Homonuclear J coupling effects in volume localized NMR spectroscopy: pitfalls and solutions

Functional Magnetic Resonance Spectroscopy of Prolonged Motor Activation using Conventional and Spectral GLM Analyses

Background

Functional MRS (fMRS) is a technique used to measure metabolic changes in response to increased neuronal activity, providing unique insights into neurotransmitter dynamics and neuroenergetics. In this study we investigate the response of lactate and glutamate levels in the motor cortex during a sustained motor task using conventional spectral fitting and explore the use of a novel analysis approach based on the application of linear modelling directly to the spectro-temporal fMRS data.

Methods

fMRS data were acquired at a field strength of 3 Tesla from 23 healthy participants using a short echo-time (28ms) semi-LASER sequence. The functional task involved rhythmic hand clenching over a duration of 8 minutes and standard MRS preprocessing steps, including frequency and phase alignment, were employed. Both conventional spectral fitting and direct linear modelling were applied, and results from participant-averaged spectra and metabolite-averaged individual analyses were compared.

Results

We observed a 20% increase in lactate in response to the motor task, consistent with findings at higher magnetic field strengths. However, statistical testing showed some variability between the two averaging schemes and fitting algorithms. While lactate changes were supported by the direct spectral modelling approach, smaller increases in glutamate (2%) were inconsistent. Exploratory spectral modelling identified a 4% decrease in aspartate, aligning with conventional fitting and observations from prolonged visual stimulation.

Conclusion

We demonstrate that lactate dynamics in response to a prolonged motor task are observed using short-echo time semi-LASER at 3 Tesla, and that direct linear modelling of fMRS data is a useful complement to conventional analysis. Future work includes mitigating spectral confounds, such as scalp lipid contamination and lineshape drift, and further validation of our novel direct linear modelling approach through experimental and simulated datasets.

Multi-echo single-shot spectroscopy combined with simultaneous 2D model fitting for fast and accurate measurement of metabolite-specific concentrations and T2 relaxation times

The purpose of the current study was to develop a novel single-voxel MR spectroscopy acquisition scheme to simultaneously determine metabolite-specific concentrations and transverse relaxation times within realistic clinical scan times. Partly truncated multi-TE data are acquired as an echo train in a single acquisition (multi-echo single-shot [MESS]). A 2D multiparametric model fitting approach combines truncated, low-resolved short TE data with fully sampled, highly resolved, longer TE data to yield concentration and T2 estimates for major brain metabolites simultaneously. Cramer-Rao lower bounds (CRLB) are used as a measure of performance. The novel scheme was compared with traditional multi-echo multi-shot methods. In silico, in vitro, and in vivo experiments support the findings. MESS schemes, requiring only 2 min 12 s for the acquisition of three echo times, provide valid concentration and relaxation estimates for multiple metabolites and outperform traditional methods for simultaneous determinations of metabolite-specific T2 s and concentrations, with improvements ranging from 5% to 30% for T2 s and from 10% to 50% for concentrations. However, substantial unsuppressed residual water signals may hamper the method's reproducibility, as observed in an initial experiment setup that prioritizes short TEs with severely truncated acquisition for the benefit of signal-to-noise ratio (SNR). Nevertheless, CRLB have been confirmed to be well suited as design criteria, and within-session repeatability approaches CRLB when residual water is removed in postprocessing by exploiting longer and less truncated data recordings. MESS MRS combined with 2D model fitting promises comparable accuracy, increased precision, or inversely shorter experimental times compared with traditional approaches. However, the optimal design must be investigated as a trade-off between SNR, the truncation factor, and TE batch selections, all of which influence the robustness of estimations.

Simulated basis sets for semi-LASER: the impact of including shaped RF pulses and magnetic field gradients

Objective

To study the need for inclusion of shaped RF pulses and magnetic field gradients in simulations of basis sets for the analysis of proton MR spectra of single voxels of the brain acquired with a semi-LASER pulse sequence.

Materials and methods

MRS basis sets where simulated at different echo times with hard RF pulses as well as with shaped RF pulses without or with magnetic field gradients included. The influence on metabolite concentration quantification was assessed using both phantom and in vivo measurements. For comparison, simulations and measurements were performed with the PRESS pulse sequence.

Results

The effect of including gradients in the simulations was smaller for semi-LASER than for PRESS, however, still noticeable. The difference was larger for strongly coupled metabolites and at longer echo times. Metabolite quantification using semi-LASER was thereby less dependent on the inclusion of gradients than PRESS, which was seen in both phantom and in vivo measurements.

Discussion

The inclusion of the shaped RF pulses and magnetic field gradients in the simulation of basis sets for semi-LASER is only important for strongly coupled metabolites. If computational time is a limiting factor, simple simulations with hard RF pulses can provide almost as accurate metabolite quantification as those that include the chemical-shift related displacement.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10334-020-00900-1.

Spectral editing in 1 H magnetic resonance spectroscopy: Experts' consensus recommendations

Spectral editing in in vivo ¹ H-MRS provides an effective means to measure low-concentration metabolite signals that cannot be reliably measured by conventional MRS techniques due to signal overlap, for example, γ-aminobutyric acid, glutathione and D-2-hydroxyglutarate. Spectral editing strategies utilize known J-coupling relationships within the metabolite of interest to discriminate their resonances from overlying signals. This consensus recommendation paper provides a brief overview of commonly used homonuclear editing techniques and considerations for data acquisition, processing and quantification. Also, we have listed the experts' recommendations for minimum requirements to achieve adequate spectral editing and reliable quantification. These include selecting the right editing sequence, dealing with frequency drift, handling unwanted coedited resonances, spectral fitting of edited spectra, setting up multicenter clinical trials and recommending sequence parameters to be reported in publications.

Magnetic resonance markers of bilateral neuronal metabolic dysfunction in patients with unilateral internal carotid artery occlusion

OBJECTIVES

To evaluate cerebral hemodynamic, metabolic and anatomic changes occurring in patients with unilateral occlusion of the internal carotid artery (ICA).

MATERIALS AND METHODS

Twenty-two patients with unilateral occlusion of ICA and twenty age and sex matched healthy subjects were included in the study. Single voxel proton magnetic resonance spectroscopy (¹H-MRS) of the centrum semiovale, semi-automated hippocampal volumetry in T1-weighted scans and transcranial Doppler examination (TCD) with calculation of Breath Holding Index (BHI) were performed in both groups. Metabolic, anatomic, and hemodynamic features were compared between the two groups.

RESULTS

The N-acetylaspartate (NAA)/choline (Cho) ratio was significantly lower in both hemispheres of enrolled patients compared to controls (p = 0.005 for the side with occlusion, p = 0.04 for the side without occlusion). The hippocampus volume was significantly reduced bilaterally in patients compared to healthy subjects (p = 0.049). A statistically significant difference in BHI values was observed between the side with occlusion and without occlusion (p = 0.037) of the patients, as well as between BHI values of the side with occlusion and healthy volunteers (p = 0.014).

DISCUSSION

Patients with unilateral ICA occlusion have reduced NAA/Cho ratio in the white matter of both hemispheres and have bilateral atrophy of hippocampus. The alteration of hemodynamics alone cannot explain these changes.

Magnetic resonance spectroscopy for the study of cns malignancies

Despite intensive research, brain tumors are amongst the malignancies with the worst prognosis; therefore, a prompt diagnosis and thoughtful assessment of the disease is required. The resistance of brain tumors to most forms of conventional therapy has led researchers to explore the underlying biology in search of new vulnerabilities and biomarkers. The unique metabolism of brain tumors represents one potential vulnerability and the basis for a system of classification. Profiling this aberrant metabolism requires a method to accurately measure and report differences in metabolite concentrations. Magnetic resonance-based techniques provide a framework for examining tumor tissue and the evolution of disease. Nuclear Magnetic Resonance (NMR) analysis of biofluids collected from patients suffering from brain cancer can provide biological information about disease status. In particular, urine and plasma can serve to monitor the evolution of disease through the changes observed in the metabolic profiles. Moreover, cerebrospinal fluid can be utilized as a direct reporter of cerebral activity since it carries the chemicals exchanged with the brain tissue and the tumor mass. Metabolic reprogramming has recently been included as one of the hallmarks of cancer. Accordingly, the metabolic rewiring experienced by these tumors to sustain rapid growth and proliferation can also serve as a potential therapeutic target. The combination of 13C tracing approaches with the utilization of different NMR spectral modalities has allowed investigations of the upregulation of glycolysis in the aggressive forms of brain tumors, including glioblastomas, and the discovery of the utilization of acetate as an alternative cellular fuel in brain metastasis and gliomas. One of the major contributions of magnetic resonance to the assessment of brain tumors has been the non-invasive determination of 2-hydroxyglutarate (2HG) in tumors harboring a mutation in isocitrate dehydrogenase 1 (IDH1). The mutational status of this enzyme already serves as a key feature in the clinical classification of brain neoplasia in routine clinical practice and pilot studies have established the use of in vivo magnetic resonance spectroscopy (MRS) for monitoring disease progression and treatment response in IDH mutant gliomas. However, the development of bespoke methods for 2HG detection by MRS has been required, and this has prevented the wider implementation of MRS methodology into the clinic. One of the main challenges for improving the management of the disease is to obtain an accurate insight into the response to treatment, so that the patient can be promptly diverted into a new therapy if resistant or maintained on the original therapy if responsive. The implementation of 13C hyperpolarized magnetic resonance spectroscopic imaging (MRSI) has allowed detection of changes in tumor metabolism associated with a treatment, and as such has been revealed as a remarkable tool for monitoring response to therapeutic strategies. In summary, the application of magnetic resonance-based methodologies to the diagnosis and management of brain tumor patients, in addition to its utilization in the investigation of its tumor-associated metabolic rewiring, is helping to unravel the biological basis of malignancies of the central nervous system.

Flexible MEGA editing scheme with asymmetric adiabatic pulses applied for T 2 measurement of lactate in human brain

PURPOSE

A flexible MEGA editing scheme which decouples the editing efficiency from TE is proposed and the utility of asymmetric adiabatic pulses for this new technique is explored. It is demonstrated that the method enables robust T 2 measurement of lactate in healthy human brain.

METHODS

The proposed variation of the MEGA scheme applies editing pulses in both acquired spectra, ensuring that the difference in J-evolution of the target resonance leads to maximal signal yield in the difference spectrum for arbitrary TE. A MEGA-sLASER sequence is augmented with asymmetric adiabatic editing pulses for enhanced flexibility and immunity to B 1 + miscalibration and inhomogeneities. The technique is validated and optimized for flexible lactate editing via a simple analytical model, numerical simulations and in vitro experiments. The T 2 relaxation constant of lactate is determined in vivo via multiple-TE measurements with the proposed method and a dedicated postprocessing and quantification approach.

RESULTS

Asymmetric adiabatic editing pulses improve robustness and facilitate efficient J-editing in sequences or protocols with strong timing constraints. Single voxel measurements using the proposed MEGA scheme in the occipital cortex of six healthy subjects yield a relaxation constant of T 2 = 171 ± 19  ms for the methyl resonance of lactate at a field strength of 3T.

CONCLUSIONS

The proposed MEGA editing scheme allows for novel kinds of J-editing experiments and promises to be an asset to robust T 2 measurement of lactate and potentially other J-coupled metabolites in vivo.

Quantitative 1 H MRI and MRS of fatty acid composition

Adipose tissue as well as other depots of fat (triglycerides) are increasingly being recognized as active contributors to the human function and metabolism. In addition to the fat concentration, also the fatty acid chemical composition (FAC) of the triglyceride molecules may play an important part in diseases such as obesity, insulin resistance, hepatic steatosis, osteoporosis, and cancer. MR spectroscopy and chemical-shift-encoded imaging (CSE-MRI) are established methods for non-invasive quantification of fat concentration in tissue. More recently, similar techniques have been developed for assessment also of the FAC in terms of the number of double bonds, the fraction of saturated, monounsaturated, and polyunsaturated fatty acids, or semi-quantitative unsaturation indices. The number of papers focusing on especially CSE-MRI-based techniques has steadily increased during the past few years, introducing a range of acquisition protocols and reconstruction algorithms. However, a number of potential sources of bias have also been identified. Furthermore, the measures used to characterize the FAC using both MRI and MRS differ, making comparisons between different techniques difficult. The aim of this paper is to review MRS- and MRI-based methods for in vivo quantification of the FAC. We describe the chemical composition of triglycerides and discuss various potential FAC measures. Furthermore, we review acquisition and reconstruction methodology and finally, some existing and potential applications are summarized. We conclude that both MRI and MRS provide feasible non-invasive alternatives to the gold standard gas chromatography for in vivo measurements of the FAC. Although both are associated with gas chromatography, future studies are warranted.

Effects of noise and linewidth on in vivo analysis of glutamate at 3 T

Magnetic resonance spectroscopy (MRS) can noninvasively detect metabolites in vivo, including glutamate (Glu). However, quantification is known to be affected by the overlaps among metabolite resonance lines and background macromolecule signals. We found that adding a moderate amount of noise or line broadening (2 Hz) caused large variations in concentration of Glu and other metabolites, when determined by LCModel analysis of in vivo short-echo time (TE) spectra. Theses variations were largely attributed to strong spectral baselines in short TE spectra, especially near 2.35 ppm, as well as overlapping metabolite resonance lines. To address this issue, we acquired in vivo data at 3 T using both short-TE and the multiple echo time J-resolved point-resolved spectroscopy (JPRESS) MRS techniques. We found that one-dimensional (1D) JPRESS, by simultaneously fitting the two cross-sections of JPRESS at J = 0 and J = 7.5 Hz, was highly resistant to variations in noise levels and spectral linewidths. Our results demonstrate that LCModel analysis of short-TE data is highly sensitive to variations in noise levels and spectral linewidths and this sensitivity is greatly reduced by 1D JPRESS given its substantially reduced baselines and enhanced spectral resolution.

Improved detection of lactate and β-hydroxybutyrate using MEGA-sLASER at 3 T

Lactate and β-hydroxybutyrate are important MRS-visible biomarkers for the energy metabolism of the human brain. A major obstacle for their unambiguous detection and quantification in vivo is their inherently low concentration and spectral overlap with resonances from lipids and macromolecules. In this work, we demonstrate the improved detectability of lactate and β-hydroxybutyrate with MEGA-sLASER compared to MEGA-PRESS at the clinical field strength of 3 T. The method is validated by numerical simulations, in vitro measurements and in vivo experiments on healthy subjects. It is demonstrated that MEGA-sLASER offers an SNR increase of approximately 70% for lactate and β-hydroxybutyrate detection compared to MEGA-PRESS in various brain regions. This increased SNR translates into reduced Cramér-Rao lower bounds for quantification and enables a more robust detection of subtle changes in the (brain) energy metabolism. The sensitivity of the method for detection of β-hydroxybutyrate concentration changes is demonstrated through measurements before and during a ketogenic diet while the sensitivity for detection of lactate concentration changes is shown by measurements before and after an intensive anaerobic exercise.

In situ postmortem ethanol quantification in the cerebrospinal fluid by non-water-suppressed proton MRS

Determination of the ethanol concentration in corpses with MRS would allow a reproducible forensic assessment by which evidence is collected in a noninvasive manner. However, although MRS has been successfully used to detect ethanol in vivo, it has not been applied to postmortem ethanol quantification in situ. The present study examined the feasibility of the noninvasive measurement of the ethanol concentration in human corpses with MRS. A total of 15 corpses with suspected alcohol consumption before demise underwent examination in a 3 T whole body scanner. To address the partial overlap of the ethanol and lactate signal in the postmortem spectrum, non-water-suppressed single voxel spectra were recorded in the cerebrospinal fluid (CSF) of the left lateral ventricle via the metabolite cycling technique. The ethanol signals were quantified using the internal water as reference standard, as well as based on a reference signal acquired in a phantom. The measured values were compared with biochemically determined concentrations in the blood (BAC) and CSF (CSFAC). In 8 of the 15 corpses a BAC above zero was determined (range 0.03-1.68 g/kg). In all of these 8 corpses, ethanol was measured in CSF with the proposed MRS protocol. The two applied MRS calibration strategies resulted in similar concentrations. However, the MRS measurements generally overestimated the ethanol concentration by 0.09 g/kg (4%) to 0.72 g/kg (45%) as compared with the CSFAC value. The presented MRS protocol allows the measurement of ethanol in the CSF in human corpses and provides an estimation of the ethanol concentration prior to autopsy. Observed deviations from biochemically determined concentrations are mainly explained by the approximate correction of the relaxation attenuation of the ethanol signal.

Functional MRS with J-edited lactate in human motor cortex at 4 T

While functional MRI (fMRI) localizes regions of brain activation, functional MRS (fMRS) provides insights into metabolic underpinnings. Previous fMRS studies detected task-induced lactate increase using short echo-time non-edited ¹H-MRS protocols, where lactate changes depended on accurate exclusion of overlapping lactate and lipid/macromolecule signals. Because long echo-time J-difference ¹H-MRS detection of lactate is less susceptible to this shortcoming, we posited if J-edited fMRS protocol could reliably detect metabolic changes in the human motor cortex during a finger-tapping paradigm in relation to a reliable measure of basal lactate. Our J-edited fMRS protocol at 4T was guided by an fMRI pre-scan to determine the ¹H-MRS voxel placement in the motor cortex. Because lactate and β-hydroxybutyrate (BHB) follow similar J-evolution profiles we observed both metabolites in all spectra, but only lactate showed reproducible task-induced modulation by 0.07 mM from a basal value of 0.82 mM. These J-edited fMRS results demonstrate good sensitivity and specificity for task-induced lactate modulation, suggesting that J-edited fMRS studies can be used to investigate the metabolic underpinning of human cognition by measuring lactate dynamics associated with activation and deactivation fMRI paradigms across brain regions at magnetic field lower than 7T.

Simultaneous detection of valine and lactate using MEGA-PRESS editing in pyogenic brain abscess

Valine and lactate have been recognized as important metabolic markers to diagnose brain abscess by means of MRS. However, in vivo unambiguous detection and quantification is hampered by macromolecular contamination. In this work, MEGA-PRESS difference editing of valine and lactate is proposed. The method is validated in vitro and applied for quantitative in vivo experiments in one healthy subject and two brain abscess patients. It is demonstrated that with this technique the overlapping lipid signal can be reduced by more than an order of magnitude and thus the robustness of valine and lactate detection in vivo can be enhanced. Quantification of the two abscess MEGA-PRESS spectra yielded valine/lactate concentration ratios of 0.10 and 0.27. These ratios agreed with the concentration ratios determined from concomitantly acquired short-T_E PRESS data and were in line with literature values. The quantification accuracy of lactate (as measured with Cramér-Rao lower bounds in LCModel processing) was better for MEGA-PRESS than for short-T_E PRESS in all acquired in vivo datasets. The Cramér-Rao lower bounds of valine were only better for MEGA-PRESS in one of the two abscess cases, while in the other case coediting of isoleucine confounded the quantification in the MEGA-PRESS analysis. MEGA-PRESS and short-T_E PRESS should be combined for unambiguous quantification of amino acids in abscess measurements. Simultaneous valine/lactate MEGA-PRESS editing might benefit the distinction of brain abscesses from tumors, and further categorization of bacteria with reasonable sensitivity and specificity.

Improved localisation for 2-hydroxyglutarate detection at 3T using long-TE semi-LASER

2-hydroxyglutarate (2-HG) has emerged as a biomarker of tumour cell IDH mutations that may enable the differential diagnosis of glioma patients. At 3 Tesla, detection of 2-HG with magnetic resonance spectroscopy is challenging because of metabolite signal overlap and a spectral pattern modulated by slice selection and chemical shift displacement. Using density matrix simulations and phantom experiments, an optimised semi-LASER scheme (TE = 110 ms) improves localisation of the 2-HG spin system considerably compared to an existing PRESS sequence. This results in a visible 2-HG peak in the in vivo spectra at 1.9 ppm in the majority of IDH mutated tumours. Detected concentrations of 2-HG were similar using both sequences, although the use of semi-LASER generated narrower confidence intervals. Signal overlap with glutamate and glutamine, as measured by pairwise fitting correlation was reduced. Lactate was readily detectable across glioma patients using the method presented here (mean CLRB: (10±2)%). Together with more robust 2-HG detection, long TE semi-LASER offers the potential to investigate tumour metabolism and stratify patients in vivo at 3T.

Echo time optimization for J-difference editing of glutathione at 3T

PURPOSE

To investigate the echo time (TE) dependence of J-difference editing of glutathione and to determine the optimal TE for in vivo measurements at 3T.

METHODS

Spatially resolved density-matrix simulations and phantom experiments were performed at a range of TEs to establish the spatial and TE modulation of glutathione signals in editing-on, editing-off, and difference spectra at 3T. In vivo data were acquired in five healthy subjects to compare a TE of 68 ms and a TE of 120 ms. At the longer TE, high-bandwidth, frequency-modulated, slice-selective refocusing pulses were also compared with conventional amplitude-modulated pulses.

RESULTS

Simulations and relaxation-corrected phantom experiments suggest that the maximum edited signal occurs at TE 160 ms, ignoring transverse relaxation. Considering in vivo T₂ relaxation times of 67-89 ms, the optimal in vivo TE is estimated to be 120 ms. In vivo measurements showed that this TE yielded 15% more signal than TE 68 ms. A further gain of 57% resulted from using improved slice-selective refocusing pulses.

CONCLUSION

J-difference editing of glutathione using TE 120 ms delivers increased signal due to improved editing efficiency that more than offsets T₂ losses. The additional TE also allows for use of improved slice-selective refocusing pulses, which results in additional signal gains. Magn Reson Med 77:498-504, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Separation and quantification of lactate and lipid at 1.3 ppm by diffusion-weighted magnetic resonance spectroscopy

PURPOSE

To separate the spectrally overlapped lactate and lipid signals at 1.3 ppm using diffusion-weighted magnetic resonance spectroscopy (DW-MRS) based on their large diffusivity difference.

METHODS

DW-MRS was applied to the gel phantoms containing lactate and lipid droplets, and to the rat brain tumors. Lactate and lipid signals and their apparent diffusion coefficients were computed from the diffusion-weighted proton spectra. Biexponential fitting and direct spectral subtraction approaches were employed and compared.

RESULTS

DW-MRS could effectively separate lactate and lipid signals both in phantoms and rat brain C6 glioma by biexponential fitting. In phantoms, lactate and lipid signals highly correlated with the known lactate concentration and lipid volume fractions. In C6 glioma, both lactate and lipid signals were detected, and the lipid signal was an order of magnitude higher than lactate signal. The spectral subtraction approach using three diffusion weightings also allowed the separation of lactate and lipid signals, yielding results comparable to those by the biexponential fitting approach.

CONCLUSION

DW-MRS presents a new approach to separate and quantify spectrally overlapped molecules and/or macromolecules, such as lactate and lipid, by using the diffusivity difference associated with their different sizes or mobility within tissue microstructure. Magn Reson Med 77:480-489, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Two-dimensional J-resolved LASER and semi-LASER spectroscopy of human brain

PURPOSE

Two-dimensional J-resolved localized and semi-localized by adiabatic selective refocusing (LASER and semi-LASER) spectroscopy, named "J-resolved LASER" and "J-resolved semi-LASER", were introduced to suppress chemical shift artifacts, additional J-refocused artifactual peaks from spatially dependent J-coupling evolution, and sensitivity to radiofrequency (RF) field inhomogeneity.

METHODS

Three pairs of adiabatic pulses were employed for voxel localization in J-resolved LASER and two pairs in J-resolved semi-LASER. The first half of t1 period was inserted between the last pair of adiabatic pulses, which was proposed in this work to obtain two-dimensional adiabatic J-resolved spectra of human brain for the first time. Phantom and human experiments were performed to demonstrate their feasibility and advantages over conventional J-resolved spectroscopy (JPRESS).

RESULTS

Compared to JPRESS, J-resolved LASER or J-resolved semi-LASER exhibited significant suppression of chemical shift artifacts and additional J-refocused peaks from spatially dependent J-coupling evolution, and demonstrated insensitivity to the change of RF frequency offset over large bandwidth.

CONCLUSION

Experiments on phantoms and human brains verified the feasibility and strengths of two-dimensional adiabatic J-resolved spectroscopy at 3T. This technique is expected to advance the application of in vivo two-dimensional MR spectroscopy at 3T and higher field strengths for more reliable and accurate quantification of metabolites.

Effect of J coupling on 1.3-ppm lipid methylene signal acquired with localised proton MRS at 3 T

The purpose of this work was to investigate the effect of J-coupling interactions on the quantification and T2 determination of 1.3-ppm lipid methylene protons at 3 T. The response of the 1.3-ppm protons of hexanoic, heptanoic, octanoic, linoleic and oleic acid was measured as a function of point-resolved spectroscopy (PRESS) and stimulated echo acquisition mode (STEAM) TE. In addition, a narrow-bandwidth refocusing PRESS sequence designed to rewind J-coupling evolution of the 1.3-ppm protons was applied to the five fatty acids, to corn oil and to tibial bone marrow of six healthy volunteers. Peak areas were plotted as a function of TE, and data were fitted to monoexponentially decaying functions to determine Mo (the extrapolated area for TE = 0 ms) and T2 values. In phantoms, rewinding J-coupling evolution resulted in 198%, 64%, 44%, 20% and 15% higher T2 values for heptanoic, octanoic, linoleic and oleic acid, and corn oil, respectively, compared with those obtained with standard PRESS. The narrow-bandwidth PRESS sequence also resulted in significant changes in Mo , namely -77%, -22%, 28%, 23% and 28% for heptanoic, octanoic, linoleic and oleic acid, and corn oil, respectively. T2 values obtained with STEAM were closer to the values measured with narrow-bandwidth PRESS. On average, in tibial bone marrow (six volunteers) rewinding J-coupling evolution resulted in 21% ± 3% and 9 % ± 1% higher Mo and T2 values, respectively. This work demonstrates that the consequence of neglecting to consider scalar coupling effects on the quantification of 1.3-ppm lipid methylene protons and their T2 values is not negligible. The linoleic and oleic acid T2 results indicate that T2 measures of lipids with standard MRS techniques are dependent on lipid composition.

Proton MRS and MRSI of the brain without water suppression

Water suppression (WS) techniques have played a vital role in the commencement and development of in vivo proton magnetic resonance spectroscopy (MRS, including spectroscopic imaging - MRSI). WS not only made in vivo proton MRS functionally available but also made its applications conveniently accessible, and it has become an indispensable tool in most of the routine applications of in vivo proton MR spectroscopy. On the other hand, WS brought forth some challenges. Therefore, various techniques of proton MRS without WS have been developed since the pioneering work in the late 1990s. After more than one and a half decades of advances in both hardware and software, non-water-suppressed proton MRS is coming to the stage of maturity and seeing increasing application in biomedical research and clinical diagnosis. In this article, we will review progress in the technical development and applications of proton MRS without WS.

Fetal MRI: A Technical Update with Educational Aspirations

Fetal magnetic resonance imaging (MRI) examinations have become well-established procedures at many institutions and can serve as useful adjuncts to ultrasound (US) exams when diagnostic doubts remain after US. Due to fetal motion, however, fetal MRI exams are challenging and require the MR scanner to be used in a somewhat different mode than that employed for more routine clinical studies. Herein we review the techniques most commonly used, and those that are available, for fetal MRI with an emphasis on the physics of the techniques and how to deploy them to improve success rates for fetal MRI exams. By far the most common technique employed is single-shot T2-weighted imaging due to its excellent tissue contrast and relative immunity to fetal motion. Despite the significant challenges involved, however, many of the other techniques commonly employed in conventional neuro- and body MRI such as T1 and T2*-weighted imaging, diffusion and perfusion weighted imaging, as well as spectroscopic methods remain of interest for fetal MR applications. An effort to understand the strengths and limitations of these basic methods within the context of fetal MRI is made in order to optimize their use and facilitate implementation of technical improvements for the further development of fetal MR imaging, both in acquisition and post-processing strategies.

The in vivo J-difference editing MEGA-PRESS technique for the detection of n-3 fatty acids

In this study, we present a method for the detection of n-3 fatty acid (n-3 FA) signals using MRS in adipose tissue in vivo. This method (called oMEGA-PRESS) is based on the selective detection of the CH3 signal of n-3 FA using the MEGA-PRESS (MEshcher-GArwood Point-RESolved Spectroscopy) J-difference editing technique. We optimized the envelope shape and frequency of spectral editing pulses to minimize the spurious co-editing and incomplete subtraction of the CH3 signal of other FAs, which normally obscure the n-3 FA CH3 signal in MR spectra acquired using standard PRESS techniques. The post-processing of the individual data scans with the phase and frequency correction before data subtraction and averaging was implemented to further improve the quality of in vivo spectra. The technique was optimized in vitro on lipid phantoms using various concentrations of n-3 FA and examined in vivo at 3 T on 15 healthy volunteers. The proportion of n-3 FA estimated by the oMEGA-PRESS method in phantoms showed a highly significant linear correlation with the n-3 FA content determined by gas chromatography. The signal attributed to n-3 FA was observed in all subjects. Comparisons with the standard PRESS technique revealed an enhanced identification of the n-3 FA signal using oMEGA-PRESS. The presented method may be useful for the non-invasive quantification of n-3 FA in adipose tissue, and could aid in obtaining a better understanding of various aspects of n-3 FA metabolism.

Assessment of diffusion tensor MR imaging (DTI) in liver fibrosis with minimal confounding effect of hepatic steatosis

PURPOSE

Given the potential confounding effect of fat on apparent diffusion coefficient (ADC) in the liver, we have assessed diffusion tensor imaging in liver fibrosis with minimal effect of fat on ADC and fractional anisotropy (FA).

METHODS

Thirty-six mice were used, among which 20 mice were CCl4 treated for fibrosis induction. Diffusion tensor imaging was performed at 9.4T using a spin-echo diffusion tensor imaging sequence with six gradient directions. Hepatic fat fraction obtained by MR spectroscopy was used as hepatic fat content. Fibrosis scores were obtained from histopathology.

RESULTS

The hepatic fat fractions of the two animal groups were below 5.5% and not different (5.3 ± 1.5 vs. 4.6 ± 1.1%; P = 0.115). Fibrosis scores were higher in CCl4 -treated mice (0.0 ± 0.0 vs. 2.1 ± 0.7; P < 0.001). Nonetheless, there was no difference in ADC between the two groups (0.711 ± 0.068 × 10(-3) vs. 0.718 ± 0.095 × 10(-3) mm(2) s(-1) ; P = 0.911). The treated group had a lower FA than control (0.552 ± 0.050 vs. 0.586 ± 0.013; P = 0.023). ADC was not correlated with hepatic fat fraction and fibrosis. FA was correlated with hepatic fat fraction (r = 0.418, P = 0.011) and fibrosis (r = -0.411, P = 0.012).

CONCLUSION

FA may be more sensitive to mild-to-moderate liver fibrosis than ADC. In addition to ADC, FA may also be sensitive to hepatic fat content, and therefore need careful interpretation in liver fibrosis with concomitant fatty liver.

Spectroscopic localization by simultaneous acquisition of the double-spin and stimulated echoes

PURPOSE

To design a proton MR spectroscopy ((1) H-MRS) localization sequence that combines the signal-to-noise-ratio (SNR) benefits of point resolved spectroscopy (PRESS) with the high pulse bandwidths, low chemical shift displacements (CSD), low specific absorption rates (SAR), short echo times (TE), and superior radiofrequency transmit field (B1+) immunity of stimulated echo acquisition mode (STEAM), by simultaneously refocusing and acquiring both the double-spin and stimulated echo coherence pathways from the volume of interest.

THEORY AND METHODS

We propose a family of (1)H-MRS sequences comprising three orthogonal spatially selective pulses with flip angles 90° < α, β, γ < 128°. The stimulated and double-spin echo are refocused in-phase simultaneously by altering the pulses' phases, flip angles and timing, as well as the interpulse gradient spoiling moments. The ≈ 90° nutations of α, β, γ provide STEAM-like advantages (lower SAR, in-plane CSD and TE; greater B1+ immunity), but with SNRs comparable with PRESS.

RESULTS

Phantom and in vivo brain experiments show that 83-100% of the PRESS SNR (metabolite-dependent) is achieved at under 75% of the SAR and 66% lower in-plane CSD.

CONCLUSION

The advantages of STEAM can be augmented with the higher SNR of PRESS by combining the spin and stimulated echoes. Quantification, especially of J-coupled resonances and intermediate and long TEs, must be carefully considered.

High-field small animal magnetic resonance oncology studies

This review focuses on the applications of high magnetic field magnetic resonance imaging (MRI) and spectroscopy (MRS) to cancer studies in small animals. High-field MRI can provide information about tumor physiology, the microenvironment, metabolism, vascularity and cellularity. Such studies are invaluable for understanding tumor growth and proliferation, response to treatment and drug development. The MR techniques reviewed here include (1)H, (31)P, chemical exchange saturation transfer imaging and hyperpolarized (13)C MRS as well as diffusion-weighted, blood oxygen level dependent contrast imaging and dynamic contrast-enhanced MRI. These methods have been proven effective in animal studies and are highly relevant to human clinical studies.

In vivo relaxation behavior of liver compounds at 7 Tesla, measured by single-voxel proton MR spectroscopy

PURPOSE

To assess the proton T1 and T2 relaxation of in vivo hepatic water, choline and lipid resonances with possible J-coupling behavior of lipids in healthy volunteers at 7 Tesla (T).

MATERIALS AND METHODS

Relaxation measurements were conducted on corn oil phantoms and on the hepatic tissue of 11 healthy volunteers at 7 T using a surface coil and a STEAM sequence. T1 's were determined by monoexponential fitting, and T2 's by both monoexponential and enhanced-exponential fitting (empirically designed to consider J-coupling of lipid resonances).

RESULTS

In vivo T1 's at 7 T were estimated as follows: water (4.70 ppm), 1362 ± 83 ms; methyl- (0.90 ppm), 1026 ± 162 ms; methylene- (1.30 ppm), 514 ± 25 ms; α-olefinic- (2.02 ppm), 488 ± 220 ms; α-carboxyl- (2.24 ppm), 476 ± 89 ms; diacyl- (2.77 ppm), 479 ± 260 ms group of lipid chains; and choline compounds (3.22 ppm), 1084 ± 52 ms. The T2 's calculated with enhanced fitting were as follows: water, 15 ± 2 ms; methyl-, 34 ± 10 ms; methylene-, 41 ± 8 ms; α-olefinic-, 44 ± 19 ms; α-carboxyl-, 39 ± 15 ms; diacyl-, 44 ± 5 ms group of lipid chains; and choline compounds, 32 ± 9 ms.

CONCLUSION

An accurate knowledge of in vivo relaxation and J-coupling behavior will significantly improve the quantification of an extended number of resolved liver metabolites at 7 T.

Two-dimensional semi-LASER correlation spectroscopy with well-maintained cross peaks

PURPOSE

To demonstrate that the limited bandwidth of the second 90° radiofrequency (RF) pulse in two-dimensional (2D) localized correlation spectroscopy (L-COSY) induces spatially dependent magnetization transfer that results in attenuated cross-peaks, and to propose a new 2D semi-adiabatically localized COSY sequence to solve this problem.

METHODS AND THEORY

A semi-localization by adiabatic selective refocusing (semi-LASER or sLASER) method was incorporated into the COSY sequence with the slice-selective first 90° RF pulse and the non-slice-selective second 90° RF pulse to form a new 2D sLASER localized COSY sequence, named "sLASER-first-COSY," to solve the problem of spatially dependent magnetization transfer. Experiments were performed to verify the feasibility and advantages of sLASER-first-COSY sequence over a recently reported other sLASER COSY sequence with a slice-selective second 90° RF pulse, named "sLASER-last-COSY".

RESULTS

Phantom, ex vivo, and in vivo human brain experiments demonstrated that sLASER-first-COSY yielded stronger cross peaks and higher ratios of cross peak volumes to diagonal peak volumes than sLASER-last-COSY.

CONCLUSION

As COSY relies on the cross peaks to obtain larger dispersion of peaks for quantification, the new sLASER-first-COSY sequence yielding well-maintained cross peaks will facilitate more reliable and accurate quantification of metabolites with coupled spin systems.

Improved efficiency on editing MRS of lactate and γ-aminobutyric acid by inclusion of frequency offset corrected inversion pulses at high fields

γ-Aminobutyric acid (GABA) and lactate are metabolites which are present in the brain. These metabolites can be indicators of psychiatric disorders or tumor hypoxia, respectively. The measurement of these weakly coupled spin systems can be performed using MRS editing techniques; however, at high field strength, this can be challenging. This is due to the low available B1 (+) field at high fields, which results in narrow-bandwidth refocusing pulses and, consequently, in large chemical shift displacement artifacts. In addition, as a result of the increased chemical shift displacement artifacts and chemical shift dispersion, the efficiency of the MRS method is reduced, even when using adiabatic refocusing pulses. To overcome this limitation, frequency offset corrected inversion (FOCI) pulses have been suggested as a mean to substantially increase the bandwidth of adiabatic pulses. In this study, a Mescher-Garwood semi-localization by adiabatic selection and refocusing (MEGA-sLASER) editing sequence with refocusing FOCI pulses is presented for the measurement of GABA and lactate in the human brain. Metabolite detection efficiencies were improved by 20% and 75% for GABA and lactate, respectively, when compared with editing techniques that employ adiabatic radiofrequency refocusing pulses. The highly efficient MEGA-sLASER sequence with refocusing FOCI pulses is an ideal and robust MRS editing technique for the measurement of weakly coupled metabolites at high field strengths.

J-refocused 1H PRESS DEPT for localized 13C MR spectroscopy

Proton point-resolved spectroscopy (PRESS) localization has been combined with distortionless enhanced polarization transfer (DEPT) in multinuclear MRS to overcome the signal contamination problem in image-selected in vivo spectroscopy (ISIS)-combined DEPT, especially for lipid detection. However, homonuclear proton scalar couplings reduce the DEPT enhancement by modifying the spin coherence distribution under J modulation during proton PRESS localization. Herein, a J-refocused proton PRESS-localized DEPT sequence is presented to obtain simultaneously enhanced and localized signals from a large number of metabolites by in vivo (13) C MRS. The suppression of J modulation during PRESS and the substantial recovery of signal enhancement by J-refocused PRESS-localized DEPT were demonstrated theoretically by product operator formalism, numerically by the spin density matrix simulations for different scalar coupling conditions, and experimentally with a glutamate phantom at various TEs, as well as a colza oil phantom. The application of the sequence for localized detection of saturated and unsaturated fatty acids in the calf bone marrow and skeletal muscle of healthy subjects yielded high signal enhancements simultaneously obtained for all components.

Utility of proton MR spectroscopy for differentiating typical and atypical primary central nervous system lymphomas from tumefactive demyelinating lesions

BACKGROUND AND PURPOSE

It may be challenging to differentiate primary CNS lymphomas, especially primary CNS lymphomas with atypical MR features, from tumefactive demyelinating lesions by the use of conventional MR. This study aimed to investigate the usefulness of (1)H-MR spectroscopy for making this discrimination.

MATERIALS AND METHODS

Forty-four patients with primary CNS lymphomas and 21 with tumefactive demyelinating lesions were enrolled. Single-voxel (TE = 144 ms) (1)H-MR spectroscopy scans with the use of the point-resolved spectroscopy sequence were retrospectively analyzed. The Cho/Cr and Cho/NAA area ratios were calculated. The lipid and/or lactate peak was visually categorized into 5 grades on the basis of comparison with the height of the Cr peak. The (1)H-MR spectroscopy findings were compared in all of the primary CNS lymphomas and the tumefactive demyelinating lesions and in the subgroup of atypical primary CNS lymphomas and tumefactive demyelinating lesions. The thresholds and added value of (1)H-MR spectroscopy to conventional MR were calculated by use of receiver operating characteristic curves.

RESULTS

Discrepancies between all of the primary CNS lymphomas and tumefactive demyelinating lesions were found in the Cho/Cr ratio (P = .000), Cho/NAA ratio (P = .000), and the lipid and/or lactate peak grade (P = .000). Lymphoma rather than tumefactive demyelinating lesions was suggested when the Cho/Cr ratio was >2.58, the Cho/NAA ratio was >1.73, and a high lipid and/or lactate peak grade (grade >3) was seen. Higher Cho/Cr ratios, Cho/NAA ratios, and lipid and/or lactate peak grades were found in atypical primary CNS lymphomas when compared with those of tumefactive demyelinating lesions. The area under the receiver operating characteristic curve of conventional MR was improved from 0.827 to 0.870 when Cho/NAA ratio was added in the uncertain cases.

CONCLUSIONS

(1)H-MR spectroscopy may be useful for differentiating primary CNS lymphomas from tumefactive demyelinating lesions. Cho/NAA ratio could provide added value to conventional MR imaging.

Slice-selective broadband refocusing pulses for the robust generation of crushed spin-echoes

A major challenge for in vivo magnetic resonance spectroscopy with point-resolved spectroscopy (PRESS) is the low signal intensity for the measurement of weakly scalar coupled spins, for example lactate. The chemical-shift displacement error between the two coupling partners of the lactate molecule leads to a signal decrease. The chemical-shift displacement error is decreased and therefore the lactate signal is increased by using refocusing pulses with a broad bandwidth. Previously, slice-selective broadband universal rotation pulses (S-BURBOP) were designed and applied as refocusing pulses in the PRESS pulse sequence (Janich MA, et al., Journal of Magnetic Resonance, 2011, 213, 126-135). However, S-BURBOP pulses leave a phase error across the slice which is superimposed on the spectra when spatially resolving the PRESS voxel. In the present novel design of slice-selective broadband refocusing pulses (S-BREBOP) this phase error is avoided. S-BREBOP pulses obtain 2.5 times the bandwidth of conventional Shinnar-Le Roux pulses and are robust against ±20% miscalibration of the B(1) amplitude. S-BREBOP pulses were validated in phantoms and in a low-grade brain tumor of a patient. Compared to conventional Shinnar-Le Roux pulses they lead to a decrease of the chemical-shift displacement error and consequently a lactate signal increase.

In vivo (1)H-MRS hepatic lipid profiling in nonalcoholic fatty liver disease: an animal study at 9.4 T

The applicability of the in vivo proton magnetic resonance spectroscopy hepatic lipid profiling (MR-HLP) technique in nonalcoholic fatty liver disease was investigated. Using magnetic resonance spectroscopy, the relative fractions of diunsaturated (fdi), monounsaturated (fmono), and saturated (fsat) fatty acids as well as total hepatic lipid content were estimated in the livers of 8 control and 23 CCl4-treated rats at 9.4 T. The mean steatosis, necrosis, inflammation, and fibrosis scores of the treated group were all significantly higher than those of the control group (P < 0.01). There was a strong correlation between the histopathologic parameters and the MR-HLP parameters (r = 0.775, P < 0.01) where both steatosis and fibrosis are positively correlated with fmono and negatively correlated with fdi. Both necrosis and inflammation, however, were not correlated with any of the MR-HLP parameters. Hepatic lipid composition appears to be changed in association with the severity of steatosis and fibrosis in nonalcoholic fatty liver disease, and these changes can be depicted in vivo by using the MR-HLP method at 9.4 T. Thus, while it may not likely be that MR-HLP helps differentiate between steatohepatitis in its early stages and simple steatosis, these findings altogether are in support of potential applicability of in vivo MR-HLP at high field in nonalcoholic fatty liver disease.

Robust slice-selective broadband refocusing pulses

Slice-selective broadband refocusing pulses are of great interest in localized MR spectroscopy for improving spatial selectivity, reducing chemical-shift displacement errors, and reducing anomalous J modulation. In practice the bandwidth of RF pulses is limited by the maximum available B1 amplitude. The goal of the present work is to design slice-selective and broadband refocusing pulses which are tolerant against B1 deviations. Pulse design is performed by numerical optimization based on optimal control theory. A comprehensive study of different cost functions and their effect on the optimization is given. The optimized slice-selective broadband refocusing pulses are compared to conventional Shinnar-Le Roux (SLR), broadband SLR, and hyperbolic secant pulses. In simulations and experiments optimized pulses were shown to fulfill broadband slice specifications over a range of ±20% B1 scalings. Experimental validation showed a reduction of chemical-shift displacement error by a factor of 3 compared to conventional SLR pulses.

Effect of J-coupling on lipid composition determination with localized proton magnetic resonance spectroscopy at 9.4 T

PURPOSE

To demonstrate, at 9.4 T, that J-coupling interactions exhibited by lipid protons affects lipid composition determination with a point resolved spectroscopy (PRESS) sequence.

MATERIALS AND METHODS

Experiments were conducted on four oils (almond, corn, sesame, and sunflower), on visceral adipose tissue of a euthanized mouse, and on pure linoleic acid at 9.4 T. The 2.1, 2.3, and 2.8 ppm resonances were measured at multiple echo times (TEs) by a standard PRESS sequence and by a PRESS sequence consisting of narrow-bandwidth refocusing pulses designed to rewind the J-coupling evolution of the target peak protons in the voxel of interest. T(2) corrections were performed on both groups of data for the three peaks and lipid compositions for the oils and for the mouse tissue were determined. Lipid compositions were also calculated from a short-TE standard PRESS spectrum.

RESULTS

A chemical analysis of the samples was not performed; however, the oil compositions calculated from resonance peaks acquired with the PRESS sequence designed to minimize J-coupling effects, following T(2) relaxation correction, closely agreed with values in the literature, which was not the case for all of the compositions determined from the regular PRESS spectra.

CONCLUSION

The presented work brings to attention the significance of J-coupling effects when calculating lipid compositions from localized proton spectra.

Noninvasive quantification of ascorbate and glutathione concentration in the elderly human brain

In this study, ascorbate (Asc) and glutathione (GSH) concentrations were quantified noninvasively using double-edited (1)H MRS at 4 T in the occipital cortex of healthy young [age (mean ± standard deviation) = 20.4 ± 1.4 years] and elderly (age = 76.6 ± 6.1 years) human subjects. Elderly subjects had a lower GSH concentration than younger subjects (p < 0.05). The Asc concentration was not significantly associated with age. Furthermore, the lactate (Lac) concentration was higher in elderly than young subjects. Lower GSH and higher Lac concentrations are indications of defective protection against oxidative damage and impaired mitochondrial respiration. The extent to which the observed concentration differences could be associated with physiological differences and methodological artifacts is discussed. In conclusion, GSH and Asc concentrations were compared noninvasively for the first time in young vs elderly subjects.

Measurement of N-acetylaspartylglutamate in the human frontal brain by 1H-MRS at 7 T

N-Acetylaspartylglutamate in human brain has been measured with difference editing at 7 T. The CH(2) proton resonances (∼ 2.5 ppm) of the aspartyl groups of N-acetylaspartylglutamate and N-acetylaspartate were difference edited (MEGA) using 20-msec gaussian radiofrequency pulses for selective 180 ° rotations of the coupling partners at 4.61 and 4.38 ppm, respectively. The echo time of the editing sequence, 108 msec, was obtained in phantom tests. Single-voxel localized in vivo measurements were conducted in the medial prefrontal and right frontal cortices of five healthy volunteers. The gray and white matter fractions within the voxels were obtained from T(1)-weighted image segmentation. Using linear regression of the metabolite concentration vs. fractional white matter contents within the voxels, the N-acetylaspartylglutamate-to-N-acetylaspartate concentration ratios in gray and white matter were estimated to be 0.13 and 0.28 by difference editing (95% confidence intervals 0.07-0.19 and 0.22-0.34), respectively, assuming identical relaxation effects between the metabolites.

If J doesn't evolve, it won't J-resolve: J-PRESS with bandwidth-limited refocusing pulses

There is increasing interest in the J-PRESS technique, an in vivo implementation of two-dimensional J-spectroscopy combined with PRESS localization, for high-field spectroscopy studies of the human brain. The experiment is designed to resolve scalar couplings in the second, indirectly detected dimension, but will only do so if the slice-selective refocusing pulses in the PRESS sequence affect all coupled spins equally. At high magnet field strengths, due to limited RF pulse bandwidth, PRESS-based localization results in spatially dependent evolution of coupling. In some regions of the localized volume, coupling evolves during the PRESS echo time, while in other regions it may be partially or fully refocused. This study investigates the impact of this effect on the appearance of the J-PRESS spectrum for coupled spins, focusing on two commonly observed metabolites, lactate and N-acetyl aspartate, showing that such behavior results in additional peaks in the J-resolved spectrum (termed J-refocused peaks). It is also demonstrated that increasing the bandwidth of refocusing pulses significantly reduces the size of such signals.

Non-invasive detection and quantification of human foetal brain lactate in utero by magnetic resonance spectroscopy

OBJECTIVE

To assess the feasibility of foetal cerebral lactate detection and quantification by proton magnetic resonance spectroscopy ((1)H-MRS) in pregnancies at increased risk of cerebral hypoxia, using a clinical 1.5 T magnetic resonance imaging (MRI) system.

METHOD

Localised (1)H-MRS was performed in four patients with pregnancies in their third trimester complicated by intrauterine growth restriction (IUGR). A long echo time (TE) of 288 ms was used to maximise detection and conspicuity of the lactate methyl resonance, together with a short TE MRS acquisition to check for the presence of lipid contamination. Individual peaks in the resulting spectra were measured, corrected for relaxation and referenced to the unsuppressed water signal to provide metabolite concentrations.

RESULTS

A resonance peak consistent with the presence of lactate was observed in all cases. In one subject, this was confounded by the identification of significant lipid contamination in the short TE MRS acquisition. The range of measured lactate concentrations was 2.0-3.3 mmol/kg and compared well with preterm neonatal MRS studies.

CONCLUSION

The non-invasive detection and quantification of foetal cerebral lactate by MRS is achievable on a clinical 1.5 T MRI system.

Quantification of ethanol methyl (1)H magnetic resonance signal intensity following intravenous ethanol administration in primate brain

In vivo(1)H magnetic resonance spectroscopy (MRS) can be used to directly monitor brain ethanol. Previously, studies of human subjects have lead to the suggestion that the ethanol methyl (1)H MRS signal intensity relates to tolerance to ethanol's intoxicating effects. More recently, the ethanol (1)H MRS signal intensity has been recognized to vary between brain gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF) due to differences in T(2) within these environments. The methods presented here extend ethanol MRS techniques to non-human primate subjects. Twelve monkeys were administered ethanol while sedated and positioned within a 3T MRI system. Chemical shift imaging (CSI) measurements were performed following intravenous infusion of 1g/kg ethanol. Magnetic resonance imaging (MRI) data were also recorded for each monkey to provide volume fractions of GM, WM, and CSF for each CSI spectrum. To estimate co-variance of ethanol MRS intensity with GM, WM, and CSF volume fractions, the relative contribution of each tissue subtype was determined following corrections for radiofrequency pulse profile non-uniformity, chemical shift artifacts, and differences between the point spread function in the CSI data and the imaging data. The ethanol MRS intensity per unit blood ethanol concentration was found to differ between GM, WM, and CSF. Individual differences in MRS intensity were larger in GM than WM. This methodology demonstrates the feasibility of ethanol MRS experiments and analysis in non-human primate subjects, and suggests GM may be a site of significant variation in ethanol MRS intensity between individuals.

PRESS echo time behavior of triglyceride resonances at 1.5T: detecting omega-3 fatty acids in adipose tissue in vivo

AIM

This study investigated the impact of fatty acid (FA) composition on the echo time behavior of triglyceride resonances in a clinical setting. The feasibility of (1)H NMR spectroscopy to detect these resonances was also evaluated in human adipose tissue in vivo.

METHOD

Ten edible oils chosen to cover a wide spectrum of FA compositions were used as phantom material. The detailed FA composition and intrinsic proton spectra of the oils were characterized by gas chromatography and high-resolution (1)H NMR spectroscopy (11.7T), respectively. The detailed echo time behavior of the oils were subsequently measured by (1)H NMR spectroscopy in a clinical scanner (1.5T) using PRESS. The effect of temperature was investigated in five oils.

RESULTS

The olefinic (5.3 ppm) and diallylic (2.8 ppm) resonances exhibited distinct J-modulation patterns independent of oil FA composition. The methylene resonance (1.3 ppm) displayed an exponential decay, with the apparent T(2) showing a weak positive correlation with oil unsaturation (R=0.628, P=0.052), probably a result of changes in viscosity. For the methyl resonance (0.9 ppm), oils high in omega-3 FA displayed a markedly different J-modulation pattern compared to non-omega-3 oils. The characteristic J-modulation of the omega-3 methyl group could be attributed to the phase behavior of the omega-3 methyl triplet signal (all triplet lines in-phase at TE of 135 ms), a result of the omega-3 methyl end forming a first order spin system. The omega-3 methyl outer triplet line at 1.08ppm of the TE=140 ms spectrum was found to be useful for determining the omega-3 content of the oils (R=0.999, standard error of estimate (SE) 0.80). The olefinic and diallylic proton resonance (measured at TE=50 ms) areas correlated with the olefinic (R=0.993, SE 0.33) and diallylic (R=0.997, SE 0.19) proton contents calculated from the GC data. Information derived from long echo time spectra (TE=200) demonstrated good correlations to GC data and showed no change with increasing temperature (and T(2)). In (1)H NMR spectra (1.5T) of adipose tissue in five healthy subjects, the analytically important olefinic and diallylic resonances were clearly resolved with a coefficient of variation of 1.6% and 8.4%, respectively, for repeated measurements. The characteristic phase behavior of the omega-3 methyl outer triplet line at 1.08 ppm could also be detected at very long echo times (470 and 540 ms).

CONCLUSION

Fatty acid composition has an impact on the echo time behavior of triglyceride resonances. Long TE spectra can resolve omega-3 FA in adipose tissue in vivo. These findings will benefit long TE studies of tissue lipids.

RF pulses for in vivo spectroscopy at high field designed under conditions of limited power using optimal control

Localized in vivo spectroscopy at high magnetic field strength (>3T) is susceptible to localization artifacts such as the chemical shift artifact and the spatial interference artifact for J-coupled spins. This latter artifact results in regions of anomalous phase for J-coupled spins. These artifacts are exacerbated at high magnetic field due to the increased frequency dispersion, coupled with the limited RF pulse bandwidths used for localization. Approaches to minimize these artifacts include increasing the bandwidth of the frequency selective excitation pulses, and the use of frequency selective saturation pulses to suppress the signals in the regions with anomalous phase. The goal of this article is to demonstrate the efficacy of optimal control methods to provide broader bandwidth frequency selective pulses for in vivo spectroscopy in the presence of limited RF power. It is demonstrated by examples that the use of optimal control methods enable the generation of (i) improved bandwidth selective excitation pulses, (ii) more efficient selective inversion pulses to be used for generation of spin echoes, and (iii) improved frequency selective saturation pulses. While optimal control also allows for the generation of frequency selective spin echo pulses, it is argued that it is more efficient to use dual inversion pulses for broadband generation of spin echoes. Finally, the optimal control routines and example RF pulses are made available for downloading.

Measurement of glutathione in human brain at 3T using an improved double quantum filter in vivo

A single voxel proton NMR double quantum filter (DQF) for measurement of glutathione (GSH) in human brain at 3T is reported. Yield enhancement for the CH(2) resonances of the cysteine moiety at 2.95ppm has been achieved by means of dual encoding. After the preparation of double quantum and zero quantum coherences (DQC and ZQC) at equal magnitude, the first DQC encoding was followed by interchange of DQC and ZQC, and another DQC encoding. The multi-quantum coherences were fully utilized to generate a GSH target signal at approximately 2.95ppm. The optimal echo time and the editing efficiency were obtained with numerical analysis of the filtering performance and phantom measurements. The dual-DQC encoding method provided GSH yield greater by a factor of 2.1 than single-DQC encoding for identical slice-selective RF pulses in phantom tests. Using the phantom relaxation times and the ratio of edited GSH to N-acetylaspartate (NAA) 2.0-ppm peak areas, the concentration of GSH in the medial parietal cortex of the healthy human brain in vivo was estimated to be 1.0+/-0.3mM (mean+/-SD, n=7), with reference to NAA at 10mM.

J-difference lactate editing at 3.0 Tesla in the presence of strong lipids

PURPOSE

To implement in vivo detection of lactate in the presence of lipids by proton magnetic resonance spectroscopy at a 3 Tesla (T) field strength for potential applications in human tumors outside of the brain.

MATERIALS AND METHODS

The BASING J-difference sequence was implemented in the presence of high lipid concentrations in phantoms and in vivo at 3 Tesla.

RESULTS

The effectiveness of the lactate editing scheme is demonstrated in phantoms containing both lactate and lipids and in vivo in ischemic induced human muscle.

CONCLUSION

The ability of the BASING J-difference technique to detect lactate in the presence of strong lipid signals outside the brain at 3T is feasible. This robust technique should permit noninvasive lactate measurements in human tumors to investigate its potential as a prognostic indicator.

Detection of glutamate and glutamine (Glx) by turbo spectroscopic imaging

Turbo spectroscopic imaging (TSI) is a spin echo spectroscopic imaging technique in which two or more echoes are acquired per excitation to reduce the acquisition time. The application of TSI has primarily been limited to the detection of uncoupled spins because the signal from coupled spins is modulated as a function of echo time. In this work we demonstrate how the TSI sequence can be modified to observe spins like the C(2) protons of Glx (approximately 3.75 ppm) which are involved solely in weak-coupling interactions. The technique exploits the chemical shift displacement effect by employing TSI refocusing pulses that have bandwidths which are less than the chemical shift difference between the target spins and the spins to which they are weakly coupled. The modified TSI sequence rewinds the J-evolution of the target protons in the slice of interest independently of the echo time or echo spacing, thereby removing any signal variation between successive echoes (apart from T(2) relaxation effects). In this study we tailored the narrow-bandwidth TSI sequence for observation of the C(2) Glx protons. The echo time was experimentally optimized to minimize signal contamination from myo-inositol, and the efficacy of the method was verified on phantom solutions of Glx and on brain in vivo.

Numerical simulations of localized high field 1H MR spectroscopy

The limited bandwidths of volume selective RF pulses in localized in vivo MRS experiments introduce spatial artifacts that complicate spectral quantification of J-coupled metabolites. These effects are commonly referred to as a spatial interference or "four compartment" artifacts and are more pronounced at higher field strengths. The main focus of this study is to develop a generalized approach to numerical simulations that combines full density matrix calculations with 3D localization to investigate the spatial artifacts and to provide accurate prior knowledge for spectral fitting. Full density matrix calculations with 3D localization using experimental pulses were carried out for PRESS (TE=20, 70 ms), STEAM (TE=20, 70 ms) and LASER (TE=70 ms) pulse sequences and compared to non-localized simulations and to phantom solution data at 4 T. Additional simulations at 1.5 and 7 T were carried out for STEAM and PRESS (TE=20 ms). Four brain metabolites that represented a range from weak to strong J-coupling networks were included in the simulations (lactate, N-acetylaspartate, glutamate and myo-inositol). For longer TE, full 3D localization was necessary to achieve agreement between the simulations and phantom solution spectra for the majority of cases in all pulse sequence simulations. For short echo time (TE=20 ms), ideal pulses without localizing gradients gave results that were in agreement with phantom results at 4 T for STEAM, but not for PRESS (TE=20). Numerical simulations that incorporate volume localization using experimental RF pulses are shown to be a powerful tool for generation of accurate metabolic basis sets for spectral fitting and for optimization of experimental parameters.

Anomalous J-modulation effects on amino acids in clinical 3T MR spectroscopy

The signal-intensity loss from anomalous J-modulation effects due to chemical-shift displacement was investigated on amino acid groups (alanine, valine, leucine, and isoleucine) at 3T by using point-resolved (1)H spectroscopy in patients with brain abscess and phantom experiments. With a larger chemical shift between methyl and methine resonances, alanine shows a greater effect of signal-intensity cancellation compared with other amino acids around 0.9 ppm, resulting in noninverted doublets at a TE of 144 ms.

Lactate and T (2) measurements of synovial aspirates at 1.5 T: differentiation of septic from non-septic arthritis

OBJECTIVE

The aim of this study was to differentiate septic from non-septic arthritis by measuring lactate concentration with (1)H magnetic resonance spectroscopy (HMRS) and by estimating total protein content with the assessment of T (2) values.

MATERIALS AND METHODS

In 30 patients with acute arthritis, synovial fluid was aspirated. Lactate concentrations were analyzed with single voxel HMRS at 1.5 T. T (2) relaxation times were mapped with a multi-spin echo sequence. All samples underwent microbiological testing and routine laboratory analysis to quantify lactate concentration and total protein content. Values obtained in septic and non-septic arthritis were compared with a Mann-Whitney U test.

RESULTS

Synovial fluid from patients with septic arthritis (n = 10) had higher concentrations of lactate (11.4 +/- 4.0 mmol/L) and higher total protein content (51.8 +/- 10.7 g/L) than fluid obtained in non-septic arthritis (n = 20; 5.2 +/- 1.1 mmol/L and 40.4 +/- 6.9 g/L, respectively, p < 0.001 and <0.01, respectively). Measured lactate concentrations and T (2) relaxation times (as an indicator of total protein content) were moderately correlated to laboratory-confirmed lactate concentration (r (2) = 0.71) and total protein content (r (2) = 0.73). Markedly increased lactate concentrations (>6 mmol/L) in combination with low T (2) values (<550 ms) identify septic arthritis with a sensitivity of 70% and a specificity of 89%.

CONCLUSION

Spectroscopic measurements of lactate concentration in combination with the estimation of protein content using T (2) may be of value in the differentiation of septic from non-septic arthritis.

Lactate detection by MRS in mitochondrial encephalopathy: optimization of technical parameters

Mitochondriopathies are a heterogeneous group of diseases with variable phenotypic presentation, which can range from subclinical to lethal forms. They are related either to DNA mutations or nuclear-encoded mitochondrial genes that affect the integrity and function of these organelles, compromising adenosine triphosphate (ATP) synthesis. Magnetic resonance (MR) is the most important imaging technique to detect structural and metabolic brain abnormalities in mitochondriopathies, although in some cases these studies may present normal results, or the identified brain abnormalities may be nonspecific. Magnetic resonance spectroscopy (MRS) enables the detection of high cerebral lactate levels, even when the brain has normal appearance by conventional MR scans. MRS is a useful tool for the diagnosis of mitochondriopathies, but must be correlated with clinical, neurophysiological, biochemical, histological, and molecular data to corroborate the diagnosis. Our aim is to clarify the most relevant issues related to the use of MRS in order to optimize its technical parameters, improving its use in the diagnosis of mitochondriopathies, which is often a challenge.

Exploiting the chemical shift displacement effect in the detection of glutamate and glutamine (Glx) with PRESS

A PRESS (Point RESolved Spectroscopy) sequence for the improved detection of the C2 protons of Glx (glutamate and glutamine) at approximately 3.75ppm is presented in this work. It is shown that for spins like the C2 protons of Glx which are involved solely in weak coupling interactions, the chemical shift displacement effect can be turned to advantage by exploiting PRESS refocusing pulses with bandwidths less than the chemical shift difference between the target spins and the spins to which they are weakly coupled. The narrow-bandwidth PRESS sequence allows refocusing of the J-coupling evolution of the target protons in the voxel of interest independently of echo time yielding signal equivalent to that which can be obtained with a one-pulse acquire sequence (assuming ideal pulses and ignoring T2 relaxation). The total echo time of PRESS was set long enough for the decay of macromolecule signal and the two echo times were empirically optimized so that the Glx signal at 3.75ppm suffered minimal contamination from myo-inositol. The efficacy of the method was verified on phantom solutions of Glx and on brain in vivo.