Multiexponential T2 relaxation in degenerating peripheral nerve

Understanding carpal tunnel syndrome

ABSTRACT

Carpal tunnel syndrome (CTS) is an entrapment neuropathy affecting the median nerve. Prevalence is estimated at 4% to 5% of the population. A solid understanding of the anatomy, presentation, and diagnostics is key to efficient diagnosis and appropriate referral. Both surgical and nonsurgical interventions have led to improved clinical outcomes. Clinicians who have an in-depth knowledge of CTS symptoms and treatment options can prepare patients and streamline referrals for improved patient outcomes.

Quantification and Proximal-to-Distal Distribution Pattern of Tibial Nerve Lesions in Relapsing-Remitting Multiple Sclerosis : Assessment by MR Neurography

PURPOSE

Recent studies suggest an involvement of the peripheral nervous system (PNS) in multiple sclerosis (MS). Here, we characterize the proximal-to-distal distribution pattern of peripheral nerve lesions in relapsing-remitting MS (RRMS) by quantitative magnetic resonance neurography (MRN).

METHODS

A total of 35 patients with RRMS were prospectively included and underwent detailed neurologic and electrophysiologic examinations. Additionally, 30 age- and sex-matched healthy controls were recruited. 3T MRN with anatomical coverage from the proximal thigh down to the tibiotalar joint was conducted using dual-echo 2‑dimensional relaxometry sequences with spectral fat saturation. Quantification of PNS involvement was performed by evaluating microstructural (proton spin density (ρ), T2-relaxation time (T2_app)), and morphometric (cross-sectional area, CSA) MRN markers in every axial slice.

RESULTS

In patients with RRMS, tibial nerve lesions at the thigh and the lower leg were characterized by a decrease in T2_app and an increase in ρ compared to controls (T2_app thigh: p < 0.0001, T2_app lower leg: p = 0.0040; ρ thigh: p < 0.0001; ρ lower leg: p = 0.0098). An additional increase in nerve CSA was only detectable at the thigh, while the semi-quantitative marker T2w-signal was not altered in RRMS in both locations. A slight proximal-to-distal gradient was observed for T2_app and T2-signal, but not for ρ.

CONCLUSION

PNS involvement in RRMS is characterized by a decrease in T2_app and an increase in ρ, occurring with proximal predominance at the thigh and the lower leg. Our results indicate microstructural alterations in the extracellular matrix of peripheral nerves in RRMS and may contribute to a better understanding of the pathophysiologic relevance of PNS involvement.

Variable echo time imaging for detecting the short T2* components of the sciatic nerve: a validation study

OBJECTIVE

The aim of this study was to develop and validate an MRI protocol based on a variable echo time (vTE) sensitive to the short T2* components of the sciatic nerve.

MATERIALS AND METHODS

15 healthy subjects (M/F: 9/6; age: 21-62) were scanned at 3T targeting the sciatic nerve at the thigh bilaterally, using a dual echo variable echo time (vTE) sequence (based on a spoiled gradient echo acquisition) with echo times of 0.98/5.37 ms. Apparent T2* (aT2*) values of the sciatic nerves were calculated with a mono-exponential fit and used for data comparison.

RESULTS

There were no significant differences in aT2* related to side, sex, age, and BMI, even though small differences for side were reported. Good-to-excellent repeatability and reproducibility were found for geometry of ROIs (Dice indices: intra-rater 0.68-0.7; inter-rater 0.70-0.72) and the related aT2* measures (intra-inter reader ICC 0.95-0.97; 0.66-0.85) from two different operators. Side-related signal-to-noise-ratio non-significant differences were reported, while contrast-to-noise-ratio measures were excellent both for side and echo.

DISCUSSION

Our study introduces a novel MR sequence sensitive to the short T2* components of the sciatic nerve and may be used for the study of peripheral nerve disorders.

In vivo investigation of the multi-exponential T2 decay in human white matter at 7 T: Implications for myelin water imaging at UHF

INTRODUCTION

Multi-component T₂ mapping using a gradient- and spin-echo (GraSE) acquisition has become standard for myelin water imaging at 3 T. Higher magnetic field strengths promise signal-to-noise ratio benefits but face specific absorption rate limits and shortened T₂ times. This study investigates compartmental T₂ times in vivo and addresses advantages and challenges of multi-component T₂ mapping at 7 T.

METHODS

We acquired 3D multi-echo GraSE data in seven healthy adults at 7 T, with three subjects also scanned at 3 T. Stimulated echoes arising from B₁⁺ inhomogeneities were accounted for by the extended phase graph (EPG) algorithm. We used the computed T₂ distributions to determine T₂ times that identify different water pools and assessed signal-to-noise and fit-to-noise characteristics of the signal estimation. We compared short T₂ fractions and T₂ properties of the intermediate water pool at 3 T and 7 T.

RESULTS

Flip angle mapping confirmed that EPG accurately determined the larger B₁⁺ inhomogeneity at 7 T. Multi-component T₂ analysis demonstrated shortened T₂ times at 7 T compared with 3 T. Fit-to-noise and signal-to-noise ratios were improved at 7 T but depended on B₁⁺ homogeneity. Adjusting the shortest T₂ to 8 ms and the T₂ threshold that separates different water compartments to 20 ms yielded short T₂ fractions at 7 T that conformed to 3 T data. Short T₂ fractions in myelin-rich white matter regions were lower at 7 T than at 3 T, and higher in iron-rich structures.

DISCUSSION

Adjusting the T₂ compartment boundaries was required due to the shorter T₂ relaxation times at 7 T. Shorter echo spacing would better sample the fast decaying signal but would increase peripheral nerve stimulation. Multi-channel transmission will improve T₂ measurements at 7 T.

CONCLUSION

We used a multi-echo 3D GraSE sequence to characterize the multi-exponential T₂ decay at 7 T. We adapted T₂ parameters for evaluation of the short T₂ fraction. Obtained 7 T multi-component T₂ maps were in good agreement with 3 T data.

Myelin Water Imaging Demonstrates Lower Brain Myelination in Children and Adolescents With Poor Reading Ability

Magnetic resonance imaging (MRI) provides a means to non-invasively investigate the neurological links with dyslexia, a learning disability that affects one’s ability to read. Most previous brain MRI studies of dyslexia and reading skill have used structural or diffusion imaging to reveal regional brain abnormalities. However, volumetric and diffusion MRI lack specificity in their interpretation at the microstructural level. Myelin is a critical neural component for brain function and plasticity, and as such, deficits in myelin may impact reading ability. MRI can estimate myelin using myelin water fraction (MWF) imaging, which is based on evaluation of the proportion of short T2 myelin-associated water from multi-exponential T2 relaxation analysis, but has not yet been applied to the study of reading or dyslexia. In this study, MWF MRI, intelligence, and reading assessments were acquired in 20 participants aged 10–18 years with a wide range of reading ability to investigate the relationship between reading ability and myelination. Group comparisons showed markedly lower MWF by 16–69% in poor readers relative to good readers in the left and right thalamus, as well as the left posterior limb of the internal capsule, left/right anterior limb of the internal capsule, left/right centrum semiovale, and splenium of the corpus callosum. MWF over the entire group also correlated positively with three different reading scores in the bilateral thalamus as well as white matter, including the splenium of the corpus callosum, left posterior limb of the internal capsule, left anterior limb of the internal capsule, and left centrum semiovale. MWF imaging from T2 relaxation suggests that myelination, particularly in the bilateral thalamus, splenium, and left hemisphere white matter, plays a role in reading abilities. Myelin water imaging thus provides a potentially valuable in vivo imaging tool for the study of dyslexia and its remediation.

Evaluation of principal component analysis image denoising on multi-exponential MRI relaxometry

PURPOSE

Multi-exponential relaxometry is a powerful tool for characterizing tissue, but generally requires high image signal-to-noise ratio (SNR). This work evaluates the use of principal-component-analysis (PCA) denoising to mitigate these SNR demands and improve the precision of relaxometry measures.

METHODS

PCA denoising was evaluated using both simulated and experimental MRI data. Bi-exponential transverse relaxation signals were simulated for a wide range of acquisition and sample parameters, and experimental data were acquired from three excised and fixed mouse brains. In both cases, standard relaxometry analysis was performed on both original and denoised image data, and resulting estimated signal parameters were compared.

RESULTS

Denoising reduced the root-mean-square-error of parameters estimated from multi-exponential relaxometry by factors of ≈3×, for typical acquisition and sample parameters. Denoised images and subsequent parameter maps showed little or no signs of spatial artifact or loss of resolution.

CONCLUSION

Experimental studies and simulations demonstrate that PCA denoising of MRI relaxometry data is an effective method of improving parameter precision without sacrificing image resolution. This simple yet important processing step thus paves the way for broader applicability of multi-exponential MRI relaxometry.

Myelin Water Fraction Imaging of the Brain in Children with Prenatal Alcohol Exposure

BACKGROUND

Prenatal alcohol exposure (PAE) is linked to alterations of cerebral white matter, including volume and nonspecific diffusion magnetic resonance imaging (MRI) indices of microstructure in humans. Some animal models of PAE have demonstrated myelination deficiencies, but myelin levels have not yet been evaluated in individuals with PAE. Multiecho T₂ MRI offers a quantitative method to estimate myelin water fraction (MWF; related to myelin content) noninvasively, which was used here to evaluate brain myelination in children with PAE.

METHODS

Participants with PAE (n = 10, 6 females, mean age 13.9 years, range 7 to 18 years) and controls (n = 14, 11 females, mean age 13.2 years, range 9 to 16 years) underwent 3T MRI of the brain. T₂ images (15 minutes acquisition for 32 echoes) were used to create MWF maps from which mean MWF was measured in 12 regions of interest (ROIs) including 8 in white matter and 4 in deep gray matter.

RESULTS

As expected, across the combined sample, MWF was highest for major white matter tracts such as the internal capsule and genu/splenium of the corpus callosum (10 to 18%) while the caudate and putamen had MWF less than 5%. Mean MWF was similar across 11/12 brain white and gray matter regions for the PAE and control groups (L/R internal capsule, major forceps, putamen, caudate nucleus, L minor forceps, genu and splenium of corpus callosum). In the PAE group, MWF was positively correlated with age in the genu of corpus callosum and right minor forceps, notably 2 frontal tracts.

CONCLUSIONS

Given comparable MRI-derived myelination fraction measures in PAE relative to controls, white matter alterations shown in other imaging studies, such as diffusion tensor imaging, may reflect microstructural anomalies related to axon caliber and density.

Measuring intra-axonal T2 in white matter with direction-averaged diffusion MRI

PURPOSE

To demonstrate how the T₂ relaxation time of intra-axonal water (T_2a ) in white matter can be measured with direction-averaged diffusion MRI.

METHODS

For b-values larger than about 4000 s/mm² , the direction-averaged diffusion MRI signal from white matter is dominated by the contribution from water within axons, which enables T_2a to be estimated by acquiring data for multiple TE values and fitting a mono-exponential decay curve. If given a value of the intra-axonal diffusivity, an extension of the method allows the extra-axonal relaxation time (T_2e ) to be calculated also. This approach was applied to estimate T_2a in white matter for 3 healthy subjects at 3 T, as well as T_2e for a selected set of assumed intra-axonal diffusivities.

RESULTS

The estimated T_2a values ranged from about 50 ms to 110 ms, with considerable variation among white matter regions. For white matter tracts with primarily collinear fibers, T_2a was found to depend on the angle of the tract relative to the main magnetic field, which is consistent with T_2a being affected by magnetic field inhomogeneities arising from spatial differences in magnetic susceptibility. The T_2e values were significantly smaller than the T_2a values across white matter regions for several plausible choices of the intra-axonal diffusivity.

CONCLUSION

The relaxation time for intra-axonal water in white matter can be determined in a straightforward manner by measuring the direction-averaged diffusion MRI signal with a large b-value for multiple TEs. In healthy brain, T_2a is greater than T_2e and varies considerably with anatomical region.

Two-dimensional magnetization-transfer - CPMG MRI reveals tract-specific signatures in fixed rat spinal cord

Multiexponential T₂ (MET₂) Relaxometry and Magnetization Transfer (MT) are among the most promising MRI-derived techniques for white matter (WM) characterization. Both techniques are shown to have histologically correlated sensitivity to myelin, but these correlations are not fully understood. Furthermore, MET₂ and MT report on different WM features, thus they can be considered specific to different (patho)physiological states. Two-dimensional studies potentially resolving interactions, such as those commonly used in NMR, have been rarely performed in this context. Here, we investigated how off-resonance irradiation affects different MET₂ components in fixed rat spinal cord white matter at 16.4 T. These 2D MT-MET₂ experiments reveal that MT affects both short and long T₂ components in a tract-specific fashion. The spatially distinct signal modulations enhanced contrast between microstructurally-distinct spinal cord tracts. Two hypotheses to explain these findings were proposed: either selective elimination of a short T₂ component through pre-saturation combines with intercompartmental water exchange effects occurring on the irradiation timescale; or, other macromolecular species that exist within the tissue - other than myelin - such as neurofilaments, may be involved in the apparent microstructural segregation of the spinal cord (SC) from MET₂. Though further investigation is required to elucidate the underlying mechanism, this phenomenon adds a new dimension for WM characterization.

Sciatic Nerve Regeneration in Wistar Albino Rats Evaluated by in vivo Conductivity and in vitro 1H NMR Relaxometry

Objective: The aim of this study was to evaluate and quantify functional and structural nerve regeneration after reconstruction using either direct suture or silicon graft.

Methods: Thirty-two adult Wistar Albino rats were divided in two equal groups. The left sciatic nerve was cross-sectioned and reconstructed using either direct suture (DS group) or a silicone graft (SG group). At 4, 6, 8 and 10 weeks two rats were randomly chosen from each group for in vivo measurement of nerve electric conductivity and subsequently sacrificed together with other two rats from the same group for in vitro 1H NMR relaxometry measurements. The T2 distributions were assigned to 1H located in different pools corresponding to the nerve structure.

Results: In the injured nerve we observed a significant increase in the stimulation threshold and a decrease in conduction velocity when compared with the healthy nerve in both groups. Whereas the conduction velocity increased progressively from 4 to 10 weeks in the DS group, the opposite evolution was observed in the SG group. In both groups, the first two peaks corresponding to water bound to collagen and epineurium had smaller transverse relaxation times in the injured nerves, while there was no change in the peaks corresponding to perineurium and free water between healthy and injured nerves.

Conclusions: Significant differences were observed between direct suture and nerve graft reconstructions from both a functional and structural point of view. In the case of direct suture reconstruction, the nerve was functionally healed at 10 weeks after injury.

Inferring brain tissue composition and microstructure via MR relaxometry

MRI relaxometry is sensitive to a variety of tissue characteristics in a complex manner, which makes it both attractive and challenging for characterizing tissue. This article reviews the most common water proton relaxometry measures, T₁, T₂, and T₂^*, and reports on their development and current potential to probe the composition and microstructure of brain tissue. The development of these relaxometry measures is challenged by the need for suitably accurate tissue models, as well as robust acquisition and analysis methodologies. MRI relaxometry has been established as a tool for characterizing neural tissue, particular with respect to myelination, and the potential for further development exists.

Effect of myelin water exchange on DTI-derived parameters in diffusion MRI: Elucidation of TE dependence

PURPOSE

Water exchange exists between different neuronal compartments of brain tissue but is often ignored in most diffusion models. The goal of the current study was to demonstrate the dependence of diffusion measurements on echo time (TE) in the human brain and to investigate the underlying effects of myelin water exchange.

METHODS

Five healthy subjects were examined with single-shot pulsed-gradient spin-echo echo-planar imaging with fixed duration (δ) and separation (Δ) of diffusion gradient pulses and a set of varying TEs. The effects of water exchange and intrinsic T₂ difference in cellular environments were investigated with Monte Carlo simulations.

RESULTS

Both in vivo measurements and simulations showed that fractional anisotropy (FA) and axial diffusivity (AD) had positive correlations with TE, while radial diffusivity (RD) showed a negative correlation, which is consistent with a previous study. The simulation results further indicated the sensitivity of TE dependence to the change of g-ratio.

CONCLUSION

The exchange between myelin and intra/extra-axonal water pools often plays a non-negligible role in the observed TE dependence of diffusion parameters, which may accompany or alter the effect of intrinsic T₂ in causing such dependence. The TE dependence may potentially serve as a biomarker for demyelination processes (e.g., in multiple sclerosis and Alzheimer's disease). Magn Reson Med 79:1650-1660, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Exploring the origins of echo-time-dependent quantitative susceptibility mapping (QSM) measurements in healthy tissue and cerebral microbleeds

Quantitative susceptibility mapping (QSM) is increasingly used to measure variation in tissue composition both in the brain and in other areas of the body in a range of disease pathologies. Although QSM measurements were originally believed to be independent of the echo time (TE) used in the gradient-recalled echo (GRE) acquisition from which they are derived; recent literature (Sood et al., 2016) has shown that these measurements can be highly TE-dependent in a number of brain regions. In this work we systematically investigate possible causes of this effect through analysis of apparent frequency and QSM measurements derived from data acquired at multiple TEs in vivo in healthy brain regions and in cerebral microbleeds (CMBs); QSM data acquired in a gadolinium-doped phantom; and in QSM data derived from idealized simulated phase data. Apparent frequency measurements in the optic radiations (OR) and central corpus callosum (CC) were compared to those predicted by a 3-pool white matter model, however the model failed to fully explain contrasting frequency profiles measured in the OR and CC. Our results show that TE-dependent QSM measurements can be caused by a failure of phase unwrapping algorithms in and around strong susceptibility sources such as CMBs; however, in healthy brain regions this behavior appears to result from intrinsic non-linear phase evolution in the MR signal. From these results we conclude that care must be taken when deriving frequency and QSM measurements in strong susceptibility sources due to the inherent limitations in phase unwrapping; and that while signal compartmentalization due to tissue microstructure and content is a plausible cause of TE-dependent frequency and QSM measurements in healthy brain regions, better sampling of the MR signal and more complex models of tissue are needed to fully exploit this relationship.

Multi-compartment T2 relaxometry using a spatially constrained multi-Gaussian model

The brain's myelin content can be mapped by T2-relaxometry, which resolves multiple differentially relaxing T2 pools from multi-echo MRI. Unfortunately, the conventional fitting procedure is a hard and numerically ill-posed problem. Consequently, the T2 distributions and myelin maps become very sensitive to noise and are frequently difficult to interpret diagnostically. Although regularization can improve stability, it is generally not adequate, particularly at relatively low signal to noise ratio (SNR) of around 100-200. The purpose of this study was to obtain a fitting algorithm which is able to overcome these difficulties and generate usable myelin maps from noisy acquisitions in a realistic scan time. To this end, we restrict the T2 distribution to only 3 distinct resolvable tissue compartments, modeled as Gaussians: myelin water, intra/extra-cellular water and a slow relaxing cerebrospinal fluid compartment. We also impose spatial smoothness expectation that volume fractions and T2 relaxation times of tissue compartments change smoothly within coherent brain regions. The method greatly improves robustness to noise, reduces spatial variations, improves definition of white matter fibers, and enhances detection of demyelinating lesions. Due to efficient design, the additional spatial aspect does not cause an increase in processing time. The proposed method was applied to fast spiral acquisitions on which conventional fitting gives uninterpretable results. While these fast acquisitions suffer from noise and inhomogeneity artifacts, our preliminary results indicate the potential of spatially constrained 3-pool T2 relaxometry.

Improved intravoxel incoherent motion analysis of diffusion weighted imaging by data driven Bayesian modeling

In addition to the diffusion coefficient, fitting the intravoxel incoherent motion model to multiple b-value diffusion-weighted MR data gives pseudo-diffusion measures associated with rapid signal attenuation at low b-values that are of use in the assessment of a number of pathologies. When summary measures are required, such as the average parameter for a region of interest, least-squares based methods give adequate estimation accuracy. However, using least-squares methods for pixel-wise fitting typically gives noisy estimates, especially for the pseudo-diffusion parameters, which limits the applicability of the approach for assessing spatial features and heterogeneity. In this article, a Bayesian approach using a shrinkage prior model is proposed and is shown to substantially reduce estimation uncertainty so that spatial features in the parameters maps are more clearly apparent. The Bayesian approach has no user-defined parameters, so measures of parameter variation (heterogeneity) over regions of interest are determined by the data alone, whereas it is shown that for the least-squares estimates, measures of variation are essentially determined by user-defined constraints on the parameters. Use of a Bayesian shrinkage prior approach is, therefore, recommended for intravoxel incoherent motion modeling.

Multimodal assessment of nervous and immune system responses following sciatic nerve injury

Subsequent to peripheral nerve compression and irritation, pathophysiological processes take place within nervous and immune systems. Here, we utilized a multimodal approach to comprehend peripheral and central soft tissue changes as well as alterations occurring in systemic analytes following unilateral chronic constriction injury (CCI) of the sciatic nerve in rodents. Using magnetic resonance imaging and [18F]-2-fluoro-2-deoxy-d-glucose (FDG) positron emission tomography, we demonstrated robust structural abnormalities and enhanced FDG uptake within the injured nerve and surrounding muscle, respectively. To assess whether central morphological changes were induced by nerve injury, diffusion tenor imaging was performed. A decrease in fractional anisotropy in primary motor cortex contralateral to the injury site was observed. Evaluation of a panel of circulating cytokines, chemokines, and growth factors showed decreased levels of interleukin-1β and Fractalkine in CCI animals. Area under the receiver operating curve (ROC) calculations of analyte levels, imaging, and behavioral end points ranged from 0.786 to 1, where behavioral and peripheral imaging end points (eg, FDG uptake in muscle) were observed to have the highest discriminatory capabilities (maximum area under ROC = 1) between nerve injury and sham conditions. Lastly, performance of correlation analysis involving all analyte, behavioral, and imaging data provided an understanding of the overall association amongst these end points, and importantly, a distinction in correlation patterns was observed between CCI and sham conditions. These findings demonstrate the multidimensional pathophysiology of sciatic nerve injury and how a combined analyte, behavioral, and imaging assessment can be implemented to probe this complexity.

In-vivo multi-exponential T2, magnetization transfer and quantitative histology in a rat model of intramyelinic edema

Two MRI methods, multi-exponential analysis of transverse relaxation (MET₂) and quantitative magnetization transfer (qMT), were used along with quantitative evaluation of histology in a study of intra-myelinic edema in rat spinal white matter. The results showed a strong linear correlation between a distinct long-T₂ signal from MET₂ analysis and the edema water volume fraction as measured by histology, although this analysis overestimated the edema water content by ≈ 100% relative to quantitative histological measurements. This overestimation was reasoned to result from the effects of inter-compartmental water exchange on observed transverse relaxation. Commonly studied MRI markers for myelin, the myelin water fraction (from MET₂ analysis) and the macromolecular pool size ratio (from qMT analysis) produced results that could not be explained purely by changes in myelin content. The results demonstrate the potential for MET₂ analysis as well as the limits of putative myelin markers for characterizing white matter abnormalities involving intra-myelinic edema.

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We studied a rat model of intra-myelinic edema induced by hexachlorophene ingestion.

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We used multi-exponential T₂ (MET₂) and quantitative magnetization transfer MRI.

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Histology was quantitatively evaluated to measure edema volume and myelin content.

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MET₂ provides a measure that correlates but overestimates with edema volume fraction.

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MET₂ measure of edema is affected by microscopic water dynamics.

Imaging of the peripheral nervous system

This chapter summarizes progress in the evaluation of peripheral nerve (PN) lesions and disorders by imaging techniques encompassing magnetic resonance imaging (MRI) and nerve ultrasound (US). Due to the radiation exposure and limited sensitivity in soft tissue contrast, computed-tomography (CT) plays no significant role in the diagnostic work-up of PN disorders. MRI and US are complementary techniques for the evaluation of peripheral nerves, each having particular advantages and disadvantages. Nerve injury induces intrinsic MRI signal alterations on T2-weighted sequences in degenerating or demyelinating nerve segments as well as in corresponding muscle groups exhibiting denervation which can be exploited diagnostically. Nerve US is based on changes in the nerve echotexture due to tumor formation or focal enlargement caused by entrapment or inflammation. Both MRI and US provide morphological information on the precise site and extent of nerve injury. While US has the advantage of easy accessibility, providing images with superior spatial resolution at low cost, MRI shows better soft tissue contrast and better image quality for deep-lying nerve structures since imaging is not hindered by bone. Recent advances have remarkably increased spatial resolution of both MRI and US making imaging indispensible for the elucidation of causes of nerve compression, peripheral nerve tumors, and focal inflammatory conditions. Both MRI and US further guide neurosurgical exploration and can simplify treatment. Importantly, imaging can reveal treatable conditions even in the absence of gross electrophysiological alterations, illustrating its increasing role in clinical practice. In experimental settings, novel molecular and cellular MRI contrast agents allow in-vivo assessment of nerve regeneration as well as monitoring of neuroinflammation. Depending on further clinical development, contrast-enhanced MRI has the potential to follow cellular responses over time in vivo and to overcome the current limitations of histological assessment of nerve afflictions. Further advances in contrast-enhanced US has the potential for developing into a tool for the assessment of nerve blood perfusion, paving the way for better assessments of ischemic neuropathies.

Magnetic resonance imaging of blood brain/nerve barrier dysfunction and leukocyte infiltration: closely related or discordant?

Unlike other organs the nervous system is secluded from the rest of the organism by the blood brain barrier (BBB) or blood nerve barrier (BNB) preventing passive influx of fluids from the circulation. Similarly, leukocyte entry to the nervous system is tightly controlled. Breakdown of these barriers and cellular inflammation are hallmarks of inflammatory as well as ischemic neurological diseases and thus represent potential therapeutic targets. The spatiotemporal relationship between BBB/BNB disruption and leukocyte infiltration has been a matter of debate. We here review contrast-enhanced magnetic resonance imaging (MRI) as a non-invasive tool to depict barrier dysfunction and its relation to macrophage infiltration in the central and peripheral nervous system under pathological conditions. Novel experimental contrast agents like Gadofluorine M (Gf) allow more sensitive assessment of BBB dysfunction than conventional Gadolinium (Gd)-DTPA enhanced MRI. In addition, Gf facilitates visualization of functional and transient alterations of the BBB remote from lesions. Cellular contrast agents such as superparamagnetic iron oxide particles (SPIO) and perfluorocarbons enable assessment of leukocyte (mainly macrophage) infiltration by MR technology. Combined use of these MR contrast agents disclosed that leukocytes can enter the nervous system independent from a disturbance of the BBB, and vice versa, a dysfunctional BBB/BNB by itself is not sufficient to attract inflammatory cells from the circulation. We will illustrate these basic imaging findings in animal models of multiple sclerosis, cerebral ischemia, and traumatic nerve injury and review corresponding findings in patients.

Characterizing inter-compartmental water exchange in myelinated tissue using relaxation exchange spectroscopy

PURPOSE

To investigate inter-compartmental water exchange in two model myelinated tissues ex vivo using relaxation exchange spectroscopy.

METHODS

Building upon a previously developed theoretical framework, a three-compartment (myelin, intra-axonal, and extra-axonal water) model of the inversion-recovery prepared relaxation exchange spectroscopy signal was applied in excised rat optic nerve and frog sciatic nerve samples to estimate the water residence time constants in myelin (τmyelin ).

RESULTS

In the rat optic nerve samples, τmyelin = 138 ± 15 ms (mean ± standard deviation) was estimated. In sciatic nerve, which possesses thicker myelin sheaths than optic nerve, a much longer τmyelin = 2046 ± 140 ms was observed.

CONCLUSION

Consistent with previous studies in rat spinal cord, the extrapolation of exchange rates in optic nerve to in vivo conditions indicates that τmyelin < 100 ms. This suggests that there is a significant effect of inter-compartmental water exchange on the transverse relaxation of water protons in white matter. The much longer τmyelin values in sciatic nerve supports the postulate that the inter-compartmental water exchange rate is mediated by myelin thickness. Together, these findings point to the potential for MRI methods to probe variations in myelin thickness in white matter.

Accumulation of non-compressive fascicular lesions underlies NF2 polyneuropathy

A distinct polyneuropathy (PNP) syndrome affects up to 66 % of patients with neurofibromatosis II (NF2). Whether this is primarily a diffuse PNP or due to single, surgically amenable mass lesions has not yet been conclusively demonstrated. We aimed to solve this question by investigating the pathomorphological MR imaging correlate of this rare disorder. Eight patients with NF2-PNP were characterized by clinical examination, electrophysiological studies, and genetic analysis. All patients additionally underwent extended peripheral nerve imaging by a novel protocol of large-coverage high-resolution MRI. Quantitative analyses were performed by separately evaluating cross-sectional images, and by categorizing lesions into non-compressive fascicular microlesions (<2 mm), intermediate lesions (2-5 mm), and compressive macrolesions (>5 mm). The predominant imaging findings were non-compressive fascicular microlesions and intermediate lesions. Proximal-to-distal cumulative lesion burden of these lesions correlated strongly with the severity of clinical symptoms of NF2-PNP. In contrast, compressive macrolesions were not found at all in several symptomatic extremities. We conclude that proximal-to-distal accumulation of non-compressive fascicular lesions instead of compressive mass lesions predominantly underlies the clinical manifestation and severity of NF2-associated PNP. Diagnostic management may now be assisted by large-coverage high-resolution imaging of plexus and peripheral nerves. Additionally, the results underscore the feasibility of this new method, which may open up new diagnostic and investigative possibilities for other disseminated disorders of the peripheral nervous system.

On the inherent precision of mcDESPOT

A statistical analysis of the mcDESPOT protocol for characterizing two exchanging water proton pools--a seven-dimensional problem that fits to multiple flip angle measurements of both spoiled and refocused gradient echoes--is presented. Theoretical calculations of the Cramér-Rao lower bounds of the variance of fitted model parameters were made using a variety of model system parameters, meant to mimic those expected in human white matter. The results, validated by Monte Carlo simulations, indicated that mcDESPOT signals acquired at feasibly attainable signal-to-noise ratios cannot provide parameter estimates with useful levels of precision. Precision can be greatly improved by constraining solutions with a priori model information, although this will generally lead to biased parameter estimates with less specificity. These results indicate that previous, apparently successful applications of mcDESPOT to human white matter may have used data fitting methods that implicitly constrained parameter solutions, or that the two-pool model of white matter may not be sufficient to describe the observed water proton signal in mcDESPOT acquisitions. In either case, mcDESPOT-derived estimates of two-pool model parameters cannot yet be unambiguously related to specific tissue characteristics.

Carpal tunnel syndrome: a review of the recent literature

Carpal Tunnel Syndrome (CTS) remains a puzzling and disabling condition present in 3.8% of the general population. CTS is the most well-known and frequent form of median nerve entrapment, and accounts for 90% of all entrapment neuropathies. This review aims to provide an overview of this common condition, with an emphasis on the pathophysiology involved in CTS. The clinical presentation and risk factors associated with CTS are discussed in this paper. Also, the various methods of diagnosis are explored; including nerve conduction studies, ultrasound, and magnetic resonance imaging.

Bayesian algorithm using spatial priors for multiexponential T₂ relaxometry from multiecho spin echo MRI

Multiexponential T₂ relaxometry is a powerful research tool for detecting brain structural changes due to demyelinating diseases such as multiple sclerosis. However, because of unusually high signal-to-noise ratio requirement compared with other MR modalities and ill-posedness of the underlying inverse problem, the T₂ distributions obtained with conventional approaches are frequently prone to noise effects. In this article, a novel multivoxel Bayesian algorithm using spatial prior information is proposed. This prior takes into account the expectation that volume fractions and T₂ relaxation times of tissue compartments change smoothly within coherent brain regions. Three-dimensional multiecho spin echo MRI data were collected from five healthy volunteers at 1.5 T and myelin water fraction maps were obtained using the conventional and proposed algorithms. Compared with the conventional method, the proposed method provides myelin water fraction maps with improved depiction of brain structures and significantly lower coefficients of variance in white matter.

Effect of intercompartmental water exchange on the apparent myelin water fraction in multiexponential T2 measurements of rat spinal cord

The myelin water fraction has been used as a quantitative measure of the amount of myelin present in tissue. However, recent work has suggested that intercompartmental exchange of water between myelin and nonmyelin compartments may cause the myelin water fraction to underestimate the true myelin content of tissue. In this work, multiexponential T(2) experiments were performed in vivo within the rat spinal cord, and a wide variation of the myelin water fraction (10-35%) was measured within four rat spinal cord tracts with similar myelin content. A numerical simulation based upon segmented histology images was used to quantitatively account for T(2) variations between tracts. The model predicts that a difference in exchange between the four spinal cord tracts, mediated by a difference in the average axon radius and myelin thickness, is sufficient to account for the variation in myelin water fraction measured in vivo.

Proximal neuropathic lesions in distal symmetric diabetic polyneuropathy: findings of high-resolution magnetic resonance neurography

OBJECTIVE

This study investigated high-resolution magnetic resonance neurography (MRN) in distal symmetric diabetic polyneuropathy (dPNP).

RESEARCH DESIGN AND METHODS

MRN comprised high-resolution transaxial imaging of peripheral nerves of the lower limbs in 20 patients with type 2 diabetes (10 with dPNP, type 2/dPNP[+], and 10 without dPNP, type 2/dPNP[-]), seven patients with type 1 diabetes (two with dPNP, type 1/dPNP[+], five without dPNP, type 1/dPNP[-]), and 10 nondiabetic control subjects. Intraneural T2 lesions, as the main diagnostic criterion of MRN, were detected visually by two independent observers and quantitatively by analysis of T2 contrast ratios.

RESULTS

Multifocal fascicular, symmetric intraneural T2 lesions occurred in the proximal trunks of sciatic nerves in four patients (three with type 2/dPNP[+] and one with type 1/dPNP[+]) but not in control subjects (type 2/dPNP[-], type 1/dPNP[-], nondiabetic control subjects), which was confirmed by quantitative analysis. Clinical severity was higher in patients with T2 lesions (neuropathy deficit score: 10 vs. 7.8; P = 0.05).

CONCLUSIONS

For the first time, proximal neuropathic lesions of dPNP are reported in vivo. This supports that accumulation of proximal, multifocal fascicular injury may be important in disease progression.

Compartment-specific enhancement of white matter and nerve ex vivo using chromium

Chromium--Cr(VI) in the form of potassium dichromate--has been shown to specifically enhance white matter signal. The proposed mechanism for this enhancement is reduction of diamagnetic Cr(VI) to paramagnetic chromium species by oxidizable myelin lipids. The purpose of the study herein was to better understand the microanatomical basis of this enhancement (i.e., the relative enhancement of myelin, intra-axonal, and extra-axonal water). Toward this end, integrated T(1)-T(2) measurements were performed in potassium dichromate loaded (hereafter referred to as chromated) rat brains, rat optic nerve samples, and frog sciatic nerve samples ex vivo. In control optic nerve and white matter, two T(1)-T(2) components were resolved, representing myelin and nonmyelin water (intra- and extra-axonal water). Following chromation, three T(1)-T(2) components were resolved in these same tissues. Results from similar measurements in sciatic nerve-all three components are resolvable in control and chromated samples-and quantitative histologic analysis suggest that this additional T(1)-T(2) component is due to a splitting of the nonmyelin water component into intra- and extra-axonal water components. This compartment-specific enhancement may provide unique contrast for MR histology, as well as allow one to probe the compartmental basis of various contrast mechanisms in neural tissue.

Multiexponential T2, magnetization transfer, and quantitative histology in white matter tracts of rat spinal cord

Quantitative MRI measures of multiexponential T(2) relaxation and magnetization transfer were acquired from six samples of excised and fixed rat spinal cord and compared with quantitative histology. MRI and histology data were analyzed from six white matter tracts, each of which possessed unique microanatomic characteristics (axon diameter and myelin thickness, in particular) but a relatively constant volume fraction of myelin. The results indicated that multiexponential T(2) relaxation characteristics varied substantially with variation of microanatomy, while the magnetization transfer characteristics remained close to constant. The most-often-cited multiexponential T(2) relaxation metric, myelin water fraction, varied by almost a factor of 2 between two regions with myelin volume fractions that differed by only approximately 12%. Based on the quantitative histology, the proposed explanation for this variation was intercompartmental water exchange, which caused the underestimation of myelin water fraction and T(2) values and is, presumably, a greater factor in white matter regions where axons are small and myelin is thin. In contrast to the multiexponential T(2) relaxation observations, magnetization transfer metrics were relatively constant across white matter tracts and concluded to be relatively insensitive to intercompartmental water exchange.

Sciatic nerve injury model in the axolotl: functional, electrophysiological, and radiographic outcomes

OBJECT

The 2 aims of this study were as follows: 1) to establish outcome measures of nerve regeneration in an axolotl model of peripheral nerve injury; and 2) to define the timing and completeness of reinnervation in the axolotl following different types of sciatic nerve injury.

METHODS

The sciatic nerves in 36 axolotls were exposed bilaterally in 3 groups containing 12 animals each: Group 1, left side sham, right side crush; Group 2, left side sham, right side nerve resected and proximal stump buried; and Group 3 left side cut and sutured, right side cut and sutured with tibial and peroneal divisions reversed. Outcome measures included the following: 1) an axolotl sciatic functional index (ASFI) derived from video swim analysis; 2) motor latencies; and 3) MR imaging evaluation of nerve and muscle edema.

RESULTS

For crush injuries, the ASFI returned to baseline by 2 weeks, as did MR imaging parameters and motor latencies. For buried nerves, the ASFI returned to 20% below baseline by 8 weeks, with motor evoked potentials present. On MR imaging, nerve edema peaked at 3 days postintervention and gradually normalized over 12 weeks, whereas muscle denervation was present until a gradual decrease was seen between 4 and 12 weeks. For cut nerves, the ASFI returned to 20% below baseline by Week 4, where it plateaued. Motor evoked potentials were observed at 2-4 weeks, but with an increased latency until Week 6, and MR imaging analysis revealed muscle denervation for 4 weeks.

CONCLUSIONS

Multiple outcome measures in which an axolotl model of peripheral nerve injury is used have been established. Based on historical controls, recovery after nerve injury appears to occur earlier and is more complete than in rodents. Further investigation using this model as a successful "blueprint" for nerve regeneration in humans is warranted.

Current concepts of carpal tunnel syndrome: pathophysiology, treatment, and evaluation

The current concepts of carpal tunnel syndrome (CTS) with respect to its pathophysiology, treatment, and evaluation are discussed. With regard to the pathophysiology of idiopathic CTS, biomechanical studies to determine the kinematics of the flexor tendon, and the median nerve inside the carpal tunnel may provide valuable insights. Different degrees of excursion between the flexor tendons and the median nerve could cause strain and microdamage to the synovial tissue; this has been microscopically observed. A biomechanical approach for elucidating the events that trigger the development of CTS seems interesting; however, there are limitations to its applications. Endoscopic carpal tunnel release (ECTR) is a useful technique for achieving median nerve decompression. However, it is not considered superior to conventional open carpal tunnel release in terms of fast recovery of hand function. Unless the effect of inserting a cannula into the diseased carpal tunnel on the median nerve function is quantitatively elucidated, ECTR will not be regarded as a standard procedure for relieving the median nerve from chronic compression. The treatment of CTS should be evaluated on the basis of patient-oriented questionnaires as well as conventional instruments because these questionnaires have been validated and found to be highly responsive to the treatment. It should be noted that nerve conduction studies exclusively evaluate the function of the median nerve, whereas patient-oriented questionnaires take into account not only the symptoms of CTS but other accompanying pathologies as well, such as flexor tenosynovitis. In Japan, the number of CTS patients is expected to rise; this may be attributed to a general increase in the life-span of the Japanese and increase in the number of diabetic patients. Thus, more efforts should be directed toward elucidating the pathophysiology of so-called idiopathic CTS, so that new treatment strategies can be established for CTS of different pathologies.

The MT pool size ratio and the DTI radial diffusivity may reflect the myelination in shiverer and control mice

A quantitative magnetization transfer (qMT) technique was employed to quantify the ratio of the sizes of the bound and free water proton pools in ex vivo mouse brains. The goal was to determine the pool size ratio sensitivity to myelin. Fixed brains from both shiverer mice and control littermates were imaged. The pool size ratio in the corpus callosum of shiverer mice was substantially lower than that in the control mice, while there was no distinguishable difference in the pool size ratio in the gray matter. These results correlate with diffusion tensor imaging (DTI) derived radial diffusivity which previously was shown to reflect myelin integrity in this animal model. Histological study reveals the presence of myelin in control mice white matter and the absence of myelin in shiverer mice white matter, supporting the qMT and DTI results. Our findings support the view that qMT may be used for estimating myelin integrity.

N,N-diethyldithiocarbamate promotes oxidative stress prior to myelin structural changes and increases myelin copper content

Dithiocarbamates are a commercially important class of compounds that can produce peripheral neuropathy in humans and experimental animals. Previous studies have supported a requirement for copper accumulation and enhanced lipid peroxidation in dithiocarbamate-mediated myelinopathy. The study presented here extends previous investigations in two areas. Firstly, although total copper levels have been shown to increase within the nerve it has not been determined whether copper is increased within the myelin compartment, the primary site of lesion development. Therefore, the distribution of copper in sciatic nerve was characterized using synchrotron X-ray fluorescence microscopy to determine whether the neurotoxic dithiocarbamate, N,N-diethyldithiocarbamate, increases copper levels in myelin. Secondly, because lipid peroxidation is an ongoing process in normal nerve and the levels of lipid peroxidation products produced by dithiocarbamate exposure demonstrated an unusual cumulative dose response in previous studies the biological impact of dithiocarbamate-mediated lipid peroxidation was evaluated. Experiments were performed to determine whether dithiocarbamate-mediated lipid peroxidation products elicit an antioxidant response through measuring the protein expression levels of three enzymes, superoxide dismutase 1, heme oxygenase 1, and glutathione transferase alpha, that are linked to the antioxidant response element promoter. To establish the potential of oxidative injury to contribute to myelin injury the temporal relationship of the antioxidant response to myelin injury was determined. Myelin structure in peripheral nerve was assessed using multi-exponential transverse relaxation measurements (MET(2)) as a function of exposure duration, and the temporal relationship of protein expression changes relative to the onset of changes in myelin integrity were determined. Initial assessments were also performed to explore the potential contribution of dithiocarbamate-mediated inhibition of proteasome function and inhibition of cuproenzyme activity to neurotoxicity, and also to assess the potential of dithiocarbamates to promote oxidative stress and injury within the central nervous system. These evaluations were performed using an established model for dithiocarbamate-mediated demyelination in the rat utilizing sciatic nerve, spinal cord and brain samples obtained from rats exposed to N,N-diethyldithiocarbamate (DEDC) by intra-abdominal pumps for periods of 2, 4, and 8 weeks and from non exposed controls. The data supported the ability of DEDC to increase copper within myelin and to enhance oxidative stress prior to structural changes detectable by MET(2). Evidence was also obtained that the excess copper produced by DEDC in the central nervous system is redox active and promotes oxidative injury.

Magnetic resonance imaging of the peripheral nervous system

The diagnostic work up of patients with peripheral neuropathy largely depends on clinical and electrophysiological investigations. In contrast to disorders of the CNS, magnetic resonance imaging (MRI) has not been widely used as a diagnostic tool in the PNS except for detection of nerve compressing mass lesions. Normal nerves appear isointense to the surrounding tissue on T1- and T2-weighted (w) MRIs, but upon injury the nerves become hyperintense and thus visible on T2-w MRI. These signal alterations can be exploited to diagnose nerve damage in vivo and to follow regeneration. In patients with peripheral nerve disorders, MRI has been especially useful in detecting focal intrinsic and extrinsic nerve lesions and may reveal treatable conditions even in the absence of gross electrophysiological alterations. This clinical review provides practical guidelines on the performance of nerve imaging by MRI and will focus on focal lesions exemplified by case presentations.

Multiexponential T2 and magnetization transfer MRI of demyelination and remyelination in murine spinal cord

Identification of remyelination is important in the evaluation of potential treatments of demyelinating diseases such as multiple sclerosis. Local injection of lysolecithin into the brain or spinal cord provides a murine model of demyelination with spontaneous remyelination. The aim of this study was to determine if quantitative, multicomponent T(2) (qT(2)) analysis and magnetization transfer ratio (MTR), both indicative of myelin content, could detect changes in myelination, particularly remyelination, of the cervical spinal cord in mice treated with lysolecithin. We found that the myelin water fraction and geometric mean T(2) value of the intra/extracellular water significantly decreased at 14 days then returned to control levels by 28 days after injury, corresponding to clearance of myelin debris and remyelination which was shown by eriochrome cyanine and oil red O staining of histological sections. The MTR was significantly decreased 14 days after lysolecithin injection, and remained low over the time course studied. Evidence of demyelination shown by both qT(2) and MTR lagged behind the histological evidence of demyelination. Myelin water fraction increased with remyelination, however MTR remained lower after 28 days. The difference between qT(2) and MTR may identify early remyelination.

“Current Pathology Techniques” Symposium Review: Advances and Issues in Neuropathology

Our understanding of the mechanisms that incite neurological diseases has progressed rapidly in recent years, mainly owing to the advent of new research instruments and our increasingly facile ability to assemble large, complex data sets acquired across several disciplines into an integrated representation of neural function at the molecular, cellular, and systemic levels. This mini-review has been designed to communicate the principal technical advances and current issues of importance in neuropathology research today in the context of our traditional neuropathology practices. Specific topics briefly addressed in this paper include correlative biology of the many facets of the nervous system; conventional and novel methods for investigating neural structure and function; theoretical and technical issues associated with investigating neuropathology end points in emerging areas of concern (developmental neurotoxicity, neurodegenerative conditions); and challenges and opportunities that will face pathologists in this field in the foreseeable future. We have organized this information in a manner that we hope will be of interest not only to professionals with a career focus in neuropathology, but also to general pathologists who occasionally face neuropathology questions.

Technology insight: visualizing peripheral nerve injury using MRI

Currently, the evaluation of peripheral nerve disorders depends on clinical examination, supplemented by electrophysiological studies. These approaches provide general information on the distribution and classification of nerve lesions-for example, axonal versus demyelinative-but nerve biopsies are still required to obtain morphological and pathophysiological details. In this article, we review recent progress in the imaging of peripheral nerve injury by magnetic resonance (MR) neurography. Axonal nerve injury leads to Wallerian degeneration, resulting in a hyperintense nerve signal on T2-weighted MR images of the distal nerve segment. This signal is lost following successful regeneration. Concomitant denervation-induced signal alterations in muscles can further help us to determine whether nerve trunks or roots are affected. These signal changes are caused by various combinations of nonspecific tissue alterations, however, and are not related to particular pathoanatomical findings, such as inflammation, demyelination or axonal injury. New experimental MR contrast agents, such as gadofluorine M and superparamagnetic iron oxide particles, allow visualization of the dynamics of peripheral nerve injury and repair. Further clinical development of these MR contrast agents should allow these functional aspects of nerve injury and repair to be assessed in humans, thereby aiding the differential diagnosis of peripheral nerve disorders.

Magnetic resonance imaging of myelin

The ability to measure myelin in vivo has great consequences for furthering our knowledge of normal development, as well as for understanding a wide range of neurological disorders. The following review summarizes the current state of myelin imaging using MR. We consider five MR techniques that have been used to study myelin: 1) conventional MR, 2) MR spectroscopy, 3) diffusion, 4) magnetization transfer, and 5) T2 relaxation. Fundamental studies involving peripheral nerve and MR/histology comparisons have aided in the interpretation and validation of MR data. We highlight a number of important findings related to myelin development, damage, and repair, and we conclude with a critical summary of the current techniques available and their potential to image myelin in vivo.

Multislice Brain Myelin Water Fractions at 3T in Multiple Sclerosis

PURPOSE

To evaluate a multislice nonlinearly-spaced 12-echo imaging sequence at 3T covering the supratentorial brain for the quantification of myelin water fraction (MWF) in multiple sclerosis (MS) patients.

METHODS

Eighty-nine patients with, or at risk of, MS (69 relapsing remitting MS [RRMS], 7 secondary progressive MS [SPMS], 13 clinically isolated syndrome [CIS]) and 28 controls were studied. Twelve-echo datasets were acquired using a multislice T2 prep spiral imaging sequence and were fitted using a nonnegative least squares algorithm. The mean MWF within normal appearing white matter (NAWM), contrast-enhancing (CE), and nonenhancing T2 lesions were calculated.

RESULTS

Mean MWF in white matter for controls was 11.3%. Mean MWF was significantly reduced in NAWM of MS patients (10.6%, P= .004) relative to controls. SPMS/RRMS patients with disease duration >5 years (10.3%) had lower MWF compared to CIS/RRMS with disease duration <or=5 years (10.8%, P= .03). Mean MWF was reduced by 26% and 29% within both CE (P < .0001) and nonenhancing T2 lesions (P < .0001) relative to controls.

CONCLUSIONS

Using a multicomponent T2 sequence at 3T, a significant decrease in the supratentorial MWF was observed in MS NAWM and lesions relative to controls. The method was sensitive to detect white matter changes early in the disease process.

In vivo visualization of focal demyelination in peripheral nerves by gadofluorine M-enhanced magnetic resonance imaging

Magnetic resonance imaging (MRI) allows assessment of axonal nerve lesions, but detection of focal demyelination is still difficult. We have recently shown that the novel micellar magnetic resonance (MR) contrast agent gadofluorine M (Gf) accumulates in nerve fibers undergoing Wallerian degeneration. In the present study, we report on the in vivo visualization of focal demyelination induced by lysolecithin. Upon appropriate intraneural injection, lysolecithin focally dissolves myelin sheaths with sparing of axons. Conventional unenhanced and gadolinium-DTPA enhanced T1-w MRI did not show signal alterations or contrast enhancement. In contrast, application of Gf led to bright contrast enhancement on T1-w images at the site of focal demyelination, but spared distal nerve segments not affected by demyelination. Gf enhancement persisted until remyelination had occurred. Our study shows that areas of focal nerve demyelination can be detected in vivo by Gf-enhanced MRI. This finding opens up a broad spectrum of applications in experimental neurology, and, depending on further clinical development of Gf, may aid in the diagnostic work up of patients with patchy, multifocal demyelinative disorders in the future.

Multicomponent T2 analysis of dithiocarbamate-mediated peripheral nerve demyelination

Standard light microscope histological evaluation of peripheral nerve lesions has been used routinely to assess peripheral nerve demyelination; however, the development of magnetic resonance (MR) methodology for assessing peripheral nerve may provide complementary information, with less expense and in less time than nerve histology methods. In this study, the utility of multicomponent NMR T(2) relaxation analysis for assessing myelin injury in toxicology studies was examined using two dithiocarbamates, N,N-diethyldithiocarbamate (DEDC) and pyrrolidine dithiocarbamate (PDTC), known to produce myelin injury and elevate copper in the nervous system. T(2) analysis was used in conjunction with standard histological methods to assess myelin injury and determine if dithiocarbamate-mediated copper accumulation in peripheral nerve was associated with more severe myelin lesions. Male Sprague-Dawley rats were administered i.p. DEDC for 8 weeks and maintained on either a diet containing normal (13 ppm) or elevated (200 ppm) copper. Another group of male Sprague-Dawley rats was administered oral PDTC and a 200 ppm copper diet, with controls given only the 200 ppm copper diet, for 47 weeks. Following exposures, the morphology of sciatic nerve was evaluated using light microscopy and multicomponent T(2) analysis of excised fixed nerves; and copper levels in sciatic nerve were determined using ICP-AES. Light microscopy demonstrated the presence of a primary myelinopathy in dithiocarbamate-exposed rats characterized by intramyelinic edema, demyelination, and secondary axonal degeneration. Both the nerve copper level and number of degenerated axons, as ascertained by ICP-AES and microscopy, respectively, were augmented by dietary copper supplementation in conjunction with administration of DEDC or PDTC. T(2) analysis revealed a decreased contribution from the shortest T(2) component in multicomponent T(2) spectra obtained from animals administered DEDC or PDTC, consistent with decreased myelin content; and the decrease of the myelin water component was inversely correlated to the levels of nerve copper and myelin lesion counts. Also, the T(2) analysis showed reduced variability compared to histological assessment. These studies support multicomponent T(2) analysis as a complementary method to light microscopic evaluations that may also be applicable to in vivo assessments.

Volumetric magnetic resonance imaging of dorsal root ganglia for the objective quantitative assessment of neuron death after peripheral nerve injury

Prevention of neuron death after peripheral nerve injury is vital to regaining adequate cutaneous innervation density and quality of sensation, and while experimentally proven neuroprotective therapies exist, there lacks suitable clinical outcome measures for translational research. Axotomized dorsal root ganglia (DRG) histologically exhibit volume reduction in proportion to the amount of neuronal death within them. Hence, this study evaluated the validity of using magnetic resonance imaging (MRI) to quantify DRG volume as a proxy measure of cell death. A high-resolution 3D MRI sequence was developed for volumetric quantification of the L4 DRG in the rat sciatic nerve model. An unoperated "control" group (n=4), and a "nerve transection" group (n=6), 4 weeks after axotomy, were scanned. Accuracy and validity of the technique were evaluated by comparison with morphological quantification of DRG volume and stereological counts of surviving neurons (optical fractionator). The technique was precise (coefficient of variation=4.3%), highly repeatable (9% variability), and sensitive (mean 15.0% volume reduction in axotomized ganglia detected with statistical significance: p<0.01). MRI showed strong and highly significant correlation with morphological measures of DRG volume loss (r=0.90, p<0.001), which in turn correlated well with neuron loss (r=0.75, p<0.05). MRI similarly exhibited direct correlation with neuron loss (r=0.67, p<0.05) with consistent agreement. MRI volumetric quantification of DRG is therefore a valid in vivo measure of neuron loss. As a non-invasive, objective measure of neuronal death after nerve trauma this technique has potential as a diagnostic modality and a quantitative tool for clinical studies of neuroprotective agents.