Single-shot diffusion trace H-1 NMR spectroscopy

Ignoring diffusion anisotropy can severely hamper the quantitative determin ation of water and metabolite diffusion in complex tissues. The measurement of the trace of the diffusion tenser provides unambiguous and rotationally invariant ADC values, but usually requires three separate experiments. A s ingle-shot technique developed earlier, originally designed for diffusion t race MR imaging (Mori and van Zijl, Magn Reson Med 1995;33:41-52), was impr oved and adapted for diffusion trace MR spectroscopy. A double spin-echo pu lse sequence was incorporated with four pairs of bipolar gradients with spe cific predetermined relative signs in each of the three orthogonal directio ns. The combination of gradient directions leads to cancellation of all off -diagonal tenser elements while constructively adding the diagonal elements , Furthermore, the pulse scheme provides complete compensation for cross-te rms between static magnetic field gradients and the applied diffusion gradi ents, while simultaneously avoiding cross-terms with localization gradients . The sequence was tested at 4.7 T in vivo on rat brain for MRI and on rat skeletal muscle and brain for MRS. It is shown that the average ADC as dete rmined from the measurement of the ADCs in the three orthogonal directions is in close agreement with the ADC obtained along the trace of the diffusio n tenser in a single acquisition, for both water and metabolite diffusion. The large differences in water and metabolite diffusion coefficients as mea sured in the individual orthogonal directions illustrate the need for diffu sion trace measurements when accurate and rotationally invariant diffusion quantitation is required. The pulse scheme presented here may be applied fo r such purposes in MRS and MRI studies. (C) 2001 Wiley-Liss, Inc.