Karthik Chary1, Mikko J. Nissi2,3, Ramón I. Rey4, Eppu Manninen1, Karin Shmueli5, Alejandra Sierra1, and Olli Gröhn1
1Department of Neurobiology, A.I. Virtanen Institute for Molecular Sciences, University of Eastern Finland, Kuopio, Finland, 2Department of Applied Physics, University of Eastern Finland, Kuopio, Finland, 3Finland Diagnostic Imaging Center, Kuopio University Hospital, Kuopio, Finland, 4Department of Neurology, Clinical Neurosciences Research Laboratory, Hospital Clínico Universitario, Health Research Institute of Santiago de Compostela (IDIS), University of Santiago de Compostela, Santiago de Compostela, Spain, 5Department of Medical Physics & Biomedical Engineering, University College London, London, United Kingdom
Synopsis
Our
aim was to test the sensitivity of QSM to demyelination, iron and
calcifications in a rat model of TBI. Ex
vivo QSM data were obtained from five injured and four sham control rats,
six months after TBI. Our results showed susceptibility changes in white matter
areas consistent with myelin staining. Perilesional cortex became more
diamagnetic after TBI. Thalamic nuclei showed variable responses as diamagnetic
calcification and paramagnetic iron accumulation occurred in the same brain
areas. Overall, QSM showed sensitivity to TBI changes. However, further studies
are required to better understand the influence of potentially counteracting
pathological processes.Purpose
The initial insult after
traumatic brain injury (TBI) is followed by a range of secondary tissue-level
changes associated with a broad spectrum of symptoms and disabilities
1.
Quantitative susceptibility mapping (QSM) is sensitive to the tissue magnetic
susceptibility and can reveal changes in tissue composition of diamagnetic and
paramagnetic substances. After TBI, iron accumulates with bleeding
2,
calcifications appear in areas of extensive neurodegeneration
3, and white
matter undergoes demyelination
2. Therefore, the purpose of this
study was to investigate changes in the susceptibility distribution in the rat
brain after TBI.
Methods
Lateral fluid percussion injury
was performed in adult male Sprague-Dawley rats (n(TBI)=5; n(sham)=4). Six
months after TBI, rats were perfused with 4% paraformaldehyde. The rat brains
were imaged
ex vivo at 9.4 T
using an Agilent console and a 3D multi-echo GRE sequence with TR=130 ms, flip
angle=25˚, BW=50 kHz, 6 echoes with TE=3:4:23 ms, matrix size=150×192×256, 100
μm isotropic resolution, and a scan time of 106 min. For QSM, field maps were
estimated by fitting the complex data over TEs
4, followed by
Laplacian unwrapping and SHARP filtering (threshold=0.05, kernel diameter=9
voxels) to remove residual wraps and background fields. Susceptibility maps were
then calculated using TKD
5 (threshold=2/3) and correction for
susceptibility underestimation
6. Regions of interest (ROIs) were
manually defined on magnitude images of 3 coronal slices in selected brain
areas. In a separate group of rats 6 months
post-TBI (n=8) typical alterations in myelin content, cellularity, iron content
and calcifications were evaluated using gold chloride, Nissl, Perls´ and
Alizarin Red stainings, respectively.
Results
Fig.
1 shows ex vivo QSM maps in a sham and
a TBI rat.
White
Matter
The
ipsilateral corpus callosum (cc) in TBI rats showed more diamagnetic susceptibility
values than sham rats (Fig.2A). Changes in susceptibility were also found in
the contralateral cc due to the continuity of this structure between
hemispheres (Fig. 3A). Histology revealed a decreased cc thickness due to loss
of myelinated axons. However, in the remaining cc myelin content was not
markedly changed. We also detected gliosis and sparse microbleeds along the
ipsilateral and contralateral cc in most of the animals after TBI (Fig.3G). One
animal showed increased cc susceptibility due to a higher number of bleeds
(Fig.2A).
The
ipsilateral internal capsule in TBI rats showed more paramagnetic susceptibility
values than in sham rats (Fig. 2B). Histology revealed thinning of this
structure together with decreased myelin density (Fig.3B) as well as iron
deposits in the ipsilateral internal capsule and neighboring areas (Fig.3D).
Cortex
Surprisingly,
the cortex close to the lesion site in TBI rats appeared to be more diamagnetic
than both the contralateral hemisphere in the same animals and in shams (Fig.2C).
As we frequently observed paramagnetic microbleeds below the perilesional
cortex, we cannot completely rule out the contribution of residual non-local field effect that was not fully resolved by the
QSM processing. There was loss of myelinated
axons throughout the cortex despite a low overall cortical myelin content (Fig.3B).
There was diffuse iron accumulation, mainly in deep cortical layers (Fig.3D). Nissl
staining also showed increased cellularity in perilesional cortex, which can be
attributed to gliosis.
Thalamic
Nuclei
The
two thalamic nuclei included in our analysis, ventrobasal complex and
laterodorsal nucleus, are composed of a mixture of white matter fibre bundles
and grey matter. Most of the TBI rats showed more paramagnetic susceptibility values
in the ipsilateral ventrobasal complex compared to the contralateral ROI (Fig.2D).
There was one animal which showed more diamagnetic values. In this thalamic
area the contribution of myelin loss (Fig. 3B), iron accumulation (Fig.3D), and
calcifications (Fig.3F) are expected to influence the susceptibility. Susceptibility
values were variable in the laterodorsal thalamic nucleus (Fig.2E). This area
showed extensive neurodegeneration, gliosis and calcifications (Fig.3F).
Discussion
& Conclusion
We tested the sensitivity
of QSM to pathological processes including demyelination, iron accumulation and
calcifications in a rat model of TBI. As expected, several pathological processes
with potentially counteracting influences on magnetic susceptibility overlap in
the same tissue. Furthermore, caution should be exercised in reconstruction and
interpretation of QSM maps as TBI pathology involves focal spots of drastically
altered magnetic susceptibility, which may lead to residual non-local effects. Validation
of the results requires detailed histological characterization at the level of
individual animals. This kind of comparative study will lead to a better
understanding of the contributions to altered magnetic susceptibility during
progressive brain pathology and will pave the way for utilization of QSM as a
part of the non-invasive imaging toolbox available to study the complex
pathology of TBI.
Acknowledgements
Academy of Finland and Charles River
Laboratories.References
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