0.017 | 0.040 | 0.000 | 0.840(0.721 to 0.959) | 0.000 | 0.448 | 0.066 | 0.046 | 0.000 | 0.806(0.676 to 0.935) | 0.000 |
Posterior corpus callosum | 0.530 | 0.186 | 0.001 | 0.000 | 0.797(0.671 to 0.923) | 0.000 | 0.403 | 0.503 | 0.016 | 0.002 | 0.750(0.603 to 0.897) | 0.002 |
Ventral midbrain | 0.979 | 0.279 | 0.016 | 0.000 | 0.796(0.66 to 0.932) | 0.000 | 0.754 | 0.744 | 0.012 | 0.009 | 0.705(0.549 to 0.860) | 0.009 |
Dorsal midbrain | 0.094 | 0.175 | 0.030 | 0.000 | 0.775(0.621 to 0.93) | 0.000 | 0.200 | 0.503 | 0.016 | 0.001 | 0.757(0.626 to 0.888) | 0.001 |
Pons | 0.619 | 0.264 | 0.002 | 0.000 | 0.813(0.670 to 0.956) | 0.000 | 0.937 | 0.440 | 0.002 | 0.001 | 0.722(0.569 to 0.874) | 0.005 |
WBGM | 0.374 | 0.279 | 0.010 | 0.001 | 0.762(0.628 to 0.896) | 0.001 | 0.638 | 0.020 | 0.434 | 0.004 | 0.724(0.558 to 0.890) | 0.004 |
Thalamus | 0.855 | 0.250 | 0.794 | 0.416 | 0.635(0.474 to 0.796) | 0.087 | 0.958 | 0.311 | 0.010 | 0.000 | 0.879(0.780 to 0.978) | 0.000 |
Cerebellar peduncles | 0.350 | 0.231 | 0.001 | 0.000 | 0.823(0.697 to 0.950) | 0.000 | 0.248 | 0.452 | 0.023 | 0.052 | 0.654(0.487 to 0.820) | 0.049 |
Cerebellar cortex | 0.762 | 0.707 | 0.002 | 0.002 | 0.740(0.603 to 0.877) | 0.002 | 0.425 | 0.321 | 0.824 | 0.064 | 0.645(0.478 to 0.811) | 0.064 |
GOS Glasgow Outcome Score, ROC = receiver operating curve, CI = confidence intervals.
A small subgroup of four patients had their conventional MR sequences reported as normal by both neuroradiologists. Their clinical characteristics are shown in
Control data are shown in grey, and patients in white. For FA only SWM was significantly lower in this subset of patients. SWM: supratentorial white matter, ACC: anterior corpus callosum, PCC: posterior corpus callosum, WBGM: whole brain grey matter. The p-value pertains to a Mann-Whitney U (exact) test between the two groups. * p<0·05; ** <0.01; NS, non-significant.
Patient | Cause of Injury | Age at Injury | Gender | Injury to MRI interval (days) | GCS at ictus | GOS |
1 | RTA | 37 | Male | 1130 | 15 | 3 |
2 | RTA | 46 | Female | 2342 | 13 | 4 |
3 | RTA | 46 | Male | 677 | 15 | 4 |
4 | RTA | 27 | Female | 1097 | 14 | 5 |
To our knowledge, this study is the first to use DTI to investigate the full spectrum of outcome of TBI patients in the chronic phase post injury, ranging from the vegetative state to minimal or no disability. We show gradations of DTI abnormality in a broad range of ROIs, with patients with worse outcomes having lower FA and higher ADCs. An eigenvalue analysis of DTI data suggested that the changes in FA were associated with increases in both radial and axial diffusivity. These findings support the inclusion of DTI in the portfolio of imaging tools used to characterize the burden of insult following TBI.
Previous studies have found little correlation between CT and/or conventional MR sequences on one hand, and cognitive and functional outcomes on the other
Significant differences in DTI parameters in the central WM, WBGM, corpus callosum (anterior and posterior) and the thalamus were found in comparisons between all patients groups. However, the midbrain and pons ROIs were only significantly different to controls in patients in the poorest outcome groups (GOS 2 and 3). This may indicate that damage to these areas is particularly important in determining whether a patient develops permanent impairments in consciousness or not. Indeed, brainstem lesions have previously been associated with unfavorable outcomes in TBI
The increase in diffusivity in both radial and axial directions may be expected in grey matter regions like the WBGM and thalamus, where cellular necrosis may result in less restricted diffusion. However, we also noted this finding in predominantly white matter ROIs, such as the central WM, pons, and the anterior corpus callosum. Such changes would not be explained by simple demyelination, which would only predictably increase radial diffusivity
The patients studied here encompassed a wide range of disability. It is difficult to find robust cognitive tasks and functional measures that are applicable across such a broad spectrum of patients, who range from the vegetative state, to those able to return to work with minimal or no impairment. In this context, the GOS has several advantages: it characterizes the entire spectrum of TBI outcomes, is easily obtained and reproducible, and is widely used. These attributes make our results more easily applicable and interpreted in the context of other cohorts of TBI patients. However, despite these advantages of the GOS, the lack of refinement in describing some outcomes may be a disadvantage. For example, in GOS category 3, patients in the minimally conscious state (patients who exhibit inconsistent, but reproducible responsiveness; MCS) are grouped with patients who, while unable to live independently, are cognitively far less disabled. Arguably, MCS patients are clinically more similar to the VS patients than those at the higher end of GOS 3, but the framework of the GOS does not permit such reallocation. In any event, a reanalysis with the MCS and VS patients grouped together produced similar results.
One approach to a more refined outcome classification would be to use the extended Glasgow outcome scale (GOSe). In 90% of our patients we had outcome data that allowed such categorization, and a reanalysis with patients categorized in this way did not materially change our inferences about the association between DTI parameters and clinical outcome (see
2 to 8 represent GOSe categories 2 to 8 and C is the control group.
The patients were also studied at varying time points after TBI, but, except for one patient who was diagnosed to be in VS, had a minimum interval between injury and imaging of approximately six months. It is possible that continuing clinical recovery may have resulted in some reclassification of functional outcome in some patients. However, many studies in TBI use a follow up time point of six months post-injury, recognising that a substantial proportion of clinical recovery occurs by this time point. Notwithstanding this, a future study that used uniform (and potentially serial) late follow up and imaging would produce useful corroboration of our findings. In addition, larger studies, particularly involving patients with little damage on conventional imaging, may allow more subtle differences in outcome and neurocognitive functioning to be correlated with DTI parameters.
Finally, our demonstration of pervasive DTI abnormalities in the cerebellum which scale with functional outcome reflect a growing understanding that cerebellar lesions may be important in defining TBI outcome. In a perceptive position paper, Ghajar and Ivry summarized the evidence for abnormalities of cerebellar function contributing to cognitive deficits in TBI
We have shown that clinical outcome relates to the burden of white matter injury, as quantified by diffusivity parameters in patients in the chronic phase post TBI. These DTI abnormalities are seen even in patients with the best outcomes, and in patients with normal conventional MRI, suggesting that they can detect subtle injury that is missed by other approaches. Our data thus provide a basis for including DTI in evaluating TBI outcome, while providing a mechanistic basis for deficits that remained unexplained by other approaches.
Median (interquartile range) for diffusivity parameters for the Central WM, WBGM, and corpus callosum (genu and splenium) by subject group.
(DOC)
The corresponding author had full access to all of the data in the study and had final responsibility for the decision to submit for publication.