Reliable detection of macromolecules in single-volume 1H NMR spectra of the human brain

Brain lipidomics: From functional landscape to clinical significance

Lipids are crucial components of cellular function owing to their role in membrane formation, intercellular signaling, energy storage, and homeostasis maintenance. In the brain, lipid dysregulations have been associated with the etiology and progression of neurodegeneration and other neurological pathologies. Hence, brain lipids are emerging as important potential targets for the early diagnosis and prognosis of neurological diseases. This review aims to highlight the significance and usefulness of lipidomics in diagnosing and treating brain diseases. We explored lipid alterations associated with brain diseases, paying attention to organ-specific characteristics and the functions of brain lipids. As the recent advances in brain lipidomics would have been impossible without advances in analytical techniques, we provide up-to-date information on mass spectrometric approaches and integrative analysis with other omic approaches. Last, we present the potential applications of lipidomics combined with artificial intelligence techniques and interdisciplinary collaborative research for treating brain diseases with clinical heterogeneities.

Measurement of regional variation of GABA in the human brain by optimized point-resolved spectroscopy at 7 T in vivo

The (1)H resonances of γ-aminobutyric acid (GABA) in the human brain in vivo are extensively overlapped with the neighboring abundant resonances of other metabolites and remain indiscernible in short-TE MRS at 7 T. Here we report that the GABA resonance at 2.28 ppm can be fully resolved by means of echo time optimization of a point-resolved spectroscopy (PRESS) scheme. Following numerical simulations and phantom validation, the subecho times of PRESS were optimized at (TE, TE2) = (31, 61) ms for detection of GABA, glutamate (Glu), glutamine (Gln), and glutathione (GSH). The in vivo feasibility of the method was tested in several brain regions in nine healthy subjects. Spectra were acquired from the medial prefrontal, left frontal, medial occipital, and left occipital brain and analyzed with LCModel. Following the gray and white matter (GM and WM) segmentation of T1 -weighted images, linear regression of metabolite estimates was performed against the fractional GM contents. The GABA concentration was estimated to be about seven times higher in GM than in WM. GABA was overall higher in frontal than in occipital brain. Glu was about twice as high in GM as in WM in both frontal and occipital brain. Gln was significantly different between frontal GM and WM while being similar between occipital GM and WM. GSH did not show significant dependence on tissue content. The signals from N-acetylaspartylglutamate were clearly resolved, giving the concentration more than 10 times higher in WM than in GM. Our data indicate that the PRESS TE = 92 ms method provides an effective means for measuring GABA and several challenging J-coupled spin metabolites in human brain at 7 T.

Semi-LASER 1 H MR spectroscopy at 7 Tesla in human brain: Metabolite quantification incorporating subject-specific macromolecule removal

PURPOSE

To develop an in vivo ¹ H short-echo-time semi-LASER spectroscopy protocol at 7 Tesla (T) incorporating subject-specific macromolecule removal.

METHODS

T₁ constants of the major metabolites were measured with little macromolecule contribution in seven healthy volunteers and used to optimize double inversion metabolite nulling. Spectra were acquired from parietal-occipital cortex of five healthy volunteers. Metabolite-nulled macromolecule spectra were subtracted from the metabolite spectra before fitting in the time domain with prior-knowledge templates. Absolute metabolite concentrations were determined by referencing to the water signal, following partial volume and relaxation corrections.

RESULTS

The average signal to noise ratio, N-acetylaspartate peak height divided by the baseline noise standard deviation, was 48 ± 6. T₁ constants for N-acetylaspartate, glutamate, creatine, and choline were 1.71 ± 0.15 s, 1.68 ± 0.19 s, 1.63 ± 0.10 s, and 1.41 ± 0.09 s, respectively. The optimal double inversion times for metabolite suppression were TI₁  = 2.09 s and TI₂  = 0.52 s. The coefficient of variation was less than 10% for N-acetylaspartate, creatine, choline, and myo-inositol, and less than 20% for glutamate and glutamine.

CONCLUSION

Short echo-time ¹ H semi-LASER spectroscopy at 7T incorporating subject-specific macromolecule removal yielded reproducible brain metabolite concentrations ideal for applications in disease conditions where macromolecule contributions may deviate from the norm. Magn Reson Med 74:4-12, 2015. © 2014 Wiley Periodicals, Inc.

Smoothness of in vivo spectral baseline determined by mean-square error

PURPOSE

A nonparametric smooth line is usually added to the spectral model to account for background signals in vivo magnetic resonance spectroscopy (MRS). The assumed smoothness of the baseline significantly influences quantitative spectral fitting. In this paper, a method is proposed to minimize baseline influences on the estimated spectral parameters.

METHODS

The nonparametric baseline function with a given smoothness was treated as a function of spectral parameters. Its uncertainty was measured by root-mean-square error (RMSE). The proposed method was demonstrated with a simulated spectrum and in vivo spectra of both short echo time and averaged echo times. The estimated in vivo baselines were compared with the metabolite-nulled spectra and the LCModel-estimated baselines. The accuracies of estimated baseline and metabolite concentrations were further verified via cross-validation.

RESULTS

An optimal smoothness condition was found that led to the minimal baseline RMSE. In this condition, the best fit was balanced against minimal baseline influences on metabolite concentration estimates.

CONCLUSION

Baseline RMSE can be used to indicate estimated baseline uncertainties and serve as the criterion for determining the baseline smoothness of in vivo MRS.

Soft constraints in nonlinear spectral fitting with regularized lineshape deconvolution

This article presents a novel method for incorporating a priori knowledge into regularized nonlinear spectral fitting as soft constraints. Regularization was recently introduced to lineshape deconvolution as a method for correcting spectral distortions. Here, the deconvoluted lineshape was described by a new type of lineshape model and applied to spectral fitting. The nonlinear spectral fitting was carried out in two steps that were subject to hard constraints and soft constraints, respectively. The hard constraints step provided a starting point and, therefore, only the changes of the relevant variables were constrained in the soft constraints step and incorporated into the linear substeps of the Levenberg-Marquardt algorithm. The method was demonstrated using localized averaged echo time point resolved spectroscopy proton spectroscopy of human brains.

Quantitative proton magnetic resonance spectroscopy and spectroscopic imaging of the brain: a didactic review

This article presents background information related to methodology for estimating brain metabolite concentration from magnetic resonance spectroscopy (MRS) and magnetic resonance spectroscopic imaging measurements of living human brain tissue. It reviews progress related to this methodology, with emphasis placed on progress reported during the past 10 years. It is written for a target audience composed of radiologists and magnetic resonance imaging technologists. It describes in general terms the relationship between MRS signal amplitude and concentration. It then presents an overview of the many practical problems associated with deriving concentration solely from absolute measured signal amplitudes and demonstrates how a various signal calibration approaches can be successfully used. The concept of integrated signal amplitude is presented with examples that are helpful for qualitative reading of MRS data as well as for understanding the methodology used for quantitative measurements. The problems associated with the accurate measurement of individual signal amplitudes in brain spectra having overlapping signals from other metabolites and overlapping nuisance signals from water and lipid are presented. Current approaches to obtaining accurate amplitude estimates with least-squares fitting software are summarized.

Improvement of resolution for brain coupled metabolites by optimized (1)H MRS at 7T

Resolution enhancement for glutamate (Glu), glutamine (Gln) and glutathione (GSH) in the human brain by TE-optimized point-resolved spectroscopy (PRESS) at 7 T is reported. Sub-TE dependences of the multiplets of Glu, Gln, GSH, γ-aminobutyric acid (GABA) and N-acetylaspartate (NAA) at 2.2-2.6 ppm were investigated with density matrix simulations, incorporating three-dimensional volume localization. The numerical simulations indicated that the C4-proton multiplets can be completely separated with (TE(1), TE(2)) = (37, 63) ms, as a result of a narrowing of the multiplets and suppression of the NAA 2.5 ppm signal. Phantom experiments reproduced the signal yield and lineshape from simulations within experimental errors. In vivo tests of optimized PRESS were conducted on the prefrontal cortex of six healthy volunteers. In spectral fitting by LCModel, Cramér-Rao lower bounds (CRLBs) of Glu, Gln and GSH were 2 ± 1, 5 ± 1 and 6 ± 2 (mean ± SD), respectively. To evaluate the performance of the optimized PRESS method under identical experimental conditions, stimulated-echo spectra were acquired with (TE, TM) = (14, 37) and (74, 68) ms. The CRLB of Glu was similar between PRESS and short-TE stimulated-echo acquisition mode (STEAM), but the CRLBs of Gln and GSH were lower in PRESS than in both STEAM acquisitions.

Measurement sequences for single voxel proton MR spectroscopy

Proton spectroscopy is easy to perform at clinically used whole body MR scanners, since the necessary hardware is identical to that of MR imaging. Only specific measurements sequences for the acquisition of volume selected spectral information and appropriate evaluation software have to be installed. Several techniques for a volume-selective spectroscopy have been proposed, but only two are widely used in clinical examinations: the double spin-echo sequence (point resolved spectroscopy sequence (PRESS)) and the stimulated echo sequence (stimulated echo acquisition mode (STEAM)). The properties of these sequences are described and additional techniques for artifact reduction and the extraction of selected signals are explained.

Short-TE localised 1H MRS of the human brain at 3 T: quantification of the metabolite signals using two approaches to account for macromolecular signal contributions

The goal of this study was to validate metabolite quantification at short TE, with particular focus on how to best account for the macromolecular signal contribution. A robust, short-TE PRESS protocol is presented, which allows reliable quantification, in vivo, of metabolite signals at 3 T in human brain. Water suppression was adapted to the experimental conditions at 3 T. Metabolite signal from the parietal white matter was quantified in the time domain using QUEST (jMRUI). The increased macromolecular signal contribution at short TE was dealt with by two approaches, based on either metabolite nulling or initial signal truncation. A detailed comparison of the two approaches was made. The first used a metabolite-nulled signal, measured either individually or averaged over different subjects. The second used the total signal, metabolites and macromolecules, from a single scan. The two approaches gave similar quantification results in terms of metabolite concentrations, but differed in their precision and the number of metabolites quantified reliably. With an average metabolite-nulled baseline, a set of seven metabolites could be reliably quantified in parietal white matter under these experimental conditions: N-acetylaspartate, myo-inositol, glucose, glutamate, glutathione, creatine and choline. When initial signal truncation was used, glucose was removed from this set. The short TE (10-11 ms) facilitated quantification of glutamate. The reliable quantification of N-acetylaspartyl glutamate at 3 T proved very difficult.

(1)H MR spectroscopy of inflammation, infection and ischemia of the brain

Different pathologic patterns in multiple sclerosis (MS) are reflected by alterations of metabolites in (1)H MR spectroscopy of the brain. Elevated choline (Cho), lactate (Lac), lipids and macromolecules are reliable markers for acute demyelination regardless of the clinical entity (also in acute disseminated encephalomyelitis). N-acetyl-aspartate (NAA) is a suitable marker for neuronal integrity. It is reduced in acute MS lesions and in normal appearing white matter, even distant to acute and chronic-lesions. Recovery from reduced NAA levels to subnormal values during remyelination, and varying time courses of NAA in normal appearing white matter during relapsing remitting disease indicate the value of this spectroscopic marker for monitoring activity and recovery. Inositol (Ins) is increased in chronic MS lesions being a marker for astrocytic gliosis. In viral disease, Cho and Ins are always increased, whereas a reduction of NAA mostly reflects an advanced or a detoriated clinical state. In bacterial brain abscesses, numerous amino acids, lipids and Lac can be elevated. In ischemia, especially the Lac/NAA in comparison with perfusion and diffusion weighted imaging seems to be a new measure for areas of metabolic need, and may help to better characterise the penumbra of the stroke and the final infarct size.

Linear discriminant analysis of brain tumour (1)H MR spectra: a comparison of classification using whole spectra versus metabolite quantification

(1)H MRS is an attractive choice for non-invasively diagnosing brain tumours. Many studies have been performed to create an objective decision support system, but there is not yet a consensus as to the best techniques of MRS acquisition or data processing to be used for optimum classification. In this study, we investigate whether LCModel analysis of short-TE (30 ms), single-voxel tumour spectra provide a better input for classification than the use of the original spectra. A total of 145 histologically diagnosed brain tumour spectra were acquired [14 astrocytoma grade II (AS2), 15 astrocytoma grade III (AS3), 42 glioblastoma (GBM), 41 metastases (MET) and 33 meningioma (MNG)], and linear discriminant analyses (LDA) were performed on the LCModel analysis of the spectra and the original spectra. The results consistently suggest improvement in classification when the LCModel concentrations are used. LDA of AS2, MNG and high-grade tumours (HG, comprising GBM and MET) correctly classified 94% using the LCModel dataset compared with 93% using the spectral dataset. The inclusion of AS3 reduced the accuracy to 82% and 78% for LCModel analysis and the original spectra, respectively, and further separating HG into GBM and MET gave 70% compared with 60%. Generally MNG spectra have profiles that are visually distinct from those of the other tumour types, but the classification accuracy was typically about 80%, with MNG with substantial lipid/macromolecule signals being classified as HG. Omission of the lipid/macromolecule concentrations in the LCModel dataset provided an improvement in classification of MNG (91% compared with 76%). In conclusion, there appears to be an advantage to performing pattern recognition on the quantitative analysis of tumour spectra rather than using the whole spectra. However, the results suggest that a two-step LDA process may help in classifying the five tumour groups to provide optimum classification of MNG with high lipid/macromolecule contributions which maybe misclassified as HG.

Concentrations and magnetization transfer ratios of metabolites in gray and white matter

The concentrations and magnetization transfer ratios (MTRs) in gray matter (GM) and white matter (WM) of N-acetyl aspartate (NAA), creatine (Cr), choline (Cho), myo-inositol (Ins), and glutamate plus glutamine (Glx) were investigated using magnetic resonance spectroscopic imaging (MRSI). The macromolecule (MM) baseline was studied separately using a metabolite-nulling inversion. Three data sets were collected from a point-resolved spectroscopy (PRESS)-selected volume (TE/TR = 30/3000 ms) of human frontal lobe in vivo: one with MT pulses applied, one with an inversion pulse to null small metabolites, and one with no inversion or MT pulses. The MM signal, which was analyzed by integrating the metabolite-nulled spectrum between 0 and 3 ppm, was estimated to be 38% higher in GM than in WM. MM subtraction decreased the signal-to-noise ratio (SNR) and also decreased the reliability of LCModel quantification of most metabolites, but may have improved the accuracy of quantification of Glx. Glx and Cr were both found to correlate strongly with the GM volume fraction of the voxels. Cr showed the highest MTR, but the other metabolites also showed some attenuation of signal when the MT pulses were applied. The MTRs did not correlate with the GM volume fraction, which implies that the local environment of metabolites does not differ markedly between GM and WM.

Primary central nervous system lymphomas (PCNSL): MRI features at presentation in 100 patients

To avoid an unnecessary extend of surgery in primary central nervous system lymphoma (PCNSL), the diagnosis should be suspected after MRI. Pre-treatment MRI examinations of 100 immunologically competent patients with biopsy-proven PCNSL were evaluated. All patients had T2- and T1-weighted images with contrast enhancement. Diffusion-weighted MRI (DW-MRI) was available in 15, proton-MR-spectroscopy (1H-MRS) in four patients. The number of lesions ranged from one (n=65 patients) to eight (n=1) with a mean of 1.7. The most frequent locations were the cerebral hemispheres (n=66), the basal ganglia (n=27) and the corpus callosum (n=24). In the 65 patients with a solitary lesion, hemispheric lesions were most frequent (n=23) followed by corpus callosum (n=18). Contrast enhancement was found in all but one patient. 1H-MRS revealed a uniformly pathologic pattern of metabolite concentrations in all patients. Characteristic imaging features of PCNSL are contrast-enhancing lesions with a diameter of at least 15 mm in contact with the subarachnoid space. DW-MRI and proton spectroscopy may aid in differential diagnosis.

A phase rotation scheme for achieving very short echo times with localized stimulated echo spectroscopy

In a single-voxel stimulated echo localization sequence in magnetic resonance spectroscopy, magnetic field gradients are inserted within the echo time (TE) to filter signals generated through coherence pathways other than that leading to the stimulated echo. There is a significant penalty for these gradients as they increase the minimum TE, thereby leading to significant signal loss from spin-spin relaxation and phase distortions in coupled spin systems. Here, an RF phase rotation technique is described for a stimulated echo localization sequence that allows removal of the gradients in the TE intervals and, subsequently, reduction of the minimum TE to only 6 ms. Experiments carried out on six healthy volunteers on a 1.5-T whole-body MR system show a significant signal increase in the metabolite concentrations when measured with a 6-ms TE (N-acetyl-aspartate, 12%, P=.002; creatine, 15%, P=.04; and glutamate+glutamine, 92%, P=.02) compared to concentrations measured with data collected at TEs of 15 and 20 ms.

MRI and proton MR spectroscopy in acute disseminated encephalomyelitis

INTRODUCTION

Acute disseminated encephalomyelitis (ADEM) is one of a group of demyelinating disorders of the central nervous system (CNS). It is said to be attributed to an overshooting immunologic response following an infection or vaccination. The clinical course and type of manifestation is heterogeneous. The early application of corticosteroids has been shown to be beneficial to outcome; thus, an early diagnosis is highly desirable.

METHODS

The potential diagnostic value of advanced MR techniques such as proton MR spectroscopy and diffusion-weighted imaging (DWI) was investigated in two paediatric patients with ADEM, one of whom had a remitting and relapsing clinical course and presented with additional cranial nerve involvement. Proton MR spectroscopy revealed typical signs of acute demyelination, such as increased macromolecules, not found in other forms of non-necrotising pathology.

CONCLUSION

The addition of proton MR spectroscopy and DWI adds to the diagnostic power of MRI in the setting of post-infectious demyelinating disorders of the CNS or ADEM and may obviate the need for biopsy.

Time-domain semi-parametric estimation based on a metabolite basis set

A novel and fast time-domain quantitation algorithm--quantitation based on semi-parametric quantum estimation (QUEST)--invoking optimal prior knowledge is proposed and tested. This nonlinear least-squares algorithm fits a time-domain model function, made up from a basis set of quantum-mechanically simulated whole-metabolite signals, to low-SNR in vivo data. A basis set of in vitro measured signals can be used too. The simulated basis set was created with the software package NMR-SCOPE which can invoke various experimental protocols. Quantitation of 1H short echo-time signals is often hampered by a background signal originating mainly from macromolecules and lipids. Here, we propose and compare three novel semi-parametric approaches to handle such signals in terms of bias-variance trade-off. The performances of our methods are evaluated through extensive Monte-Carlo studies. Uncertainty caused by the background is accounted for in the Cramér-Rao lower bounds calculation. Valuable insight about quantitation precision is obtained from the correlation matrices. Quantitation with QUEST of 1H in vitro data, 1H in vivo short echo-time and 31P human brain signals at 1.5 T, as well as 1H spectroscopic imaging data of human brain at 1.5 T, is demonstrated.

Variability of metabolite yield using STEAM or PRESS sequences in vivo at 3.0 T, illustrated with myo-inositol

Using as an example the myo-inositol (mI) band at approximately 3.6 ppm in the proton spectrum from brain, an evaluation is presented that highlights the difficulties of quantifying metabolites with strongly coupled spins with either STEAM or PRESS and demonstrates some advantages of prospective sequence analysis when measuring their concentrations. The analysis emphasizes the variation in coupled-spin signal yield and lineshape, compared with that of uncoupled singlets such as N-acetylaspartate, a variation that differs from one metabolite spin system to another. This difference in variation between a target metabolite (e.g., mI) and its contaminating background metabolites (e.g., glutamate and taurine, etc.) is shown to provide in certain circumstances a substantial reduction in background contamination (both metabolite and macromolecule) while maintaining sufficient signal-to-noise ratio for precise quantification. For example, sequence times are demonstrated, both for STEAM and for PRESS, that, relative to the short echo-time sequences typical in the literature, enhance the signal to metabolite background of the 3.6-ppm band of mI by factors of 1.7 and 1.3, respectively, essentially eliminate the macromolecular baseline, and yet in vivo retain an S/N approximately 10 in both cases.

High-field localized 1H NMR spectroscopy in the anesthetized and in the awake monkey

Localized cerebral in vivo 1H NMR spectroscopy (MRS) was performed in the anesthetized as well as the awake monkey using a novel vertical 7 T/60 cm MR system. The increased sensitivity and spectral dispersion gained at high field enabled the quantification of up to 16 metabolites in 0.1- to 1-ml volumes. Quantification was accomplished by using simulations of 18 metabolite spectra and a macromolecule (MM) background spectrum consisting of 12 components. Major cerebral metabolites (concentrations >3 mM) such as glutamate (Glu), N-acetylaspartate (NAA), creatine (Cr)/phosphocreatine (PCr) and myo-inositol (Ins) were identified with an error below 3%; most other metabolites were quantified with errors in the order of 10%. Metabolite ratios were 1.39:1 for total NAA, 1.38:1 for glutamate (Glu)/glutamine (Gln) and 0.09:1 for cholines (Cho) relative to total Cr. Taurine (Tau) was detectable at concentrations lower than 1 mM, while lactate (Lac) remained below the detection limit. The spectral dispersion was sufficient to separate metabolites of similar spectral patterns, such as Gln and Glu, N-acetylaspartylglutamate (NAAG) and NAA, and PCr-Cr. MRS in the awake monkey required the development and refinement of acquisition and correction strategies to minimize magnetic susceptibility artifacts induced by respiration and movement of the mouth or body. Periods with major motion artifacts were rejected, while a frequency/phase correction was performed on the remaining single spectra before averaging. In resting periods, both spectral amplitude and line width, that is, the voxel shim, were unaffected permitting reliable measurements. The corrected spectra obtained from the awake monkey afforded the reliable detection of 6-10 cerebral metabolites of 1-ml volumes.

Quantitation of simulated short echo time1H human brain spectra by LCModel and AMARES

LCModel and AMARES, two widely used quantitation tools for magnetic resonance spectroscopy (MRS) data, were employed to analyze simulated spectra similar to those typically obtained at short echo times (TEs) in the human brain at 1.5 T. The study focused mainly on the influence of signal-to-noise ratios (SNRs) and different linewidths on the accuracy and precision of the quantification results, and their effectiveness in accounting for the broad signal contribution of macromolecules and lipids (often called the baseline in in vivo MRS). When applied in their standard configuration (i.e., fitting a spline as a baseline for LCModel, and weighting the first data points for AMARES), both methods performed comparably but with their own characteristics. LCModel and AMARES quantitation benefited considerably from the incorporation of baseline information into the prior knowledge. However, the more accurate quantitation of the sum of glutamate and glutamine (Glx) favored the use of LCModel. Metabolite-to-creatine ratios estimated by LCModel with extended prior knowledge are more accurate than absolute concentrations, and are nearly independent of SNR and line broadening.

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.

The role of relaxation time corrections for the evaluation of long and short echo time 1H MR spectra of the hippocampus by NUMARIS and LCModel techniques

1H MR spectroscopy is routinely used for lateralization of epileptogenic lesions. The present study deals with the role of relaxation time corrections for the quantitative evaluation of long (TE=135 ms) and short echo time (TE=10 ms) 1H MR spectra of the hippocampus using two methods (operator-guided NUMARIS and LCModel programs). Spectra of left and right hippocampi of 14 volunteers and 14 patients with epilepsy were obtained by PRESS (TR/TE=5000/135 ms) and STEAM (TR/TE=5000/10 ms) sequences with a 1.5-T imager. Evaluation was carried out using Siemens NUMARIS software and the results were compared with data from LCModel processing software. No significant differences between the two methods of processing spectra with TE=135 ms were found. The range of relaxation corrections was determined. Metabolite concentrations in hippocampi calculated from spectra with TE=135 ms and 10 ms after application of correction coefficients did not differ in the range of errors and agreed with published data (135 ms/10 ms: NAA=10.2+/-0.6/10.4+/-1.3 mM, Cho=2.4+/-0.1/2.7+/-0.3 mM, Cr=12.2+/-1.3/11.3+/-1.3 mM). When relaxation time corrections were applied, quantitative results from short and long echo time evaluation with LCModel were in agreement. Signal intensity ratios obtained from long echo time spectra by NUMARIS operator-guided processing also agreed with the LCModel results.

PRESS-based proton single-voxel spectroscopy and spectroscopic imaging with very short echo times using asymmetric RF pulses

Modified point-resolved spectroscopy (PRESS) sequences for single voxel spectroscopy (MRS) and spectroscopic imaging (SI) with very short echo time ( T(E)) are described using asymmetric radio-frequency (RF) pulses as well as an optimized design and timing of the PRESS sequence. The proposed sequences were implemented on a standard 4.7 T imaging system yielding a T(E) of 6.0 ms only. Simulations and experimental data measured on phantoms and the rat brain in vivo are presented for MRS and SI showing a high signal-to-noise ratio and hardly any phase distortions caused by J-coupling.

Differentiating primary central nervous system lymphoma from glioma in humans using localised proton magnetic resonance spectroscopy

In order to characterise primary central nervous system lymphomas (PCNSL) and to evaluate if 1H spectroscopy improves the preoperative differential diagnosis of PCNSL and glioma, seven immunocompetent patients with PCNSL and 21 patients with glioma were examined using single voxel, short echo time magnetic resonance spectroscopy (MRS; 1.5 T, STEAM 1500/20). All PCNSL demonstrated massively elevated lipid resonances and markedly elevated choline. Similarly increased lipid resonances were only found in seven necrotic glioblastomas, PCNSL differing from all solid astrocytomas by massively elevated lipid resonances. Additionally, PCNSL had higher Cho/Cr ratios than all grades of astrocytoma. In conclusion, we found that massively elevated lipid resonances are a hallmark of PCNSL in immunocompetent patients. Together with a markedly elevated Cho/Cr ratio, MRS provides metabolic information which may improve the preoperative differentiation of PCNSL and glioma.

Quantitative proton short-echo-time LASER spectroscopy of normal human white matter and hippocampus at 4 Tesla incorporating macromolecule subtraction

Accurate quantification of in vivo short-echo-time (TE) (1)H spectra must account for contributions from both mobile metabolites and less mobile macromolecules, which can fluctuate in disease. The purpose of this study was to develop an approach for the acquisition and processing of macromolecule information to optimize metabolite quantification accuracy and precision. Human parietal white matter (8-cm(3) voxel) and posterior hippocampus (1.7-cm(3) voxel) metabolite levels were quantified, following manomolecule subtraction, from short-echo-time spectra (TE = 46 ms) acquired at 4.0 Tesla with localization by adiabatic selective refocusing (LASER). Nineteen metabolites were fit using a time domain Levenberg-Marquardt minimization that incorporated prior knowledge of metabolite lineshapes. The macromolecule contribution to the spectrum was reduced by 87% (P < 0.05) when the acquisition of single averages of the full spectrum and macromolecule spectrum were interleaved to reduce subtraction errors due to motion. Subtracting the Hankel Lanczos singular value decomposition (HLSVD) fit of the macromolecule spectrum, which contained no random noise, did not alter quantified metabolite levels but did not increase metabolite quantification precision. Several metabolites had higher concentrations in the posterior hippocampus compared to parietal white matter, which emphasizes the need to carefully control for partial volume contamination in hippocampal spectroscopy studies.

Parameterized evaluation of macromolecules and lipids in proton MR spectroscopy of brain diseases

Short echo time (TE) proton MR spectra of the brain include signals of several metabolites as well as macromolecules. In various pathologies, such as brain tumors and multiple sclerosis (MS), the presence of mobile lipids or pathologically altered macromolecules may provide useful additional diagnostic information. A reliable quantitation of these resonances, however, is often not possible due to the lack of adequate prior knowledge. Furthermore, even if advanced fitting procedures are used, a reliable evaluation of metabolites in the presence of pathological lipids or macromolecules often fails if the latter are omitted in the spectral evaluation. In this study, a method is presented for the simultaneous evaluation of all visible components, including metabolites, lipids, and macromolecules, by the use of the fitting procedure LCModel. A standard basis set of brain metabolites was extended by inclusion of parameterized components for macromolecules and lipids that were derived from metabolite-nulled in vivo spectra of normal brain and high-grade gliomas, respectively. The improved spectral quantitation is demonstrated in glial brain tumors and MS lesions as well as in normal brain. It is pointed out that both macromolecules and lipids must be included to provide a proper spectral evaluation.

Proton magnetic resonance spectroscopy with metabolite nulling reveals regional differences of macromolecules in normal human brain

PURPOSE

To quantify the macromolecular content in different anatomic brain regions and to evaluate an age dependency of the macromolecular concentrations.

MATERIAL AND METHODS

A short echo time Stimulated Echo Acquisition Mode (STEAM) sequence was used without and with inversion recovery metabolite nulling in 8-12 healthy volunteers. Quantitation was achieved by an extended LCModel, and macromolecular resonances at 0.9, 1.4, 2.1, and 3.0 ppm were evaluated.

RESULTS

In the cerebellum, the 1.4, 2.1, and 3.0 ppm resonances were highest compared to all other regions (P < 0.02); the 0.9 ppm resonance was significantly higher than that of pons (P < 0.01). In the motor cortex, the 0.9, 1.4, and 2.1 ppm resonances were higher than those of white matter and pons (P < 0.02). Pons and white matter did not differ significantly from each other. A significant correlation of the macromolecular concentrations with the age could not be found.

CONCLUSION

There were higher macromolecular concentrations in the cerebellum and motor cortex than in pons or white matter. These were probably due to the higher portions of gray matter in these volumes of interest (VOIs) than in the other regions.

Proton magnetic resonance spectroscopy in the brain: report of AAPM MR Task Group #9

AAPM Magnetic Resonance Task Group #9 on proton magnetic resonance spectroscopy (MRS) in the brain was formed to provide a reference document for acquiring and processing proton (1H) MRS acquired from brain tissue. MRS is becoming a common adjunct to magnetic resonance imaging (MRI), especially for the differential diagnosis of tumors in the brain. Even though MR imaging is an offshoot of MR spectroscopy, clinical medical physicists familiar with MRI may not be familiar with many of the common practical issues regarding MRS. Numerous research laboratories perform in vivo MRS on other magnetic nuclei, such as 31P, 13C, and 19F. However, most commercial MR scanners are generally only capable of spectroscopy using the signals from protons. Therefore this paper is of limited scope, giving an overview of technical issues that are important to clinical proton MRS, discussing some common clinical MRS problems, and suggesting how they might be resolved. Some fundamental issues covered in this paper are common to many forms of magnetic resonance spectroscopy and are written as an introduction for the reader to these methods. These topics include shimming, eddy currents, spatial localization, solvent saturation, and post-processing methods. The document also provides an extensive review of the literature to guide the practicing medical physicist to resources that may be useful for dealing with issues not covered in the current article.

Proton MRS in Kennedy disease: absolute metabolite and macromolecular concentrations

PURPOSE

To investigate whether glutamine and glutamate (Glx) were elevated in Kennedy Disease (KD), and whether pathological proteins were spectroscopically visible as altered macromolecular (MM) resonances.

MATERIALS AND METHODS

Ten patients with KD and twelve healthy volunteers were investigated using a stimulated echo acquisition mode (STEAM) spectroscopy sequence with metabolite-nulling.

RESULTS

The concentrations of Glx remained unchanged in KD. An increased myo-inositol (Ins), and elevated MM at 0.9 ppm were found within the motor area. The N-acetyl-aspartate (NAA)/creatine (Cr) ratio was negatively correlated to the number of cytosine adenosine guanine (CAG) repeats in the motor area.

CONCLUSION

The elevated MM at 0.9 ppm may be attributed to a pathologically altered protein in KD. Additionally, the changes of Ins point to a clinically unexpected involvement of the motor cortex. The correlation of NAA/Cr with the number of CAG repeats indicates a link between metabolites and genetic failure.