Structural changes in glutamate cell swelling followed by multiparametric q-space diffusion MR of excised rat spinal cord

Microstructural and functional correlates of glutamate concentration in the posterior cingulate cortex

Glutamate is the major excitatory neurotransmitter in the human brain and has a central role in both intrinsic and stimulus-induced activity. We conducted a study in a cohort of healthy, male volunteers in which glutamate levels were measured in the posterior cingulate cortex (PCC) using 1H magnetic resonance spectroscopy at 3T. The advantages of simultaneous electroencephalography and magnetic resonance imaging (EEG-MRI) were exploited and the subjects were measured in the same session and under the same physiological conditions. Diffusion tensor imaging (DTI), functional MRI (fMRI) and EEG were measured in order to investigate the functional and microstructural correlates of glutamate. The concentration of glutamate (institute units) was calculated and those values were tested for correlation with the metrics of resting state fMRI, DTI, and EEG electrical sources. Our results showed that the concentration of glutamate in the PCC had a significant negative correlation with the tissue mean diffusivity in the same area. The analysis of resting state networks did not show any relationship between the concentration of glutamate and the intrinsic activity of the resting state networks. The concentration of glutamate showed a positive correlation with the electrical generators of α-1 frequency and a negative correlation with the generators of α-2 and β-1 electrical generators. © 2017 Wiley Periodicals, Inc.

Diffusion in realistic biophysical systems can lead to aliasing effects in diffusion spectrum imaging

PURPOSE

Diffusion spectrum imaging (DSI) is an imaging technique that has been successfully applied to resolve white matter crossings in the human brain. However, its accuracy in complex microstructure environments has not been well characterized.

THEORY AND METHODS

Here we have simulated different tissue configurations, sampling schemes, and processing steps to evaluate DSI performances' under realistic biophysical conditions. A novel approach to compute the orientation distribution function (ODF) has also been developed to include biophysical constraints, namely integration ranges compatible with axial fiber diffusivities.

RESULTS

Performed simulations identified several DSI configurations that consistently show aliasing artifacts caused by fast diffusion components for both isotropic diffusion and fiber configurations. The proposed method for ODF computation showed some improvement in reducing such artifacts and improving the ability to resolve crossings, while keeping the quantitative nature of the ODF.

CONCLUSION

In this study, we identified an important limitation of current DSI implementations, specifically the presence of aliasing due to fast diffusion components like those from pathological tissues, which are not well characterized, and can lead to artifactual fiber reconstructions. To minimize this issue, a new way of computing the ODF was introduced, which removes most of these artifacts and offers improved angular resolution. Magn Reson Med 76:1837-1847, 2016. © 2015 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Modeling of the diffusion MR signal in calibrated model systems and nerves

Diffusion NMR is a powerful tool for gleaning microstructural information on opaque systems. In this work, the signal decay in single-pulsed-field gradient diffusion NMR experiments performed on a series of phantoms of increasing complexity, where the ground truth is known a priori, was modeled and used to identify microstructural features of these complex phantoms. We were able to demonstrate that, without assuming the number of components or compartments, the modeling can identify the number of restricted components, detect their sizes with an accuracy of a fraction of a micrometer, determine their relative populations, and identify and characterize free diffusion when present in addition to the components exhibiting restricted diffusion. After the accuracy of the modeling had been demonstrated, this same approach was used to study fixed nerves under different experimental conditions. It seems that this approach is able to characterize both the averaged axon diameter and the relative population of the different diffusing components in the neuronal tissues examined.

Advances in MR imaging for cervical spondylotic myelopathy

PURPOSE

To outline the pathogenesis of cervical spondylotic myelopathy (CSM), the correlative abnormalities observed on standard magnetic resonance imaging (MRI), the biological implications and current status of diffusion tensor imaging (DTI), and MR spectroscopy (MRS) as clinical tools, and future directions of MR technology in the management of CSM patients.

METHODS

A systematic review of the pathogenesis and current state-of-the-art in MR imaging technology for CSM was performed.

RESULTS

CSM is caused by progressive, degenerative, vertebral column abnormalities that result in spinal cord damage related to both primary mechanical and secondary biological injuries. The T2 signal change on conventional MRI is most commonly associated with neurological deficits, but tends not to be a sensitive predictor of recovery of function. DTI and MRS show altered microstructure and biochemistry that reflect patient-specific pathogenesis.

CONCLUSION

Advanced imaging techniques, including DTI and MRS, show higher sensitivity to microstructural and biochemical changes within the cord, and may aid in management of CSM patients.

A viable isolated tissue system: a tool for detailed MR measurements and controlled perturbation in physiologically stable tissue

In vivo magnetic resonance imaging (MRI) assessment of neuronal tissue is prone to artifacts such as movement, pulsatile flow, and tissue susceptibility. Furthermore, stable in vivo scans of over 3 h are difficult to achieve, experimental design is therefore limited. Using isolated tissue maintained in a viable physiological state can mitigate many of these in vivo issues. This work describes the fabrication and validation of an MRI compatible viable isolated tissue maintenance chamber. Parameters measured from maintained rat optic nerves did not change significantly over 10 h: (i) mean axon radius [electron microscopy--0 h: 0.75±0.46; 5 h: 0.74±0.35; 10 h: 0.76±0.35 μm (P>>0.05, t-test], (ii) action potentials [grease-gap electrophysiology--4.89±0.16 mv, (P>>0.05, Pearson test], and (iii) diffusion tensor imaging parameters [fractional anisotropy: 0.86±0.02 (P>>0.05, Pearson test), mean diffusivity: 1.48E-06±9.74E-08 cm2/s, (P>>0.05, Pearson test)]. In addition, a thorough diffusion-weighted MR protocol demonstrated the comparable stability of viable isolated and chemically fixed rat optic nerve. This MRI compatible viable isolated tissue system allows researchers to probe neuronal physiology in a controlled environment by limiting in vivo artifacts and allowing extended MRI acquisitions.

WITHDRAWN: First observation of diffusion-diffraction pattern in neuronal tissue by double-pulsed-field-gradient NMR

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q-space and conventional diffusion imaging of axon and myelin damage in the rat spinal cord after axotomy

Parallel and perpendicular diffusion properties of water in the rat spinal cord were investigated 3 and 30 days after dorsal root axotomy, a specific insult resulting in early axonal degeneration followed by later myelin damage in the dorsal column white matter. Results from q-space analysis (i.e., the diffusion probability density function) obtained with strong diffusion weighting were compared to conventional anisotropy and diffusivity measurements at low b-values, as well as to histology for axon and myelin damage. q-Space contrasts included the height (return to zero displacement probability), full width at half maximum, root mean square displacement, and kurtosis excess of the probability density function, which quantifies the deviation from gaussian diffusion. Following axotomy, a significant increase in perpendicular diffusion (with decreased kurtosis excess) and decrease in parallel diffusion (with increased kurtosis excess) were found in lesions relative to uninjured white matter. Notably, a significant change in abnormal parallel diffusion was detected from 3 to 30 days with full width at half maximum, but not with conventional diffusivity. Also, directional full width at half maximum and root mean square displacement measurements exhibited different sensitivities to white matter damage. When compared to histology, the increase in perpendicular diffusion was not specific to demyelination, whereas combined reduced parallel diffusion and increased perpendicular diffusion was associated with axon damage.

Spatial And Temporal Damage Evolution after Hemi-Crush Injury in Rat Spinal Cord Obtained by High b-Value q-Space Diffusion Magnetic Resonance Imaging

Spinal cord injury (SCI) is a major cause of disability for many living persons. Therefore, several experimental models and handful of techniques were developed to study and characterize the damage evolution following SCI. In the present study, high b-value q-space diffusion-weighted imaging (DWI) was used to follow the spatial and temporal damage evolution in excised rat spinal cords following hemi-crush injury. The DWI results were correlated with behavioral testing. It was found that the damage depends, as expected, on the severity of the insult. Significant spontaneous recovery was observed, six weeks following the insult, only for the mild hemi-crush injury but not following the severe injury. The damage was found to be more severe in the area caudal to the trauma site as compared to the rostral section of the cord.

Transient signal changes in diffusion-weighted stimulated echoes during neuronal stimulation at 3T

PURPOSE

To develop a sensitive method for detecting minute transient signal changes that can arise due to variations in the extravascular apparent self-diffusion coefficient, D, during neuronal activation.

MATERIALS AND METHODS

A three-pulse sequence that reads out a moderately diffusion-weighted (DW) primary echo (PRE) and a heavily DW stimulated echo (STE) was employed to investigate whether small transient signal changes in extravascular D occur in response to a visual stimulus. Contributions to signal changes caused by subtle differences in the transient variations of the apparent transverse relaxation constant, T(2), between the PRE and STE were also quantified.

RESULTS

On z-maps obtained from the STE, more voxels showed significant stimulus-related signal changes compared to maps of the PRE. The average maximum signal change of the STE was larger than that of the PRE. The observed increase in the relative signal change was independent of the strength of the diffusion weighting.

CONCLUSION

The STE is more sensitive to neuronal activity than the PRE. The discrepancy between the two echoes does not arise from transient changes in D, but from subtle differences in stimulus-related variations of T(2) between the two echoes.

Maturation-dependent microstructure length scale in the corpus callosum of fixed rat brains by magnetic resonance diffusion-diffraction

Techniques capable of assessing microstructure length scale are potentially useful in probing the integrity of biologic tissue at the microscopic level. Although the magnetic resonance (MR) diffusion-diffraction technique has been proposed for years, its realization in an undissected brain has not been reported on. In this study, validation of this method in a phantom simulating a series of repeated sheets of water with regular spacing was first performed. The same technique was applied to the corpus callosum of fixed rat brains of different ages (range, 21-84 days). The phantom was constructed with a pile of transparencies immersed in water doped with Gd-DTPA. The measured signal showed diffraction-like coherence peaks, the modulation of which was influenced by the gap distance and the center-to-center distance of the adjacent gaps. The measured distances were consistent with the actual values. In five 84-day-old rats, the diffusion length scale derived from the diffractogram was highly reproducible. In the course of brain maturation, the measured size decreased with age. Electron microscopy showed that axons on day 21 were smaller in diameter and less myelinated as compared with those on day 84. Progressive decrease in the diffusion length scale observed during brain maturation might reflect a gradual decrease in transmembrane permeability due to myelination. In conclusion, MR diffusion-diffraction can be observed in the corpus callosum of fixed rat brains. This technique might be useful in probing the status of myelination in the development of disease.

High b-value q-space diffusion MRI in myelin-deficient rat spinal cords

In this study, we explore the effect of the lack of myelin on the diffusion characteristics and diffusion anisotropy obtained from high b-value q-space diffusion-weighted MRI (q-space DWI) in excised rat spinal cords. Twenty-one-day-old myelin-deficient (md) mutant (N=6) and control rats (N=6) were used in this study. The MRI protocol included multi-slice T(1), T(2), proton density (PD) MR images and high b-value q-space diffusion MRI measured perpendicular and parallel to the fibers of the spine. q-Space displacement and probability maps, in both directions, as well as displacement anisotropy maps, were computed from the diffusion data. At the end of the MRI protocol, representative spinal cords from both groups were subjected to electron microscopy (EM). The md spinal cords show different gray/white matter contrast in the T(1), T(2) and PD MR images as compared with controls. In addition, the mean displacement extracted from the high b-value q-space diffusion data was found to be dramatically higher in the white matter (WM) of the md spinal cords than the controls when diffusion was measured perpendicular and parallel to the fibers of the spine. However, interestingly, at the diffusion time used in the present study, the difference in the WM displacement anisotropies of the two groups was not found to be statistically significant. Myelin was found to have a pronounced effect on the diffusion characteristics of water in WM but less so on the diffusion anisotropy observed at the diffusion time used in the present study.

Improved detectability of experimental allergic encephalomyelitis in excised swine spinal cords by high b-value q-space DWI

Experimental allergic encephalomyelitis (EAE) is the primary experimental model of multiple sclerosis (MS), which involves both inflammation and demyelination and is known to be species-dependent. Spinal cord abnormalities were found in more than 80% of postmortem specimens of MS patients. In the present study, T1, T2 and high b-value q-space diffusion-weighted magnetic resonance imaging (MRI) were used, for the first time, to characterize the EAE model in excised swine spinal cords. The MR images were compared with histological staining and clinical scoring. Although all spinal cords were excised from swine with severe or very severe (clinical score between 3 to 5 on a scale of 5) motor impairments, T1- and T2-weighted MRI revealed white matter (WM) abnormalities in only five of the ten EAE diseased spinal cords studied, while high b-value q-space diffusion weighted MRI (q-space DWI) detected WM abnormalities in all diseased spinal cords studied. Interestingly, high b-value q-space DWI was able to detect abnormalities in the normal appearing white matter (NAWM) even in spinal cords where no plaques were identified by the T1- and T2-weighted MR images. Good anatomical correlation was observed between the high b-value q-space MR images and histology. The extent of DWI abnormalities paralleled the clinical scoring and correlated with histology. In addition, areas classified as NAWM by the T1- and T2-weighted MR images that showed abnormalities in the q-space DWI were also found to have abnormal histology. This improved detection level of the EAE model by high b-value q-space DWI over conventional T1-, and T2-weighted MRI is briefly discussed.

Effect of experimental parameters on high b -value q -space MR images of excised rat spinal cord

The influence of diffusion time (delta), gradient duration (delta), and TE on the appearance of high b-value q-space diffusion MR images of excised rat spinal cord (SC) was evaluated. The water signal decays in the white (WM) and gray matter (GM) were analyzed when the diffusion was measured perpendicular ( perpendicular) and parallel to the fibers of the SC, using three different approaches: single-component q-space analysis, the biexponential model, and the bi-Gaussian fit of the displacement distribution profile. Probability and displacement contrast and anisotropy indices were calculated for the WM and GM. It was found that WM/GM contrast increases as the diffusion time is increased when diffusion is measured perpendicular to the long axis of the SC. At a diffusion time of 50 ms, when diffusion was measured parallel to the fibers of the SC, the displacement was found to be higher for GM as compared to WM. For this direction the WM/GM contrast increased when diffusion time was increased, although here the changes were much less pronounced than for the perpendicular direction. The WM/GM displacement contrast nearly disappears for a diffusion time of 150 ms, when diffusion is measured parallel to the fibers of the SC. As expected, the anisotropy indices were found to be higher in WM than in GM, and increased with the increase in diffusion time. Both delta and TE affected the extracted parameters. It was found that long delta and long TE overemphasizes the apparent slow-diffusing water component of the SC, which is also the more restricted one. It is demonstrated that the single-component q-space analysis best describes diffusion in WM when diffusion is measured perpendicular to the fibers of the SC. In other cases, a more complete description is obtained by using two-component models.