In vivo diffusion tensor imaging of rat spinal cord at 7 T

Effects of Photon versus Carbon-Ion Irradiation in the Rat Cervical Spinal Cord - a Serial T2 and Diffusion-weighted Magnetic Resonance Imaging Study

Carbon-ion irradiation is increasingly used at the skull base and spine near the radiation-sensitive spinal cord. To better characterize the in vivo radiation response of the cervical spinal cord, radiogenic changes in the high-dose area were measured in rats using magnetic resonance imaging (MRI) diffusion measurements in comparison to conventional photon irradiations. In this longitudinal MRI study, we examined the gray matter (GM) of the cervical spinal cord in 16 female Sprague-Dawley rats after high-dose photon (n = 8) or carbon-ion (12C) irradiation (n = 8) and in 6 sham-exposed rats until myelopathy occurred. The differences in the diffusion pattern of the GM of the cervical spinal cord were examined until the endpoint of the study, occurrence of paresis grade II of both forelimbs was reached. In both radiation techniques, the same order of the occurrence of MR-morphological pathologies was observed - from edema formation to a blood spinal cord barrier (BSCB) disruption to paresis grade II of both forelimbs. However, carbon-ion irradiation showed a significant increase of the mean apparent diffusion coefficient (ADC; P = 0.031) with development of a BSCB disruption in the GM. Animals with paresis grade II as a late radiation response had a highly significant increase in mean ADC (P = 0.0001) after carbon-ion irradiation. At this time, a tendency was observed for higher mean ADC values in the GM after 12C irradiation as compared to photon irradiation (P = 0.059). These findings demonstrated that carbon-ion irradiation leads to greater structural damage to the GM of the rat cervical spinal cord than photon irradiation due to its higher linear energy transfer (LET) value.

Myelin Water Fraction and Intra/Extracellular Water Geometric Mean T2 Normative Atlases for the Cervical Spinal Cord from 3T MRI

BACKGROUND AND PURPOSE

Acquiring and interpreting quantitative myelin-specific MRI data at an individual level is challenging because of technical difficulties and natural myelin variation in the population. To overcome these challenges, we used multiecho T₂ myelin water imaging (MWI) to create T₂ metric healthy population atlases that depict the mean and variation of myelin water fraction (MWF), and intra- and extracellular water mobility as described by geometric mean T₂ (IEGMT₂ ).

METHODS

Cervical cord MWI was performed at 3T on 20 healthy individuals (10M/10F, mean age: 36 years) and 3 relapsing remitting multiple sclerosis (RRMS) participants (1M/2F, age: 39/42/37 years). Anatomical data were collected for the purpose of image segmentation and registration. Atlases were created by coregistering and averaging T₂ metrics from all controls. Voxel-wise z-score maps from 3 RRMS participants were produced to demonstrate the preliminary utility of the MWF and IEGMT₂ atlases.

RESULTS

The average MWF atlas provides a representation of myelin in the spinal cord consistent with well-known spinal cord anatomical characteristics. The IEGMT₂ atlas also depicted structural variations in the spinal cord. Z-score analysis illustrated distinct abnormalities in MWF and IEGMT₂ in the 3 RRMS cases.

CONCLUSIONS

Our findings highlight the potential for using a quantitative T₂ relaxation metric atlas to visualize and detect pathology in spinal cord. Our MWF and IEGMT₂ atlases (URL: https://sourceforge.net/projects/mwi-spinal-cord-atlases/) can serve as normative references in the cervical spinal cord for other studies.

Cervical Spine Prospective Feasibility Study : Dynamic Flexion-Extension Diffusion-Tensor Weighted Magnetic Resonance Imaging

PURPOSE

Diffusion tensor imaging (DTI) in flexion-extension may serve as a diagnostic tool to improve the sensitivity for detection of myelopathy. In this study, the feasibility and reproducibility of dynamic DTI in the cervical spinal cord was assessed in healthy volunteers and patients.

METHODS

All subjects were examined in maximum neck flexion-extension in a 3T magnetic resonance imaging (MRI) scanner. Range of motion, space available for the spinal cord, fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were measured and compared between the neck positions.

RESULTS

Volunteers showed no variation in ADC and FA. In patients, extension produced higher ADC in the diseased than in the control segments (p = 0.0045). The ADC of the affected segments was higher in extension than in the neutral position (p = 0.0030) or in flexion (p = 0.0002). The FA was significantly lower in extension in patients at both the control level C2/3 (p = 0.0154) and the affected segment (p = 0.0187).

CONCLUSIONS

Dynamic DTI of the cervical spine is feasible and ADC increased in the patient group in extension. This finding may open a previously unexplored avenue to attempt an earlier identification of myelopathy.

Imaging of Spinal Cord Injury: Acute Cervical Spinal Cord Injury, Cervical Spondylotic Myelopathy, and Cord Herniation

We review the pathophysiology and imaging findings of acute traumatic spinal cord injury (SCI), cervical spondylotic myelopathy, and briefly review the much less common cord herniation as a unique cause of myelopathy. Acute traumatic SCI is devastating to the patient and the costs to society are staggering. There are currently no "cures" for SCI and the only accepted pharmacologic treatment regimen for traumatic SCI is currently being questioned. Evaluation and prognostication of SCI is a demanding area with significant deficiencies, including lack of biomarkers. Accurate classification of SCI is heavily dependent on a good clinical examination, the results of which can vary substantially based upon the patient׳s condition or comorbidities and the skills of the examiner. Moreover, the full extent of a patients׳ neurologic injury may not become apparent for days after injury; by then, therapeutic response may be limited. Although magnetic resonance imaging (MRI) is the best imaging modality for the evaluation of spinal cord parenchyma, conventional MR techniques do not appear to differentiate edema from axonal injury. Recently, it is proposed that in addition to characterizing the anatomic extent of injury, metrics derived from conventional MRI and diffusion tensor imaging, in conjunction with the neurological examination, can serve as a reliable objective biomarker for determination of the extent of neurologic injury and early identification of patients who would benefit from treatment. Cervical spondylosis is a common disorder affecting predominantly the elderly with a potential to narrow the spinal canal and thereby impinge or compress upon the neural elements leading to cervical spondylotic myelopathy and radiculopathy. It is the commonest nontraumatic cause of spinal cord disorder in adults. Imaging plays an important role in grading the severity of spondylosis and detecting cord abnormalities suggesting myelopathy.

Timing of diffusion tensor imaging in the acute spinal cord injury of rats

The aim of this study was to evaluate the characteristics of magnetic resonance diffusion tensor imaging (DTI) in acute spinal cord following a thoracic spinal cord injury (SCI), and to determine the optimal time of examination. Sprague-Dawley rats were used as experimental animals and contusion injuries were made at the T10 vertebral level. The rats were divided into control, mild injury, moderate injury, and severe injury groups. Spinal magnetic resonance DTI was scheduled at 6, 24 and 72 hours (h) post-SCI, and the DTI parameters such as fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were calculated, and the diffusion tensor tractography (DTT) of the spinal cord was also generated. We observed a significant decrease of FA in all the three injured groups, and the FA at 24 h post-SCI exhibited the greatest decrease among different set times. For ADC, only the group of severely injured rats saw a significant decrease at 24 and 72 h compared with the control group. DTT showed interruption of nerve fiber tracking in the injured groups. This study demonstrates that FA can differentiate various grades of SCI in the early stage, and 24 h after injury might be the optimal time for identifying injury severity.

The current state-of-the-art of spinal cord imaging: methods

A first-ever spinal cord imaging meeting was sponsored by the International Spinal Research Trust and the Wings for Life Foundation with the aim of identifying the current state-of-the-art of spinal cord imaging, the current greatest challenges, and greatest needs for future development. This meeting was attended by a small group of invited experts spanning all aspects of spinal cord imaging from basic research to clinical practice. The greatest current challenges for spinal cord imaging were identified as arising from the imaging environment itself; difficult imaging environment created by the bone surrounding the spinal canal, physiological motion of the cord and adjacent tissues, and small cross-sectional dimensions of the spinal cord, exacerbated by metallic implants often present in injured patients. Challenges were also identified as a result of a lack of "critical mass" of researchers taking on the development of spinal cord imaging, affecting both the rate of progress in the field, and the demand for equipment and software to manufacturers to produce the necessary tools. Here we define the current state-of-the-art of spinal cord imaging, discuss the underlying theory and challenges, and present the evidence for the current and potential power of these methods. In two review papers (part I and part II), we propose that the challenges can be overcome with advances in methods, improving availability and effectiveness of methods, and linking existing researchers to create the necessary scientific and clinical network to advance the rate of progress and impact of the research.

Diffusion tensor imaging of the spinal cord: a review

Diffusion tensor imaging (DTI) is a magnetic resonance technique capable of measuring the magnitude and direction of water molecule diffusion in various tissues. The use of DTI is being expanded to evaluate a variety of spinal cord disorders both for prognostication and to guide therapy. The purpose of this article is to review the literature on spinal cord DTI in both animal models and humans in different neurosurgical conditions. DTI of the spinal cord shows promise in traumatic spinal cord injury, cervical spondylotic myelopathy, and intramedullary tumors. However, scanning protocols and image processing need to be refined and standardized.

Diffusion tensor imaging of the normal pediatric spinal cord using an inner field of view echo-planar imaging sequence

BACKGROUND AND PURPOSE

DTI in the brain has been well established, but its application in the spinal cord, especially in pediatrics, poses several challenges. The small cord size has inherent low SNR of the diffusion signal intensity, respiratory and cardiac movements induce artifacts, and EPI sequences used for obtaining diffusion indices cause eddy-current distortions. The purpose of this study was to 1) evaluate the accuracy of cervical spinal cord DTI in children using a newly developed iFOV sequence with spatially selective 2D-RF excitations, and 2) examine reproducibility of the DTI measures.

MATERIALS AND METHODS

Twenty-five typically developing subjects were imaged twice using a 3T scanner. Axial DTI images of the cervical spinal cord were acquired with this sequence. After motion correction, DTI indices were calculated using regions of interest manually drawn at every axial section location along the cervical spinal cord for both acquisitions. Various DTI indices were calculated: FA, AD, RD, MD, RA, and VR. Geometric diffusion measures were also calculated: Cp, Cl, and Cs.

RESULTS

The following average values for each index were obtained: FA = 0.50 ± 0.11; AD = 0.97 ± 0.20 × 10(-3)mm(2)/s; RD = 0.41 ± 0.13 × 10(-3)mm(2)/s; MD = 0.59 ± 0.15 × 10(-3)mm(2)/s; RA = 0.35 ± 0.08; VR = 0.03 ± 0.00; Cp = 0.13 ± 0.07; Cl = 0.29 ± 0.09; and Cs = 0.58 ± 0.11. The reproducibility tests showed moderate to strong ICC in all subjects for all DTI parameters (ICC>0.72).

CONCLUSIONS

This study showed that accurate and reproducible DTI parameters can be estimated in the pediatric cervical spinal cord using an iFOV EPI sequence.

Wallerian degeneration after spinal cord lesions in cats detected with diffusion tensor imaging

One goal of in vivo neuroimaging is the detection of neurodegenerative processes and anatomical reorganizations after spinal cord (SC) injury. Non-invasive examination of white matter fibers in the living SC can be conducted using magnetic resonance diffusion-weighted imaging. However, this technique is challenging at the spinal level due to the small cross-sectional size of the cord and the presence of physiological motion and susceptibility artifacts. In this study, we acquired in vivo high angular resolution diffusion imaging (HARDI) data at 3T in cats submitted to partial SC injury. Cats were imaged before, 3 and 21 days after injury. Spatial resolution was enhanced to 1.5 × 1.5 × 1 mm(3) using super-resolution technique and distortions were corrected using the reversed gradient method. Tractography-derived regions of interest were generated in the dorsal, ventral, right and left quadrants, to evaluate diffusion tensor imaging (DTI) and Q-Ball imaging metrics with regards to their sensitivity in detecting primary and secondary lesions. A three-way ANOVA tested the effect of session (intact, D3, D21), cross-sectional region (left, right, dorsal and ventral) and rostrocaudal location. Significant effect of session was found for FA (P<0.001), GFA (P<0.05) and radial diffusivity (P<0.001). Post-hoc paired T-test corrected for multiple comparisons showed significant changes at the lesion epicenter (P<0.005). More interestingly, significant changes were also found several centimeters from the lesion epicenter at both 3 and 21 days. This decrease was specific to the type of fibers, i.e., rostrally to the lesion on the dorsal aspect of the cord and caudally to the lesion ipsilaterally, suggesting the detection of Wallerian degeneration.

Spinal cord - MR of rodent models

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

Fractional Anisotropy Levels Derived From Diffusion Tensor Imaging in Cervical Syringomyelia

BACKGROUND:

Syringomyelia can result in major functional disability. Conventional imaging techniques frequently fail to detect the underlying cause of syringomyelia. The prediction as to whether syringomyelia might lead to neurological deficits is still challenging.

OBJECTIVE:

We hypothesized that fractional anisotropy (FA) derived from diffusion tensor imaging (DTI) is a parameter to detect dynamic forms of syringomyelia.

METHODS:

Six patients with cervical syringomyelia, all comparable in size, shape, and location, were examined, along with 2 volunteers. Patients underwent electrophysiological recordings (somatosensory evoked potentials, motor evoked potentials, silent periods). Magnetic resonance imaging (1.5 T) was performed with a 6-element spine coil. Anatomic images were acquired with a 3-dimensional, constructive interference in steady-state sequence, and DTI with an echo-planar imaging sequence (5-mm thickness, b value 800 s/mm2) using the generalized autocalibrating partially parallel acquisitions technique. The positions were centered on the syrinx in the volunteers between the C2 and Th1. DTI data were interpolated to a spatial resolution of 0.5 mm. After calculation of a diffusion tensor in each pixel, an FA map was calculated and profiles of the FA values across the spinal cord were calculated in all slices.

RESULTS:

FA values were lower at the level of all examined syrinxes and reached normal values beyond them. Electrophysiological results correlated with the decrease in FA value. There were no presyrinx changes in the white matter tracts in terms of signs of FA changes beneath the syrinx.

CONCLUSION:

DTI of syringomyelia can demonstrate white matter fiber tracts around and beyond the syrinx consistent with electrophysiological values. DTI of the cervical spine can provide quantitative information about the pathological characteristics beyond the abnormalities visible on magnetic resonance imaging.

Evaluating regional blood spinal cord barrier dysfunction following spinal cord injury using longitudinal dynamic contrast-enhanced MRI

BACKGROUND

In vivo preclinical imaging of spinal cord injury (SCI) in rodent models provides clinically relevant information in translational research. This paper uses multimodal magnetic resonance imaging (MRI) to investigate neurovascular pathology and changes in blood spinal cord barrier (BSCB) permeability following SCI in a mouse model of SCI.

METHODS

C57BL/6 female mice (n = 5) were subjected to contusive injury at the thoracic T11 level and scanned on post injury days 1 and 3 using anatomical, dynamic contrast-enhanced (DCE-MRI) and diffusion tensor imaging (DTI). The injured cords were evaluated postmortem with histopathological stains specific to neurovascular changes. A computational model was implemented to map local changes in barrier function from the contrast enhancement. The area and volume of spinal cord tissue with dysfunctional barrier were determined using semi-automatic segmentation.

RESULTS

Quantitative maps derived from the acquired DCE-MRI data depicted the degree of BSCB permeability variations in injured spinal cords. At the injury sites, the damaged barriers occupied about 70% of the total cross section and 48% of the total volume on day 1, but the corresponding measurements were reduced to 55% and 25%, respectively on day 3. These changes implied spatio-temporal remodeling of microvasculature and its architecture in injured SC. Diffusion computations included longitudinal and transverse diffusivities and fractional anisotropy index. Comparison of permeability and diffusion measurements indicated regions of injured cords with dysfunctional barriers had structural changes in the form of greater axonal loss and demyelination, as supported by histopathologic assessments.

CONCLUSION

The results from this study collectively demonstrated the feasibility of quantitatively mapping regional BSCB dysfunction in injured cord in mouse and obtaining complementary information about its structural integrity using in vivo DCE-MRI and DTI protocols. This capability is expected to play an important role in characterizing the neurovascular changes and reorganization following SCI in longitudinal preclinical experiments, but with potential clinical implications.

Focal reversible deactivation of cerebral metabolism affects water diffusion

The underlying biophysical mechanisms which affect cerebral diffusion contrast remain poorly understood. We hypothesized that cerebral metabolism may affect cerebral diffusion contrast. The purpose of this study was to develop the methodology to reversibly deactivate cerebral metabolism and measure the effect on the diffusion MRI signal. We developed an MRI-compatible cortical cooling system to reversibly deactivate cortical metabolism in rhesus monkey area V1 and used MR thermometry to calculate three-dimensional temperature maps of the brain to define the extent of deactivated brain in vivo. Significant changes in the apparent diffusion coefficient (ADC) were only observed during those experiments in which the cortex was cooled below the metabolic cutoff temperature of 20 degrees C. ADC decreases (12-20%) were observed during cortical cooling in regions where the temperature did not change. The normalized in vivo ADC as function of temperature was measured and found to be equivalent to the normalized ADC of free water at temperatures above 20 degrees C, but was significantly decreased below 20 degrees C (20-25% decrease). No changes in fractional anisotropy were observed. In future experiments, we will apply this methodology to quantify the effect of reversible deactivation on neural activity as measured by the hemodynamic response and compare water diffusion changes with hemodynamic changes.

Rapid, postmortem 9.4 T MRI of spinal cord injury: correlation with histology and survival times

High field magnetic resonance imaging (MRI) has been increasingly used to assess experimental spinal cord injury (SCI). In the present investigation, after partial spinal cord injury and excision of the whole spine, pathological changes of the spinal cord were studied in spinal cord-spine blocks, from the acute to the chronic state (24 h to 5 months). Using proton density (PD) weighted imaging parameters at a magnetic field strength of 9.4 tesla (T), acquisition times ranging from <1 to 10 h per specimen were used. High in-plane pixel resolution (68 and 38 microm, respectively) was obtained, as well as high signal-to-noise ratio (SNR), which is important for optimal contrast settings. The quality of the resulting MR images was demonstrated by comparison with histology. The cord and the lesion were shown in their anatomical surroundings, detecting cord swelling in the acute phase (24 h to 1 week) and cord atrophy at the chronic stage. Haemorrhage was detected as hypo-intense signal. Oedema, necrosis and scarring were hyper-intense but could not be distinguished. Histology confirmed that the anatomical delimitation of the lesion extent by MRI was precise, both with high and moderate resolution. The present investigation thus demonstrates the precision of spinal cord MRI at different survival delays after compressive partial SCI and establishes efficient imaging parameters for postmortem PD MRI.

Longitudinal comparison of two severities of unilateral cervical spinal cord injury using magnetic resonance imaging in rats

Magnetic resonance imaging (MRI) should be a powerful tool for characterization of spinal cord pathology in animal models. We evaluated the utility of medium-field MRI for the longitudinal assessment of progression of spinal cord injury (SCI) in a rat model. Thirteen adult rats were subjected to a 6.25 or 25 g-cm unilateral cervical SCI, and underwent MRI and behavioral tests during a 3-week study period. MRI was also performed post-mortem. Quantification of cord swelling, hypointense and hyperintense signal, and lesion length were the most valuable parameters to determine and were highly correlated to behavioral and histopathological measures. Immediately after injury, MRI showed loss of gray matter-white matter differentiation, presence of scattered hyperintense signal and local hypointense signal, and cord swelling in both groups. At 7 days after injury, the spinal cord in the 25 g-cm group was significantly larger than that of the 6.25 g-cm group (p = 0.02). Contrast enhancement of the lesion was seen at 24 h in the 6.25 g-cm group, and at 24 h and 7 days in the 25 g-cm group. The volume of hypointense signal, representing hemorrhage, throughout the lesion region was significantly larger in the 25 g-cm compared to the 6.25 g-cm group at both 14 and 21 days after SCI (p, </= 0.04). The appearance of the scattered hyperintense signal, initially representing edema, at later time points changed to a rim of hyperintense signal surrounding the lesion cavity. Significant correlations were found between cord swelling at 7 days after SCI, and lesion length and gray and white matter sparing as determined by histopathology. Other parameters that were highly correlated with histopathology were quantity of hyperintense and hypointense signal, and in vivo lesion length. Hypointense signal and in vivo lesion length were highly correlated to behavior. Significant correlation was also found between parameters determined by MRI: swelling, hypointense signal, hyperintense signal, and lesion length. MRI is a valuable imaging modality to assess the temporal evolution of SCI and to distinguish different severities of cervical SCI in rats. In future, MRI could be applied as a screening tool to either administer goal-directed therapies, or enable even group distribution, prior to therapeutic intervention for example through quantification and matching of swelling and edema.

Diffusion tensor MR imaging in cervical spine trauma

BACKGROUND AND PURPOSE

Our aim was to investigate the extent and severity of changes in spinal cord diffusion tensor imaging (DTI) parameters in patients with cervical cord injury.

MATERIALS AND METHODS

DTI was performed in 20 symptomatic patients (mean, 45.7 +/- 17.7 years of age; 2 women, 18 men) with cervical spine trauma and 8 volunteers (mean, 34.2 +/- 10.7 years of age; 6 men, 2 women). The whole cord and regional apparent diffusion coefficient (ADC), fractional anisotropy (FA), relative anisotropy (RA), and volume ratio (VR) of patients and volunteers were compared. DTI parameters were calculated in 16 patients. MR imaging demonstrated hemorrhagic cord contusions (n = 6), nonhemorrhagic cord contusions (n = 4), and soft-tissue injury (n = 6). Medical records were reviewed for extent of neurologic deficit.

RESULTS

Regional ADC values differed significantly between upper and mid and upper and lower (both, P < .004) cervical cord sections. FA was significantly different between upper and lower sections (P < .03). Whole cord ADC values were significantly lower in patients than in volunteers (P < .0001). Whole spine FA was not significantly decreased in patients (P < .06). ADC and FA values were significantly decreased at injury sites when compared with volunteers (P < .031 and .0001, respectively). The greatest differences in whole cord ADC, FA, RA, and VR were in patients with hemorrhagic cord contusions compared with healthy volunteers (P < .0001, .003, .0005, and .008, respectively).

CONCLUSION

DTI parameters are sensitive markers of cervical cord injury, with ADC showing the greatest sensitivity. Changes in DTI parameters are most marked at injury sites and reflect the severity of cord injury.

In vivo DTI of the healthy and injured cat spinal cord at high spatial and angular resolution

Spinal cord diffusion tensor imaging (DTI) is challenging in many ways: the small size of the cord, physiological motion and susceptibility artifacts pose daunting obstacles to the acquisition of high-quality data. Here, we present DTI results computed from in vivo studies of the healthy and injured spinal cord of five cats. Both high spatial (1.1 mm3) and angular (55 directions) resolutions were used to optimise modelling of the diffusion process. Also, particular effort was directed towards a strategy that limits susceptibility artifacts. For validation purposes, acquisitions were repeated in two cats before and after making a spinal lesion. As a result, various axonal trajectories were identified by tractography including dorsal and ventral columns as well as lateral tracts. Also, fibre bundles showed robust disruption at the site of spinal cord injuries (partial and complete) via tractography, accompanied with significantly lower fractional anisotropy values at the site of lesions. Important outcomes of this work are (i) tractography-based localisation of anatomical tracts in the thoraco-lumbar spinal cord and (ii) in vivo assessment of axonal integrity following experimental spinal cord injury.

MR diffusion tensor imaging and fiber tracking in spinal cord arteriovenous malformations: a preliminary study

BACKGROUND AND PURPOSE

Diffusion tensor imaging (DTI) of the spinal cord in patients harboring spinal arteriovenous malformations (AVMs) was carried out to evaluate the feasibility of this new technique to determine the displacement of the spinal cord tracts and to correlate morphologic and functional DTI data (fractional anisotropy [FA] and apparent diffusion coefficient [ADC]) with the clinical symptoms.

MATERIALS AND METHODS

Nine patients with spinal cord AVMs were investigated at 1.5T using a sagittal spin-echo single-shot echo-planar generalized autocalibrating partially parallel acquisition diffusion-weighted imaging sequence. ADC and FA maps were computed in different regions of interest (both above and below the nidus), and tractography was used to visualize the course of the tracts. The data were correlated with the clinical symptoms and compared with 12 healthy control subjects.

RESULTS

At the level of the nidus, tracts were normal, shifted, separated, or interrupted but not intermingled with the nidus. Interruption of the tracts was coherent with the clinical symptoms. In patients with severe neurologic deficits, FA values caudal to the nidus showed a reduced anisotropy consistent with loss of white matter tracts.

CONCLUSIONS

We demonstrate that AVMs may interrupt, displace, or separate the fiber tracts and that clinical symptoms may be reflected by the quantitative FA results and the morphologic loss of fibers distant to the lesion. DTI with fiber tracking offers a novel approach to image spinal cord AVMs and may open a window to understand the complex pathophysiology of these lesions.

Diffusion tensor magnetic resonance imaging and fiber tracking in spinal cord lesions: current and future indications

Diffusion-weighted imaging and fractional anisotropy may be more sensitive than other conventional magnetic resonance imaging techniques to detect, characterize, and map the extent of spinal cord lesions. Fiber tracking offers the possibility of visualizing the integrity of white matter tracts surrounding some lesions, and this information may help in formulating a differential diagnosis and in planning biopsies or resection. Fractional anisotropy measurements may also play a role in predicting the outcome of patients who have spinal cord lesions. In this article, we address several conditions in which diffusion-weighted imaging and fiber tracking is known to be useful and speculate on others in which we believe these techniques will be useful in the near future.

Postmortem delay does not change regional diffusion anisotropy characteristics in mouse spinal cord white matter

It has been demonstrated previously that water diffusion anisotropy in vivo is equivalent to that observed ex vivo after perfusion fixation in the mouse brain. This finding supports the practice of ex vivo diffusion tensor imaging (DTI) measurement on perfusion-fixed tissues. However, the validity of extrapolating ex vivo DTI measurements from immersion-fixed autopsy specimens to the in vivo state is questionable because of variable postmortem delays often encountered before fixation. In this study, we investigated the effect of postmortem delay on the water diffusion anisotropy of ventrolateral spinal cord white matter from mice. Mouse spinal cords, each from the same animal, were examined using DTI in vivo, in situ after death before fixation, and ex vivo immersion fixed 10 h after death. Our results suggest that diffusion anisotropy in mouse spinal cord is preserved up to 10 h after death. Regional characteristics of diffusion anisotropy in mouse spinal cord white matter are equivalent in vivo, in situ after death (up to 10 h before fixation), and ex vivo 15 weeks after immersion fixation.

Diffusion tensor imaging (DTI) of rodent brains in vivo using a 1.5T clinical MR scanner

PURPOSE

To evaluate the feasibility of using a clinical 1.5T MR scanner to perform magnetic resonance (MR) diffusion tensor imaging (DTI) on in vivo rodent brains and to trace major rodent neuronal bundles with anatomical correlation.

MATERIALS AND METHODS

Two normal adult Sprague Dawley (SD) rats were anesthetized and imaged in a 1.5T MR scanner with a microscopic coil. DTI was performed at a resolution of 0.94 mm x 0.94 mm x 0.5 mm (reconstructed to 0.47 mm x 0.47 mm x 0.5 mm, with b-factors of 600 seconds/mm2 and 1000 seconds/mm2) and a higher resolution of 0.63 mm x 0.63 mm x 0.5 mm (reconstructed to 0.235 mm x 0.235 mm x 0.5 mm, with a b-factor of 1500 seconds/mm2). The fiber-tracking results were correlated with corresponding anatomical sections stained to visualize neuronal fibers. The apparent diffusion coefficient (ADC) and fractional anisotropy (FA) of the neuronal fibers were measured and compared with results in published reports.

RESULTS

Several major neuronal fiber tracts, including the corticospinal cord, corpus callosum, and anterior commissure, were identified in all DTI data sets. Stained anatomical sections obtained from the rats confirmed the location of these fibers. The ADC values (0.6-0.8 +/- 10(-3) mm2/second) of the fibers were similar to published figures. However, the FA values (0.3-0.35) were lower than those obtained in previous studies of white matter in rodent spinal cord.

CONCLUSION

We have demonstrated the feasibility of using a 1.5T clinical MR scanner for neuronal fiber tracking in rodent brains. The technique will be useful in rodent neuroanatomy studies. Further investigation is encouraged to verify the FA values generated by DTI with such techniques.

In vivo isotropic 3D diffusion tensor mapping of the rat brain using diffusion-weighted 3D MP-RAGE MRI

The purpose of this study was to examine the potential of diffusion-weighted (DW) three-dimensional (3D) MP-RAGE MRI for diffusion-tensor mapping of the rat brain in vivo. A DW-3D-MP-RAGE (3D-DWI) sequence was implemented at 2.0 T using six gradient orientations and a b value of 1000 s/mm2. In this sequence, the preparation sequence with a "90 degrees RF-motion proving gradient (MPG): MPG-180 degrees RF-MPG-90 degrees RF" pulse train (DW driven equilibrium Fourier transform) was used to sensitize the magnetization to diffusion. A centric k-space acquisition order was necessary to minimize saturation effects (T1 contamination) from tissues with short relaxation time. The image matrix was 128x128x128 (interpolated from 64x64x64 acquisitions), which resulted in small isotropic DW image data (voxel size: 0.273x0.273x0.273 mm3). Moreover, 3D-DWI-derived maps of the fractional anisotropy (FA), relative anisotropy (RA) and main-diffusion direction were completely free of susceptibility-induced signal losses and geometric distortions. Two well-known commissural fibers, the corpus callosum and anterior commissure, were indicated and shown to be in agreement with the locations of these known stereotaxic atlases. The experiment took 45 min, and shorter times should be possible in clinical application. The 3D-DWI sequence allows for in vivo 3D diffusion-tensor mapping of the rat brain without motion artifacts and susceptibility to distortion.

In vivo DTI evaluation of white matter tracts in rat spinal cord

PURPOSE

To determine whether differences in specific spinal cord white matter (WM) tracts can be detected with in vivo DTI.

MATERIALS AND METHODS

In vivo DTI was performed on six rats at the lower thoracic region using a 4.7T magnet. Axial diffusion images were obtained with diffusion gradients applied in six independent directions, with low and high b-values equal to 0 and 692 seconds/mm(2), respectively. Regions of interest (ROIs) were selected corresponding to the major spinal cord tracts, including the dorsal cortical spinal tract (dCST), fasciculus gracilis (FG), rubrospinal tract (RST), vestibulospinal tract (VST), and reticulospinal tract (ReST).

RESULTS

ANOVA demonstrated overall differences between tracts for all of the DTI parameters, including fractional anisotropy (FA), trace diffusion (Tr), longitudinal diffusivity (EL = lambda(1)), and transverse diffusivity (ET = (lambda(2) + lambda(3))/2). Similarly to previous ex vivo analyses, the spinal cord tract with the largest and most widely spaced axons (VST) had the largest EL and ET.

CONCLUSION

The principal diffusivities appear to reflect axon morphologic differences between the WM tracts that are not as well appreciated with FA and Tr.

Detecting axon damage in spinal cord from a mouse model of multiple sclerosis

In the current study, the feasibility and reproducibility of in vivo diffusion tensor imaging (DTI) of the spinal cord in normal mice are illustrated followed by its application to mice with experimental allergic encephalomyelitis (EAE) to detect and differentiate axon and myelin damage. Axial diffusivity, describing water movement along the axonal fiber tract, in all regions of spinal cord white matter from EAE-affected C57BL/6 mice was significantly decreased compared to normal mice, whereas there was no statistically significant change in radial diffusivity, describing water movement across the fiber tract. Furthermore, a direct comparison between DTI and histology from a single mouse demonstrated a decrease in axial diffusivity that was supported by widespread staining of antibody against beta-amyloid precursor protein. Regionally elevated radial diffusivity corresponded with locally diminished Luxol fast blue staining in the same tissue from the EAE mouse cord. Our findings suggest that axonal damage is more widespread than myelin damage in the spinal cord white matter of mice with EAE and that in vivo DTI may provide a sensitive and specific measure of white matter injury.

Ex vivo magnetic resonance imaging of rat spinal cord at 9.4 T

The magnetic resonance (MR) properties of the rat spinal cord were characterized at the T9 level with ex vivo experiments performed at 9.4 T. The inherent endogenous contrast parameters, proton density (PD), longitudinal and transverse relaxation times T1 and T2, and magnetization transfer ratio (MTR) were measured separately for the grey matter (GM) and white matter (WM). Analysis of the measurements indicated that these tissues have statistically different proton densities with means PD(GM)=54.8+/-2.5% versus PD(WM)=45.2+/-2.4%, and different T1 values with means T1GM=2.28+/-0.23 s versus T1WM=1.97+/-0.21 s. The corresponding values for T2 were T2GM=31.8+/-4.9 ms versus T2WM=29.5+/-4.9 ms, and the difference was insignificant. The difference between MTR(GM)=31.2+/-6.1% and MTR(WM)=33.1+/-5.9% was also insignificant. These results collectively suggest that PD and T1 are the two most important parameters that determine the observed contrast on spinal cord images acquired at 9.4 T. Therefore, in MR imaging studies of spinal cord at this field strength, these parameters need to be considered not only in optimizing the protocols but also in signal enhancement strategies involving exogenous contrast agents.

Microneurography of human median nerve

PURPOSE

To examine the possibility of performing high-resolution MRI (microneurography) on peripheral nerves.

MATERIALS AND METHODS

A specific radio frequency (RF) coil was developed to probe the human median nerve at a magnetic field strength of 9.4 T and tested on three excised samples by acquiring microneurograms.

RESULTS

The microneurograms revealed neuronal tissue constituents at subfascicular level. The contrast features on proton-density and T1- and T2-weighted images were described and compared. The microscopic water movement was quantified using diffusion weighting parallel and orthogonal to the neuronal fiber orientation. The characteristics of anisotropic diffusion in the median nerve were comparable to those reported from other biological tissues (white matter and kidney).

CONCLUSION

The results overall suggest that microneurography might provide new noninvasive insights into microscopic gross anatomy of the peripheral nerve, injury evaluation, and efficacy of repair, although the feasibility at current clinically relevant field strengths is yet to be determined.

Manganese-enhanced MRI of rat spinal cord injury

The potential of the manganese-enhanced MRI (MEI) technique in labeling the intact neuronal circuitry of rat spinal cord was examined. Experiments were conducted on normal and injured cords at 9.4-T magnetic field strength using an implantable rf coil. The contrast agent manganese (Mn) was locally delivered within the parenchyma at a dose of 25 mmol/L in 10 nL. The transport, uptake and accumulation of Mn in tissue were then followed remotely on T1-weighted images that were acquired serially from the cord. In MEIs of normal cord, Mn was observed to be transported in directions both rostral and caudal to the site of injection. In the cord that was subjected to hemisection, signal enhancement was on the contralesional side of the cord, but not at the ipsilesional side. The sensitivity and specificity of the MEI technique in labeling the neurons that are functional were also validated with a traditional track-tracing method using biotinylated dextran amine.

In vivo measurements of T1 relaxation times in mouse brain associated with different modes of systemic administration of manganese chloride

PURPOSE

To measure regional T1 and T2 values for normal C57Bl/6 mouse brain and changes in T1 after systemic administration of manganese chloride (MnCl2) at 9.4 T.

MATERIALS AND METHODS

C57Bl/6 mice were anesthetized and baseline T1 and T2 measurements obtained prior to measurement of T1 after administration of MnCl2 at 9.4 T. MnCl2 was administered systemically either by the intravenous (IV), intraperitoneal (IP), or subcutaneous (SC) routes. T1 and T2 maps for each MRI transverse slice were generated using commercial software, and T1 and T2 values of white matter (WM), gray matter (GM), pituitary gland, and lateral ventricle were obtained.

RESULTS

When compared with baseline values at low-field, significant lengthening of the T1 values was shown at 9.4 T, while no significant change was seen for T2 values. Significant T1 shortening of the normal mouse brain was observed following IV, IP, and SC administration of MnCl2, with IV and IP showing similar acute effects. Significant decreases in T1 values were seen for the pituitary gland and the ventricles 15 minutes after either IV or IP injection. GM showed greater uptake of the contrast agent than WM at 15 and 45 minutes after either IV or IP injections. Although both structures are within the blood-brain barrier (BBB), GM and WM revealed a steady decrease in T1 values at 24 and 72 hours after MnCl2 injection regardless of the route of administration.

CONCLUSION

Systemic administration of MnCl2 by IV and IP routes induced similar time-course of T1 changes in different regions of the mouse brain. Acute effects of MnCl2 administration were mainly influenced by either the presence or absence of BBB. SC injection also provided significant T1 change at subacute stage after MnCl2 administration.

MRI diffusion coefficients in spinal cord correlate with axon morphometry

Following spinal cord injury, diffusion MRI (DWI) has been shown to detect injury and functionally significant neuroprotection following treatment that otherwise would go undetected with conventional MRI. The underlying histologic correlates to directional apparent diffusion coefficients (ADC) obtained with DWI have not been determined, however, and we address this issue by directly correlating ADC values with corresponding axon morphometry in the normal rat cervical spinal cord. ADC values transverse (perpendicular) and longitudinal (parallel) to axons both correlate with axon counts, however each directional ADC reflects distinct histologic parameters. DWI may therefore be capable of providing specific histologic data regarding the integrity of white matter.

Magnetic resonance imaging of mouse spinal cord

The feasibility of performing high-resolution in vivo MRI on mouse spinal cord (SC) at 9.4 T magnetic field strength is demonstrated. The MR properties of the cord tissue were measured and the characteristics of water diffusion in the SC were quantified. The data indicate that the differences in the proton density (PD) and transverse relaxation time between gray matter (GM) and white matter (WM) dominate the contrast seen on the mouse SC images at 9.4 T. However, on heavily T(2)-weighted images these differences result in a reversal of contrast. The diffusion of water in the cord is anisotropic, but the WM exhibits greater anisotropy and principal diffusivity than the GM. The quantitative data presented here should establish a standard for comparing similar measurements obtained from the SCs of genetically engineered mouse or mouse models of SC injury (SCI).

Acute and subacute ischemic stroke at high-field-strength (3.0-T) diffusion-weighted MR imaging: intraindividual comparative study

PURPOSE

To compare signal-to-noise ratios (SNRs), contrast-to-noise ratios (CNRs), image quality, and confidence in diagnosis between 1.5- and 3.0-T diffusion-weighted (DW) magnetic resonance (MR) imaging of ischemic stroke lesions.

MATERIALS AND METHODS

The study design was approved by the institutional review board, and all patients gave informed consent. In a prospective intraindividual study, 25 patients who had clinical symptoms consistent with ischemic stroke underwent DW MR imaging at both 1.5 T and 3.0 T. The 3.0- or 1.5-T examination was performed immediately one after the other, in random order. Two readers in consensus recorded the presence and number of ischemic lesions and rated image quality and lesion conspicuity. The image SNR and the CNR of the ischemic lesions were quantified. Paired Student t and Wilcoxon matched-pairs signed rank tests were used to test for statistical significance.

RESULTS

Image quality at 3.0-T DW MR imaging was consistently lower than that at 1.5-T DW MR imaging owing to greater image distortions (P < .05). Yet, overall SNR and lesion CNR at 3.0 T increased significantly; mean increases were 48.8% (P < .001) and 96.3% (P < .01), respectively. The higher overall SNR and lesion CNR translated into a significantly higher sensitivity in the detection of ischemic lesions at 3.0 T than at 1.5 T. Of the total of 48 lesions that were identified in 19 of the 25 patients, 47 (98%) were diagnosed at 3.0 T and 36 (75%) were diagnosed at 1.5 T. In addition, the conspicuity of the lesions that were visible with both systems was significantly higher at 3.0 T (P < .001).

CONCLUSION

Although 3.0-T DW MR imaging generates greater image distortions, it yields increased SNR and CNR compared with DW MR imaging at 1.5 T. The increased CNR at 3.0 T translates into a significantly improved diagnostic confidence in the detection of focal apparent diffusion coefficient changes in the setting of subacute and acute ischemic stroke.

High-resolution MR imaging of the rat spinal cord in vivo in a wide-bore magnet at 17.6 Tesla

The objective was to demonstrate the feasibility and to evaluate the performance of high-resolution in vivo magnetic resonance (MR) imaging of the rat spinal cord in a 17.6-T vertical wide-bore magnet. A probehead consisting of a surface coil that offers enlarged sample volume suitable for rats up to a weight of 220 g was designed. ECG triggered and respiratory-gated gradient echo experiments were performed on a Bruker Avance 750 wide-bore spectrometer for high-resolution imaging. With T*2 values between 5 and 20 ms, good image contrast could be obtained using short echo times, which also minimizes motion artifacts. Anatomy of healthy spinal cords and pathomorphological changes in traumatically injured rat spinal cord in vivo could be visualized with microscopic detail. It was demonstrated that imaging of the rat spinal cord in vivo using a vertical wide-bore high-magnetic-field system is feasible. The potential to obtain high-resolution images in short scan times renders high-field imaging a powerful diagnostic tool.

Nuclear magnetic resonance microimaging of mouse spinal cord in vivo

The spinal cord is the site of traumatic injuries, the devastating consequences of which constitute a public health problem in our societies. So far, there is no efficient repair therapeutic approach, and this is mainly due to the great difficulty for elaborating predictive experimental models of this pathology. Up to now, most pathophysiological studies were based on postmortem evaluation of the quantity and extent of the lesions, and their comparison in-between human and rodent specimen. Recent progresses of magnetic resonance imaging provide new tools to examine in vivo rodent central nervous system, and eventually to monitor the progression of lesions. However, up to now, mice spinal cord has been inaccessible to such studies, due to specific physiological characteristics and to the small size of the cord. In this study, the first diffusion-weighted images depicting the mouse thoracic spinal cord in vivo are shown. Motion-related artifacts are significantly reduced by respiratory gating using a dedicated sensor. By changing the direction of diffusion-sensitizing gradients, different contrasts were obtained that are compared with ex vivo MRI and histological preparations. In addition, preliminary results obtained on pathological cords are presented.

Trace of diffusion tensor differentiates the Parkinson variant of multiple system atrophy and Parkinson's disease

We have recently shown that diffusion-weighted magnetic resonance (MR) imaging (DWI) discriminates patients with the Parkinson variant of multiple system atrophy (MSA-P) from those with Parkinson's disease (PD) by regional apparent diffusion coefficients (rADC) in the putamen. Because rADCs measured in one direction may underestimate diffusion-related pathologic processes, we investigated the diffusivity in different brain areas by trace of diffusion tensor (Trace(D)) in a new cohort of patients with MSA-P and PD. We studied 11 MSA-P, 17 PD patients, and 10 healthy volunteers matched for age and disease duration. Regional ADCs in three orthogonal directions and Trace(D) values were determined in selected brain regions including the basal ganglia, gray matter, white matter, substantia nigra, and pons. MSA-P patients had significantly higher putaminal and pallidal rTrace(D) values as well as rADCs in y- and z-direction than both PD patients and healthy volunteers. Moreover, putaminal Trace(D) discriminated completely MSA-P from both PD and healthy volunteers. The rADCs in the y- and z-direction provided a complete or near complete separation. In conclusion, our study confirms the results of previous studies of our group that patients with MSA-P show an increased putaminal diffusivity due to neuronal loss and gliosis. Because rADCs in one direction are dependent on the slice orientation relative to the directions of fiber tracts, Trace(D) imaging appears to be more accurate in the separation of MSA-P from PD.

Biexponential diffusion tensor analysis of human brain diffusion data

Several studies have shown that in tissues over an extended range of b-factors, the signal decay deviates significantly from the basic monoexponential model. The true nature of this departure has to date not been identified. For the current study, line scan diffusion images of brain suitable for biexponential diffusion tensor analysis were acquired in normal subjects on a clinical MR system. For each of six noncollinear directions, 32 images with b-factors ranging from 5 to 5000 s/mm2 were collected. Biexponential fits yielded parameter maps for a fast and a slow diffusion component. A subset of the diffusion data, consisting of the images obtained at the conventional range of b-factors between 5 and 972 s/mm2, was used for monoexponential diffusion tensor analysis. Fractional anisotropy (FA) of the fast-diffusion component and the monoexponential fit exhibited no significant difference. FA of the slow-diffusion biexponential component was significantly higher, particularly in areas of lower fiber density. The principal diffusion directions for the two biexponential components and the monoexponential solution were largely the same and in agreement with known fiber tracts. The second and third diffusion eigenvector directions also appeared to be aligned, but they exhibited significant deviations in localized areas.

Diffusion tensor imaging of in vivo and excised rat spinal cord at 7 T with an icosahedral encoding scheme

Regional values of fractional anisotropy (FA) and mean diffusivity (D(av)) of in vivo and excised rat spinal cords were measured using an iscosahedral encoding scheme that is based on 21 uniformly distributed and alternating gradient directions with an echo planar imaging (EPI) readout. Based on the water phantom studies, this scheme was shown to provide unbiased estimation of FA. The stability of the scanner during the acquisition of diffusion tensor imaging (DTI) data was evaluated. Repeated measurements of the FA values demonstrated excellent reproducibility, as assessed by the Bland-Altman analysis. These studies demonstrated a reduced anisotropy in excised samples relative to in vivo cords. Diffusion in the spinal cord gray matter was shown to be anisotropic. The FA value in the dorsal white matter (WM) was found to be higher relative to the ventral WM. Results from these studies should provide the necessary baseline data for serial in vivo DTI of injured spinal cord.

Mohr diagram interpretation of anisotropic diffusion indices in MRI

Descriptions of diffusion anisotropy in MR Diffusion Tensor Imaging are often based on scalar indices such as surface-to-volume ratio, volume ratio, fractional anisotropy and rotational anisotropy. Recently, Mohr diagram was introduced to visualize the anisotropic diffusion information, but in a graphical form. Even though both scalar indices and the Mohr diagram are derived from the elements of the diffusion tensor, so far, the relationships between these measures and the key aspects of the Mohr diagram have not been established. This paper demonstrates these relationships both qualitatively and quantitatively for two microscopically heterogeneous biologic tissues, kidney and spinal cord.