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

doi:10.1016/S0730-725X(02)00493-9

Magnetic Resonance Imaging

Volume 20, Issue 3, April 2002, Pages 243-247

https://doi.org/10.1016/S0730-725X(02)00493-9 Get rights and content

Abstract

In vivo diffusion tensor imaging of normal rat spinal cord was performed using a multi-segmented, blipped EPI sequence at 7 T field strength. At high diffusion weighting, the signal exhibited a non-monoexponential decay that was fitted to a biexponential function, associated with the fast and slow components of diffusion in the cord tissue, using a nonlinear regression analysis along with a constrained optimization procedure. From the measured tensors, the eigenvalues and the maps of invariant scalar measures (fractional anisotropy, relative anisotropy, volume ratio, and trace) were calculated and analyzed statistically. The results were combined to quantitatively characterize the anisotropic properties of the fast and slow diffusions in white- and gray matter of live spinal cords.

Introduction

Diffusion tensor imaging (DTI) has the potential to provide important information about the status of neuronal fiber tracts in injured spinal cord that is not evident on conventional MRI [1], [2]. Much of our knowledge about the spatial and temporal evolution of SCI and the evaluation of various treatments to reverse the deleterious pathobiological events, following the primary mechanical trauma, are based on experimental SCI, mostly on rodents. Therefore, in vivo MRI, including DTI, of spinal cord (SC) in rodents has recently attracted considerable interest [3]. Before evaluating the role of DTI in experimental SCI, however, it is essential to characterize the anisotropic water diffusion in normal cord tissue.

Diffusion tensor imaging of SC in live rodents is technically challenging because of the low signal-to-noise ratio (SNR) associated with the small cord size (2 to 3 mm in diameter), breathing related image artifacts, and small field-of-view requirements. Because of these reasons, hitherto most diffusion studies have been performed either on excised cords with very long acquisition times [4], [5], [6], [7], [8] or limited to just simple diffusion-weighted imaging, mainly for demonstrating the anisotropic nature of diffusion [9]. The altered natural biophysical environment of the tissue upon its excision and fixation could have profound effects on its diffusion properties. Quantitative differences from such effects were first reported by Fenyes and Narayana [10], [11], based on the DTI measurements at 2 T field strength under relatively weak diffusion weighting that resulted in a monoexponential signal decay. A number of recent studies, however, have shown that diffusion in CNS tissues, especially at high diffusion weighting, exhibits non-monoexponential behavior [12], [13], [14], [15], which can be approximated by a biexponential decay representing slow and fast diffusion components [16], [17]. To the best of our knowledge, in vivo DTI of SC in rodents at high diffusion weighting has not been reported so far. Therefore, in these studies, we performed in vivo DTI of normal rat spinal cord at 7 T using high b-values of up to 6200 s/mm² by employing implanted RF coils to improve the SNR and restrict the field-of-view, characterized the diffusion tensors, and quantified various invariant, scalar diffusion measures for the fast and slow components in both gray matter (GM) and white matter (WM).

Section snippets

Methods

These studies were performed on three male Sprague-Dawley rats weighing 300–350 g on a 7 T, 30 cm horizontal bore BioSpec scanner (Bruker, Billerica, MA), fitted with a 116 mm diameter gradient coil capable of generating a maximum gradient amplitude of 200 mT/m with less than 160 μs settling time. An RF coil was implanted over the spinal cord, centered at the T7 level, for improving the SNR. Details of the design, fabrication, and coupling of the RF coils usable at 7 T field strength and their

Results and discussion

The variations of signal intensity with diffusion weighting applied along the three orthogonal directions are shown on semi-log plots in Fig. 1 for pixels located in GM and WM regions of a normal SC. The dashed lines in these plots were calculated using a monoexponential function to fit to the data for b < 1000 s/mm², and can be seen as describing the experimental data reasonably well up to the b-values of 1600 s/mm². For higher b-values, however, the calculations based on this fit deviated

Acknowledgements

This work is supported in part by Grants from NIH (1 S10 RR14804 and NS30821) and Christopher Reeve Paralysis Foundation BA1-0001-2. The authors thank Dr. Shi-Jie Liu for his assistance with animal surgery.

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

Abstract

Introduction

Section snippets

Methods

Results and discussion

Acknowledgements

Biophsy J

Magn Reson Imag

Magn Reson Imag

Magn Reson Imag

Magnetic resonance of spinal cord injury

Multiple component diffusion tensor imaging in excised fixed CNS tissue

Apparent diffusion tensor measurements in myelin-deficient rat spinal cords

Magn Reson Med

A multiple echo pulse sequence for diffusion tensor imaging and its application in excised rat spinal cords

Magn Reson Med

Diffusion anisotropy MRI for quantitative assessment of recovery in injured rat spinal cord

Magn Reson Med

In vivo quantitative microimaging of rat spinal cord at 7T

Magn Reson Med

In vivo diffusion tensor imaging of rat spinal cord with echo planar imaging

Magn Reson Med

Water diffusion compartmentation and anisotropy at high b values in the human brain

Magn Reson Med