In vivo diffusion tensor imaging of the rat spinal cord at 9.4T

PURPOSE: To determine differences in diffusion measurements in white matter (WM) and gray matter (GM) regions of the rat cervical, thoracic, and cauda equina spinal cord using in vivo diffusion tensor imaging (DTI) with a 9.4T MR scanner. MATERIALS AND METHODS: DTI was performed on seven rats in three slices at the cervical, thoracic, and cauda equina regions of the spinal cord using a 9.4T magnet. Axial diffusion weighted images (DWIs) were collected at a b-value of 1000 seconds/mm(2) in six directions. Regions of interest were identified via T2-weighted images for the lateral, dorsal, and ventral funiculi, along with GM regions. RESULTS: Analysis of variance (ANOVA) results indicated significant differences between every WM funiculus compared to GM for longitudinal apparent diffusion coefficient (lADC), transverse apparent diffusion coefficient (tADC), fractional anisotropy (FA), measured longitudinal anisotropy (MA1), and anisotropy index (AI). A significant difference in mean diffusivity (MD) between regions of the spinal cord was not found. Diffusion measurements were significantly different at each spinal level. In general, GM regions were significantly different than WM regions; however, there were few significant differences between individual WM regions. CONCLUSION: In vivo DTI of the rat spinal cord at 9.4T appears sensitive to the architecture of neural structures in the rat spinal cord and may be a useful tool in studying trauma and pathologies in the spinal cord.     

Gray and white matter delineation in the human spinal cord using diffusion tensor imaging and fuzzy logic

RATIONALE AND OBJECTIVES: Diffusion tensor imaging (DTI) has been used extensively in determining morphology and connectivity of the brain; however, similar analysis in the spinal cord has proven difficult. The objective of this study was to improve the delineation of gray and white matter in the spinal cord by applying signal processing techniques to the eigenvalues of the diffusion tensor. Our approach involved creating anisotropy indices based on the difference between eigenvalues and mean diffusivity then using a fuzzy inference system (FIS) to delineate between gray and white matter in the human cervical spinal cord. MATERIALS AND METHODS: DTI was performed on the cervical spinal cord in five neurologically intact subjects. Distributions were extracted for regions of gray and white matter through the use of a digitized histologic template. Fuzzy membership functions were created based on these distributions. Detectability index and receiver operating characteristic (ROC) analysis was performed on traditional DTI indices and FIS classified regions. RESULTS: A significantly higher contrast between gray and white matter was observed using fuzzy classification compared with traditionally used DTI indices based on the detectability index (P < .001) and trends in the ROC analysis. Reconstructed images from the FIS qualitatively showed a better anatomical representation of the spinal cord compared with traditionally used DTI indices. CONCLUSIONS: Diffusion tensor imaging using an FIS for tissue classification provides high contrast between spinal gray and white matter compared with traditional DTI indices and may provide a noninvasive technique to quantify the integrity and morphology of the human spinal cord following injury.     

Water diffusion anisotropy in white and gray matter of the human spinal cord

PURPOSE: To develop a reliable technique for diffusion imaging of the human spinal cord at 1.5 Tesla and to assess potential differences in diffusion anisotropy in cross-sectional images. MATERIALS AND METHODS: A single-shot echo-planar imaging sequence with double spin-echo diffusion preparation was optimized regarding cerebrospinal fluid artifacts, effective resolution, and contrast-to-noise ratios. Eleven healthy volunteers participated in the study for quantitative characterization of diffusion anisotropy in white matter (WM) and gray matter (GM) by means of two diffusion encoding schemes: octahedral-six-directions for fractional anisotropy (FA) evaluation and orthogonal-three-directions for anisotropy index (AI) calculation. RESULTS: Pulse-trigger gated sequences with optimal matrix size (read x phase = 64 x 32) and b-value (700 s/mm(2)) allowed the acquisition of high-resolved images (voxel size = 0.9 x 0.9 x 5.0 mm(3)). The GM butterfly shape was recognizable in both AI and FA maps. Both encoding schemes yielded high diffusion anisotropy in dorsal WM (FA = 0.79 +/- 0.07; AI = 0.39 +/- 0.04). Lateral WM showed slightly lower anisotropy (FA = 0.69 +/- 0.08; AI = 0.35 +/- 0.03) than dorsal WM. Clearly smaller anisotropy was found in regions containing GM (FA = 0.45 +/- 0.06; AI = 0.21 +/- 0.05). CONCLUSION: Diffusion anisotropy data of the spinal cord can be obtained in a clinical setting. Its application seems promising for the assessment of neurological disorders.     

Ex vivo diffusion tensor imaging and quantitative tractography of the rat spinal cord during long-term recovery from moderate spinal contusion

Purpose

To characterize DTI metric changes throughout the length of the entire spinal cord from the acute through chronic stages of spinal cord injury (SCI).

Materials and Methods

Ex vivo DTI was performed at 9.4 Tesla to examine changes in water diffusion throughout the entire spinal cord (7‐cm) up to 25 weeks after injury in a rat model of contusive SCI. Animals were grouped according to recovery times after injury (2, 5, 15, 20, or 25 weeks), and various DTI metrics were evaluated including transverse and longitudinal apparent diffusion coefficient (tADC and lADC), mean diffusivity (MD), and fractional anisotropy (FA).

Results

An overall decrease in lADC throughout the cord and decreases in MD remote from the lesion site were observed, along with an increase in tADC within fiber tracts throughout the recovery period. These trends were statistically significant at P < 0.05 and were found in both white and gray matter regions. tADC and lADC distributions in fiber bundles extracted using DTI tractography were well fit by an exponential model (R = 0.998) with time constants of 4.6 and 3.3 days, respectively.

Conclusion

Results from the current study support the hypothesis that the spinal cord undergoes continual changes during recovery from SCI. J. Magn. Reson. Imaging 2008;28:1068–1079. © 2008 Wiley‐Liss, Inc.     

In vivo waveguide elastography of white matter tracts in the human brain

White matter is composed primarily of myelinated axons which form fibrous, organized structures and can act as waveguides for the anisotropic propagation of sound. The evaluation of their elastic properties requires both knowledge of the orientation of these waveguides in space, as well as knowledge of the waves propagating along and through them. Here, we present waveguide elastography for the evaluation of the elastic properties of white matter tracts in the human brain, in vivo, using a fusion of diffusion tensor imaging, magnetic resonance elastography, spatial‐spectral filtering, a Helmholtz decomposition, and anisotropic inversions, and apply this method to evaluate the material parameters of the corticospinal tracts of five healthy human volunteers. We begin with an Orthotropic inversion model and demonstrate that redundancies in the solution for the nine elastic coefficients indicate that the corticospinal tracts can be approximated by a Hexagonal model (transverse isotropy) comprised of five elastic coefficients representative of a medium with fibers aligned parallel to a central axis, and provides longitudinal and transverse wave velocities on the order of 5.7 m/s and 2.1 m/s, respectively. This method is intended as a new modality to assess white matter structure and health by means of the evaluation of the anisotropic elasticity tensor of nerve fibers. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Effects of cord motion on diffusion imaging of the spinal cord.

Measurement of diffusion and its dependence on direction has become an important tool for clinical and research studies of the brain. Diffusion imaging of the spinal cord may likewise prove useful as an indicator of tissue damage and axonal integrity; however, it is more challenging to perform diffusion imaging in the cord than in the brain. Here we report a study of the effects of motion on single-shot fast spin echo (FSE) diffusion tensor imaging (DTI) of the spinal cord. Diffusion imaging was performed at four different times in the cardiac cycle both without and with velocity compensation of the diffusion gradients. Uncompensated diffusion images demonstrated substantial signal loss artifacts in the cord that were strongly dependent on the delay after the pulse-oximeter trigger. Quantitative diffusion analysis was also strongly affected by this motion artifact. The use of flow-compensated gradients helped to restore normal signal in the cord, especially at particular trigger delays. Theoretical arguments suggest that improved spatial resolution may help eliminate this signal loss. Even with higher spatial resolution, motion-related signal attenuation may still occur in diffusion imaging of pathologies that alter the motion of the cord. However, this same cord motion may contain diagnostically valuable information when probed using appropriate diffusion imaging approaches.     

脊髓肿瘤的扩散张量成像研究

目的：探讨磁共振扩散张量成像(DTI)在评估脊髓肿瘤患者神经功能方面的应用价值。方法对22例脊髓肿瘤患者和22例健康志愿者进行脊髓常规磁共振成像(MRI)和 DTI 扫描,分析各向异性分数(FA 值)、表观扩散系数(ADC)值和纤维束比值(FTR)的组间差异,及其与 McCormik 评分的相关性。结果2组病灶层的 FA 值(t =-5.587,P =0.000)、ADC 值(t =7.232, P =0.000)有显著差异;病灶上层的 FA 值(t=-0.438,P =0.666)、ADC 值(t =0.303,P =0.765)无显著差异;病灶下层的 FA 值(t=-1.777,P =0.090)无显著差异,ADC 值(t=2.113,P =0.047)有显著差异;2组的 FTR 值(t =-7.902,P =0.000)有显著差异。髓外肿瘤与髓内肿瘤的 FA 值、ADC 值和 FTR 值均有显著差异(P <0.05)。脊髓肿瘤组病灶层 FA 值、ADC 值、FTR 值与McCormik 分级均存在直线相关关系。结论DTI 能够直观地反映脊髓肿瘤患者的脊髓神经功能和损伤情况。     

In vivo quantitative microimaging of rat spinal cord at 7T.

In vivo T(2), ADC, and MT properties of the GM and WM of the rat spinal cord were measured at 7T in the cervical region. The GM T(2), T(2GM) = 43.2 +/- 1.0 msec is significantly reduced compared to the WM T(2), T(2WM) = 57.0 +/- 1.6 msec. Diffusion is anisotropic for both GM and WM, with a larger ADC value along the cord axis (ADC(GM//) = 1.05 +/- 0.09 10(-9) m(2)sec(-1) and ADC(WM//) = 1.85 +/- 0.18 10(-9) m(2)sec(-1)) than perpendicular to this plane (ADC(GM)( perpendicular) approximately 0.50 * 10(-9) m(2)sec(-1) and ADC(WM)( perpendicular) approximately 0.18 * 10(-9) m(2)sec(-1)). The MT properties do not significantly differ between the WM and the GM, but allow one to distinguish the thin CSF layer from the WM. DWI with the sensitizing gradient perpendicular to the cord axis leads to the best contrast between GM and WM in the cervical region.     

Diffusion tensor tractography demonstration of partially injured spinal cord tracts in a patient with posttraumatic Brown Sequard syndrome

The authors report the utility of diffusion tensor tractography in demonstrating the partially severed spinal cord tracts on one side with normal, intact, distally traceable tracts on the opposite side in a patient with posttraumatic Brown Sequard syndrome. A 30-year-old man presented with typical clinical features of a hemisection injury of the thoracic spinal cord, 2 months after he had sustained a back stab injury. Routine MRI showed T2 hyperintense zones in the thoracic spinal cord at the level of T5. We did axial single shot echo planar diffusion tensor imaging with a 1.5 Tesla MR machine. Tractography effectively depicted the injured spinal cord tracts on the left side with normal intact tracts on the right side, which could be traced distally. The fractional anisotropy and apparent diffusion coefficient values showed significant changes at the level of injury. Tractographic demonstration of human spinal cord injury is reported for the first time.     

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.     

A diffusion tensor imaging group study of the spinal cord in multiple sclerosis patients with and without T2 spinal cord lesions

Purpose

To examine the T₂‐normal appearing spinal cord of patients with multiple sclerosis (MS) using diffusion tensor imaging.

Materials and Methods

Diffusion tensor images of the spinal cord were acquired from 21 healthy subjects, 11 MS patients with spinal cord lesions, and 10 MS patients without spinal cord lesions on the T₂‐weighted MR images. Different diffusion measures were evaluated using both a region of interest (ROI) ‐based and a diffusion tensor tractography‐based segmentation approach.

Results

It was observed that the FA, the transverse diffusivity λ_?, and the ratio of the longitudinal and transverse diffusivities (λ_∥/λ _?) were significantly lower in the spinal cord of MS patients with spinal cord lesions compared with the control subjects using both the ROI method (P = 0.014, P = 0.028, and P = 0.039, respectively) and the tractography‐based approach (P = 0.006, P = 0.037, and P = 0.012, respectively). For both image analysis methods, the FA and the λ _∥/λ _? values were significantly different between the control group and the MS patient group without T₂ spinal cord lesions (P = 0.013).

Conclusion

Our results suggest that the spinal cord may still be affected by MS, even when lesions are not detected on a conventional MR scan. In addition, we demonstrated that diffusion tensor tractography is a robust tool to analyze the spinal cord of MS patients. J. Magn. Reson. Imaging 2009;30:25–34. © 2009 Wiley‐Liss, Inc.

     

快速 DTI 在急性颈髓损伤的临床应用

目的：研究扩散张量成像(DTI)在急性颈髓损伤(CSCI)的成像特点,评估其临床应用价值。方法本组8例 CSCI 患者(发病72 h 内)均采用3.0T 磁共振仪进行快速颈髓 DTI 扫描,并在工作站进行扩散张量纤维束成像(DTT)。同时,在工作站分别测量并计算颈髓病变区及上下相对正常区的各向异性(FA)值和表观扩散系数(ADC)值,之后进行统计学组间配对 t 检验分析(SPSS 13.0)。结果急性 CSCI 以 C5~C6节段(占4/8)和 C4~C5节段(占3/8)多见,且快速 DTI 均获得了较好的图像质量。急性 CSCI 时病变区 FA 值和 ADC 值均明显低于相对正常区域数值(P <0.01),相应在 FA 图和 ADC 图均表现为低信号,而上下相对正常区 FA 值和 ADC 值间无明显区别;同时,DTT 有利于显示刀刺伤导致的颈髓纤维束断裂,颈髓闭合伤则主要表现为脊髓纤维束紊乱等。结论3.0T 快速 DTI 序列可以在2 min 扫描时间内获得临床较为满意的诊断图像,并通过 FA 值和 ADC 值更敏感地反映急性 CSCI 后髓鞘损伤导致的 FA 改变及细胞毒性水肿和血管源性水肿导致的水分子扩散的变化。     

Automatic segmentation of rodent spinal cord diffusion MR images

MRI, is a key tool for noninvasive spinal cord lesion analysis; however, accurate, quantitative methods for this analysis are lacking. A new, multistep, multidimensional approach, utilizing the classification expectation maximization algorithm, is proposed for MRI segmentation of spinal cord tissues. Diffusion tensor imaging is used to generate multiple images of each spinal slice, with different diffusion direction weightings. The maximum likelihood tissue classifications are then jointly estimated to produce a binary classification image, corresponding to voxels containing either spinal cord or background. Edge detection is employed to find a nonparametric curve encapsulating the entire spinal cord. The algorithm is evaluated using data from in vivo diffusion tensor imaging of control and injured mouse spinal cords. The algorithm is shown to remain accurate for whole spinal cord, white matter, and hemorrhage segmentation in the presence of significant injury. The results of the method are shown to be at least on par with expert manual segmentation. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

In vivo magnetic resonance microscopy of rat spinal cord at 7 T using implantable RF coils.

An inductively coupled implanted coil was designed for high-resolution magnetic resonance (MR) studies of rat spinal cord (SC) in vivo at 7 T. The practical issues involved in implementation of the coil at high fields are discussed, and the adjustment of various parameters for optimizing coil performance are described. The utility of the coil was demonstrated with anatomical, magnetization transfer, diffusion tensor imaging, and proton MR spectroscopy (MRS).     

In vivo DTI assessment of hepatic ischemia reperfusion injury in an experimental rat model

Purpose

To investigate hepatic ischemia reperfusion injury (IRI) using diffusion tensor imaging (DTI).

Materials and Methods

Ten Sprague‐Dawley rats were scanned at 7 Tesla (T) with DTI using b‐value of 1000 s/mm² and 6 gradient directions before, 2 h, and 1 day after 30‐min total hepatic IRI. Apparent diffusion coefficient or mean diffusivity (MD), directional diffusivities and fractional anisotropy (FA) were measured. Seven of the animals were also examined with spin‐echo echo‐planar diffusion‐weighted imaging (DWI) with seven b‐values up to 2000 s/mm² to estimate the true diffusion coefficient (D), blood pseudodiffusion coefficient (D*), and perfusion fraction (f) using a bi‐compartmental model.

Results

MD 2 h after IRI (0.77 ± 0.07 × 10^?3 mm²/s) was significantly lower (P < 0.01) than that before (1.03 ± 0.07 × 10^?3 mm²/s) and 1 day after IRI (1.01 ± 0.05 × 10^?3 mm²/s). Meanwhile, FA 2 h after IRI (0.33 ± 0.03) was significantly higher (P < 0.01) than that before (0.21 ± 0.02) and 1 day after IRI (0.20 ± 0.02). The bi‐compartmental model analysis revealed the transient decrease in D, D* and f 2 h after IRI. Liver histology showed the multifocal cell swelling 3 h after IRI and widespread cell necrosis/apoptosis 1 day after IRI. Sinusoidal narrowing and congestion of erythrocytes were also observed 3 h and 1 day after IRI.

Conclusion

DTI can characterize hepatic IRI by detecting the transient change in both MD and FA. J. Magn. Reson. Imaging 2009;30:890–895. © 2009 Wiley‐Liss, Inc.

     

脑卒中后白质纤维重塑的DTI研究进展

脑卒中是发病率及致残率均较高的急性脑血管疾病,亚急性期及慢性期的白质纤维重塑可促进脑卒中后的神经功能恢复,降低致残率。扩散张量成像(DTI)是一种可无创、动态观察卒中后脑内白质纤维微观结构改变的影像学手段。就DTI用于脑卒中后脑内不同结构水平的白质纤维重塑方面的研究进展及其局限性和方法学发展进行综述。     

Noninvasive diffusion tensor imaging of evolving white matter pathology in a mouse model of acute spinal cord injury.

We examined in vivo measurements of directional diffusivity derived from diffusion tensor imaging (DTI) to study the evolution of ventrolateral white matter (VWM) changes following contusive spinal cord injury (SCI) in C57BL/6 mice at 1, 3, 7, and 14 days postinjury. Relative anisotropy maps provided excellent gray matter (GM)/white matter (WM) contrast for characterization of evolving WM injury at all time points. Longitudinal DTI measurements clearly demonstrated rostral-caudal injury asymmetry. Axial diffusivity provided a sensitive, noninvasive measure of axonal integrity within the injury epicenter and at remote levels. Quantitative measurements of axial and radial diffusivities in VWM showed a trend of acute primary axonal injury followed by delayed, subacute myelin damage at the impact site, with good histological correlation.