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Diffuse axonal injury has a specific multidimensional MRI signature in traumatically injured corpus callosum
Dan Benjamini1, Diego Iacono2, Michal Komlosh1, Daniel Perl2, David Brody2, and Peter Basser1
1National Institute of Child Health and Human Development, Bethesda, MD, United States, 2Uniformed Services University of the Health Sciences, Bethesda, MD, United States

Synopsis

Can microscopic diffuse axonal injury lesions following trauma be seen non-invasively? We report that simultaneously integrating multiple MRI dimensions - T1, T2, and diffusion - can be targeted to image microscopic damage. Corpora Callosa derived from eight subjects that sustained TBI and three healthy control brain donors underwent post-mortem ex-vivo MRI at 7T. Multidimensional-, diffusion tensor-, and quantitative T1- and T2-MRI data were acquired and processed, along with corresponding pathohistological data. Although invisible using the conventional MRI modalities, multidimensional MRI provided images of the microscopic injuries, suggesting that it can be used for the detection of microscopic axonal injury.

Introduction

Traumatic brain injury (TBI) presents major medical, social and economic challenges world-wide and is associated with high mortality, co-morbidities, and long-term disabilities.1 Among the various pathological brain lesions produced by impact, diffuse axonal injury (DAI) is caused by mechanical damage to axons multifocally throughout the white matter (WM).2 Axonal transport is disrupted following axonal injury and pathologic accumulation of different axonal proteins causes axonal swelling, which leads to axonal degeneration.3 Amyloid precursor protein (APP) is one of those accumulated proteins, and it can be detected immunohistochemically within a few hours post injury.4 Because of that, and due to its high sensitivity and robustness, APP remains the gold-standard for the identification of axonal injury.3

Conventional neuroradiological tools, such as CT and MRI, are insensitive to DAI caused by trauma. Diffusion tensor imaging (DTI) has been established as a sensitive tool for depicting axonal integrity in regions with TBI-associated lesions.5 In particular, the fractional anisotropy (FA) had stood out as being a sensitive biomarker for TBI. Unfortunately, despite DTI’s high sensitivity, conflicting findings of either decreases6-7 or increases5,8 of the FA in TBI groups compared with control subjects have been reported.

The fundamental obstacle for using MRI to detect microscopic tissue alterations is the averaging that occurs across the voxels. Voxel-averaged images can only provide macroscopic information with respect to the voxel size (typically on the order of 1 mm3), thus limited by MRI’s relatively low image resolution. Multidimensional MRI is an emerging approach that combines T1, T2, and diffusion and replaces voxel-averaged values with distributions,9,10 which allows selective isolation of specific potential abnormal components.11-13 By performing a combined post-mortem multidimensional MRI and histopathology study, we aimed to investigate T1-T2-diffusion changes linked to DAI and to define their histopathological correlates.

Methods

Corpora Callosa derived from eight subjects that sustained TBI, and three control brain donors underwent post-mortem ex vivo MRI at 7 T. Multidimensional-, diffusion tensor-, and quantitative T1- and T2-MRI data were acquired and processed. Following MRI acquisition, slices from the same tissue were tested for APP immunoreactivity to define DAI severity. A robust image co-registration method was applied to accurately match MRI-derived parameters and histopathology, after which 12 regions of interest per tissue block were selected based on APP density, but blind to MRI. In total, 132 regions of interest from 11 cases were included in this study. Abnormal multidimensional T1-T2, diffusion-T2, and diffusion-T1 components that are strongly associated with DAI were identified using a spectral thresholding algorithm,14 and were then used to generate axonal injury images.

Results

We observed a consistent and gradual change in the way in which T1, T2, and MD were distributed as the APP-based severity of the DAI was increasing. Figure 1 shows five cases with increased injury severity and their respective T1-T2 (A-E), MD-T2 (F-J), and MD-T1 (K-O) distributions. Qualitatively, the magnified regions show a gradual shortening of T1 and T2 as the severity of the injury is increased (Fig. 1, left to right).

We grouped the 132 tissue regions into controls, normal-appearing WM in TBI patients, and DAI lesions in TBI patients. We further partitioned the TBI patients into mild and severe traumatic axonal injury (mTAI, sTAI) according to the APP staining-based severity. We found that compared to control white matter, mild and severe DAI lesions contained significantly larger abnormal T1-T2 component (P=0.005 and P<0.001, respectively), and significantly larger abnormal diffusion-T2 component (P=0.005 and P<0.001, respectively). Furthermore, within patients with TBI the multidimensional MRI biomarkers differentiated normal-appearing white matter from mild and severe DAI lesions, with significantly larger abnormal T1-T2 and diffusion-T2 components (P=0.003 and P<0.001, respectively for T1-T2, and P=0.022 and P<0.001, respectively for diffusion-T2). Conversely, none of the conventional quantitative MRI parameters were able to differentiate lesions and normal-appearing white matter (Fig. 3).

Lastly, we found that the abnormal T1-T2, diffusion-T1, and diffusion-T2 components and their axonal damage images were strongly correlated with quantitative APP staining (r = 0.876, P < 0.001; r = 0.727, P < 0.001; and r = 0.743, P < 0.001, respectively), while producing negligible intensities in gray matter and in normal-appearing white matter (Fig. 4).

Discussion

This study crucially identified potential imaging biomarkers of DAI pathology from the joint analysis of multidimensional MRI and histopathological data in the Corpus Callosum. The ability to selectively extract a specific T1-T2-MD spectral range, or spectral signature, and provide its corresponding image, allows multidimensional MRI to achieve good separation between subjects and microscopic lesion and non-lesion regions. We found that the multidimensional MRI axonal injury images are significantly and strongly correlated with histological evidence of DAI.

We believe that the improved sensitivity of these novel injury biomarkers towards DAI is advancing the neuroimaging field closer towards noninvasive quantitative “histology”. This neuroimaging tool provides an injury-only image that may help clinicians and clinical investigators to detect and visualize microscopic DAI lesions in the brain. As evidence that indicates DAI is a likely pathological substrate for concussion is mounting,15 the importance of developing a means to detect it clinically becomes clear.

Acknowledgements

We thank the subjects’ families that consented for brain donations for the better understanding of TBI consequences. The authors thank Dr. Jessica Ettedgui for fruitful discussions, and Dr. Thaddeus Haight for insights into the statistical analysis. We also thank Mrs. Patricia Lee, Mrs. Nichelle Gray and Mr. Paul Gegbeh for their valuable technical work. We are grateful to Mrs. Stacey Gentile, Mrs. Deona Cooper and Mr. Harold Kramer Anderson for their administrative assistance. We thank the TRACK-TBI Investigators.

References

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Figures

Multidimensional spectra of the DAI lesions in representative cases with increasing degree of severity (left to right). (A)-(E) T1-T2, (F)-(J) MD-T2, and (K)-(O) MD-T1. A T1-T2-MD range, T1=[91.03, 339.32] ms, T2=[6.70, 34.85] ms, and MD=[0.004, 0.146] µm2/ms is highlighted as pink rectangles. Below each distribution, a magnification of the highlighted spectral ROI is shown. The progressive shift towards shorter T1, shorter T2, and slower diffusivity as the severity of the injury increases is evident.

Control brain (A) APP histological images co-registered to MRIs. Deconvolved histological image: red = APP stain. (B) Conventional MRI and (C) multidimensional injury maps do not show visible abnormalities. Nonfatal TBI brain (D) APP images show visible DAI lesions in the CC, while (E) conventional MRI do not. (F) Multidimensional injury maps show significant injury in WM. Fatal TBI brain (G) APP histological images show DAI lesions, while none are observed with (H) conventional MRI. (I) Multidimensional injury maps show substantial injury along the WM/GM interface.

Between-group comparisons of voxel-averaged and multidimensional biomarkers and histopathological measures. Box plots showing between-group differences among control CC, normal-appearing WM in mTAI CC, mTAI lesions, normal-appearing WM in sTAI CC, and sTAI lesions. All parameters are averaged across their corresponding spatial ROIs. Note that all adjusted voxel-averaged parameters were obtained after dividing them by the mean for that parameter across all WM within the subject, to correct for possible between-subject differences arising from postmortem effects.


APP density (% area) from 132 tissue regions, consisting of 4 APP-positive regions from each TAI case (total of 32, blue dots), 4 to 6 normal-appearing WM regions from all cases (total of 56, red dots), and 4 cortical GM regions from all cases (total of 44, yellow dots), and the corresponding MR parameter correlations. Individual data points represent the mean ROI value from each post-mortem tissue sample. Scatterplots of the mean (with 95% confidence interval error bars) % area APP and all MR parameters.

Summary of the findings, illustrated by comparing the FA with the T1-T2 injury image.

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)
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