Response of metabolites with coupled spins to the STEAM sequence

Abstract

This article demonstrates that a numerical solution of the full quantum mechanical equations for all metabolites with coupled spins is an efficient and accurate means, first, of predicting the optimum STEAM sequence design for quantifying any target metabolite in brain, and, second, for providing the basis lineshapes and yields of these metabolites to facilitate their accurate quantification. Using as illustrations the weakly coupled AX 3 system of lactate, the ABX aspartyl group of N-acetylaspartate, which has only two strongly coupled spins, and the much larger strongly coupled AMNPQ glutamyl group of glutamate, the numerical solutions for the response to STEAM highlight the principal source of response variability, namely, the evolution of and transfer between zero quantum terms during the mixing time, TM. These highlights include the rapid oscillations of zero quantum terms due to the chemical shift difference of the coupled spins, the proliferation of oscillating zero order terms due to strong coupling, and the serendipitous smoothing of the response as the number of strongly coupled spins increases. The numerical solutions also demonstrate that the design of the selective 90°pulses is a far less critical factor in determining the response than was the case for the selective 180°pulses Metabolite detection and quantification in vivo with proton spectroscopic methods is most commonly carried out using the spatial localization provided either by the single voxel PRESS (point resolved spectroscopy) (1,2) or STEAM (stimulated echo acquisition mode) (3,4) sequences. When both sequences are available, the sequence of choice is often determined by the metabolite target. For example, the observation of uncoupled singlet resonances in brain, namely, N-acetylaspartate (NAA) at 2.02 ppm, creatine (Cr) at 3.05 ppm, and choline (Cho) at 3.2 ppm, are routinely observed with a long-echo PRESS sequence. This stems from the factor of two advantages in uncoupled-spin signal yield over STEAM and the relatively long T 2 s of these resonances compared with either the macromolecular signals or the J-modulation envelopes of coupled spins. However, when the target metabolites contain coupledspin systems, short-echo STEAM spectroscopy has gained popularity in the clinical setting, due principally to its robustness at echo times from ϳ10 to ϳ50 ms. Nonetheless, the response of certain key, coupled-spin metabolites to the STEAM sequence, not to mention the background metabolite spectrum, exhibit significant variability. The purpose of this article is to demonstrate the efficacy of numerical calculations in the determination of the sources of that variability. The range of echo and mixing times considered (TE and TM, respectively) is that which is typically regarded as falling into the short-echo experimental range, namely, TE and TM less than ϳ50 ms.The short-echo-time limit is regarded as desirable for the observation of coupled spins because the longer echo-time modulations of the metabolite lineshapes lead, more often than not, to...

Simultaneous detection of resolved glutamate, glutamine, and γ-aminobutyric acid at 4T

“…In the schematic diagram of three spectral regions of a pseudo-triplet, a and c represent the outer-wings and b the central peak (for simplicity, only shown in the Glu spectrum). It has been found in the previous study [24] and in our own spectral simulation that the central peaks of the target resonances maintain relatively stable amplitudes while the outer-wings exhibit marked variation with TE and TM, and at some TE and TM parameters the amplitudes of these outer-wings are reduced significantly. We hypothesized that it is possible to find the optimized TE and TM parameters with which the outer-wings of the two (Gln and Glu) or three (Gln, Glu, and GABA) target pseudo-triplets can be significantly suppressed simultaneously.…”

“…The first one arises from the J-evolution of strongly-coupled spins with echo time(s). Although J-modulation did not produce monotonical amplitude-damping of the central peaks [24], these peaks of the target resonances at TE = 82 ms/TM = 48 ms, as shown in Fig. 3b, did decrease with J-evolution, compared with those at TE = 10 ms/TM = 10 ms in Fig.…”

“…Thompson and Allen have made a detailed study of the responses of weakly (such as lactate) and strongly coupled spin systems (such as the aspartyl group of NAA and Glu) to the STEAM sequence in a typical short-echo range, namely, TE and TM less than 50 ms [24]. Their study suggested that these metabolites exhibit significant variability in response to the STEAM sequence parameters.…”

“…As the spectral ranges of these triple-peak structures are quite stable [24], we defined the spectral ranges of [a] , [b], and [c] based on the simulated spectra at TE = 10 ms/TM = 10 ms, and the boundaries were chosen at the local minima in the simulated spectrum. The spectral ranges were 2.178-2.259 ppm, 2.259-2.327 ppm, and 2.327-2.408 ppm, respectively, for GABA; 2.247-2.327 ppm, 2.327-2.385 ppm, and 2.385-2.465 ppm, respectively, for Glu; and 2.350-2.419 ppm, 2.419-2.488 ppm, and 2.488-2.557 ppm, respectively, for Gln.…”

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.…”