MR properties of rat sciatic nerve following trauma

Magnetic resonance microneurography of rabbit sciatic nerve on a 1.5-T clinical MR system correlated with gross anatomy

This study was performed to investigate the feasibility and accuracy of magnetic resonance (MR) microneurography of the rabbit sciatic nerve on a 1.5-T clinical MR system by correlation with the gross anatomy. The 3D T2-weighted imaging (3D-T2WI), 3D T2-weighted imaging plus spectral presaturation with inversion recovery (SPIR), and T1-weighted imaging (T1WI) of the sciatic nerve in ten rabbits were performed on a 1.5-T MR system. The radiological anatomy of the sciatic nerve was observed and correlated with the gross anatomy. The anterior-posterior diameter of the sciatic nerve trunk was measured on 3D T2WI and on gross anatomy. The T1 and T2 relaxation times were also measured with multiecho spin echo and mixed sequence, respectively. The tibial nerve and peroneal nerve in the sciatic nerve trunk in all ten rabbits could be clearly displayed on T2WI and T2WI. The fine branches of the gastrocnemius nerve, posterior femoral cutaneous nerve, and the posterior gluteal nerve could be clearly depicted on T2WI. The T1 and T2 relaxation times of the sciatic nerves were 915 and 40 ms, respectively. The anterior-posterior diameter of sciatic nerve trunk was measured grossly, and on T2WI was 3.17 +/- 0.21 mm and 3.15 +/- 0.19 mm, respectively. There was no statistically significant difference (t = 0.768, P = 0.462). With the 1.5-T clinical MR system, the microneurography of the sciatic nerve could be revisualized, and the finer structure of the sciatic nerve trunk could be clearly and accurately delineated.

Diffusion-tensor MR imaging and tractography: exploring brain microstructure and connectivity

Diffusion magnetic resonance (MR) imaging is evolving into a potent tool in the examination of the central nervous system. Although it is often used for the detection of acute ischemia, evaluation of directionality in a diffusion measurement can be useful in white matter, which demonstrates strong diffusion anisotropy. Techniques such as diffusion-tensor imaging offer a glimpse into brain microstructure at a scale that is not easily accessible with other modalities, in some cases improving the detection and characterization of white matter abnormalities. Diffusion MR tractography offers an overall view of brain anatomy, including the degree of connectivity between different regions of the brain. However, optimal utilization of the wide range of data provided with directional diffusion MR measurements requires careful attention to acquisition and postprocessing. This article will review the principles of diffusion contrast and anisotropy, as well as clinical applications in psychiatric, developmental, neurodegenerative, neoplastic, demyelinating, and other types of disease.

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.

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.

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.

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.

Diffusion tensor imaging of time-dependent axonal and myelin degradation after corpus callosotomy in epilepsy patients

Axonal degeneration of white matter fibers is a key consequence of neuronal or axonal injury. It is characterized by a series of time-related events with initial axonal membrane collapse followed by myelin degradation being its major hallmarks. Standard imaging cannot differentiate these phenomena, which would be useful for clinical investigations of degeneration, regeneration and plasticity. Animal models suggest that diffusion tensor magnetic resonance imaging (DTI) is capable of making such distinction. The applicability of this technique in humans would permit inferences on white matter microanatomy using a non-invasive technique. The surgical bisection of the anterior 2/3 of the corpus callosum for the palliative treatment of certain types of epilepsy serves as a unique opportunity to assess this method in humans. DTI was performed on three epilepsy patients before corpus callosotomy and at two time points (1 week and 2-4 months) after surgery. Tractography was used to define voxels of interest for analysis of mean diffusivity, fractional anisotropy and eigenvalues. Diffusion anisotropy was reduced in a spatially dependent manner in the genu and body of the corpus callosum at 1 week and remained low 2-4 months after the surgery. Decreased anisotropy at 1 week was due to a reduction in parallel diffusivity (consistent with axonal fragmentation), whereas at 2-4 months, it was due to an increase in perpendicular diffusivity (consistent with myelin degradation). DTI is capable of non-invasively detecting, staging and following the microstructural degradation of white matter following axonal injury.

Functional pathway-defined MRI diffusion measures reveal increased transverse diffusivity of water in multiple sclerosis

The diffusion properties of water are sensitive to microscopic changes in the white matter of multiple sclerosis (MS) patients. Typical MRI measures of disease burden in MS demonstrate modest to poor correlation with disability. Functional MRI and DTI-based fiber tracking were used to define the interhemispheric white matter pathway connecting bilateral supplementary motor areas (SMA) in 16 MS patients sand 16 control subjects. Fractional anisotropy (FA), mean diffusivity (MD), longitudinal (lambda(1)) and transverse diffusivity (lambda(2)) were measured along this pathway in all subjects. Mean FA was 0.587 +/- 0.032 for patients and 0.608 +/- 0.020 for controls (P < 0.02). Mean MD was (0.821 +/- 0.055) x 10(-3) mm(2) s(-1) for patients and (0.770 +/- 0.020) x 10(-3) mm(2) s(-1) for controls (P < 0.004). Mean lambda(1) values were (1.462 +/- 0.099) x 10(-3) mm(2) s(-1) for patients and (1.400 +/- 0.034) x 10(-3) mm(2) s(-1) for controls (P < 0.02). Mean lambda(2) values were (0.500 +/- 0.047) x 10(-3) mm(2) s(-1) for patients and (0.454 +/- 0.027) x 10(-3) mm(2) s(-1) for controls (P < 0.001). In addition, the correlation between the Multiple Sclerosis Functional Composite (MSFC) and transverse diffusivity was -0.341 (P < 0.05). The component test of the MSFC most related to the SMA pathway studied with our MRI method (Nine-hole Peg Test) showed significant correlation with transverse diffusivity (r = 0.392, P < 0.02), indicating that probing functional pathways with MRI measures can lead to a better reflection of disease status.

Catheter confocal fluorescence imaging and functional magnetic resonance imaging of local and systems level recovery in the regenerating rodent sciatic nerve

The goal of the present work was to develop minimally invasive imaging techniques to monitor local regeneration of peripheral nerves and to determine the extent of return to function of brain cortical regions associated with that nerve. The sciatic nerve crush model was applied to Sprague-Dawley rats and conventional histological staining for myelin, axons and cell architecture was carried out, as well as traditional behavioral testing, to verify that nerve regeneration was occurring. The rate of sciatic nerve regeneration was measured by determining the distance a lipophilic, fluorescence probe (DiO) would move along the nerve's membrane following a direct injection into the sciatic nerve. This movement was monitored using a catheter based, confocal fluorescence microscope. Two to five days after the crush, the dye moved 1.4 + 0.6 mm/day, as compared to a distance of 5.3 + 0.5 mm/day in the normal nerve. Between 9 and 13 days following the crush, the distance the dye moved increases to 5.5 + 0.5 mm/day, similar to the control, and by 15 days following the crush, the distance increased to 6.5 + 0.9 mm/day. Functional Magnetic Resonance Imaging (fMRI) measurements were performed on alpha-chloralose anesthetized rats to monitor the return of somatosensory cortical functions, which were activated by the stimulation of the lesioned peripheral nerve. fMRI results showed the return of cortical activation around 15 days following the crush procedure. However, the somatosensory cortical region activated by stimulating the crushed hindpaw was significantly smaller in extent than the intact hindpaw stimulation. These findings demonstrate that fluorescence imaging and fMRI can integrate local and system level correlates of nerve regeneration in a non-destructive manner, thus enabling serial imaging of individual animals.

Assessment of lesion evolution in experimental autoimmune neuritis by gadofluorine M-enhanced MR neurography

Experimental autoimmune neuritis (EAN) represents an animal model of acute inflammatory nerve injury mirroring pathophysiological aspects of the human Guillain-Barré syndrome. In the present study, we for the first time visualized the spatiotemporal evolution of autoimmune nerve injury and recovery by magnetic resonance imaging (MRI) by use of the novel micellar magnetic resonance (MR) contrast agent gadofluorine M (Gf). EAN was induced in Lewis rats by T-cell transfer (AT-EAN) leading to severe axonal damage, and Gf was applied intravenously at various disease stages mostly 24 h before MRI. In naive rats, Gf enhancement was present solely in the vascular compartment. In AT-EAN, clinically asymptomatic rats already showed consistent Gf uptake in spinal nerves on day 3, while sciatic nerves were spared. The cauda equina correspondingly exhibited massive T-cell infiltration. Gf enhancement further extended to the plexus lumbosacralis on day 4. On days 5 and 6, the entire peripheral neuraxis from the cauda equina, along the sciatic down to the tibial and peroneal nerves, showed strong Gf enhancement. Spinal and peripheral nerves now exhibited massive inflammation and axonal injury on parallel histological analysis. Gf enhancement persisted in the afflicted nerves until complete recovery and disappeared with a proximodistal gradient. In conclusion, Gf-enhanced MR neurography opens a new avenue for monitoring nerve damage in-vivo during an immune attack.

Diffusion-tensor MR imaging of corticospinal tract in amyotrophic lateral sclerosis and progressive muscular atrophy

PURPOSE

To prospectively evaluate several diffusion-tensor magnetic resonance (MR) imaging indexes (mean diffusivity [MD], fractional anisotropy [FA], and eigenvalues) of corticospinal tract impairment in patients with progressive muscular atrophy (PMA) and patients with amyotrophic lateral sclerosis (ALS).

MATERIALS AND METHODS

This study had institutional review board approval, and written informed consent was obtained from all subjects. Eight male patients with PMA (mean age, 63 years +/- 13 [standard deviation]), eighteen patients with ALS (14 men and four women; mean age, 64 years +/- 7), and twelve control subjects (four men and eight women; mean age, 65 years +/- 6) underwent diffusion-tensor MR imaging at which 25 spin-echo echo-planar imaging diffusion-weighted images (b = 1000 sec/mm2) were acquired along noncollinear directions. MD and FA were measured along the corticospinal tracts in each patient and subject. Changes in diffusion along and orthogonal to fiber bundles in patients were evaluated by using diffusion-tensor eigenvalues. Differences in diffusion-tensor imaging indexes between patients with PMA and those with ALS, as compared with these indexes in control subjects, were evaluated with Mann-Whitney testing. Correlations between diffusion-tensor imaging indexes and clinical variables were estimated with Pearson and Spearman rank correlation testing.

RESULTS

As compared with MD (697.1 x 10(-6) mm2/sec +/- 28.1) and FA (0.585 +/- 0.032) in control subjects, MD was typically significantly increased (734.7 x 10(-6) mm2/sec +/- 41.2, P = .035) and FA significantly decreased (0.534 +/- 0.053, P = .037) along the corticospinal tracts in patients with ALS, while these parameters showed no significant change in patients with PMA (MD, 707.0 x 10(-6) mm2/sec +/- 44.2; FA, 0.559 +/- 0.028). Estimation of diffusion-tensor eigenvalues revealed normal diffusion along fiber tracts in all patients, while diffusion was increased orthogonal to fiber tracts only in patients with typical ALS. In patients with ALS, MD correlated with disease duration while FA correlated with disease severity.

CONCLUSION

Diffusion-tensor MR imaging reveals corticospinal tract impairment in ALS but not in PMA.

MR properties of excised neural tissue following experimentally induced demyelination

Changes in the magnetic resonance (MR) parameters of demyelinated neural tissue were measured in vitro using an experimental animal model. A tellurium (Te) diet was applied to weanling rats to induce the demyelination process in the sciatic nerve. The quantitative MR parameters, such as T(1), T(2) relaxation time constants and magnetization transfer (MT) were measured each day after applying the Te diet (up to 7 days) and were found to be substantially different from those of normal nerves. An increase in the average T(1) and T(2) was observed along with a decrease in the MT ratio (MTR) and the quantitative MT parameter M(0B), which describes the semisolid pool of protons. Most of the MR parameters correlated very well with the myelin fraction of neural tissue evaluated by quantitative histopathology. The T(2) relaxation spectrum provided the most efficient quantitative assessment of changes in neural tissue microstructure and its analysis resulted in a powerful tool to distinguish the processes of demyelination and inflammation. In comparison, the MT measurements were less successful.

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.

Histological and magnetic resonance analysis of sciatic nerves in the tellurium model of neuropathy

Ingestion of tellurium (Te), a toxic element, produces paralysis of the hind limbs in weanling rats that is due to temporary, segmental demyelination of the sciatic nerves bilaterally. Weanling rats were fed a 1.1% elemental Te diet and sacrificed at various time points for histological and magnetic resonance (MR) analysis of the sciatic nerves. No controls exhibited impairments of the hind limbs, whereas Te-treated animals became progressively impaired with increased Te exposure. Toluidine blue-stained nerve sections of Te-treated animals showed widened endoneurial spaces, disrupted myelin sheaths, swollen Schwann cells, and a few instances of axonal degeneration. Te decreased healthy myelin by 68% and increased percent extracellular matrix by 45% on day 7. MR experiments showed a decrease in the area of the short T2 component, an increase in average T1, and an increase in the position of the intermediate T2 component in Te-treated nerves. The correlation coefficient for healthy myelin and average T1 was 0.88 and that for healthy myelin and the area underneath the short T2 component was 0.77. The area of the short T2 component has been postulated as the best measure of the process of demyelination.

Assessment of nerve degeneration by gadofluorine M-enhanced magnetic resonance imaging

Nerve injury represents a major cause of disability. In the peripheral nervous system, nerves have the capacity to regrow but within weeks after injury, it is impossible to clarify whether proper regeneration is under way or is failing. In this experimental study, we report on a novel tool to assess nerve outgrowth in vivo. After systemic application, the novel gadolinium-based magnetic resonance (MR) contrast agent Gadofluorine M (Gf) selectively accumulated and persisted in nerve fibers undergoing Wallerian degeneration causing bright contrast on T1-weighted MR images. Gf enhancement on MR imaging was present already at 48 hours within the entire nerve segments undergoing Wallerian degeneration, and subsequently disappeared from proximal to distal parts in parallel to regrowth of nerve fibers. Most importantly, Gf enhancement persisted in nonregenerating, permanently transected nerves. Our novel Gf-based MR imaging methodology holds promise for clinical use to bridge the diagnostic gap between nerve injury and completed nerve regeneration, and to determine the necessity for neurolysis and engraftment if spontaneous regeneration is not successful.

Diffusion-weighted EPI with magnetization transfer contrast

Capabilities of diffusion-weighted (DW) and magnetization transfer (MT) imaging are well established for tissue characterization in various pathologies individually. However, the effect of suppression of macromolecules on applying MT pulse on signals associated with DW imaging and resulting change in the apparent diffusion coefficient (ADC) of water molecules has not been demonstrated previously. In the present study, we have performed DW echo planar imaging (EPI) with and without MT preparation pulse to see the effect of macromolecular signal suppression on ADC. A total of 10 normal volunteers and 20 patients with different intracranial cystic lesions [abscesses (n=10), cystic tumors (n=5), arachnoid cysts (n=5)] were subjected to DW imaging (b=0 and 1000 s/mm(2)) with and without MT saturation pulse. Analysis of region of interest (ROI) from different areas of white matter in normal volunteers and in the wall and cavity of cystic lesions in patients was carried out for calculating the ADC values. We found a significant increase (P<.05) in the ADC values in brain parenchyma and cavity of those intracranial cystic lesions having considerable amount of proteins after the application of MT preparation pulse except for arachnoid cysts. This is due to the size of the macromolecules present in the normal and abnormal tissue. Our studies suggest that this technique is likely to give a novel image contrast and may be of value in improving the tissue specificity in pathologies associated with variable macromolecular size.

Mechanisms of normal appearing corpus callosum injury related to pericallosal T1 lesions in multiple sclerosis using directional diffusion tensor and 1H MRS imaging

OBJECTIVES

To investigate the extent of tissue damage in a region of normal appearing corpus callosum (NACC) for different forms of multiple sclerosis (MS) using diffusion tensor and proton magnetic resonance (MR) spectroscopic imaging.

METHODS

A total of 47 patients with MS and 15 controls were included. Regions of interest from the NACC were manually segmented using high resolution anatomical images. Diffusion tensor eigenvalues and metabolite ratio of N-acetyl-aspartate (NAA) to creatine/phosphocreatine (Cr) were calculated in the NACC region.

RESULTS

Increased apparent diffusion coefficients (ADCs) and decreased anisotropy were observed in the NACC for patients with MS relative to the control subjects. These resulted from increased diffusion tensor eigenvalues perpendicular to the maximum diffusion direction. The NAA:Cr ratio was decreased in the NACC for patients with MS relative to the control subjects. Significant correlations between pericallosal T1 lesion load and MR modalities in the NACC were observed for patients with relapsing remitting/secondary progressive MS (RR/SPMS), but not for patients with primary progressive MS (PPMS).

CONCLUSION

This study provides further insight into changes in the ADC and diffusion anisotropy based on the diffusion tensor eigenvalues for patients with MS. The changes in the diffusion tensor eigenvalues and NAA:Cr ratio in the NACC for patients with RR/SPMS suggest axonal injury and/or dysfunction induced by wallerian degeneration. The lack of correlation between these variables in the NACC and focal MS lesions for patients with PPMS further supports intrinsic differences related to tissue injury between these subtypes of MS.

MRI of peripheral nerve degeneration and regeneration: correlation with electrophysiology and histology

Acute axonal nerve lesions cause a hyperintense signal on T2-weighted (T2-w) magnetic resonance imaging (MRI) at the nerve lesion site and distal to it. The aim of this experimental study was to investigate the spatiotemporal evolution and resolution of MR nerve signal changes following denervation and reinnervation, and to relate these findings to electrophysiology and histology. The proximal sciatic nerve of adult rats was ligated by a tight suture that was removed 1 week later to induce complete axotomy and nerve regeneration upon release. Serial electromyography (EMG) and motor nerve conduction studies were performed parallel to MRI at multiple points of time. Moreover, sciatic nerves were taken for quantitative histological evaluation. Nerve hyperintensity on T2-w MRI was present distal to the lesion at thigh level 24 h after denervation preceding the occurrence of spontaneous activity on EMG by 24 h. After 48 h, the entire sciatic nerve and its branches showed an increased signal down to the level of the lower leg. The increased nerve signal regressed with a proximo-distal gradient beginning from week 2 after onset of nerve regeneration in the thigh. On EMG, the first reinnervation potentials were detected at that time at the respective level. Compound muscle action potential (CMAP) in the foot muscle fully recovered 12 weeks after onset of nerve regeneration, that is, 2 weeks after resolution of the hyperintensity along the entire nerve on MRI. Histology revealed axonal degeneration in the acute phase and later nerve oedema parallel to the increased nerve signal on MRI. MR signal alterations occur as early as 24 h after an axonal nerve lesion and correlate with nerve fiber degeneration and later with nerve oedema on histology. MR findings in denervation and reinnervation parallel the electrophysiological changes. Thus, MRI is a promising diagnostic tool for the early detection of acute axonal nerve lesions and monitoring of nerve regeneration.

Regional variations in normal brain shown by quantitative magnetization transfer imaging

A quantitative magnetization transfer imaging (qMTI) study, based on a two-pool model of magnetization transfer, was performed on seven normal subjects to determine, on a regional basis, normal values for the pool sizes, exchange, and relaxation parameters that characterize the MT phenomenon. Regions were identified on high-resolution anatomical scans using a combination of manual and automatic methods. Only voxels identified as pure tissue at the resolution of the quantitative scans were considered for analysis. While no left/right differences were observed, significant differences were found among white-matter regions and gray-matter regions. These regional differences were compared with existing cytoarchitectural data. In addition, the pattern and magnitude of the regional differences observed in white matter was found to be different from that reported previously for an alternative putative MRI measure of myelination, the 10-50-ms T2 component described as myelin water.

Caught in the act: in vivo mapping of macrophage infiltration in nerve injury by magnetic resonance imaging

In vivo tracking of hematogenous macrophages has been a major challenge because these cells are key players in nerve injury and repair. We visualized the spatiotemporal course of macrophage infiltration after acute peripheral nerve injury in living rats by using superparamagnetic iron oxide (SPIO) particles and magnetic resonance imaging (MRI). A signal loss on MR images indicating iron accumulation was present in degenerating sciatic nerves between days 1 and 8 after a crush lesion, ceased thereafter, and corresponded to the transient presence of iron-labeled ED1-positive macrophages in tissue sections. In contrast, no SPIO accumulation was seen after optic nerve crush, which revealed microglial activation but lacked macrophage infiltration. SPIO-enhanced MRI provides a new tool to selectively visualize active periods of macrophage transmigration into the nervous system, thus enabling dynamic views on a fundamental process in a multitude of nerve disorders.

Directional diffusion in relapsing-remitting multiple sclerosis: A possible in vivo signature of Wallerian degeneration

PURPOSE

To examine the role of directional dependence of the apparent diffusion coefficients in the evaluation of normal-appearing brain regions of patients with relapsing-remitting multiple sclerosis.

MATERIALS AND METHODS

The role of diffusion tensor eigenvalues was investigated in the normal-appearing brain regions for 18 patients with relapsing-remitting multiple sclerosis and 15 age-matched normal controls.

RESULTS

The isotropic apparent diffusion was increased in all regions. However, reduced anisotropy was significant only in regions with high anisotropy, including the corpus callosum and the internal capsule, and was due to increased diffusion tensor eigenvalues corresponding to diffusion transverse to the fibers without significant increase along the fibers. This characteristic pattern of changes in diffusion tensor eigenvalues has been observed previously in cases of Wallerian degeneration. Low-anisotropy regions corresponded to gray matter and gray/white interface regions. Since fiber tract orientations are not determined for regions of low anisotropy, this characteristic pattern of diffusion change is not detectable in these regions.

CONCLUSION

Examination of diffusion tensor eigenvectors may provide insight into the changes observed in diffusion and a signature of Wallerian degeneration in the normal-appearing white matter of relapsing-remitting multiple sclerosis patients.

High-resolution magnetic resonance imaging is a noninvasive method of observing injury and recovery in the peripheral nervous system

OBJECTIVE

Noninvasive observation of degenerating and regenerating peripheral nerves could improve the diagnosis and treatment of nerve injuries. We constructed a novel phased-array radiofrequency coil to permit magnetic resonance imaging (MRI) observation of the sciatic nerve and its target muscles in rats after injury.

METHODS

Adult male Lewis rats underwent either crushing (n = 18) or cutting and capping (n = 17) of their right sciatic nerves and then underwent serial MRI. Serial gait track analysis was performed to assess behavioral recovery. Animals from both groups were killed at several time points for histological evaluation of the nerves, with axon counting.

RESULTS

Crushed sciatic nerves demonstrated increased T2-weighted signals, followed by signal normalization as axonal regeneration and behavioral recovery occurred. Cut sciatic nerves prevented from regenerating displayed a prolonged phase of increased signal intensity. Acutely denervated muscles exhibited hyperintense T2-weighted signals, which normalized with reinnervation and behavioral recovery. Chronically denervated muscles demonstrated persistently increased T2-weighted signals and atrophy.

CONCLUSION

In this study, we demonstrated the ability of MRI to noninvasively monitor injury and recovery in the peripheral nervous system, by demonstrating changes in nerve and muscle that correlated with histological and behavioral evidence of axonal degeneration and regeneration. This study demonstrates the potential of MRI to distinguish traumatic peripheral nerve injuries that recover through axonal regeneration (i.e., axonotmetic grade) from those that do not and therefore require surgical repair (i.e., neurotmetic grade). This diagnostic modality could improve treatment by providing earlier and more accurate diagnoses of nerve damage, as well as reducing the need for exploratory surgery.

Is multicomponent T2 a good measure of myelin content in peripheral nerve?

Multicomponent T(2) relaxation of normal and injured rat sciatic nerve was measured. The T(2) relaxation was multiexponential, indicating the multicompartmental nature of T(2) decay in nerve tissue. The size of the short, observed T(2) component correlated very well with quantitative assessment of myelin using computer-assisted histopathological image analysis of myelin. Specifically, the size of the short T(2) component reflected the processes of myelin loss and remyelination accompanying Wallerian degeneration and regeneration following trauma. However, it represented all myelin present in the sample and did not distinguish between intact myelin and myelin debris. Other changes in T(2) spectra were also observed and could be correlated with axonal loss and inflammation. The study also questions the validity of previously offered interpretations of T(2) spectra of nerve.

The basis of anisotropic water diffusion in the nervous system - a technical review

Anisotropic water diffusion in neural fibres such as nerve, white matter in spinal cord, or white matter in brain forms the basis for the utilization of diffusion tensor imaging (DTI) to track fibre pathways. The fact that water diffusion is sensitive to the underlying tissue microstructure provides a unique method of assessing the orientation and integrity of these neural fibres, which may be useful in assessing a number of neurological disorders. The purpose of this review is to characterize the relationship of nuclear magnetic resonance measurements of water diffusion and its anisotropy (i.e. directional dependence) with the underlying microstructure of neural fibres. The emphasis of the review will be on model neurological systems both in vitro and in vivo. A systematic discussion of the possible sources of anisotropy and their evaluation will be presented followed by an overview of various studies of restricted diffusion and compartmentation as they relate to anisotropy. Pertinent pathological models, developmental studies and theoretical analyses provide further insight into the basis of anisotropic diffusion and its potential utility in the nervous system.

q-Space high b value diffusion MRI of hemi-crush in rat spinal cord: evidence for spontaneous regeneration

The development of the damage following hemi-crush trauma in rat spinal cord was studied ex vivo using high b value (bmax = 1 x 10(7) s cm(-2)) q-space diffusion weighted MRI (DWI) at five days, ten days and six weeks post-trauma. Rat spinal cord trauma, produced by hemi-crush of 15s and 60s duration, was studied. The water signal decay in these diffusion experiments was found to be non mono-exponential and was analyzed using the q-space approach. The q-space MRI parameters were compared with T1 and T2 MR images, behavioral tests and histopathological osmium staining. A very good anatomical correlation was found between the q-space MRI parameters and the osmium staining. Interestingly, we found that in the 15s hemi-crush model significant recovery was observed in both the q-space MR images and the osmium staining six weeks post-trauma. However, in the 60s hemi-crush trauma model very little recovery was observed. These results paralleled those obtained from behavioral tests demonstrating that partial spontaneous recovery seems to occur in the 15s hemi-crush spinal cord model, which should be taken in consideration when using it to evaluate new therapies.

Why does MTR change with neuronal depolarization?

T1 and T2 relaxation, and magnetization transfer (MT) of the rat brain were measured during experimentally induced spreading depression (SD). All measured MR parameters changed during SD: T1 relaxation increased by approximately 13%, whereas the T2 increase was substantially larger (88%). MT results showed an MT ratio (MTR) decrease of 9%. The lack of change in the MT exchange rate indicated that the MT processes between water and macromolecular protons are not affected by neuronal depolarization. The observed decrease in MTR was only caused by changes in T1 and T2 relaxation.

Current awareness

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of NMR in biomedicine. Each bibliography is divided into 9 sections: 1 Books, Reviews ' Symposia; 2 General; 3 Technology; 4 Brain and Nerves; 5 Neuropathology; 6 Cancer; 7 Cardiac, Vascular and Respiratory Systems; 8 Liver, Kidney and Other Organs; 9 Muscle and Orthopaedic. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted.