Spatially Resolved Two-Dimensional MR Spectroscopy in vivo

Acquisition of three-dimensional (3D) spatially localized, water-suppressed one-dimensional (1D) 1H MR spectra has been optimized in human tissues over the last two decades [1–6]. Introduction of automated MRS a decade ago has enabled a 1–2 min pre-scan to calibrate the flipangle using the transmitter power, to adjust the static field homogeneity (B0) using the linear gradient shim coils, and to optimize water suppression, which used to be an at least 15 min operation, facilitating the integration of clinical MRS protocols into routine MRI examinations [7]. In 1H MR spectroscopy of human brain, spectral resonances due to methyl, methylene, and methine protons of N-acetylaspartate (NAA), glutamate/glutamine (Glx), creatine (Cr), choline (Ch), myo-inositol (mI), GABA, aspartate (Asp), N-acetylaspartyl-glutamate (NAAG), and other metabolites have been identified in the region of 0–4.5 ppm [2–12]. One-dimensional MRS in vivo is hampered by the fact that it is difficult to resolve a multitude of peaks existing over a small spectral range of approximately 300 Hz at 1.5 T. Spectral overlap of macromolecules, GABA, and glutathione with the methyl resonances of NAA, Cr, NAAG, and methylene resonances of Glx has also been reported [8,14–15]. Attempts have been made to quantify Glx [16], glucose [17], and other metabolites with only minimal success due to the difficulty in extracting this information from a region with many overlapping resonances. Using spectral-editing techniques, it is possible to select a particular J -coupled metabolite [2,14,18– 20]. However, one major drawback of editing techniques is that only one metabolite is optimized at a time assuming that the multiplets of the J -coupled metabolites are well separated. Using the LC-Model post-processing algorithm, quantification of several metabolites in human brain in vivo has been reported recently using a basis-set of 10–20 different metabolite spectra in vitro [21–23]. Unlike the spectral editing techniques, which target one metabolite at a time, two-dimensional (2D) MRS can unambiguously resolve many overlapping peaks nonselectively in vitro as shown by Ernst and co-workers two decades ago using the vertical bore NMR spectrometers [24–25]. 2D-MR spectroscopy enables converting a crowded overlapping 1D MR spectrum to a betterresolved 2D spectrum through the addition of a spectral dimension. Instead of a standard 1D spectrum plotting intensity vs. a single-axis (i.e. chemical shift), 2D MR spectroscopy techniques produce a 2D spectrum plotting intensity vs. two axes, the dimensions of which depend on the specific 2D MR technique [25]. Better dispersion of several metabolite peaks and improved spectral assignment make the proposed technique more attractive. It seems natural to explore the clinical potentials of 2D MRS techniques of human tissues in vivo. There have been several attempts during the last 15 years in the implementation and evaluation of 2D NMR spectroscopy on in vivo MRI scanners and MR spectrometers [26–30]. The aim of this chapter is to discuss the recent progress on the implementation of selected localized 2D MRS sequences on the whole body 1.5 T and higher field MRI scanners. This includes a summary of 1D spectroscopy with and without chemical shift imaging (CSI), followed by singleand multi-voxel based 2D spectroscopy dealing with coherent interactions such as chemical shift and J -coupling. A brief overview of potential artefacts in 2D spectroscopy and simulation is also included.