Observation of coupled1H metabolite resonances at long TE (original) (raw)
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Magnetic Resonance in Medicine, 2010
Quantification of short-echo time proton magnetic resonance spectroscopy results in >18 metabolite concentrations (neurochemical profile). Their quantification accuracy depends on the assessment of the contribution of macromolecule (MM) resonances, previously experimentally achieved by exploiting the several fold difference in T 1. To minimize effects of heterogeneities in metabolites T 1 , the aim of the study was to assess MM signal contributions by combining inversion recovery (IR) and diffusion-weighted proton spectroscopy at high-magnetic field (14.1 T) and short echo time (58 msec) in the rat brain. IR combined with diffusion weighting experiments (with d/D 5 1.5/200 msec and b-value 5 11.8 msec/ mm 2) showed that the metabolite nulled spectrum (inversion time 5 740 msec) was affected by residuals attributed to creatine, inositol, taurine, choline, N-acetylaspartate as well as glutamine and glutamate. While the metabolite residuals were significantly attenuated by 50%, the MM signals were almost not affected (<8%). The combination of metabolite-nulled IR spectra with diffusion weighting allows a specific characterization of MM resonances with minimal metabolite signal contributions and is expected to lead to a more precise quantification of the neurochemical profile. Magn Reson Med 64:939-946, 2010. V
Journal of Magnetic Resonance, 2001
While most proton (1 H) spectra acquired in vivo utilize selective suppression of the solvent signal for more sensitive detection of signals from the dilute metabolites, recent reports have demonstrated the feasibility and advantages of collecting in vivo data without solvent attenuation. When these acquisitions are performed at short echo times, the presence of frequency modulations of the water resonance may become an obstacle to the identification and quantitation of metabolite resonances. The present report addresses the characteristics, origin, and elimination of these sidebands. Sideband amplitudes were measured as a function of delay time between gradient pulse and data collection, as a function of gradient pulse amplitude, and as a function of spatial location of the sample for each of the three orthogonal gradient sets. Acoustic acquisitions were performed to demonstrate the correlation between mechanical vibration resonances and the frequencies of MR sidebands. A mathematical framework is developed and compared with the experimental results. This derivation is based on the theory that these frequency modulations are induced by magnetic field fluctuations generated by the transient oscillations of gradient coils.
Toward quantitative short-echo-time in vivo proton MR spectroscopy without water suppression
Magnetic Resonance in Medicine, 2006
A methodological development for quantitative short-echotime (TE) in vivo proton MR spectroscopy (MRS) without water suppression (WS) is described that integrates experimental and software approaches. Experimental approaches were used to eliminate frequency modulation sidebands and first-order phase errors. The dominant water signal was modeled and extracted by the matrix pencil method (MPM) and was used as an internal reference for absolute metabolite quantification. Spectral fitting was performed by combining the baseline characterization by a wavelet transform (WT)-based technique and time-domain (TD) parametric spectral analysis using full prior knowledge of the metabolite model spectra. The model spectra were obtained by spectral simulation instead of in vitro measurements. The performance of the methodology was evaluated by Monte Carlo (MC) studies, phantom measurements, and in vivo measurements on rat brains. More than 10 metabolites were quantified from spectra measured at TE ؍ 20 ms on a 4.7 T system.
Improvement of resolution for brain coupled metabolites by optimized 1H MRS at 7 T
NMR in Biomedicine, 2010
Resolution enhancement for glutamate (Glu), glutamine (Gln) and glutathione (GSH) in the human brain by TE-optimized point-resolved spectroscopy (PRESS) at 7 T is reported. Sub-TE dependences of the multiplets of Glu, Gln, GSH, g-aminobutyric acid (GABA) and N-acetylaspartate (NAA) at 2.2-2.6 ppm were investigated with density matrix simulations, incorporating three-dimensional volume localization. The numerical simulations indicated that the C4-proton multiplets can be completely separated with (TE 1 , TE 2) ¼ (37, 63) ms, as a result of a narrowing of the multiplets and suppression of the NAA 2.5 ppm signal. Phantom experiments reproduced the signal yield and lineshape from simulations within experimental errors. In vivo tests of optimized PRESS were conducted on the prefrontal cortex of six healthy volunteers. In spectral fitting by LCModel, Cramé r-Rao lower bounds (CRLBs) of Glu, Gln and GSH were 2 W 1, 5 W 1 and 6 W 2 (mean W SD), respectively. To evaluate the performance of the optimized PRESS method under identical experimental conditions, stimulated-echo spectra were acquired with (TE, TM) ¼ (14, 37) and (74, 68) ms. The CRLB of Glu was similar between PRESS and short-TE stimulated-echo acquisition mode (STEAM), but the CRLBs of Gln and GSH were lower in PRESS than in both STEAM acquisitions.
NMR in Biomedicine, 1999
Short echo 1 H in-vivo brain MR spectra are difficult to quantify for several reasons: low signal to noise ratio, the severe overlap of spectral lines, the presence of macromolecule resonances beneath the resonances of interest, and the effect of resonances adjacent to the spectral region of interest (SRI). This paper outlines several different quantification strategies and the effect of each on the precision of in-vivo metabolite measurements. In-vivo spectra were quantified with no operator interaction using a template of prior knowledge determined by mathematically modeling separate in-vitro metabolite spectra. Metabolite level estimates and associated precision were compared before and after the inclusion of macromolecule resonances as part of the prior knowledge, and following two different methods of handling resonances adjacent to the SRI. The effects of rectangular and exponential filters were also investigated. All methods were tested using repeated in-vivo spectra from one individual acquired at 1.5 T using stimulated echo acquisition mode (STEAM, TE = 20 ms) localization. The results showed that the inclusion of macromolecules in the prior knowledge was necessary to obtain metabolite levels consistent with the literature, while the fitting of resonances adjacent to the SRI concurrent with modeled metabolites optimized the precision of metabolite estimates. Metabolite levels and precision were also affected by rectangular and exponential filtering, suggesting caution must be taken when such filters are used.
Metabolite‐specific NMR spectroscopy in vivo
NMR in Biomedicine, 1997
An outline is presented of metabolite-specific in vivo NMR spectroscopy (particularly in brain). It reviews from a physical spectroscopist's perspective, the need for and the methods of observation of, individual metabolite resonances. © Abbreviations used: AX, Two single spins weakly coupled; AB, Two single spins strongly coupled; AX 3 , A single spin weakly coupled to a group of three spins; A 2 M 2 X 2 , Three weakly coupled spin pairs; AMNPQ, A single spin weakly coupled to two internally strongly coupled pairs of spins, the coupling between which is also strong; B 0 , Static magnetic field . A limited region (2 ppm to 3 ppm) of the 300 MHz proton spectrum from an acid extract of cat brain, reproduced by kind permission of Dr C. C. Hanstock.
NMR in Biomedicine, 2013
We present a method for the robust and accurate estimation of brain metabolite transverse relaxation times (T 2 ) from multiple spin-echo data acquired with a single-shot Carr-Purcell-Meiboom-Gill (CPMG) spectroscopic sequence. Each acquired echo consists of a small number of complex time-domain data points. The amplitudes of the spectral components in each echo are calculated by solving a set of linear equations in which previously estimated frequencies and linewidths serve as prior information. These priors are obtained from a short MRS experiment in which a large number of time-domain data points are acquired, and are subsequently estimated using linear prediction with singular value decomposition (LPSVD) processing. We show that this process can be used to accurately and rapidly measure the T 2 values for the main singlet resonances in single-volume MRS measurements in the brain. The proposed method can be generalized to any set of MRS experiments comprising repeated measurements of amplitude changes, e.g. as a function of an experimental parameter, such as TE, inversion time or diffusion weighting.
Proton magnetic resonance spectroscopy
2002
Although conventional MRI shows great sensitivity in detecting MS lesions, it is unable to detect dysfunction of neurons and their axonal processes directly. On the other hand, as recently reviewed by Arnold et al. [15], the various approaches to in vivo proton magnetic resonance spectroscopy (IH-MRS) can provide specific information regarding neuronal and axonal integrity. These approaches include: (1) single-voxel1H-MRS studies (in which proton spectra are acquired from a single volume) and (2) lH-MRS imaging studies PH-MRSI) in which proton spectra are obtained from multiple volume elements (i.e., voxels) at the same time]. Metabolites Measured Although fundamentally similar to conventional MRI (which is based on the mapping of protons associated with water), IH-MRS is different in that it records signals from other metabolites that are present in brain tissue but that, generally, can only be measured when the signal from water is suppressed. Importantly, this metabolic information can provide chemical and pathological specificity that is not available from conventional MRI [16]. As shown in Fig. 1, the water-suppressed, localized IH-MRS spectrum of the normal human brain reveals three major resonance peaks, the locations of which are expressed as the difference in parts per million (ppm) between the resonance frequency of the compound of interest and that of a standard (tetramethylsilane). These peaks are commonly ascribed to the following metabolites: (1) tetramethyl amines (Cho), which resonate at 3.2 ppm and are mostly choline-containing phospholipids that participate in membrane synthesis and degradation; (2) creatine and phosphocreatine (Cr), which resonate at 3.0 ppm and play an important role in energy metabolism; and (3} N-acetyl groups (NA), which resonate at 2.0 ppm and are comprised primarily of the neuronally-localized compound N-acetylaspartate (NAA). Methods of Quantification Whereas precise absolute quantification of these resonance intensities is more complicated in vivo than it is with in vitro IH-MRS, various methods have been developed to provide semiabsolute quantification of lH-MRS and lH-MRSI data acquired in vivo. These include (1) the use of an external reference (e.g., a phantom) with known metabolite concentrations-the most widely-used approach being the LCModel method, which considers the lH-MR spectra arising from tissues acquired in vivo as a linear combination of spectra arising from known metabolite solutions acquired in vitro [19]; and (2) the use of an internal reference to correct for various external inhomogeneities that affect metabolite resonance intensities-the most widely used reference being the water signal arising
Proton echo-planar spectroscopic imaging ofJ-coupled resonances in human brain at 3 and 4 Tesla
Magnetic Resonance in Medicine, 2007
In this multicenter study, 2D spatial mapping of J-coupled resonances at 3T and 4T was performed using short-TE (15 ms) proton echo-planar spectroscopic imaging (PEPSI). Water-suppressed (WS) data were acquired in 8.5 min with 1-cm 3 spatial resolution from a supraventricular axial slice. Optimized outer volume suppression (OVS) enabled mapping in close proximity to peripheral scalp regions. Constrained spectral fitting in reference to a non-WS (NWS) scan was performed with LCModel using correction for relaxation attenuation and partial-volume effects. The concentrations of total choline (tCho), creatine ؉ phosphocreatine (Cr؉PCr), glutamate (Glu), glutamate ؉ glutamine (Glu؉Gln), myo-inositol (Ins), NAA, NAA؉NAAG, and two macromolecular resonances at 0.9 and 2.0 ppm were mapped with mean Cramer-Rao lower bounds (CRLBs) between 6% and 18% and ϳ150-cm 3 sensitive volumes. Aspartate, GABA, glutamine (Gln), glutathione (GSH), phosphoethanolamine (PE), and macromolecules (MMs) at 1.2 ppm were also mapped, although with larger mean CRLBs between 30% and 44%. The CRLBs at 4T were 19% lower on average as compared to 3T, consistent with a higher signal-to-noise ratio (SNR) and increased spectral resolution. Metabolite concentrations were in the ranges reported in previous studies. Glu concentration was significantly higher in gray matter GM) compared to white matter (WM), as anticipated. The short acquisition time makes this methodology suitable for clinical studies. Magn Reson Med 58:236 -244, 2007.
Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine, 1995
The potential of two-dimensional (2D)-J NMR for in vivo proton MRS is examined. Single voxel measurements on the rat brain were performed at 4.7 T using point-resolved spectrocopy localization with a voxel size of 64 microliter and total measuring times of 10-15 min. It is shown that a series of measurements with only 16 or fewer different echo times (TE) enables good signal localization in the f1 axis corresponding to the coupling patterns. For data evaluation, the 2D-J NMR spectrum as well as cross-sections at given f1 values and projections onto the f2 axis are used. A comparison between cross-section spectra taken at different f1 values may help to solve problems of peak assignment. The projection of the 2D magnitude spectrum onto the f2 axis corresponds to a homonuclear decoupled 1D proton spectrum. Because the T2 relaxation times of several coupled resonances (e.g., myo-inositol and glutamate) are rather long, only minor losses in the quality of the projection spectra occur if...