Yield enhancement of a double-quantum filter sequence designed for the edited detection of GABA

Evaluation of lactate detection using selective multiple quantum coherence in phantoms and brain tumours

Lactate is a product of glucose metabolism. In tumour tissues, which exhibit enhanced glycolytic metabolism, lactate signals may be elevated, making lactate a potential useful tumour biomarker. Methods of lactate quantitation are complicated because of overlap between the lactate methyl doublet CH3 resonance and a lipid resonance at 1.3 ppm. This study presents the use of a selective homonuclear multiple quantum coherence transfer sequence (SelMQC-CSI), at 1.5 T, to better quantify lactate in the presence of lipids. Work performed on phantoms showed good lactate detection (49%) and lipid suppression (98%) efficiencies. To evaluate the method in the brain, the sequence was tested on a group of 23 patients with treated brain tumours, either glioma (N=20) or secondary metastases in the brain (N=3). Here it was proved to be of use in determining lactate concentrations in vivo. Lactate was clearly seen in SelMQC spectra of glioma, even in the presence of lipids, with high grade glioma (7.3 ± 1.9 mM, mean ± standard deviation) having higher concentrations than low grade glioma (1.9 ± 1.5 mM, p=0.048). Lactate was not seen in secondary metastases in the brain. SelMQC-CSI is shown to be a useful technique for measuring lactate in tumours whose signals are otherwise contaminated by lipid.

Detection of oncogenic IDH1 mutations using magnetic resonance spectroscopy of 2-hydroxyglutarate

The investigation of metabolic pathways disturbed in isocitrate dehydrogenase (IDH) mutant tumors revealed that the hallmark metabolic alteration is the production of D-2-hydroxyglutarate (D-2HG). The biological impact of D-2HG strongly suggests that high levels of this metabolite may play a central role in propagating downstream the effects of mutant IDH, leading to malignant transformation of cells. Hence, D-2HG may be an ideal biomarker for both diagnosing and monitoring treatment response targeting IDH mutations. Magnetic resonance spectroscopy (MRS) is well suited to the task of noninvasive D-2HG detection, and there has been much interest in developing such methods. Here, we review recent efforts to translate methodology using MRS to reliably measure in vivo D-2HG into clinical research.

Enhancement of spectral editing efficacy of multiple quantum filters in in vivo proton magnetic resonance spectroscopy

The performance of multiple quantum filters (MQFs) can be disappointing when the background signal also arises from coupled spins. Moreover, at 3.0 T and even higher fields the majority of the spin systems of key brain metabolites fall into the strong-coupling regime. In this manuscript we address comprehensively, the importance of the phase of the multiple quantum coherence-generating pulse (MQ-pulse) in the design of MQFs, using both product operator and numerical analysis, in both zero and double quantum filter designs. The theoretical analyses were experimentally validated with the examples of myo-inositol editing and the separation of glutamate from glutamine. The results demonstrate that the phase of the MQ-pulse per se provides an additional spectral discrimination mechanism based on the degree of coupling beyond the conventional level-of-coherence approach of MQFs. To obtain the best spectral discrimination of strongly-coupled spin systems, therefore, the phase of the MQ-pulse must be included in the portfolio of the sequence parameters to be optimized.

Selective homonuclear polarization transfer for spectroscopic imaging of GABA at 7T

We develop and implement a selective homonuclear polarization transfer method for the detection of 3.0 ppm C-4 GABA resonance by spectroscopic imaging in the human brain at 7T. This single shot method is demonstrated with simulations and phantoms, which achieves comparable efficiency of detection to that of J-difference editing. The macromolecule resonance that commonly co-edits with GABA is suppressed at 7T through use of a narrow band preacquisition suppression pulse. This technique is implemented in humans with an eight channel transceiver array and high degree B(0) shimming to measure supplementary motor area and thalamic GABA in controls (n = 8) and epilepsy patients (n = 8 total). We find that the GABA/N-acetyl aspartate ratio in the thalamus of control volunteers, well controlled and poorly controlled epilepsy patients are 0.053 ± 0.012 (n = 8), 0.090 ± 0.012 (n = 2), and 0.038 ± 0.009 (n = 6), respectively.

Efficient spectral editing at 7 T: GABA detection with MEGA-sLASER

At high field (7 T) spectral editing of γ-aminobutyric acid with MEGA-point-resolved spectroscopy is inefficient due to the large chemical shift displacement error. In this article, a new pulse sequence is designed which has minimal chemical shift displacement error to perform an efficient spectral editing of the γ-aminobutyric acid 3.0 ppm resonance at 7 T. The sequence consists of the conventional MEGA editing pulses and a semi-localized by adiabatic selective refocusing sequence. Phantom and in vivo measurements demonstrated an efficient detection of γ-aminobutyric acid. Using ECG triggering, excellent in vivo performance of the MEGA-semi-localized by adiabatic selective refocusing (MEGA-sLASER) provided well-resolved γ-aminobutyric acid signals in 27 mL volumes in the human brain at an echo time of 74 ms within a relatively short acquisition time (5 min). Furthermore, the high efficiency of the MEGA-sLASER was demonstrated by acquiring small volumes (8 mL) at an echo time of 74 ms, as well as long echo time measurements (222 ms in 27 mL volume).

Field strength dependence of PRESS timings for simultaneous detection of glutamate and glutamine from 1.5 to 7T

An optimization of the PRESS sequence for magnetic resonance spectroscopy is presented to simultaneously detect the important brain metabolites of glutamate (Glu) and glutamine (Gln) at field strengths of 1.5, 3, 4.7, and 7T. Standard, clinical examinations typically use short echo times which in general are not ideal for the separation of Glu and Gln. The optimization procedure is based on numerical product operator simulations to produce yield and overlap measurements for all possible practical choices of PRESS inter-echo timings. The simulations illustrate the substantial modulations in Glu and Gln with field strength. At all field strengths, the optimized timings demonstrate a significant reduction in overlap compared to short echo PRESS, while maintaining a high metabolite signal, with Glu and Gln yields >90% when excluding T2 relaxation losses. Minimal overlap was attained at 7T (0.3% Gln contamination in the Glu signal), and 4.7T (1.2%). The optimized timings were applied in vivo on healthy volunteers at field strengths of 1.5 and 4.7T.

Spectral editing of weakly coupled spins using variable flip angles in PRESS constant echo time difference spectroscopy: application to GABA

A general in vivo magnetic resonance spectroscopy editing technique is presented to detect weakly coupled spin systems through subtraction, while preserving singlets through addition, and is applied to the specific brain metabolite gamma-aminobutyric acid (GABA) at 4.7 T. The new method uses double spin echo localization (PRESS) and is based on a constant echo time difference spectroscopy approach employing subtraction of two asymmetric echo timings, which is normally only applicable to strongly coupled spin systems. By utilizing flip angle reduction of one of the two refocusing pulses in the PRESS sequence, we demonstrate that this difference method may be extended to weakly coupled systems, thereby providing a very simple yet effective editing process. The difference method is first illustrated analytically using a simple two spin weakly coupled spin system. The technique was then demonstrated for the 3.01 ppm resonance of GABA, which is obscured by the strong singlet peak of creatine in vivo. Full numerical simulations, as well as phantom and in vivo experiments were performed. The difference method used two asymmetric PRESS timings with a constant total echo time of 131 ms and a reduced 120 degrees final pulse, providing 25% GABA yield upon subtraction compared to two short echo standard PRESS experiments. Phantom and in vivo results from human brain demonstrate efficacy of this method in agreement with numerical simulations.

A practical guide to robust detection of GABA in human brain by J-difference spectroscopy at 3 T using a standard volume coil

gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in human brain and has been implicated in several neuropsychiatric disorders. In vivo human brain GABA concentrations are near the detection limit for magnetic resonance spectroscopy ( approximately 1 mM), and because of overlap with more abundant compounds, spectral editing is generally necessary to detect GABA. In previous reports, GABA spectra edited by J-difference spectroscopy vary considerably in appearance. We have evaluated the factors that affect GABA spectra and the conditions necessary for robust acquisition of J-difference spectra from arbitrary brain regions. In particular, we demonstrate that variations in spectral quality can be explained in part by the incoherent addition of transients that results from shot to shot frequency and phase variations. An automated time-domain spectral alignment strategy that enables reproducible acquisition of high-quality GABA spectra at 3 T with a standard 30-cm T/R volume coil is presented. Representative GABA spectra from human frontal lobe, an area where susceptibility-induced frequency and phase variations are especially troublesome, that demonstrate the robustness of the acquisition and data handling strategy used in this study are presented.

In vivo detection of gray and white matter differences in GABA concentration in the human brain

A novel selective multiple quantum filtering-based chemical shift imaging method was developed for acquiring GABA images in the human brain at 3 T. This method allows a concomitant acquisition of an interleaved total creatine image with the same spatial resolution. Using T(1)-based image segmentation and a nonlinear least square regression analysis of GABA-to-total creatine concentration ratios in frontal and parietal lobes of healthy adult volunteers as a function of the tissue gray matter fraction, the mean GABA concentration in gray and white matter was determined to be 1.30+/-0.36 micromol/g and 0.16+/-0.16 micromol/g (mean+/-SD, n=13), respectively. It is expected that this method will become a useful tool for studying GABAergic function in the human brain in vivo.

Echo-time independent signal modulations using PRESS sequences: a new approach to spectral editing of strongly coupled AB spin systems

In clinical MR spectroscopy, double spin-echo point resolved spectroscopy (PRESS) sequences are routinely used for volume selection. For strongly coupled AB spin systems under PRESS excitation, the dependence of the signal on the echo time TE has been thoroughly investigated, whereas less attention has been paid to the signal modulation which occurs at constant TE with varying interpulse delays. A substantial TE-independent J modulation is here predicted from analytical solutions of the Liouville equation and density matrix simulations, and verified with experiments on citrate at 1.5 and 3T. It is also shown that this modulation effect could be exploited for editing of strongly coupled AB resonances or for removal of singlets in spectra-by means of difference spectroscopy-just using a standard PRESS sequence. The applicability in vivo of this new spectral editing approach is also demonstrated, with selective detection of citrate resonances in the human prostate. This novel approach has the advantages of being simple, and directly applicable on standard clinical MR scanners, provided that the exact behavior of the resonance is known.

Brain γ-aminobutyric acid measurement by proton double-quantum filtering with selectiveJ rewinding

An optimized single-shot proton double-quantum (DQ) filter for the quantification of gamma-aminobutyric acid (GABA) levels in human brain is reported. It is demonstrated that creation of DQ coherences following dual-resonance-selective refocusing gives a theoretical editing efficiency of 50% for the detection of the GABA resonance at 3.01 ppm. The sequence times are optimized with both numerical and experimental analyses of the editing performance, giving an experimental editing efficiency of 42%. It is acknowledged that homocarnosine is partially coedited, leading to a 20% contribution to the edited signal; however, macromolecule contamination is negligible in vivo under these experimental conditions. The GABA concentration in human prefrontal cortex is estimated to be 0.8 +/- 0.1 micromol/g (mean +/- SD, n = 6), with reference to the internal standard creatine at 9 micromol/g.

Strategy for the spectral filtering of myo-inositol and other strongly coupled spins

A multiple quantum filter strategy is presented for spectrally discriminating metabolites with strongly coupled spins from those whose spins are either uncoupled or weakly coupled. The strategy also includes a means for selectively suppressing the background multiplets of metabolites that also have strongly coupled spins. As a demonstration of its efficacy at 3.0 T, the strategy is shown to enhance by a factor of approximately 5 the signal-to-background ratio of the myo-inositol band at 3.6 ppm relative to that in response to a PRESS sequence with the same sequence timings. This is done by eliminating the uncoupled resonance of glycine and the weakly coupled multiplets of glutamate and glutamine, and by selectively suppressing the strongly coupled taurine multiplet 3-fold. The macromolecular background was effectively removed through its transverse decay over 105 ms. The associated cost of gaining the signal to background enhancement is a drop in signal yield by a factor of 0.75 relative to PRESS at the same timings. The myo-inositol signal to noise ratio was nevertheless maintained by the filter at approximately 12.

Single-shot two-echo technique for simultaneous measurement of GABA and creatine in the human brain in vivo

A single-shot, two-echo method for the simultaneous detection of multiple-quantum (MQ)-filtered gamma-aminobutyric acid (GABA) and creatine (Cr) was developed and demonstrated in the human brain in vivo at 3 Tesla. The simultaneously measured Cr singlet served as a navigator for the spectral phase of GABA and any frequency shift during measurements due to drift in the static magnetic field (B(0)) or subject movement, as well as an internal concentration reference. In addition, the use of a double-band frequency-selective MQ filter for C(3) and C(4) methylene protons of GABA provided a very robust measurement of GABA, with excellent suppression of overlapping metabolites such as Cr and glutathione (GSH) in each single scan. Contamination from overlapping macromolecules was also demonstrated to be negligible with this method. The GABA-to-Cr ratio was 0.09 +/- 0.03 (mean +/- SD, N = 17) and the estimated concentration of GABA in the frontoparietal region of the human brain in vivo was 0.66 +/- 0.19 micromol/g (mean +/- SD, N = 17) with the internal reference method, and 0.69 +/- 0.18 micromol/g (mean +/- SD, N = 17) with the external reference method. The observed pattern of GABA doublet was consistent among all subjects, with a frequency separation of approximately 13 Hz.

In vivo GABA detection with improved selectivity and sensitivity by localized double quantum filter technique at 4.1T

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter for the normal function of mammal and human brain. It is difficult to detect GABA signal with the conventional single quantum technique due to its relatively low concentration and overlapping with other signals from creatine (Cr), glutathione (GSH), as well as macromolecules. Using a high-selective read pulse, DANTE, and at the facility of increased sensitivity and chemical shift resolution at high-field 4.1T, GABA editing by double quantum filter (DQF) with robust suppression of Cr and GSH was achieved. Our editing efficiency of 40-50% was achievable on a GABA phantom (50 mM GABA and 61 mM choline). Furthermore, GABA editing spectra were acquired with echo time TE = 77 ms, and any possible macromolecular contamination to GABA editing spectra was found to be negligible. This high-field DQF setup was applied to 11 healthy volunteers, and the mean GABA level was measured to be 1.12 +/- 0.15 mM in the occipital lobe in reference to 7.1 mM Cr concentration.

In vivo 1H NMR spectroscopy of individual human brain metabolites at moderate field strengths

This article reviews spectral editing techniques for in vivo 1H NMR spectroscopy of human brain tissue at moderate field strengths of 1.5-3 Tesla. Various aspects of 1H NMR spectroscopy are discussed with regard to in vivo applications. The parameter set [delta, J, n] (delta being the relative chemical shift, J the scalar coupling constant and n the number of coupled spins) is used to characterize the spin systems under investigation and to classify the editing techniques that are used in in vivo 1H NMR spectroscopy.

In vivo measurement of brain metabolites using two-dimensional double-quantum MR spectroscopy--exploration of GABA levels in a ketogenic diet

A localized proton 2D double-quantum (DQ) spin-echo spectroscopy technique was implemented on 1.5 T clinical MRI scanners for the detection of gamma-aminobutyrate (GABA) in the brain. The 2D approach facilitates separation of peaks overlapping with GABA in 1D DQ-filtered (DQF) spectra. This technique was applied to four normal adult volunteers and four children with intractable epilepsy. The coefficient of variation of the level of GABA and overlapping macromolecules at F2 = 3.0 ppm and F1 = 4.8 ppm was 0.08 in normal subjects. Three patients received 2D MRS scans before and after initiation of the ketogenic diet (KD): one patient showed a trend of decreasing GABA throughout the study, and two patients showed low initial GABA levels that increased over time. In addition to major metabolites and GABA, low-level metabolites (valine, leucine, and glutathione) were also identified in the 2D spectra.

GABA X2 multiplet measured pre- and post-administration of vigabatrin in human brain

This work demonstrates, in solution and in human brain at 3 tesla, that the X(2)-multiplet of the A(2)M(2)X(2) proton spin system of GABA at 2.315 ppm can be readily resolved from that of the overlapping background, particularly the glutamate multiplet, i.e., the PQ multiplet of the glutamate AMNPQ spin system. Prior to experiment, the values of the stimulated echo acquisition mode (STEAM) sequence parameters TE and TM that maximized the GABA-X(2) discrimination from its background (i.e., 168 ms and 28 ms, respectively) were determined numerically. The determination was made by calculating the spectral response of all contributing metabolites to the STEAM sequence throughout TE/TM space. A baseline GABA concentration (mean +/- standard deviation (SD) of the mean) of 0.78 +/- 0.04 mM was estimated from spectra acquired from a 3 x 3 x 3 cm(3) volume in the parieto-occipital cortex of eight normal control subjects. Five of the eight control subjects were also studied 24 hr post-administration of a single dose of 50 mg.kg(-1) vigabatrin. Four of the five showed increases in GABA in the range of 15-120% of their baseline level.

In vivo GABA+ measurement at 1.5T using a PRESS-localized double quantum filter

A point-resolved spectroscopy (PRESS)-localized double quantum filter was implemented on a 1.5T clinical scanner for the estimation of gamma-amino butyric acid (GABA) concentrations in vivo. Several calibrations were found to be necessary for consistent results to be obtained. The apparent filter yield was approximately 38%; filter strength was sufficient to reduce the singlet metabolite peaks in vivo to below the level of the noise. Metabolite-nulled experiments were performed, which confirmed that significant overlap occurred between macromolecule signals and the GABA resonance at 3.1 ppm. Although the multiplet arm at 2.9 ppm was confirmed to be relatively free of contamination with macromolecules, some contribution from these and from peptides is likely to remain; therefore, the term GABA+ is used. GABA+ concentrations were estimated relative to creatine (Cr) at the same echo time (TE) in a group of controls, studied on two occasions. The GABA+ concentration in 35-ml regions of interest (ROIs) in the occipital lobe was found to be 1.4 +/- 0.2 mM, with scan-rescan repeatability of 38%.

Direct in vivo measurement of human cerebral GABA concentration using MEGA-editing at 7 Tesla

Spectral editing of the GABA spin system is hampered by coediting of macromolecule (MM) coherences. To reduce contamination arising from MMs in spectra edited for GABA, the highest field strength currently available for human experimentation (7 Tesla) and MEGA-based editing were used. Despite judicious choice of experimental parameters, MM contamination was found to arise from field drifts. When the MM contribution was accounted for, [GABA] = 0.75 +/- 0.14 micromol/g (mean +/- SD, N = 16) relative to 8 micromol/g creatine (Cr), whereas without accounting for the MM signal [GABA*] = 0.88 +/- 0.23 micromol/g (mean +/- SD, N = 16). Incorporating the direct experimental assessment of MM contamination to the edited GABA signal substantially reduced the variance of the measurement, resulting in concentrations that were in excellent agreement with previous (13)C labeling experiments.

In vivo magnetic resonance spectroscopy and its application to neuropsychiatric disorders

In vivo magnetic resonance spectroscopy (MRS) is the only noninvasive imaging technique that can directly assess the living biochemistry in localized brain regions. In the past decade, spectroscopy studies have shown biochemical alterations in various neuropsychiatric disorders. These first-generation studies have, in most cases, been exploratory but have provided insightful biochemical information that has furthered our understanding of different brain disorders. This review provides a brief description of spectroscopy, followed by a literature review of key spectroscopy findings in schizophrenia, affective disorders, and autism. In schizophrenia, phosphorus spectroscopy studies have shown altered metabolism of membrane phospholipids (MPL) during the early course of the illness, which is consistent with a neurodevelopmental abnormality around the critical period of adolescence when the illness typically begins. Children and adolescents who are at increased genetic risk for schizophrenia show similar MPL alterations, suggesting that schizophrenia subjects with a genetic predisposition may have a premorbid neurodevelopmental abnormality. Independent of medication status, bipolar subjects in the depressive state tended to have higher MPL precursor levels and a deficit of high-energy phosphate metabolites, which also is consistent with major depression, though these results varied. Further bipolar studies are needed to investigate alterations at the early stage. Lastly, associations between prefrontal metabolism of high-energy phosphate and MPL and neuropsychological performance and reduced N-acetylaspartate in the temporal and cerebellum regions have been reported in individuals with autism. These findings are consistent with developmental alterations in the temporal lobe and in the cerebellum of persons with autism. This paper discusses recent findings of new functions of N-acetylaspartate.

Brain GABA editing without macromolecule contamination

A new scheme is proposed to edit the 3.0 ppm GABA resonance without macromolecule (MM) contamination. Like previous difference spectroscopy approaches, the new scheme manipulates J-modulation of this signal using a selective editing pulse. The elimination of undesirable MM contribution at 3.0 ppm is obtained by applying this pulse symmetrically about the J-coupled MM resonance, at 1.7 ppm, in the two steps of the editing scheme. The effectiveness of the method is demonstrated in vitro, using lysine to mimic MM, and in vivo. As compared to the most commonly used editing scheme, which necessitates the acquisition and processing of two distinct difference spectroscopy experiments, the new scheme offers a reduction in experimental time (-33%) and an increase in accuracy. Magn Reson Med 45:517-520, 2001.

Assessment of GABA concentration in human brain using two-dimensional proton magnetic resonance spectroscopy

A quantitative method to assess in vivo brain gamma-aminobutyric acid (GABA) levels is proposed using a J-resolved, two-dimensional (2D) magnetic resonance spectroscopy (MRS) technique. Localized, J-resolved 2D MR spectra were obtained from a 12-cm(3) voxel in the occipital lobe of 36 healthy volunteers (18 male and 18 female, age: 25.1+/-4.8 years). Based on phantom measurements, a GABA resonance peak located at 2.94 ppm, 7.45 Hz, in J-resolved 2D MRS overlaps the least with other resonance peaks which arise from N-acetylaspartate, choline, creatine, glutamate and glutamine. Measurements of this resonance peak yield in vivo GABA concentrations of 1.01+/-0.36 micromol/cm(3) for male and 1.16+/-0.43 micromol/cm(3) for female volunteers, without correction for T1 and T2 relaxation effects. These results are in good agreement with previously reported data and suggest that, with further development, 2D MRS may provide a practical means to estimate the concentration of this important neurotransmitter.

Magnetic resonance spectroscopy in schizophrenia: methodological issues and findings--part I

Our knowledge of the biological basis of schizophrenia has significantly increased with the contribution of in vivo proton and phosphorus magnetic resonance spectroscopy (MRS), a noninvasive tool that can assess the biochemistry from a localized region in the human body. Studies thus far suggest altered membrane phospholipid metabolism at the early stage of illness and reduced N-acetylaspartate, a measure of neuronal volume/viability in chronic schizophrenia. Inconsistencies remain in the literature, in part due to the complexities in the MRS methodology. These complexities of in vivo spectroscopy make it important to understand the issues surrounding the design of spectroscopy protocols to best address hypotheses of interest. This review addresses these issues, including 1) understanding biochemistry and the physiologic significance of metabolites; 2) the influence of acquisition parameters combined with spin-spin and spin-lattice relaxation effects on the MRS signal; 3) the composition of spectral peaks and the degree of overlapping peaks, including the broader underlying peaks; 4) factors affecting the signal-to-noise ratio; 5) the various types of localization schemes; and 6) the objectives to produce accurate and reproducible quantification results. The ability to fully exploit the potentials of in vivo spectroscopy should lead to a protocol best optimized to address the hypotheses of interest.

Off-resonance effects of the radiofrequency pulses used in spectral editing with double-quantum coherence transfer

Spectral editing using gradient selected double-quantum (DQ) coherence transfer is often used for the selective observation of metabolites in vivo. In attempting to optimize the detection sensitivity of a conventional DQ spectral editing sequence, the effects of using radiofrequency (RF) pulses that are not at the resonance frequency of the observed peaks were investigated both theoretically and experimentally. The results show that spectral editing using pulses at the frequency of the observed resonance does not necessarily give the optimal detection sensitivity. At 7 T, the detection sensitivity of lactate observed using a DQ editing method can be increased by up to 30% by setting the RF pulses off resonance at the proper frequency. The results also suggest that slice selective RF pulses used in DQ spectral editing combined with PRESS localization may have slice profiles different from those when the same pulses are used for standard PRESS spatial localization.

Proton NMR chemical shifts and coupling constants for brain metabolites

Proton NMR chemical shift and J-coupling values are presented for 35 metabolites that can be detected by in vivo or in vitro NMR studies of mammalian brain. Measurements were obtained using high-field NMR spectra of metabolites in solution, under conditions typical for normal physiological temperature and pH. This information is presented with an accuracy that is suitable for computer simulation of metabolite spectra to be used as basis functions of a parametric spectral analysis procedure. This procedure is verified by the analysis of a rat brain extract spectrum, using the measured spectral parameters. In addition, the metabolite structures and example spectra are presented, and clinical applications and MR spectroscopic measurements of these metabolites are reviewed.

Simultaneous spectral editing for gamma-aminobutyric acid and taurine using double quantum coherence transfer

Conventional double quantum (DQ) editing techniques recover resonances of one metabolite at a time and are thus inefficient for monitoring metabolic changes involving several metabolites. A DQ coherence transfer double editing sequence using a dual-band DQ coherence read pulse is described here. The sequence permits simultaneous spectral editing for two metabolites with similar J coupling constants in a single scan. Simultaneous editing for taurine and gamma-aminobutyric acid (GABA) is demonstrated using solution phantoms and rat brain tissue. Selectivity of the double editing sequence for the target metabolites is as good as that achieved using conventional DQ editing which selects each metabolite individually. With experimental parameters of the double editing sequence chosen to optimize GABA editing, the sensitivity for GABA detection is the same as that with GABA editing only, while the sensitivity for taurine detection is decreased slightly compared to that with taurine editing only.

Detection of glutathione in the human brain in vivo by means of double quantum coherence filtering

The feasibility of selective in vivo detection of glutathione (L-gamma-glutamyl-L-cysteinyl-glycine, GSH) in the human brain by means of (1)H magnetic resonance spectroscopy (MRS) at 1.5 T is demonstrated. A double quantum coherence (DQC) filtering sequence was used in combination with PRESS volume selection. The strongly coupled cysteinyl CH(2) compound of GSH was found to be the most suitable target for spectral editing. Analytical calculations employing a product operator description of the cysteinyl ABX three-spin system were made in order to optimize the inherent yield of the sequence. A pulse phase calibration procedure, which precedes the spectrum acquisition, secures maximal signal yield independently of the spatial localization of the volume of interest and thus comparability between individual examinations. In vitro tests show that the DQC filtering method provides good discrimination between the GSH signal at 2.9 ppm and the interfering resonances of creatine, gamma-aminobutyric acid (GABA) and aspartate. In measurements in the frontal lobe of 12 healthy volunteers a mean ratio of GSH signal to tissue water signal of 5.7 +/- 2.3 x 10(-5) was found, corresponding to a mean GSH tissue concentration of 2-5 mmol/L. The proposed technique allows for the detection of a biologically highly relevant metabolite at moderate field strength. Magn Reson Med 42:283-289, 1999.

Molecular aspects of magnetic resonance imaging and spectroscopy

Magnetic resonance imaging (MRI) is a well known diagnostic tool in radiology that produces unsurpassed images of the human body, in particular of soft tissue. However, the medical community is often not aware that MRI is an important yet limited segment of magnetic resonance (MR) or nuclear magnetic resonance (NMR) as this method is called in basic science. The tremendous morphological information of MR images sometimes conceal the fact that MR signals in general contain much more information, especially on processes on the molecular level. NMR is successfully used in physics, chemistry, and biology to explore and characterize chemical reactions, molecular conformations, biochemical pathways, solid state material, and many other applications that elucidate invisible characteristics of matter and tissue. In medical applications, knowledge of the molecular background of MRI and in particular MR spectroscopy (MRS) is an inevitable basis to understand molecular phenomenon leading to macroscopic effects visible in diagnostic images or spectra. This review shall provide the necessary background to comprehend molecular aspects of magnetic resonance applications in medicine. An introduction into the physical basics aims at an understanding of some of the molecular mechanisms without extended mathematical treatment. The MR typical terminology is explained such that reading of original MR publications could be facilitated for non-MR experts. Applications in MRI and MRS are intended to illustrate the consequences of molecular effects on images and spectra.

In vivo chemical shift imaging of gamma-aminobutyric acid in the human brain

A gradient-based multiple quantum filtering method is presented for in vivo chemical shift imaging of gamma-aminobutyric acid (GABA) in the human brain, which provides effective suppression of the overlapping creatine singlet with close to optimal detection efficiency. It is shown by product operator calculations and coherence pathway analysis that under conditions of no B1 and B0 inhomogeneity gradient filtering retains 75% of the two outer resonance lines of the GABA-4 triplet with no creatine contamination. A variation of the method with 100% retention of the GABA-4 outer resonance lines but higher sensitivity to B1 inhomogeneity is also discussed. By using a localized version of the sequence with an 8-cm surface coil for transmission and detection, it was found in phantom experiments at 2.1 T that a 69% signal retention of the two outer resonance lines of the GABA-4 triplet was achieved relative to a spin echo sequence with inhibition of GABA J modulation. A creatine suppression ratio of 2000:1 was measured. The use of the method for chemical shift imaging of GABA is demonstrated by coronal images obtained from phantoms and from the occipital lobe of a healthy volunteer.

A new multiple quantum filter design procedure for use on strongly coupled spin systems found in vivo: its application to glutamate

A numerical procedure is outlined that is appropriate for the design of multiple quantum filter sequences targeted for the strongly coupled, multiple spin systems that occur in metabolites present in brain. The procedure uses numerical methods of solution of the density matrix equations, first, to establish the most appropriate resonance to target with the filter; second, to provide contour plots of a performance index of the filter in terms of critical sequence parameters; and third, to produce the response signals of the target and the background metabolites to the optimized filter. The procedure is exemplified for the AMNPQ spin system of the amino acid glutamate at a field strength of 3 T. The 2.3 ppm peak of the PQ multiplet of glutamate was identified as the target resonance, and the performance of the filter so derived was evaluated experimentally on phantom solutions and in human brain. These experiments clearly demonstrate that a linewidth of <or=4 Hz is required for full resolution of glutamate from glutamine at 3 T using this double quantum filter. Nevertheless, even at a linewidth of approximately 7 Hz in vivo, the 2.3 ppm peak of glutamate dominates the filter response and thereby removes a significant cause of uncertainty in measuring changes in glutamate by eliminating most of the background observed in unedited spectra obtained using PRESS or STEAM.

A localized double-quantum filter for the in vivo detection of brain glucose

A double-quantum filter (DQF) sequence with PRESS localization was developed for in vivo detection of the glucose resonances in the 3.85-ppm region of the brain proton spectrum. The efficiency and spectral editing characteristics were studied in phantom and animal experiments. Approximately 45% detection efficiency was achieved at 4.7 T with TE = 68 ms. Since the efficiency of the DQF method is dependent on the relative phases of the RF pulses, a phase calibration procedure was used to correct for phase shifts induced by the spatial localization. In addition to detecting the 3.85-ppm glucose resonances with approximately 45% efficiency, the DQF sequence simultaneously detects 1.3-ppm lactate resonance with approximately 20% efficiency. The use of the DQF technique for simultaneously monitoring both the input and output of anaerobic glycolysis in the brain was demonstrated by detecting brain glucose and lactate in the same acquisition after iv injection of glucose followed by the induction of global ischemia.

Measurements of human cerebral GABA at 4.1 T using numerically optimized editing pulses

The goal of this work was to develop and evaluate a numerically optimized inversion pulse to be used with a homonuclear editing sequence to measure human cerebral GABA in vivo at 4.1 T in the occipital lobe. The optimized pulse was constructed using pallindromic symmetry with 30 pulses and 29 delays. The optimized pulse provided greater selectivity than the equivalent bandwidth matched DANTE pulse and sinc shaped DANTE. The improved selectivity reduced the co-editing of the macromolecule resonance, permitting the GABA edited doublet to be resolved in vivo. Using cerebral creatine as a reference, 7.1 mM, the measured GABA level was 1.15 +/- 0.13 mM in the occipital lobe.

Metabolite-specific NMR spectroscopy in vivo

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.

In vivo detection of GABA in human brain using a localized double-quantum filter technique

A proton MR spectral editing technique employing a spatially localized, double-quantum filter (DQF) was used to measure gamma-aminobutyric acid (GABA) in the human brain at 1.5 T. The double-quantum method provided robust, single-shot suppression of uncoupled resonances from choline, creatine, and NAA and allowed detection of the gamma CH2 GABA (3.0 ppm) resonance with 30% efficiency. Spatial localization of the GABA measurement was achieved by incorporating PRESS localization within the double-quantum excitation and detection sequence. A calibration technique was developed to adjust the relative phases of the RF pulses to maximize the in vivo double-quantum detection efficiency for an arbitrary voxel location. The sequence efficiency, degree of suppression of uncoupled reasonances, and characterization of the in vivo DQF technique was examined in phantom experiments and in a study of the occipital lobe of 10 normal subjects. The ratio of the 3.0-ppm GABA resonance to the 3.0-ppm creatine resonance was found to be 0.20 +/- 0.05 (SD).