Localized two‐dimensional shift correlated MR spectroscopy of human brain

Abstract

A two-dimensional (2D) chemical shift correlated MR spectroscopic (COSY) sequence integrated into a new volume localization technique (90°-180°-90°) is proposed for whole-body MR spectroscopy (MRS).Due to the recent improvements in the design of B 0 gradient and RF coils, 1 H MR spectra have been recorded in human brain with excellent water suppression using short TE, as short as 15 ms, and several cerebral metabolites have been identified (1-4). During the past decade alterations in several metabolites, namely, N-acetylaspartate (NAA), glutamate/glutamine (Glx), choline (Ch), creatine (Cr), myo-inositol (mI), and ␥-aminobutyrate (GABA) have been reported in different pathologic states involving the central nervous system (CNS) (5-10). Absolute quantitation of cerebral metabolites in vivo has also been reported for only a few metabolites, albeit with limited success (11-13). Due to severe overlap of these metabolites, an unambiguous assignment of J-coupled metabolite multiplets is severely hindered at 1.5 T field strength.One-dimensional (1D) MR spectral editing techniques to unravel the overlapping resonances rely on J-coupled proton metabolites that have well-separated multiplets. A technique based on subtraction methodology is very sensitive to motion artifacts leading to subtraction errors. An additional drawback is that only one metabolite can be identified at a time. Successful attempts in editing GABA and glutamate using whole-body MRI/MRS scanners have been presented by other researchers (13,14). Single-shotbased multiple-quantum filtered MR spectroscopic sequences have also been implemented on whole-body scanners, but a severe signal loss associated with various coherence transfer pathways made it less attractive to human applications (15)(16)(17).A localized version of a two-dimensional (2D) J-resolved MR spectroscopic (JPRESS) sequence using the PRESS sequence for volume localization was recently proposed (18 -20). Even though the JPRESS sequence retains 100% of the magnetization from a localized volume of interest (VOI), the strong coupling effect inherent at 1.5 T field strength resulted in a complex 2D cross-peak pattern for NAA, glutamate/glutamine, GABA, and other cerebral metabolites (19). Also, some of the 2D cross-peaks were heavily T 2 -weighted during the long incremental delays necessitated by the second dimension of the JPRESS spectrum. An oversampled J-resolved sequence has also been proposed recently (21).Compared to the 2D J-resolved spectra, a COSY spectrum produces a better dispersion of J-cross-peaks, although it requires a larger spectral window to be sampled during the evolution period (22). Different versions of the localized COSY sequence have been implemented by other researchers (23-33). McKinnon and Bosiger (23) proposed a conventional COSY sequence with hard RF pulses (90°-t 1 -90°) followed by three volume selective 180°RF pulses. Haase et al. (24) implemented a COSY combined with an outer volume suppressing sequence, namely, LOCUS. Many previous attempts to develop localiz...

GAVA: Spectral simulation for in vivo MRS applications

“…For in vivo measurements, MR spectroscopic data is complicated by the inherent overlap and complex shapes of the signals of interest. Methods to improve detection of specific spectral contributions include spectral editing [4,18,28,33,[46][47][48]; spectral fitting using constrained parametric model optimization and incorporating a priori metabolic information [6,7,23,24,31,35,40,45,50]; and use of multidimensional spectral acquisition methods [42][43][44]. In all of these areas of research, spectral simulation methods are becoming an increasingly important tool.…”

Localized two‐dimensional 1H magnetic resonance exchange spectroscopy: A preliminary evaluation in human muscle

“…However, Leifbritz et al hypothesized that water bound to a protein or located in its boundary transfers magnetization to bulk water via proton chemical exchange or cross relaxation to bulk water, which transfers magnetization to the amino group via chemical exchange and by dipolar relaxation to the methyl group (10). It was also concluded that both creatine and phosphocreatine contribute to the MT effect since it does not change after death in animal studies (11).Nonselective excitation/detection of several metabolites using spatially resolved two-dimensional (2D) MRS techniques has been successfully demonstrated recently on whole-body 1.5-and 3-T MRI scanners (12)(13)(14)(15). Coherent interactions such as chemical shift and J-coupling between protons of metabolites and lipids lead to different 2D diagonal and cross peaks showing the connectivities between protons of tissue metabolites.…”

“…Nonselective excitation/detection of several metabolites using spatially resolved two-dimensional (2D) MRS techniques has been successfully demonstrated recently on whole-body 1.5-and 3-T MRI scanners (12)(13)(14)(15). Coherent interactions such as chemical shift and J-coupling between protons of metabolites and lipids lead to different 2D diagonal and cross peaks showing the connectivities between protons of tissue metabolites.…”

Brain Metabolites and Cognitive Function among Older Depressed and Healthy Individuals Using 2D MR Spectroscopy

“…The 3 in coil was placed directly on the forehead of the subject and a 3 Â 3 Â 3 cm 3 voxel was positioned to the left side of the forehead, just above the eye to be closer to the left frontal white matter region. The 2D L-COSY sequence consisted of three slice-selective rf pulses (901, 1801, and 901) for the volume localization and the coherence transfer necessary for 2D-correlated spectroscopy (Thomas et al, 2003(Thomas et al, , 2001. A CHESS sequence was used for global water suppression prior to volume localization.…”

“…The PCh peak overlaps with glycerylphosphocholine, and the Pe peak overlaps with glycerophosphethanolamine. The assignments of these peaks were guided by the 2D L-COSY spectra of individual phantom solutions of several metabolites (Thomas et al, 2001) and a previous report on the 1D MRS chemical shifts (Govindaraju et al, 2000). …”