Diffusion Tensor Spectroscopic Imaging of Rat Brains

Introduction Diffusion-weighted spectroscopy and diffusion-weighted spectroscopic imaging (DWSI) are expected to provide useful information about tissue microstructures and functions [1]. The diffusion tensor of metabolites is expected to provide more specific microstructures of such neurons; however, only a few studies using diffusion tensor spectroscopy (DTS) [2,3] and no studies using diffusion tensor spectroscopic imaging (DTSI) have been done. The reason that DTSI has not yet been used is mostly that an accurate DWSI technique that overcomes the issues of long measurement time, low signal-to-noise ratio, and large motion artifacts has not yet been developed. To overcome these issues, we developed diffusion-weighted echo-planar spectroscopic imaging with a pair of bipolar diffusion gradients (DW-EPSI with BPGs) and showed its effectiveness in reducing motion artifacts in normal rat brains [4]. In the present study, we developed DTSI using DW-EPSI with BPGs to obtain diffusion tensor (DT) images of N-acetylaspartate (NAA) in rat brains. To the best of our knowledge, the DT images of NAA were measured for the first time. The measured DT images of NAA and DT images of water, hereafter, DTINAA and DTIwater, were analyzed by using a tensor correlation coefficient (TCC), which was defined to estimate the similarity of two tensors, and analyzed by using the difference in fractional anisotropy (FA) of two tensors. The analysis result showed that DT of NAA is similar to DT of water, and that DT of NAA has higher FA than DT of water in most regions except near the base, corpus callosum (CC), and cortex of the brain. Further investigation, from the view of biophysical meaning, is needed; however, this result demonstrates DTSI may become a useful tool for investigating microstructures of nerves.