Full tensor diffusion imaging is not required to assess the white-matter integrity in mouse contusion spinal cord injury

Diffusion basis spectrum imaging detects subclinical traumatic optic neuropathy in a closed-head impact mouse model of traumatic brain injury

Introduction

Traumatic optic neuropathy (TON) is the optic nerve injury secondary to brain trauma leading to visual impairment and vision loss. Current clinical visual function assessments often fail to detect TON due to slow disease progression and clinically silent lesions resulting in potentially delayed or missed treatment in patients with traumatic brain injury (TBI).

Methods

Diffusion basis spectrum imaging (DBSI) is a novel imaging modality that can potentially fill this diagnostic gap. Twenty-two, 16-week-old, male mice were equally divided into a sham or TBI (induced by moderate Closed-Head Impact Model of Engineered Rotational Acceleration device) group. Briefly, mice were anesthetized with isoflurane (5% for 2.5 min followed by 2.5% maintenance during injury induction), had a helmet placed over the head, and were placed in a holder prior to a 2.1-joule impact. Serial visual acuity (VA) assessments, using the Virtual Optometry System, and DBSI scans were performed in both groups of mice. Immunohistochemistry (IHC) and histological analysis of optic nerves was also performed after in vivo MRI.

Results

VA of the TBI mice showed unilateral or bilateral impairment. DBSI of the optic nerves exhibited bilateral involvement. IHC results of the optic nerves revealed axonal loss, myelin injury, axonal injury, and increased cellularity in the optic nerves of the TBI mice. Increased DBSI axon volume, decreased DBSI λ||, and elevated DBSI restricted fraction correlated with decreased SMI-312, decreased SMI-31, and increased DAPI density, respectively, suggesting that DBSI can detect coexisting pathologies in the optic nerves of TBI mice.

Conclusion

DBSI provides an imaging modality capable of detecting subclinical changes of indirect TON in TBI mice.

In vivo imaging in experimental spinal cord injury – Techniques and trends

Introduction

Traumatic Spinal Cord Injury (SCI) is one of the leading causes of disability in the world. Treatment is limited to supportive care and no curative therapy exists. Experimental research to understand the complex pathophysiology and potential mediators of spinal cord regeneration is essential to develop innovative translational therapies. A multitude of experimental imaging methods to monitor spinal cord regeneration in vivo have developed over the last years. However, little literature exists to deal with advanced imaging methods specifically available in SCI research.

Research Question

This systematic literature review examines the current standards in experimental imaging in SCI allowing for in vivo imaging of spinal cord regeneration on a neuronal, vascular, and cellular basis.

Material and Methods

Articles were included meeting the following criteria: experimental research, original studies, rodent subjects, and intravital imaging. Reviewed in detail are microstructural and functional Magnetic Resonance Imaging, Micro-Computed Tomography, Laser Speckle Imaging, Very High Resolution Ultrasound, and in vivo microscopy techniques.

Results

Following the PRISMA guidelines for systematic reviews, 689 articles were identified for review, of which 492 were sorted out after screening and an additional 104 after detailed review. For qualitative synthesis 93 articles were included in this publication.

Discussion and Conclusion

With this study we give an up-to-date overview about modern experimental imaging techniques with the potential to advance the knowledge on spinal cord regeneration following SCI. A thorough knowledge of the strengths and limitations of the reviewed techniques will help to optimally exploit our current experimental armamentarium in the field.

•

In vivo imaging is essential to enhance the understanding of SCI pathophysiology.

•

Multiple experimental imaging methods have evolved over the past years.

•

Detailed review of in vivo (f)MRI, μCT, VHRUS, and Microcopy in experimental SCI.

•

Experimental imaging allows for longitudinal examination to the cellular level.

•

Knowledge of the strengths and limitations is essential for future research.

MRI metrics at the epicenter of spinal cord injury are correlated with the stepping process in rhesus monkeys

Clinical evaluations of long-term outcomes in the early-stage spinal cord injury (SCI) focus on macroscopic motor performance and are limited in their prognostic precision. This study was designed to investigate the sensitivity of the magnetic resonance imaging (MRI) indexes to the data-driven gait process after SCI. Ten adult female rhesus monkeys were subjected to thoracic SCI. Kinematics-based gait examinations were performed at 1 (early stage) and 12 (chronic stage) months post-SCI. The proportion of stepping (PS) and gait stability (GS) were calculated as the outcome measures. MRI metrics, which were derived from structural imaging (spinal cord cross-sectional area, SCA) and diffusion tensor imaging (fractional anisotropy, FA; axial diffusivity, λ_//), were acquired in the early stage and compared with functional outcomes by using correlation analysis and stepwise multivariable linear regression. Residual tissue SCA at the injury epicenter and residual tissue FA/remote normal-like tissue FA were correlated with the early-stage PS and GS. The extent of lesion site λ_///residual tissue λ_// in the early stage after SCI was correlated with the chronic-stage GS. The ratios of lesion site λ_// to residual tissue λ_// and early-stage GS were predictive of the improvement in the PS at follow-up. Similarly, the ratios of lesion site λ_// to residual tissue λ_// and early-stage PS best predicted chronic GS recovery. Our findings demonstrate the predictive power of MRI combined with the early data-driven gait indexes for long-term outcomes. Such an approach may help clinicians to predict functional recovery accurately.

Repairing and Analgesic Effects of Umbilical Cord Mesenchymal Stem Cell Transplantation in Mice with Spinal Cord Injury

Transplantation of human umbilical cord mesenchymal stem cells (hUC-MSCs) into spinal cord injury (SCI) may alleviate neuropathic pain and promote functional recovery. The underlying mechanism likely involves activation of glial cells and regulation of inflammatory factors but requires further validation. SCI was induced in 16 ICR mice using an SCI compression model, followed by injection of lentiviral vector-mediated green fluorescent protein- (GFP-) labeled hUC-MSCs 1 week later. Behavioral tests, histological evaluation, and inflammatory factor detection were performed in the treatment (SCI+hUC-MSCs) and model (SCI) groups. Histological evaluation revealed GFP expression in the spinal cord tissue of the treatment group, implying that the injected MSCs successfully migrated to the SCI. The Basso, Beattie, and Bresnahan (BBB) scores showed that motor function gradually recovered over time in both groups, but recovery speed was significantly higher in the treatment group than in the model group. The pain threshold in mice decreased after SCI but gradually increased over time owing to the self-repair function of the body. The corresponding pain threshold of the treatment group was significantly higher than that of the model group, indicating the therapeutic and analgesic effects of hUC-MSCs. Expression of IL-6 and TNF-α in the spinal cord tissue of the treated group decreased, whereas glial cell line-derived neurotrophic factor (GDNF) expression along with ED1 expression increased compared with those in the model group, suggesting that SCI activated ED1 inflammatory macrophages/microglia, which were subsequently reduced by hUC-MSC transplantation. hUC-MSCs are speculated to enhance the repair of the injured spinal cord tissue and exert an analgesic effect by reducing the secretion of inflammatory factors IL-6 and TNF-α and upregulating the expression of GDNF.

Noninvasive Quantification of Axonal Loss in the Presence of Tissue Swelling in Traumatic Spinal Cord Injury Mice

Neuroimaging plays an important role in assessing axonal pathology after traumatic spinal cord injury. However, coexisting inflammation confounds imaging assessment of the severity of axonal injury. Herein, we applied diffusion basis spectrum imaging (DBSI) to quantitatively differentiate and quantify underlying pathologies in traumatic spinal cord injury at 3 days post-injury. Results reveal that DBSI was capable of detecting and differentiating axonal injury, demyelination, and inflammation-associated edema and cell infiltration in contusion-injured spinal cords. DBSI was able to detect and quantify axonal loss in the presence of white matter tract swelling. The DBSI-defined apparent axonal volume correlated with the corresponding histological markers. DBSI-derived pathological metrics could serve as neuroimaging biomarkers to differentiate and quantify coexisting white matter pathologies in spinal cord injury, providing potential surrogate outcome measures to assess spinal cord injury progression and response to therapies.

Testing Pathological Variation of White Matter Tract in Adult Rats after Severe Spinal Cord Injury with MRI

The purpose of this study was to assess the pathological variation in white matter tracts in the adult severe thoracic contusion spinal cord injury (SCI) rat models combined with in vivo magnetic resonance imaging (MRI), as well as the effect of spared white matter (WM) quantity on hindlimb motor function recovery. 7.0T MRI was conducted for all experimental animals before SCI and 1, 3, 7, and 14 days after SCI. The variation in the white matter tract in different regions of the spinal cord after SCI was examined by luxol fast blue (LFB) staining, NF200 immunochemistry, and diffusion tensor imaging (DTI) parameters, including fraction anisotropy, mean diffusivity, axial diffusion, and radial diffusivity. Meanwhile, Basso-Beattie-Bresnahan (BBB) open-field scoring was performed to evaluate the behavior of the paraplegic hind limbs. The quantitative analysis showed that spared white matter measures assessed by LFB and MRI had a close correlation (R² = 0.8508). The percentage of spared white matter area was closely correlated with BBB score (R² = 0.8460). After SCI, spared white matter in the spinal cord, especially the ventral column WM, played a critical role in motor function restoration. The results suggest that the first three days provides a key time window for SCI protection and treatment; spared white matter, especially in the ventral column, plays a key role in motor function recovery in rats. Additionally, DTI may be an important noninvasive technique to diagnose acute SCI degree as well as a tool to evaluate functional prognosis. During the transition from nerve protection toward clinical treatment after SCI, in vivo DTI may serve as an emerging noninvasive technique to diagnose acute SCI degree and predict the degree of spontaneous functional recovery after SCI.

Reproducibility, temporal stability, and functional correlation of diffusion MR measurements within the spinal cord in patients with asymptomatic cervical stenosis or cervical myelopathy

OBJECTIVE The purpose of this study was to quantify the reproducibility, temporal stability, and functional correlation of diffusion MR characteristics in the spinal cord in patients with cervical stenosis with or without myelopathy. The association between longitudinal diffusion tensor imaging (DTI) measurements and serial neurological function assessment was explored at both the group and individual level. METHODS Sixty-six nonoperatively treated patients with cervical stenosis were prospectively followed (3 months to > 5 years) using synchronous serial MRI and functional outcome assessment. A total of 183 separate MRI examinations were performed, separated by at least 3 months, and each patient had a minimum of 2 MRI scans (range 2-5 scans). Anatomical and DTI measurements were performed within the spinal cord at the C1-2 region as well as at the area of highest compression. Coefficients of variance (COVs) were compared across measurements in both reference tissue and areas of compression for anatomical measurements, fractional anisotropy (FA), and mean diffusivity (MD). The correlation between diffusion MR measures at the site of compression and evaluations of neurological function assessed using the modified Japanese Orthopaedic Association (mJOA) scale at multiple time points was evaluated. RESULTS The COVs for anatomical measurements (Torg ratio and canal diameter) were between 7% and 10%. The median COV for FA measurements at the site of compression was 9%, and for reference tissue at C1-2 it was 6%. The median COV for MD at the site of compression was approximately 12%, and for reference tissue at C1-2 it was 10%. The FA and MD measurements of C1-2 averaged 0.61 and 0.91 μm²/msec, respectively, whereas the FA and MD measurements at the site of compression averaged 0.51 and 1.26 μm²/msec, respectively. Both FA (slope = 0.037; R² = 0.3281, p < 0.0001) and MD (slope = -0.074; R² = 0.1101, p = 0.0084) were significantly correlated with the mJOA score. The FA decreased by approximately 0.032 units per mJOA unit decrease (R² = 0.2037, p < 0.0001), whereas the MD was increased by approximately 0.084 μm²/msec for every mJOA unit decrease (R² = 0.1016, p < 0.0001). CONCLUSIONS Quantitative DTI measurements of the spinal cord in patients with cervical stenosis with or without myelopathy have a median COV of 5%-10%, similar to anatomical measurements. The reproducibility of these measurements and significant correlation with functional outcome status suggest a potential role in the evaluation and longitudinal surveillance of nonoperatively treated patients. With respect to the specific DTI measurements, FA within the spinal cord appears slightly more sensitive to neurological function and more stable than measures of MD. Therefore, DTI of the spinal cord may be a clinically feasible imaging technique for longitudinally monitoring patients with cervical spondylotic myelopathy.

Optimizing Filter-Probe Diffusion Weighting in the Rat Spinal Cord for Human Translation

Diffusion tensor imaging (DTI) is a promising biomarker of spinal cord injury (SCI). In the acute aftermath, DTI in SCI animal models consistently demonstrates high sensitivity and prognostic performance, yet translation of DTI to acute human SCI has been limited. In addition to technical challenges, interpretation of the resulting metrics is ambiguous, with contributions in the acute setting from both axonal injury and edema. Novel diffusion MRI acquisition strategies such as double diffusion encoding (DDE) have recently enabled detection of features not available with DTI or similar methods. In this work, we perform a systematic optimization of DDE using simulations and an in vivo rat model of SCI and subsequently implement the protocol to the healthy human spinal cord. First, two complementary DDE approaches were evaluated using an orientationally invariant or a filter-probe diffusion encoding approach. While the two methods were similar in their ability to detect acute SCI, the filter-probe DDE approach had greater predictive power for functional outcomes. Next, the filter-probe DDE was compared to an analogous single diffusion encoding (SDE) approach, with the results indicating that in the spinal cord, SDE provides similar contrast with improved signal to noise. In the SCI rat model, the filter-probe SDE scheme was coupled with a reduced field of view (rFOV) excitation, and the results demonstrate high quality maps of the spinal cord without contamination from edema and cerebrospinal fluid, thereby providing high sensitivity to injury severity. The optimized protocol was demonstrated in the healthy human spinal cord using the commercially-available diffusion MRI sequence with modifications only to the diffusion encoding directions. Maps of axial diffusivity devoid of CSF partial volume effects were obtained in a clinically feasible imaging time with a straightforward analysis and variability comparable to axial diffusivity derived from DTI. Overall, the results and optimizations describe a protocol that mitigates several difficulties with DTI of the spinal cord. Detection of acute axonal damage in the injured or diseased spinal cord will benefit the optimized filter-probe diffusion MRI protocol outlined here.

Diffusion-relaxation correlation spectroscopic imaging: A multidimensional approach for probing microstructure

PURPOSE

To propose and evaluate a novel multidimensional approach for imaging subvoxel tissue compartments called Diffusion-Relaxation Correlation Spectroscopic Imaging.

THEORY AND METHODS

Multiexponential modeling of MR diffusion or relaxation data is commonly used to infer the many different microscopic tissue compartments that contribute signal to macroscopic MR imaging voxels. However, multiexponential estimation is known to be difficult and ill-posed. Observing that this ill-posedness is theoretically reduced in higher dimensions, diffusion-relaxation correlation spectroscopic imaging uses a novel multidimensional imaging experiment that jointly encodes diffusion and relaxation information, and then uses a novel constrained reconstruction technique to generate a multidimensional diffusion-relaxation correlation spectrum for every voxel. The peaks of the multidimensional spectrum are expected to correspond to the distinct tissue microenvironments that are present within each macroscopic imaging voxel.

RESULTS

Using numerical simulations, experiment data from a custom-built phantom, and experiment data from a mouse model of traumatic spinal cord injury, diffusion-relaxation correlation spectroscopic imaging is demonstrated to provide substantially better multicompartment resolving power compared to conventional diffusion- and relaxation-based methods.

CONCLUSION

The diffusion-relaxation correlation spectroscopic imaging approach provides powerful new capabilities for resolving the different components of multicompartment tissue models, and can be leveraged to significantly expand the insights provided by MRI in studies of tissue microstructure. Magn Reson Med 78:2236-2249, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Comparison of Diffusion MRI Acquisition Protocols for the In Vivo Characterization of the Mouse Spinal Cord: Variability Analysis and Application to an Amyotrophic Lateral Sclerosis Model

Diffusion-weighted Magnetic Resonance Imaging (dMRI) has relevant applications in the microstructural characterization of the spinal cord, especially in neurodegenerative diseases. Animal models have a pivotal role in the study of such diseases; however, in vivo spinal dMRI of small animals entails additional challenges that require a systematical investigation of acquisition parameters. The purpose of this study is to compare three acquisition protocols and identify the scanning parameters allowing a robust estimation of the main diffusion quantities and a good sensitivity to neurodegeneration in the mouse spinal cord. For all the protocols, the signal-to-noise and contrast-to noise ratios and the mean value and variability of Diffusion Tensor metrics were evaluated in healthy controls. For the estimation of fractional anisotropy less variability was provided by protocols with more diffusion directions, for the estimation of mean, axial and radial diffusivity by protocols with fewer diffusion directions and higher diffusion weighting. Intermediate features (12 directions, b = 1200 s/mm²) provided the overall minimum inter- and intra-subject variability in most cases. In order to test the diagnostic sensitivity of the protocols, 7 G93A-SOD1 mice (model of amyotrophic lateral sclerosis) at 10 and 17 weeks of age were scanned and the derived diffusion parameters compared with those estimated in age-matched healthy animals. The protocols with an intermediate or high number of diffusion directions provided the best differentiation between the two groups at week 17, whereas only few local significant differences were highlighted at week 10. According to our results, a dMRI protocol with an intermediate number of diffusion gradient directions and a relatively high diffusion weighting is optimal for spinal cord imaging. Further work is needed to confirm these results and for a finer tuning of acquisition parameters. Nevertheless, our findings could be important for the optimization of acquisition protocols for preclinical and clinical dMRI studies on the spinal cord.

Diffusion-Weighted Magnetic Resonance Imaging Characterization of White Matter Injury Produced by Axon-Sparing Demyelination and Severe Contusion Spinal Cord Injury in Rats

Alterations in magnetic resonance imaging (MRI)-derived measurements of water diffusion parallel (D∥) and perpendicular (D⊥) to white matter tracts have been specifically attributed to pathology of axons and myelin, respectively. We test the hypothesis that directional diffusion measurements can distinguish between axon-sparing chemical demyelination and severe contusion spinal cord white matter injury. Adult rats received either unilateral ethidium bromide (EB) microinjections (chemical demyelination) into the lateral funiculus of the spinal cord at C5 or were subjected to unilateral severe contusion spinal cord injury (SCI). Diffusion MRI metrics in the lateral funiculus were analyzed at early and late time-points following injury and correlated with histology. Early EB-demyelination resulted in a significant elevation in D⊥ and significant reduction in D∥ at the injury epicenter, with histological evidence of uniform axon preservation. Alterations in D⊥ and D∥ at the epicenter of early EB-demyelination were not significantly different from those observed with severe contusion at the epicenter, where histology demonstrated severe combined axonal and myelin injury. Diffusion abnormalities away from the injury epicenter were seen with contusion injury, but not with EB-demyelination. Chronic EB lesions underwent endogenous remyelination with normalization of diffusion metrics, whereas chronic contusion resulted in persistently altered diffusivities. In the early setting, directional diffusion measurements at the injury epicenter associated with chemical demyelination are indistinguishable from those seen with severe contusive SCI, despite dramatic pathologic differences between injury models. Caution is advised in interpretation of diffusion metrics with respect to specific white matter structural alterations. Diffusion analysis should not be limited to the epicenter of focal spinal lesions as alterations marginal to the epicenter are useful for assessing the nature of focal white matter injury.

Detection of acute nervous system injury with advanced diffusion-weighted MRI: a simulation and sensitivity analysis

Diffusion-weighted imaging (DWI) is a powerful tool to investigate the microscopic structure of the central nervous system (CNS). Diffusion tensor imaging (DTI), a common model of the DWI signal, has a demonstrated sensitivity to detect microscopic changes as a result of injury or disease. However, DTI and other similar models have inherent limitations that reduce their specificity for certain pathological features, particularly in tissues with complex fiber arrangements. Methods such as double pulsed field gradient (dPFG) and q-vector magic angle spinning (qMAS) have been proposed to specifically probe the underlying microscopic anisotropy without interference from the macroscopic tissue organization. This is particularly important for the study of acute injury, where abrupt changes in the microscopic morphology of axons and dendrites manifest as focal enlargements known as beading. The purpose of this work was to assess the relative sensitivity of DWI measures to beading in the context of macroscopic fiber organization and edema. Computational simulations of DWI experiments in normal and beaded axons demonstrated that, although DWI models can be highly specific for the simulated pathologies of beading and volume fraction changes in coherent fiber pathways, their sensitivity to a single idealized pathology is considerably reduced in crossing and dispersed fibers. However, dPFG and qMAS have a high sensitivity for beading, even in complex fiber tracts. Moreover, in tissues with coherent arrangements, such as the spinal cord or nerve fibers in which tract orientation is known a priori, a specific dPFG sequence variant decreases the effects of edema and improves specificity for beading. Collectively, the simulation results demonstrate that advanced DWI methods, particularly those which sample diffusion along multiple directions within a single acquisition, have improved sensitivity to acute axonal injury over conventional DTI metrics and hold promise for more informative clinical diagnostic use in CNS injury evaluation.

Prediction of Neurological Impairment in Cervical Spondylotic Myelopathy using a Combination of Diffusion MRI and Proton MR Spectroscopy

PURPOSE

In the present study we investigated a combination of diffusion tensor imaging (DTI) and magnetic resonance spectroscopic (MRS) biomarkers in order to predict neurological impairment in patients with cervical spondylosis.

METHODS

Twenty-seven patients with cervical spondylosis were evaluated. DTI and single voxel MRS were performed in the cervical cord. N-acetylaspartate (NAA) and choline (Cho) metabolite concentration ratios with respect to creatine were quantified, as well as the ratio of choline to NAA. The modified mJOA scale was used as a measure of neurologic deficit. Linear regression was performed between DTI and MRS parameters and mJOA scores. Significant predictors from linear regression were used in a multiple linear regression model in order to improve prediction of mJOA. Parameters that did not add value to model performance were removed, then an optimized multiparametric model was established to predict mJOA.

RESULTS

Significant correlations were observed between the Torg-Pavlov ratio and FA (R2 = 0.2021, P = 0.019); DTI fiber tract density and FA, MD, Cho/NAA (R2 = 0.3412, P = 0.0014; R2 = 0.2112, P = 0.016; and R2 = 0.2352, P = 0.010 respectively); along with FA and Cho/NAA (R2 = 0.1695, P = 0.033). DTI fiber tract density, MD and FA at the site of compression, along with Cho/NAA at C2, were significantly correlated with mJOA score (R2 = 0.05939, P < 0.0001; R2 = 0.4739, P < 0.0001; R2 = 0.7034, P < 0.0001; R2 = 0.4649, P < 0.0001). A combination biomarker consisting of DTI fiber tract density, MD, and Cho/NAA showed the best prediction of mJOA (R2 = 0.8274, P<0.0001), with post-hoc tests suggesting fiber tract density, MD, and Cho/NAA were all significant contributors to predicting mJOA (P = 0.00053, P = 0.00085, and P = 0.0019, respectively).

CONCLUSION

A linear combination of DTI and MRS measurements within the cervical spinal cord may be useful for accurately predicting neurological deficits in patients with cervical spondylosis. Additional studies may be necessary to validate these observations.

Diffusion imaging in the rat cervical spinal cord

Magnetic resonance imaging (MRI) is the state of the art approach for assessing the status of the spinal cord noninvasively, and can be used as a diagnostic and prognostic tool in cases of disease or injury. Diffusion weighted imaging (DWI), is sensitive to the thermal motion of water molecules and allows for inferences of tissue microstructure. This report describes a protocol to acquire and analyze DWI of the rat cervical spinal cord on a small-bore animal system. It demonstrates an imaging setup for the live anesthetized animal and recommends a DWI acquisition protocol for high-quality imaging, which includes stabilization of the cord and control of respiratory motion. Measurements with diffusion weighting along different directions and magnitudes (b-values) are used. Finally, several mathematical models of the resulting signal are used to derive maps of the diffusion processes within the spinal cord tissue that provide insight into the normal cord and can be used to monitor injury or disease processes noninvasively.

Imaging of spontaneous ventriculomegaly and vascular malformations in Wistar rats: implications for preclinical research

Wistar rats are widely used in biomedical research and commonly serve as a model organism in neuroscience studies. In most cases when noninvasive imaging is not used, studies assume a consistent baseline condition in rats that lack visible differences. While performing a series of traumatic brain injury studies, we discovered mild spontaneous ventriculomegaly in 70 (43.2%) of 162 Wistar rats that had been obtained from 2 different vendors. Advanced magnetic resonance (MR) imaging techniques, including MR angiography and diffusion tensor imaging, were used to evaluate the rats. Multiple neuropathologic abnormalities, including presumed arteriovenous malformations, aneurysms, cysts, white matter lesions, and astrogliosis were found in association with ventriculomegaly. Postmortem microcomputed tomography and immunohistochemical staining confirmed the presence of aneurysms and arteriovenous malformations. Diffusion tensor imaging showed significant decreases in fractional anisotropy and increases in mean diffusivity, axial diffusivity, and radial diffusivity in multiple white matter tracts (p < 0.05). These results could impact the interpretation, for example, of a pseudo-increase of axon integrity and a pseudo-decrease of myelin integrity, based on characteristics intrinsic to rats with ventriculomegaly. We suggest the use of baseline imaging to prevent the inadvertent introduction of a high degree of variability in preclinical studies of neurologic disease or injury in Wistar rats.

Diffusion tensor imaging predicts functional impairment in mild-to-moderate cervical spondylotic myelopathy

BACKGROUND CONTEXT

Magnetic resonance imaging (MRI) is the standard imaging modality for the assessment of cervical spinal cord; however, MRI assessment of the spinal cord in cervical spondylotic myelopathy patients has not demonstrated a consistent association with neurologic function or outcome after surgical or medical intervention. Thus, there is a need for sensitive imaging biomarkers that can predict functional impairment in patients with advanced cervical spondylosis.

PURPOSE

To implement diffusion tensor imaging (DTI) as an imaging biomarker for microstructural integrity and functional impairment in patients with cervical spondylosis.

STUDY DESIGN

Nonrandomized, single institution study.

PATIENT SAMPLE

Forty-eight cervical spondylosis patients with or without spinal cord signal change underwent DTI of the spinal cord along with functional assessment.

OUTCOME MEASURES

Functional measures of neurologic function via modified Japanese Orthopedic Association (mJOA) score.

METHODS

A zoomed-echoplanar imaging technique and two-dimensional spatially selective radiofrequency excitation pulse were used for DTI measurement. Fractional anisotropy (FA), mean diffusivity (MD), radial and axial diffusion (AD) coefficient, AD anisotropy, ψ, defined as AD-MD, and the standard deviation (SD) of primary eigenvector orientation were evaluated at the site of compression.

RESULTS

Results suggest average FA, transverse apparent diffusion coefficient, ψ, and SD of primary eigenvector orientation at the spinal level of highest compression were linearly correlated with mJOA score. Receiver-operator characteristic analysis suggested FA and ψ could identify stenosis patients with mild-to-moderate symptoms with a relatively high sensitivity and specificity.

CONCLUSIONS

The results of this study support the potential use of DTI as a biomarker for predicting functional impairment in patients with cervical spondylosis.

Phase-aligned multiple spin-echo averaging: a simple way to improve signal-to-noise ratio of in vivo mouse spinal cord diffusion tensor image

PURPOSE

To improve signal-noise-ratio of in vivo mouse spinal cord diffusion tensor imaging using-phase aligned multiple spin-echo technique.

MATERIAL AND METHODS

In vivo mouse spinal cord diffusion tensor imaging maps generated by multiple spin-echo and conventional spin-echo diffusion weighting were examined to demonstrate the efficacy of multiple spin-echo diffusion sequence to improve image quality and throughput. Effects of signal averaging using complex, magnitude and phased images from multiple spin-echo diffusion weighting were also assessed. Bayesian probability theory was used to generate phased images by moving the coherent signals to the real channel to eliminate the effect of phase variation between echoes while preserving the Gaussian noise distribution. Signal averaging of phased multiple spin-echo images potentially solves both the phase incoherence problem and the bias of the elevated Rician noise distribution in magnitude image. The proposed signal averaging with Bayesian phase-aligned multiple spin-echo images approach was compared to the conventional spin-echo data acquired with doubling the scan time. The diffusion tensor imaging parameters were compared in the mouse contusion spinal cord injury. Significance level (p-value) and effect size (Cohen's d) were reported between the control and contused spinal cord to inspect the sensitivity of each approach in detecting white matter pathology.

RESULTS

Compared to the spin-echo image, the signal-noise-ratio increased to 1.84-fold using the phased image averaging and to 1.30-fold using magnitude image averaging in the spinal cord white matter. Multiple spin-echo phased image averaging showed improved image quality of the mouse spinal cord among the tested methods. Diffusion tensor imaging metrics obtained from multiple spin-echo phased images using three echoes and two averages closely agreed with those derived by spin-echo magnitude data with four averages (two times more in acquisition time). The phased image averaging correctly reflected pathological features in contusion spinal cord injury.

CONCLUSION

Our in vivo imaging results indicate that averaging the phased multiple spin-echo images yields an 84% signal-noise-ratio increase over the spin-echo images and a 41% gain over the magnitude averaged multiple spin-echo images with equal acquisition time. Current results from the animal model of spinal cord injury suggest that the phased multiple spin-echo images could be used to improve signal-noise-ratio.

Multishot diffusion-weighted MR imaging features in acute trauma of spinal cord

OBJECTIVES

To analyse diffusion-weighted MRI of acute spinal cord trauma and evaluate its diagnostic value.

METHODS

Conventional MRI and multishot, navigator-corrected DWI were performed in 20 patients with acute spinal cord trauma using 1.5-T MR within 72 h after the onset of trauma.

RESULTS

Twenty cases were classified into four categories according to the characteristics of DWI: (1) Oedema type: ten cases presented with variable hyperintense areas within the spinal cord. There were significant differences in the apparent diffusion coefficients (ADCs) between lesions and unaffected regions (t = -7.621, P < 0.01). ADC values of lesions were markedly lower than those of normal areas. (2) Mixed type: six cases showed heterogeneously hyperintense areas due to a mixture of haemorrhage and oedema. (3) Haemorrhage type: two cases showed lesions as marked hypointensity due to intramedullary haemorrhage. (4) Compressed type (by epidural haemorrhage): one of the two cases showed an area of mild hyperintensity in the markedly compressed cord due to epidural haematoma.

CONCLUSIONS

Muti-shot DWI of the spinal cord can help visualise and evaluate the injured spinal cord in the early stage, especially in distinguishing the cytotoxic oedema from vasogenic oedema. It can assist in detecting intramedullary haemorrhage and may have a potential role in the evaluation of compressed spinal cord.

KEY POINTS

• Multishot, navigator-corrected diffusion-weighted MRI is helpful when evaluating spinal cord injury (SCI). • Four types of SCI may be classified according to the DWI characteristics. • DWI differentiates cytotoxic from vasogenic oedema, thereby determining the centre of SCI. • DWI can assist in detecting intramedullary haemorrhage. • DWI can help evaluate the degree of compressed spinal cord.

The impact of myelination on axon sparing and locomotor function recovery in spinal cord injury assessed using diffusion tensor imaging

The dysmyelinated axons of shiverer mice exhibit impaired conduction characteristics, similar to early postnatal axons before myelination, whereas the patterns of neuronal activity and connectivity are relatively comparable with those of wild-type myelinated axons. This unique dysmyelination pattern is exploited in the present study to determine the role of compact myelin in the loss and recovery of function following traumatic spinal cord injury (SCI). We applied in vivo diffusion tensor imaging (DTI) and post-mortem immunohistochemistry analysis to examine changes in myelin and axonal integrity, and evaluated these changes in concert with the analysis of locomotor function from 1 to 4 weeks following a mid-thoracic contusion injury in homozygous shiverer and heterozygous littermate mice. The DTI biomarkers, axial and radial diffusivities, are noninvasive indicators of axon and myelin integrity in response to SCI of both myelinated and dysmyelinated spinal cord. We show that myelin is critical for normal hind limb function in open field locomotion. However, when the functional outcome is limited during chronic SCI, the extent of recovery is associated with residual axonal integrity and independent of the extent of intact myelin at the lesion epicenter.

Longitudinal assessment of white matter pathology in the injured mouse spinal cord through ultra-high field (16.4 T) in vivo diffusion tensor imaging

This study examined the sensitivity of ultra-high field (16.4 T) diffusion tensor imaging (DTI; 70 μm in-plane resolution, 1mm slice thickness) to evaluate the spatiotemporal development of severe mid-thoracic contusive spinal cord injury (SCI) in mice. In vivo imaging was performed prior to SCI, then again at 2h, 1 day, 3 days, 7 days, and 30 days post-SCI using a Bruker 16.4 T small animal nuclear magnetic resonance spectrometer. Cross-sectional spinal cord areas were measured in axial slices and various DTI parameters, i.e. fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (λ||) and radial diffusivity (λ⊥), were calculated for the total spared white matter (WM), ventral funiculi (VF), lateral funiculi (LF) and dorsal columns (DCs) and then correlated with histopathology. Cross-sectional area measurements revealed significant atrophy (32% reduction) of the injured spinal cord at the lesion epicentre in the chronic phase of injury. Analysis of diffusion tensor parameters further showed that tissue integrity was most severely affected in the DCs, i.e. the site of immediate impact, which demonstrated a rapid and permanent decrease in FA and λ||. In contrast, DTI parameters for the ventrolateral white matter changed more gradually with time, suggesting that these regions are undergoing more delayed degeneration in a manner that may be amenable to therapeutic intervention. Of all the DTI parameters, λ⊥ was most closely correlated to myelin content whereas changes in FA and λ|| appeared more indicative of axonal integrity, Wallerian degeneration and associated presence of macrophages. We conclude that longitudinal DTI at 16.4T provides a clinically relevant, objective measure for assessing white matter pathology following contusive SCI in mice that may aid the translation of putative neuroprotective strategies into the clinic.

Neuronal activity significantly reduces water displacement: DWI of a vital rat spinal cord with no hemodynamic effect

Changes in the diffusion weighted MRI (DWI) signal were observed to be correlated with neuronal activity during chemically induced brain activity, epileptic seizures, or visual stimulation. These changes suggest a possible reduction in water displacement that accompanies neuronal activity, but were possibly affected by other physiological mechanisms such as blood oxygenation level and blood flow. We developed an imaging experiment of an excised and vital newborn rat spinal cord to examine the effect of neuronal function on the displacement of water molecules as measured by DWI signal. This approach provides a DWI experiment of a vital mammalian CNS tissue in the absence of some of the systemic sources of noise. We detected a significant and reproducible drop with an average value of 19.5 ± 1.6% (mean ± SE) upon activation. The drop repeated itself in three orthogonal directions. ADC values corresponded to an oblate anisotropy. This result was validated by high resolution DWI of a fixed tissue, imaged with an ultra-high field MRI. The results support our working hypothesis that water displacement is affected by neuronal activation. These results further imply that water displacement might serve as a potential marker for brain function, and that, although commonly viewed as wholly electrochemical, neuronal activity includes a significant mechanical dimension that affects water displacement.

Spinal cord - MR of rodent models

Different MR techniques, such as relaxation times, diffusion, perfusion, and spectroscopy have been employed to study rodent spinal cord. In this chapter, a description of these methods is given, along with examples of normal metrics that can be derived from the MR acquisitions, as well as examples of applications to pathology.