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

Exploring Proton-Only NMR Experiments and Filters for Daphnia In Vivo: Potential and Limitations

Environmental metabolomics provides insight into how anthropogenic activities have an impact on the health of an organism at the molecular level. Within this field, in vivo NMR stands out as a powerful tool for monitoring real-time changes in an organism's metabolome. Typically, these studies use 2D ¹³C-¹H experiments on ¹³C-enriched organisms. Daphnia are the most studied species, given their widespread use in toxicity testing. However, with COVID-19 and other geopolitical factors, the cost of isotope enrichment increased ~6-7 fold over the last two years, making ¹³C-enriched cultures difficult to maintain. Thus, it is essential to revisit proton-only in vivo NMR and ask, "Can any metabolic information be obtained from Daphnia using proton-only experiments?". Two samples are considered here: living and whole reswollen organisms. A range of filters are tested, including relaxation, lipid suppression, multiple-quantum, J-coupling suppression, 2D ¹H-¹H experiments, selective experiments, and those exploiting intermolecular single-quantum coherence. While most filters improve the ex vivo spectra, only the most complex filters succeed in vivo. If non-enriched organisms must be used, then, DREAMTIME is recommended for targeted monitoring, while IP-iSQC was the only experiment that allowed non-targeted metabolite identification in vivo. This paper is critically important as it documents not just the experiments that succeed in vivo but also those that fail and demonstrates first-hand the difficulties associated with proton-only in vivo NMR.

Diagnosis of multiple system atrophy

Multiple system atrophy (MSA) may be difficult to distinguish clinically from other disorders, particularly in the early stages of the disease. An autonomic-only presentation can be indistinguishable from pure autonomic failure. Patients presenting with parkinsonism may be misdiagnosed as having Parkinson disease. Patients presenting with the cerebellar phenotype of MSA can mimic other adult-onset ataxias due to alcohol, chemotherapeutic agents, lead, lithium, and toluene, or vitamin E deficiency, as well as paraneoplastic, autoimmune, or genetic ataxias. A careful medical history and meticulous neurological examination remain the cornerstone for the accurate diagnosis of MSA. Ancillary investigations are helpful to support the diagnosis, rule out potential mimics, and define therapeutic strategies. This review summarizes diagnostic investigations useful in the differential diagnosis of patients with suspected MSA. Currently used techniques include structural and functional brain imaging, cardiac sympathetic imaging, cardiovascular autonomic testing, olfactory testing, sleep study, urological evaluation, and dysphagia and cognitive assessments. Despite advances in the diagnostic tools for MSA in recent years and the availability of consensus criteria for clinical diagnosis, the diagnostic accuracy of MSA remains sub-optimal. As other diagnostic tools emerge, including skin biopsy, retinal biomarkers, blood and cerebrospinal fluid biomarkers, and advanced genetic testing, a more accurate and earlier recognition of MSA should be possible, even in the prodromal stages. This has important implications as misdiagnosis can result in inappropriate treatment, patient and family distress, and erroneous eligibility for clinical trials of disease-modifying drugs.

Detection of lactate in the striatum without contamination of macromolecules by J-difference editing MRS at 7T

Lactate levels are measurable by MRS and are related to neural activity. Therefore, it is of interest to accurately measure lactate levels in the basal ganglia networks. If sufficiently stable, lactate measurements may be used to investigate alterations in dopaminergic signalling in the striatum, facilitating the detection and diagnosis of metabolic deficits. The aim of this study is to provide a J-difference editing MRS technique for the selective editing of lactate only, thus allowing the detection of lactate without contamination of overlapping macromolecules. As a validation procedure, macromolecule nulling was combined with J-difference editing, and this was compared with J-difference editing with a new highly selective editing pulse. The use of a high-field (7T) MR scanner enables the application of editing pulses with very narrow bandwidth, which are selective for lactate. We show that, despite the sensitivity to B0 offsets, the use of a highly selective editing pulse is more efficient for the detection of lactate than the combination of a broad-band editing pulse with macromolecule nulling. Although the signal-to-noise ratio of uncontaminated lactate detection in healthy subjects is relatively low, this article describes the test-retest performance of lactate detection in the striatum when using highly selective J-difference editing MRS at 7 T. The coefficient of variation, σw and intraclass correlation coefficients for within- and between-subject differences of lactate were determined. Lactate levels in the left and right striatum were determined twice in 10 healthy volunteers. Despite the fact that the test-retest performance of lactate detection is moderate with a coefficient of variation of about 20% for lactate, these values can be used for the design of new studies comparing, for example, patient populations with healthy controls.

Multidimensional MR spectroscopic imaging of prostate cancer in vivo

Prostate cancer (PCa) is the second most common type of cancer among men in the United States. A major limitation in the management of PCa is an inability to distinguish, early on, cancers that will progress and become life threatening. One-dimensional (1D) proton ((1)H) MRS of the prostate provides metabolic information such as levels of choline (Ch), creatine (Cr), citrate (Cit), and spermine (Spm) that can be used to detect and diagnose PCa. Ex vivo high-resolution magic angle spinning (HR-MAS) of PCa specimens has revealed detection of more metabolites such as myo-inositol (mI), glutamate (Glu), and glutamine (Gln). Due to the J-modulation and signal overlap, it is difficult to quantitate Spm and other resonances in the prostate clearly by single- and multivoxel-based 1D MR spectroscopy. This limitation can be minimized by adding at least one more spectral dimension by which resonances can be spread apart, thereby increasing the spectral dispersion. However, recording of multivoxel-based two-dimensional (2D) MRS such as J-resolved spectroscopy (JPRESS) and correlated spectroscopy (L-COSY) combined with 2D or three-dimensional (3D) magnetic resonance spectroscopic imaging (MRSI) using conventional phase-encoding can be prohibitively long to be included in a clinical protocol. To reduce the long acquisition time required for spatial encoding, the echo-planar spectroscopic imaging (EPSI) technique has been combined with correlated spectroscopy to give four-dimensional (4D) echo-planar correlated spectroscopic imaging (EP-COSI) as well as J-resolved spectroscopic imaging (EP-JRESI) and the multi-echo (ME) variants. Further acceleration can be achieved using non-uniform undersampling (NUS) and reconstruction using compressed sensing (CS). Earlier versions of 2D MRS, theory of 2D MRS, spectral apodization filters, newer developments and the potential role of multidimensional MRS in PCa detection and management will be reviewed here.

In vivo neurochemistry of primary focal hand dystonia: a magnetic resonance spectroscopic neurometabolite profiling study at 3T

The neurochemical basis of dystonia is unknown. The purpose of this study was to assess the differences of the inhibitory neurotransmitter, gamma amino butyric acid (GABA), in the sensorimotor cortex and the basal ganglia using magnetic resonance spectroscopy with optimized GABA sensitivity. Twenty-two patients with focal hand dystonia and 22 healthy controls were studied. No significant differences in GABA were observed between the groups in either the sensorimotor cortex or in the basal ganglia.

MRI for the differential diagnosis of neurodegenerative parkinsonism in clinical practice

Parkinson's disease (PD) is the most common neurodegenerative cause of parkinsonism, followed by progressive supranuclear palsy (PSP) and multiple system atrophy (MSA). Despite the publication of consensus operational criteria for the diagnosis of PD and the various atypical parkinsonian disorders (APD) such as PSP, MSA and corticobasal degeneration, an accurate diagnosis of neurodegenerative parkinsonian syndromes remains a challenge for each neurologist. Particularly in the early disease stages the clinical separation of APDs from PD carries a high rate of misdiagnosis. However, an early differentiation between APD and PD, each characterized by completely different natural histories, is crucial for determining the prognosis and choosing a treatment strategy. MRI plays an important role in the exclusion of symptomatic parkinsonism due to other pathologies. Over the past two decades, conventional MRI and advanced MRI techniques, including proton magnetic resonance spectroscopy (1H-MRS), diffusion-weighted imaging (DWI), magnetization transfer imaging (MTI), and magnetic resonance volumetry (MRV) have shown abnormalities in the substantia nigra and basal ganglia, especially in APD. Furthermore, in accordance with neuropathological studies suggesting that the olfactory system is an early target of the disease, recent studies using advanced MRI techniques have shown abnormalities in the olfactory system in the early disease stages of patients with PD. Given that olfactory deficits may be a premotor marker of the disease, such methods may eventually evolve into an early screening tool for PD.

Two-dimensional MR spectroscopy of healthy and cancerous prostates in vivo

OBJECTIVES

A major goal of this article is to summarize the current status of evaluating prostate metabolites non-invasively using spatially resolved two-dimensional (2D) MR Spectroscopy (MRS).

MATERIALS AND METHODS

Due to various technical challenges, the spatially resolved versions of 2D MRS techniques are currently going through the developmental stage. During the last decade, four different versions of 2D MRS sequences have been successfully implemented on 3T and 1.5T MRI scanners manufactured by three different vendors. These sequences include half and maximum echo sampled J-resolved spectroscopy (JPRESS), S-PRESS and L-COSY, which are single volume localizing sequences, and the multi-voxel based JPRESS sequence.

RESULTS

Even though greater than 1ml voxels have been used, preliminary evaluations of 2D JPRESS, S-PRESS and L-COSY sequences have demonstrated unambiguous detection of citrate, creatine, choline, spermine and more metabolites in human prostates. ProFIT-based quantitation of JPRESS and L-COSY data clearly shows the superiority of 2D MRS over conventional one-dimensional (1D) MRS and more than six metabolites have been successfully quantified. These sequences have been evaluated in a small group of prostate pathologies and pilot investigations using these sequences show promising results in prostate pathologies.

CONCLUSION

Implementation of the state-of-the-art 2D MRS techniques and preliminary evaluation in prostate pathologies are discussed in this review. Even though these techniques are going through developmental and early testing phases, it is evident that 2D MRS can be easily added on to any clinical Magnetic Resonance Imaging (MRI) protocol to non-invasively record the biochemical contents of the prostate.

GAVA: spectral simulation for in vivo MRS applications

An application that provides a flexible and easy to use interface to the GAMMA spectral simulation package is described that is targeted at investigations using in vivo MR spectroscopic methods. The program makes available a number of widely used spatially localized MRS pulse sequences and NMR parameters for commonly observed tissue metabolites, enabling spectra to be simulated for any pulse sequence parameter and viewed in an integrated display. The application is interfaced with a database for storage of all simulation parameters and results of the simulations. This application provides a convenient method for generating a priori spectral information used in parametric spectral analyses and for visual examination of the effects of difference pulse sequences and parameter settings.

Evaluation of treatment effects in Alzheimer's and other neurodegenerative diseases by MRI and MRS

Neurodegeneration refers to a large clinically and pathologically heterogeneous disease entity associated with slowly progressive neuronal loss in different anatomical and functional systems of the brain. Neurodegenerative diseases often affect cognition, e.g. Alzheimer's disease (AD), dementia with Lewy bodies and vascular dementia, or different aspects of the motor system, e.g., amyotrophic lateral sclerosis, Parkinson's disease and ataxic disorders. Owing to increasing knowledge about the mechanisms leading to neurodegeneration, the development of treatments able to modify the neurodegenerative process becomes possible for the first time. Currently, clinical outcome measures are used to assess the efficacy of such treatments. However, most clinical outcome measures have a low test-retest reliability and thus considerable measurement variance. Therefore, large patient populations and long observation times are needed to detect treatment effects. Furthermore, clinical outcome measures cannot distinguish between symptomatic and disease-modifying treatment effects. Therefore, alternative biomarkers including neuroimaging may take on a more important role in this process. Because MR scanners are widely available and allow for non-invasive detection and quantification of changes in brain structure and metabolism, there is increasing interest in the use of MRI/MRS to monitor objectively treatment effects in clinical trials of neurodegenerative diseases. Particularly volumetric MRI has been used to measure atrophy rates in treatment trials of AD because the relationship between atrophic changes and neuron loss is well established and correlates well with clinical measures. More research is needed to determine the value of other MR modalities, i.e. diffusion, perfusion and functional MRI and MR spectroscopy, for clinical trials with neuroprotective drugs.

MR spectroscopy and spectroscopic imaging: comparing 3.0 T versus 1.5 T

In vivo magnetic resonance spectroscopy (MR spectroscopy) offers the unique possibility to monitor human brain metabolism in a noninvasive way. At 3.0 T, MR spectroscopy not only profits from higher available signal compared with 1.5 T, but from increased chemical shift dispersion as well. These gains may be exchanged into increased spatial resolution or speed in MR spectroscopic imaging. However, some adverse effects related to the higher field strength, such as increased field inhomogeneities and sequence restrictions caused by safety limitations need to be considered. These require protocol adaptations and technical advances that have not yet fully found their way onto the clinical platform. If neglected, effects such as chemical shift misregistration at higher field strength can lead to wrong localizations or loss of signals of certain metabolites, which can intervene with the diagnostic value of a spectrum. This article tries to give an understanding of the potentials and challenges of MR spectroscopy at the higher field strength of 3.0 T, and to give insight into new techniques that hopefully soon will become available in daily clinical routine to fully exploit all benefits of the higher field strength.

1H MR spectroscopy of the brain: absolute quantification of metabolites

Hydrogen 1 (1H) magnetic resonance (MR) spectroscopy enables noninvasive in vivo quantification of metabolite concentrations in the brain. Currently, metabolite concentrations are most often presented as ratios (eg, relative to creatine) rather than as absolute concentrations. Despite the success of this approach, it has recently been suggested that relative quantification may introduce substantial errors and can lead to misinterpretation of spectral data and to erroneous metabolite values. The present review discusses relevant methods to obtain absolute metabolite concentrations with a clinical MR system by using single-voxel spectroscopy or chemical shift imaging. Important methodological aspects in an absolute quantification strategy are addressed, including radiofrequency coil properties, calibration procedures, spectral fitting methods, cerebrospinal fluid content correction, macromolecule suppression, and spectral editing. Techniques to obtain absolute concentrations are now available and can be successfully applied in clinical practice. Although the present review is focused on 1H MR spectroscopy of the brain, a large part of the methodology described can be applied to other tissues as well.

ProFit: two-dimensional prior-knowledge fitting of J-resolved spectra

A two-dimensional fitting procedure is introduced, capable of extracting the full amount of information from 2D J-resolved magnetic resonance spectroscopic data. The fitting procedure uses a linear combination of 2D model spectra. For reducing the degrees of freedom and increasing robustness, it is divided into a non-linear outer loop and an inner linear least-squares fit for the concentrations. In vitro and in vivo experiments on a whole-body 3 T MR scanner show the detectability of a wide range of metabolites in the human brain, namely total creatine, N-acetylaspartate, N-acetylaspartylglutamate, choline-containing compounds, glutamate, myo-inositol, glutathione, scyllo-inositol, gamma-aminobutyric acid, alanine and ascorbic acid.

In vivo magnetic resonance spectroscopy in cancer

Magnetic resonance spectroscopy (MRS) has been used for more than two decades to interrogate metabolite distributions in living cells and tissues. Techniques have been developed that allow multiple spectra to be obtained simultaneously with individual volume elements as small as 1 uL of tissue (i.e., 1 x 1 x 1 mm(3)). The most common modern applications of in vivo MRS use endogenous signals from (1)H, (31)P, or (23)Na. Important contributions have also been made using exogenous compounds containing (19)F, (13)C, or (17)O. MRS has been used to investigate cardiac and skeletal muscle energetics, neurobiology, and cancer. This review focuses on the latter applications, with specific reference to the measurement of tissue choline, which has proven to be a tumor biomarker that is significantly affected by anticancer therapies.

High-energy phosphate metabolism in the calf muscle during moderate isotonic exercise under different degrees of cuff compression: A phosphorus 31 magnetic resonance spectroscopy study

BACKGROUND

The purpose of this study was to investigate phosphocreatine (PCr) and inorganic phosphate levels as well as pH changes in exercising muscle at a workload of 4.5 W under progressive cuff stenoses, whereby the flow reduction due to cuff compression was quantified by flow-sensitive magnetic resonance imaging.

METHODS

By using a whole-body 1.5-T magnetic resonance scanner and an exercise bench, serial phosphorus 31 (31P) magnetic resonance spectroscopy with a time resolution of 30 seconds was performed in 10 healthy men. Percentage changes in PCr, inorganic phosphate (Pi), and pH were statistically evaluated in comparison with baseline. The exercise protocol was characterized by a constant workload level of 4.5 W. Ischemic conditions were achieved by a cuff that was placed at the upper leg. Consecutively, increments of 0, 60, 90, 120, and 150 mm Hg were applied. Each increment lasted for 3 minutes. The following rest period was 10 minutes.

RESULTS

Blood flow increased significantly immediately after the onset of muscle exercise. No significant changes in blood flow were detected as long as the air pressure of the pneumatic cuff was 60 to 90 mm Hg. Significant reductions in blood flow were observed immediately after inflation of the cuff to 120 and 150 mm Hg. PCr passed into a steady state during the first increment with 0 mm Hg and showed no substantial changes during the increment with 60, 90, and 120 mm Hg. PCr hydrolysis seemed progressive during the 150-mm Hg increment. Pi passed into a plateau level at the onset of exercise and increased significantly at the increment of 150 mm Hg. The pH turned into a steady state with no significant changes during the increments up to 120 mm Hg. At 150 mm Hg, pH decreased progressively. PCr levels at the end of the 150-mm Hg increment correlated significantly and moderately with the reduction in blood flow.

CONCLUSIONS

Our study shows that the ischemic condition during constant muscle exercise is clearly characterized by PCr and Pi kinetics, as well as by pH changes. The correlation between the degree of blood flow reduction and PCr levels in the exercising muscle groups, which are supplied by the stenosed arteries, is the first essential of using 31P magnetic resonance spectroscopy in the assessment of the effect of arterial stenoses on muscle function in claudicants.

An update on conventional and advanced magnetic resonance imaging techniques in the differential diagnosis of neurodegenerative parkinsonism

PURPOSE OF REVIEW

The clinical differentiation between Parkinson's disease and atypical parkinsonian disorders (APD) remains a challenge for every neurologist. Conventional magnetic resonance imaging (MRI) and different advanced MRI techniques offer the potential for objective criteria in the differential diagnosis of neurodegenerative parkinsonism. The aim of this article is to review the recent literature on the role of conventional and advanced MRI techniques in the differential diagnosis of neurodegenerative parkinsonian disorders.

RECENT FINDINGS

An important role of MRI is the exclusion of symptomatic parkinsonism due to other pathologies. Over the past two decades, conventional MRI and different advanced MRI techniques, including proton magnetic resonance spectroscopy (1H-MRS), diffusion-weighted imaging (DWI), magnetization transfer imaging (MTI) and magnetic resonance volumetry (MRV) have been found to show abnormalities in the substantia nigra and basal ganglia, especially in APD. Recent studies using MRV, MTI, DWI and 1H-MRS to discriminate Parkinson's disease from APD are discussed extensively.

SUMMARY

Research findings suggest that novel MRI techniques such as MTI, DWI and MRV have superior sensitivity compared to conventional MRI in detecting abnormal features in neurodegenerative parkinsonian disorders. Whether these techniques will emerge as standard investigations in the work-up of patients presenting with parkinsonism requires further prospective magnetic resonance studies during early disease stages.

Chemical-shift-selective filter for the in vivo detection of J-coupled metabolites at 3T

A chemical-shift-selective filter (CSSF) was applied to the detection of J-coupled metabolites in the human brain. This filter is an acquisition-based technique that requires the chemical shifts (CS's) of different metabolites, but not their whole multiplet structures, to be resolved. The sequence is based on the 2D constant-time spin-echo experiment, which yields pure CS spectra in the indirect dimension. Localization is achieved through point-resolved spectroscopy (PRESS). The method enables unequivocal detection of glutamate and myo-inositol, both in vitro and in vivo in the human brain, at 3T.

In vivo quantitative 1H MRS of cerebellum and evaluation of quantitation reproducibility by simulation of different levels of noise and spectral resolution

A quantitative analysis of cerebellar metabolites in normal subjects has been performed by proton MR spectroscopy (MRS) with relaxation time correction. Quantitation was carried out in seven healthy human subjects with the well-established LCModel program. The prior knowledge utilized for quantitation was obtained from solutions containing the major brain metabolites and MRS investigated under the same experimental conditions. The tissue water signal was used as an internal standard for the in vivo studies. Both in vitro (for the prior knowledge template) and in vivo data were acquired separately at 1.5 T by PRESS sequence (TR, 1500 ms; TE, 30 ms). The absolute concentration of main cerebellar metabolites was corrected for relaxation time effects. Different noise and line broadening conditions were considered and simulated in the spectral processing in order to evaluate the effect of spectral quality on the concentration estimates.