Abdalla Z Mohamed1 and Fatima Abdalla Nasrallah1
1Queensland Brain institute, The University of Queensland, Brisbane, Australia
Synopsis
Traumatic brain injury (TBI) is a
severe problem worldwide. The non-invasive investigation of the microstructural
alterations is of significant benefit for early diagnosis and interventions. In
this study, we applied diffusion tensor imaging (DTI) to monitor the longitudinal
microstructural changes in a controlled cortical impact rodent model of TBI from
2 hours and up to 6 months post-injury. Using DTI, we observe ongoing white
matter changes following TBI, that initiate in corpus callosum and injury
location at early timepoints and persists to exist up to 6 months, suggesting
the temporal sensitivity of DTI to detect ongoing microstructural changes following
TBI.
Background
Traumatic brain
injury (TBI) is an external force applied to the brain causing a shearing force
between white and grey matter initiating microstructural changes and
neuroinflammatory responses1–4. Several studies aimed to develop
non-invasive techniques to monitor microstructural changes following TBI for
better diagnosis and possible treatment interventions. Diffusion tensor imaging
(DTI) was successfully used to investigate the microstructural changes
following the controlled cortical impact (CCI) rodent model of TBI5, showing reduced fractional
anisotropy (FA) at widespread
ipsilateral regions that persist to 4 weeks, in association with demyelination
and microglial activity6. To date, no previous studies investigated
the temporal profile of TBI in the CCI model using MRI up to 6 months. The
primary objective of this study is to detect the spatiotemporal profile of
microstructural alterations following CCI-TBI.
Methods
A
total of 37 male Sprague-Dawley rats underwent sham or severe-TBI using CCI
surgery (speed: 5m/sec, depth: 2mm, time:200msec). The animals were randomly divided
as a subset of n=10 TBI and n=10 sham scanned at 2 hours, 1, 3, 7, 14, 30, and
60 days post-TBI; and subset of n=9 TBI and n=8 sham scanned at 6 months
post-TBI. Animals were scanned using a 9.4T MRI system (Bruker, Germany), with T2
weighted (rapid-relaxation-with-enhancement, TR/TE=5900/65ms, RARE-factor=8,
FOV=32×25x20 mm, and matrix=256×256×40), and DTI to investigate the
microstructural changes following TBI (axial, TR/TE/FA=10000ms/29ms/90o,
FOV=24.8×24.8 mm, matrix=108×108×41, slice thickness/gap=0.5/0.1mm, 32-directions
with b-values=750 and 1500s/mm2, and 4 b0-volumes). DTI
data were corrected for eddy current, skull stripped7, and field bias; and DTI
parameters were generated using FSL-DTIFIT. T2 and DTI images
were normalized to the Schwarz template8 non-linearly using ANTS9. At each timepoint, differences between TBI and sham animals were
calculated using unpaired t-test performed with permutation test (1000
permutations; corrected using family-wise error (a<0.05)).
Results
Our T2 images showed
increased lesion volume with time with increased amounts of edema accumulation
at the injury location (Fig
1). DTI showed alterations in the white-matter tracts at all timepoints initiating
in the ipsilateral hemisphere and propagating to affect the contralateral
hemisphere. Our results revealed reduced fractional anisotropy (FA) in the corpus
callosum (CC) at 2hours; in CC, cingulum, and external capsule (EC) at 1 and 3 days;
in CC, EC, internal capsule (IC), anterior commissure (AC), and cingulum at 7 and 14 days; with more FA reduction at
30 days in most of the white-matter tracts including CC, IC, EC, AC, and
optic-tracts; reduced FA at the CC and
optic tracts at 60 days; and in CC, AC, EC, and IC at 6 months (Fig 2). Mean diffusivity
(MD) was reduced in AC, EC, and IC at 1 and 7 days; and increased MD in CC, IC,
EC and OP at 6 months (Fig
3). Axial diffusivity (AD) was reduced in CC, AC, EC, and IC at 1 and 7 days;
and in EC at 30 days; increased AD in CC, IC, EC and OP at 6 months; with no
significant differences at the white-matter tracts at 2 hours, 3, 14, and 60
days (Fig 4). Radial
diffusivity (DR) was persistently increased in CC from 1 day to 6 months; and
in EC, AC, and IC at 6 months (Fig 5).
The lesion area
showed reduced FA from 1 day to 6months; while the MD and RD showed no
difference at 2 hours, reduced at 1 day, and increased from 3 days to 6months; and
the AD showed no difference at 2 hours, reduced at 1, 3, and 7 days, and
increased from 14 days to 6months.
Furthermore, FA
was increased in the cortex, thalamus, and hippocampus at 1 day, followed by
reduced FA values from 7 days to 6 months in cortex, thalamus, and hippocampus.
The FA changes were associated with increased AD in the hippocampus, thalamus,
hypothalamus, and brainstem at 1 and 60 days and 6 months; and reduced AD in
cortex at 1,7 and 30 days. Furthermore, MD and RD were reduced in cortex,
hippocampus, amygdala, and thalamus at 1 and 7 days; and MD and RD were
increased in cortex at 3 days which extended to hippocampus, amygdala,
hypothalamus, basal ganglia, and thalamus at 6 months.
Discussion
To the best of
our knowledge, this is the first longitudinal study to show ongoing changes in DTI
up to six months in the wild type rat brain following a CCI injury. In the
acute phase, increased FA in the grey-matter
and reduced FA in the white-matter was a prominent feature; however, at the
subacute and chronic phases (7 days up to 6months )of the injury progression, a
reduction in FA in both grey- and white-matter was observed. The changes we see
may be representative of increased primary microglial activity at the early
timepoints associated with apoptosis and the start of demyelination followed by
secondary microglial peak at the chronic phase associated with more
demyelination4,6.
These findings inform on the temporal
dynamics of an injury in the rat brain revealed by DTI suggesting that
persistent changes in the brain may be associated with long-term consequences. Further
study to identify the associated underlying pathology at six months is under
investigation which will promote better interpretation of human DTI findings
following injury. Acknowledgements
This work as
supported by Motor Accident Insurance Commission (MAIC) (Grant:2014000857), the
Queensland Government, Australia for the research grant to FN. We thank the Australian Government support through NCRIS and the National
Imaging Facility for the operation of 9.4T MRI at Centre of Advanced Imaging,
University of Queensland, Brisbane, Australia.References
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