In vivo magnetic resonance imaging of spinal cord at 7T

In vivo diffusion tensor magnetic resonance imaging (DT-MRI) was implemented on a 7T magnetic resonance (MR) scanner to characterize the diffusion properties of normal and injured cords in rodents. An implanted RF coil was used for improving the signal to noise ratio. Complete diffusion tensor was determined by acquiring images with diffusion gradients applied along seven independent directions, each incremented in sixteen gradient steps. The diffusion gradient pulse width was chosen to be 12 ms, with maximum gradient amplitude of 200 mT/m resulting in a large b value of 6200 s/mm(2). The attenuation of diffusion-weighted images with b was characterized by a biexponential decay, suggesting the presence of two components of water diffusion: fast diffusion and slow diffusion components. This is the first time that demonstrated the biexponential nature of diffusion in vivo spinal cord. The images were analyzed using a nonlinear regression with constrained minimization for calculating the trace and fractional anisotropy, two rotationally invariant diffusion indices. These two indices are useful for evaluating the pathology and integrity of fiber tracts. Both trace and fractional anisotropy are significantly affected by injury, This suggests that DT-MRI provides unique information about fiber tracts in injured cord.