High-resolution diffusion-weighted MR of fresh and fixed cat spinal cords: evaluation of diffusion coefficients and anisotropy.

PURPOSE: To use high-resolution diffusion-weighted and calculated apparent diffusion coefficient (ADC) MR imaging to determine whether fixation and storage influence diffusion anisotropy in white matter tracts of cat spinal cord specimens.
METHODS: Four cat cord specimens were imaged using a diffusion-weighted spin-echo sequence. Diffusion encoding was applied in the section-select axis (parallel to white matter tracts) and in the read axis (perpendicular to white matter tracts). Five sets of axial diffusion-weighted images were acquired with b values ranging from 0 to 800 s/mm2 and used to obtain calculated ADC images and to determine diffusion coefficients in different regions of the white matter tracts.
RESULTS: After cord fixation, a decrease in T2 relaxation and spin density in the white matter caused the signal intensity to appear similar on diffusion-weighted images when the diffusion-probing gradient was applied along both the section-select and read axes. On the calculated ADC images, however, distinct differences in signal intensities were seen in the section-select and read axes.
CONCLUSION: Although there is little difference in signal intensity in the white matter tracts on diffusion-weighted images when diffusion encoding is applied in the section-select or read axis in the fixed specimens, calculated ADC images confirm that diffusion anisotropy is maintained. Therefore, calculated ADC images may be helpful in the evaluation of fixed spinal cord specimens.