WET, a T1- and B1-insensitive water-suppression method for in vivo localized 1H NMR spectroscopy

Conformational change following conversion of inactive rhinophrynin-33 to bioactive rhinophrynin-27 in the skin of the frog Rhinophrynus dorsalis

Skin secretions of the Mexican burrowing toad Rhinophrynus dorsalis (Rhinophrynidae) contain the proline-arginine-rich peptide, rhinophrynin-27 (RP-27; ELRLPEIARPVPEVLPARLPLPALPRN) with insulinotropic and immunomodulatory properties, together with a higher concentration of the biologically inactive form, rhinophrynin-33 (RP-33) that constitutes RP-27 extended from its C-terminus by the hexapeptide KMAKNQ. Determination of the conformation of RP-33 by NMR demonstrates that in both water and in a solvent that promotes protein folding (50% trifluoroethanol-water), the majority of the proline residues are found in a polyproline type II helical region. The peptide adopts a horseshoe (U-shaped) conformation with pronounced bends in the molecule of around 100°-120° at Glu¹³ and Arg¹⁸. The hexapeptide extension adopts a α-helical conformation. When the hexapeptide is excised to generate RP-27, the molecule adopts an L-shaped conformation with a single bend at Glu¹³. A search of protein sequence databases indicated the P-X-P-XXX-P-XXX-P-X-P motif found in RP-27 and RP-33 occurs in a number of proteins although its functional implications are unclear. The data suggest that RP-33 represents a biosynthetic precursor of RP-27 that is activated by a protease cleaving at a single lysine residue of the type previously identified in Xenopus laevis skin secretions.

Lipid-suppressed and tissue-fraction corrected metabolic distributions in human central brain structures using 2D 1 H magnetic resonance spectroscopic imaging at 7 T

INTRODUCTION

Magnetic resonance spectroscopic imaging (MRSI) has the potential to add a layer of understanding of the neurobiological mechanisms underlying brain diseases, disease progression, and treatment efficacy. Limitations related to metabolite fitting of low signal-to-noise ratios data, signal variations due to partial-volume effects, acquisition and extracranial lipid artifacts, along with clinically relevant aspects such as scan time constraints, are among the challenges associated with in vivo MRSI.

METHODS

The aim of this work was to address some of these factors and to develop an acquisition, reconstruction, and postprocessing pipeline to derive lipid-suppressed metabolite values of central brain structures based on free-induction decay measurements made using a 7 T MR scanner. Anatomical images were used to perform high-resolution (1 mm³ ) partial-volume correction to account for gray matter, white matter (WM), and cerebral-spinal fluid signal contributions. Implementation of automatic quality control thresholds and normalization of metabolic maps from 23 subjects to the Montreal Neurological Institute (MNI) standard atlas facilitated the creation of high-resolution average metabolite maps of several clinically relevant metabolites in central brain regions, while accounting for macromolecular distributions. Partial-volume correction improved the delineation of deep brain nuclei. We report average metabolite values including glutamate + glutamine (Glx), glycerophosphocholine, choline and phosphocholine (tCho), (phospo)creatine, myo-inositol and glycine (mI-Gly), glutathione, N-acetyl-aspartyl glutamate(and glutamine), and N-acetyl-aspartate in the basal ganglia, central WM (thalamic radiation, corpus callosum) as well as insular cortex and intracalcarine sulcus.

CONCLUSION

MNI-registered average metabolite maps facilitate group-based analysis, thus offering the possibility to mitigate uncertainty in variable MRSI data.

Multi-organ comparison of flow-based arterial spin labeling techniques: Spatially non-selective labeling for cerebral and renal perfusion imaging

Purpose

Flow‐based arterial spin labeling (ASL) techniques provide a transit‐time insensitive alternative to the more conventional spatially selective ASL techniques. However, it is not clear which flow‐based ASL technique performs best and also, how these techniques perform outside the brain (taking into account eg, flow‐dynamics, field‐inhomogeneity, and organ motion). In the current study we aimed to compare 4 flow‐based ASL techniques (ie, velocity selective ASL, acceleration selective ASL, multiple velocity selective saturation ASL, and velocity selective inversion prepared ASL [VSI‐ASL]) to the current spatially selective reference techniques in brain (ie, pseudo‐continuous ASL [pCASL]) and kidney (ie, pCASL and flow alternating inversion recovery [FAIR]).

Methods

Brain (n = 5) and kidney (n = 6) scans were performed in healthy subjects at 3T. Perfusion‐weighted signal (PWS) maps were generated and ASL techniques were compared based on temporal SNR (tSNR), sensitivity to perfusion changes using a visual stimulus (brain) and robustness to respiratory motion by comparing scans acquired in paced‐breathing and free‐breathing (kidney).

Results

In brain, all flow‐based ASL techniques showed similar tSNR as pCASL, but only VSI‐ASL showed similar sensitivity to perfusion changes. In kidney, all flow‐based ASL techniques had comparable tSNR, although all lower than FAIR. In addition, VSI‐ASL showed a sensitivity to B₁‐inhomogeneity. All ASL techniques were relatively robust to respiratory motion.

Conclusion

In both brain and kidney, flow‐based ASL techniques provide a planning‐free and transit‐time insensitive alternative to spatially selective ASL techniques. VSI‐ASL shows the most potential overall, showing similar performance as the golden standard pCASL in brain. However, in kidney, a reduction of B₁‐sensitivity of VSI‐ASL is necessary to match the performance of FAIR.

In vitro singlet state and zero-quantum encoded magnetic resonance spectroscopy: Illustration with N-acetyl-aspartate

Magnetic resonance spectroscopy (MRS) allows the analysis of biochemical processes non-invasively and in vivo. Still, its application in clinical diagnostics is rare. Routine MRS is limited to spatial, chemical and temporal resolutions of cubic centimetres, mM and minutes. In fact, the signal of many metabolites is strong enough for detection, but the resonances significantly overlap, exacerbating identification and quantification. Besides, the signals of water and lipids are much stronger and dominate the entire spectrum. To suppress the background and isolate selected signals, usually, relaxation times, J-coupling and chemical shifts are used. Here, we propose methods to isolate the signals of selected molecular groups within endogenous metabolites by using long-lived spin states (LLS). We exemplify the method by preparing the LLSs of coupled protons in the endogenous molecules N-acetyl-L-aspartic acid (NAA). First, we store polarization in long-lived, double spin states, followed by saturation pulses before the spin order is converted back to observable magnetization or double quantum filters to suppress background signals. We show that LLS and zero-quantum coherences can be used to selectively prepare and measure the signals of chosen metabolites or drugs in the presence of water, inhomogeneous field and highly concentrated fatty solutions. The strong suppression of unwanted signals achieved allowed us to measure pH as a function of chemical shift difference.

Early stages of sensorimotor map acquisition: neurochemical signature in primary motor cortex and its relation to functional connectivity

The earliest stages of sensorimotor learning involve learning the correspondence between movements and sensory results-a sensorimotor map. The present exploratory study investigated the neurochemical underpinnings of map acquisition by monitoring 25 participants as they acquired a new association between movements and sounds. Functional magnetic resonance spectroscopy was used to measure neurochemical concentrations in the left primary motor cortex during learning. Resting-state functional magnetic resonance imaging data were also collected before and after training to assess learning-related changes in functional connectivity. There were monotonic increases in γ-aminobutyric acid (GABA) and decreases in glucose during training, which extended into the subsequent rest period and, importantly, in the case of GABA correlated with the amount of learning: participants who showed greater behavioral learning showed greater GABA increase. The GABA change was furthermore correlated with changes in functional connectivity between the primary motor cortex and a cluster of voxels in the right intraparietal sulcus: greater increases in GABA were associated with greater strengthening of connectivity. Transiently, there were increases in lactate and reductions in aspartate, which returned to baseline at the end of training, but only lactate showed a statistical trend to correlate with the amount of learning. In summary, during the earliest stages of sensorimotor learning, GABA levels are linked on a subject-level basis to both behavioral learning and a strengthening of functional connections that persists beyond the training period. The findings are consistent with the idea that GABA-mediated inhibition is linked to maintenance of newly learned information.NEW & NOTEWORTHY Learning the mapping between movements and their sensory effects is a necessary step in the early stages of sensorimotor learning. There is evidence showing which brain areas are involved in early motor learning, but their role remains uncertain. Here, we show that GABA, a neurotransmitter linked to inhibitory processing, rises during and after learning and is involved in ongoing changes in resting-state networks.

Alcohol Binge Drinking: Negative and Positive Valence System Abnormalities

BACKGROUND

Each year, 3 million deaths occur owing to alcohol misuse. Translational studies are crucial to translate preclinical findings to patients. Preclinical studies have highlighted abnormalities in specific brain systems, with these forming the basis of allostasis theory. However, few studies have tested predictions in humans using neuroimaging.

METHODS

We used a Research Domain Criteria approach to test allostasis theory predictions of blunted positive valence system (PVS) and abnormally increased negative valence system (NVS) responses in 57 binge alcohol drinking subjects and healthy control subjects who completed an instrumental task during functional magnetic resonance imaging.

RESULTS

As hypothesized, binge alcohol drinkers showed abnormally increased activity in NVS-linked regions, such as the hippocampus and dorsal cingulate, and abnormally blunted activity in PVS-linked regions, such as the striatum, compared with control subjects. Higher measures of problematic alcohol use were associated with more abnormal brain activity only for binge drinkers who had been most recently drinking.

CONCLUSIONS

These results support allostasis theory predictions of abnormally increased NVS and blunted PVS responses in binge alcohol drinkers. Further similar translational neuroimaging studies are indicated, particularly focusing on the NVS.

High-resolution metabolic imaging of high-grade gliomas using 7T-CRT-FID-MRSI

OBJECTIVES

Successful neurosurgical intervention in gliomas depends on the precision of the preoperative definition of the tumor and its margins since a safe maximum resection translates into a better patient outcome. Metabolic high-resolution imaging might result in improved presurgical tumor characterization, and thus optimized glioma resection. To this end, we validated the performance of a fast high-resolution whole-brain 3D-magnetic resonance spectroscopic imaging (MRSI) method at 7T in a patient cohort of 23 high-grade gliomas (HGG).

MATERIALS AND METHODS

We preoperatively measured 23 patients with histologically verified HGGs (17 male, 8 female, age 53 ± 15) with an MRSI sequence based on concentric ring trajectories with a 64 × 64 × 39 measurement matrix, and a 3.4 × 3.4 × 3.4 mm3 nominal voxel volume in 15 min. Quantification used a basis-set of 17 components including N-acetyl-aspartate (NAA), total choline (tCho), total creatine (tCr), glutamate (Glu), glutamine (Gln), glycine (Gly) and 2-hydroxyglutarate (2HG). The resultant metabolic images were evaluated for their reliability as well as their quality and compared to spatially segmented tumor regions-of-interest (necrosis, contrast-enhanced, non-contrast enhanced + edema, peritumoral) based on clinical data and also compared to histopathology (e.g., grade, IDH-status).

RESULTS

Eighteen of the patient measurements were considered usable. In these patients, ten metabolites were quantified with acceptable quality. Gln, Gly, and tCho were increased and NAA and tCr decreased in nearly all tumor regions, with other metabolites such as serine, showing mixed trends. Overall, there was a reliable characterization of metabolic tumor areas. We also found heterogeneity in the metabolic images often continued into the peritumoral region. While 2HG could not be satisfyingly quantified, we found an increase of Glu in the contrast-enhancing region of IDH-wildtype HGGs and a decrease of Glu in IDH1-mutant HGGs.

CONCLUSIONS

We successfully demonstrated high-resolution 7T 3D-MRSI in HGG patients, showing metabolic differences between tumor regions and peritumoral tissue for multiple metabolites. Increases of tCho, Gln (related to tumor metabolism), Gly (related to tumor proliferation), as well as decreases in NAA, tCr, and others, corresponded very well to clinical tumor segmentation, but were more heterogeneous and often extended into the peritumoral region.

An integrated RF-receive/B0-shim array coil boosts performance of whole-brain MR spectroscopic imaging at 7 T

Metabolic imaging of the human brain by in-vivo magnetic resonance spectroscopic imaging (MRSI) can non-invasively probe neurochemistry in healthy and disease conditions. MRSI at ultra-high field (≥ 7 T) provides increased sensitivity for fast high-resolution metabolic imaging, but comes with technical challenges due to non-uniform B0 field. Here, we show that an integrated RF-receive/B0-shim (AC/DC) array coil can be used to mitigate 7 T B0 inhomogeneity, which improves spectral quality and metabolite quantification over a whole-brain slab. Our results from simulations, phantoms, healthy and brain tumor human subjects indicate improvements of global B0 homogeneity by 55%, narrower spectral linewidth by 29%, higher signal-to-noise ratio by 31%, more precise metabolite quantification by 22%, and an increase by 21% of the brain volume that can be reliably analyzed. AC/DC shimming provide the highest correlation (R2 = 0.98, P = 0.001) with ground-truth values for metabolite concentration. Clinical translation of AC/DC and MRSI is demonstrated in a patient with mutant-IDH1 glioma where it enables imaging of D-2-hydroxyglutarate oncometabolite with a 2.8-fold increase in contrast-to-noise ratio at higher resolution and more brain coverage compared to previous 7 T studies. Hence, AC/DC technology may help ultra-high field MRSI become more feasible to take advantage of higher signal/contrast-to-noise in clinical applications.

Designing and comparing optimized pseudo-continuous Arterial Spin Labeling protocols for measurement of cerebral blood flow

Arterial Spin Labeling (ASL) is a non-invasive, non-contrast, perfusion imaging technique which is inherently SNR limited. It is, therefore, important to carefully design scan protocols to ensure accurate measurements. Many pseudo-continuous ASL (PCASL) protocol designs have been proposed for measuring cerebral blood flow (CBF), but it has not yet been demonstrated which design offers the most accurate and repeatable CBF measurements. In this study, a wide range of literature PCASL protocols were first optimized for CBF accuracy and then compared using Monte Carlo simulations and in vivo experiments. The protocols included single-delay, sequential and time-encoded multi-timepoint protocols, and several novel protocol designs, which are hybrids of time-encoded and sequential multi-timepoint protocols. It was found that several multi-timepoint protocols produced more confident, accurate, and repeatable CBF estimates than the single-delay protocol, while also generating maps of arterial transit time. Of the literature protocols, the time-encoded protocol with T₁-adjusted label durations gave the most confident and accurate CBF estimates in vivo (16% and 40% better than single-delay), while the sequential multi-timepoint protocol was the most repeatable (20% more repeatable than single-delay). One of the novel hybrid protocols, Hybrid_T1-adj, was found to produce the most confident, accurate and repeatable CBF estimates out of all the protocols tested in both simulations and in vivo (24%, 47%, and 28% more confident, accurate, and repeatable than single-delay in vivo). The Hybrid_T1-adj protocol makes use of the best aspects of both time-encoded and sequential multi-timepoint protocols and should be a useful tool for accurately and efficiently measuring CBF.

Terminology and concepts for the characterization of in vivo MR spectroscopy methods and MR spectra: Background and experts' consensus recommendations

With a 40-year history of use for in vivo studies, the terminology used to describe the methodology and results of magnetic resonance spectroscopy (MRS) has grown substantially and is not consistent in many aspects. Given the platform offered by this special issue on advanced MRS methodology, the authors decided to describe many of the implicated terms, to pinpoint differences in their meanings and to suggest specific uses or definitions. This work covers terms used to describe all aspects of MRS, starting from the description of the MR signal and its theoretical basis to acquisition methods, processing and to quantification procedures, as well as terms involved in describing results, for example, those used with regard to aspects of quality, reproducibility or indications of error. The descriptions of the meanings of such terms emerge from the descriptions of the basic concepts involved in MRS methods and examinations. This paper also includes specific suggestions for future use of terms where multiple conventions have emerged or coexisted in the past.

Selective magnetic resonance signal suppression by colored Frank excitation

Inspired by the growing interest in miniaturized NMR devices and their applications in material science as well as in chemical and biological research, low power rf excitation is explored. ¹H NMR spectra have been measured with low power Frank excitation and are compared to spectra obtained by single-pulse excitation. Frank excitation consists of a large number of phase-modulated, constant-amplitude rf-pulses. A Frank sequence is divided into packages of discrete phase wavelets that correspond to a scan across a spectral frequency range. The largely coherent excitation is found experimentally to require less power than white noise excitation. The package structure suggests that individual wavelets can be omitted to skip individual frequency regions in the excitation, converting the white Frank excitation into colored Frank excitation. This work explores different approaches of colored, selective Frank excitation for spectroscopy and imaging. It is motivated by the aim to eliminate the rf amplifier from the NMR spectrometer so as to enable further miniaturization of NMR instruments. Colored Frank excitation bears promise as a low-power modality for solvent signal suppression in spectroscopy and motion tagging in magnetic resonance imaging.

Feasibility of free-breathing quantitative myocardial perfusion using multi-echo Dixon magnetic resonance imaging

Dynamic contrast-enhanced quantitative first-pass perfusion using magnetic resonance imaging enables non-invasive objective assessment of myocardial ischemia without ionizing radiation. However, quantification of perfusion is challenging due to the non-linearity between the magnetic resonance signal intensity and contrast agent concentration. Furthermore, respiratory motion during data acquisition precludes quantification of perfusion. While motion correction techniques have been proposed, they have been hampered by the challenge of accounting for dramatic contrast changes during the bolus and long execution times. In this work we investigate the use of a novel free-breathing multi-echo Dixon technique for quantitative myocardial perfusion. The Dixon fat images, unaffected by the dynamic contrast-enhancement, are used to efficiently estimate rigid-body respiratory motion and the computed transformations are applied to the corresponding diagnostic water images. This is followed by a second non-linear correction step using the Dixon water images to remove residual motion. The proposed Dixon motion correction technique was compared to the state-of-the-art technique (spatiotemporal based registration). We demonstrate that the proposed method performs comparably to the state-of-the-art but is significantly faster to execute. Furthermore, the proposed technique can be used to correct for the decay of signal due to T2* effects to improve quantification and additionally, yields fat-free diagnostic images.

Non-water-excitation MR spectroscopy techniques to explore exchanging protons in human brain at 3 T

PURPOSE

To develop localization sequences for in vivo MR spectroscopy (MRS) on clinical scanners of 3 T to record spectra that are not influenced by magnetization transfer from water.

METHODS

Image-selected in vivo spectroscopy (ISIS) localization and chemical-shift-selective excitation (termed I-CSE) was combined in two ways: first, full ISIS localization plus a frequency-selective spin-echo and second, two-dimensional (2D) ISIS plus a frequency-selective excitation and slice-selective refocusing. The techniques were evaluated at 3 T in phantoms and human subjects in comparison to standard techniques with water presaturation or metabolite-cycling. ISIS included gradient-modulated offset-independent adiabatic (GOIA)-type adiabatic inversion pulses; echo times were 8-10 ms.

RESULTS

The novel 2D and 3D I-CSE methods yield upfield spectra that are comparable to those from standard MRS, except for shorter echo times and a limited frequency range. On the downfield/high-frequency side, they yield much more signal for exchangeable protons when compared to MRS with water presaturation or metabolite-cycling and longer echo times.

CONCLUSION

Novel non-water-excitation MRS sequences offer substantial benefits for the detection of metabolite signals that are otherwise suppressed by saturation transfer from water. Avoiding water saturation and using very short echo times allows direct observation of faster exchanging moieties than was previously possible at 3 T and additionally makes the methods less susceptible to fast T₂ relaxation.

Increased myoinositol in the anterior cingulate cortex of veterans with a history of traumatic brain injury: a proton magnetic resonance spectroscopy study

In this study of veterans, we used a state-of-the-art neuroimaging tool to probe the neurometabolic profile of the anterior cingulate cortex in veterans with traumatic brain injury (TBI). We report significantly elevated myoinositol levels in veterans with TBI compared with those without TBI.

Advances in Pediatric Neuroimaging. MR Spectroscopy

The basic principles of proton magnetic resonance spectroscopy are presented in this work to briefly familiarize the clinician and to distinguish spectroscopy from magnetic resonance imaging. For those knowledgeable about proton magnetic resonance spectroscopy, this article will also provide the reader an update on recent technical and translational developments relevant to pediatric neurologic conditions. These developments were selected for their potential impact towards the clinical care of patients in pediatric-based practices. At this point in time, these new spectroscopic approaches are currently applied to established populations with known diseases. This information will inform our knowledge about diseases and guide therapeutic options for the future.

Clinical High-Resolution 3D-MR Spectroscopic Imaging of the Human Brain at 7 T

OBJECTIVES

Available clinical magnetic resonance spectroscopic imaging (MRSI) sequences are hampered by long scan times, low spatial resolution, strong field inhomogeneities, limited volume coverage, and low signal-to-noise ratio. High-resolution, whole-brain mapping of more metabolites than just N-acetylaspartate, choline, and creatine within clinically attractive scan times is urgently needed for clinical applications. The aim is therefore to develop a free induction decay (FID) MRSI sequence with rapid concentric ring trajectory (CRT) encoding for 7 T and demonstrate its clinical feasibility for mapping the whole cerebrum of healthy volunteers and patients.

MATERIALS AND METHODS

Institutional review board approval and written informed consent were obtained. Time-efficient, 3-dimensional encoding of an ellipsoidal k-space by in-plane CRT and through-plane phase encoding was integrated into an FID-MRSI sequence. To reduce scan times further, repetition times were shortened, and variable temporal interleaves were applied. Measurements with different matrix sizes were performed to validate the CRT encoding in a resolution phantom. One multiple sclerosis patient, 1 glioma patient, and 6 healthy volunteers were prospectively measured. For the healthy volunteers, brain segmentation was performed to quantify median metabolic ratios, Cramér-Rao lower bounds (CRLBs), signal-to-noise ratios, linewidths, and brain coverage among all measured matrix sizes ranging from a 32 × 32 × 31 matrix with 6.9 × 6.9 × 4.2 mm nominal voxel size acquired in ~3 minutes to an 80 × 80 × 47 matrix with 2.7 × 2.7 × 2.7 mm nominal voxel size in ~15 minutes for different brain regions.

RESULTS

Phantom structures with diameters down to 3 to 4 mm were visible. In vivo MRSI provided high spectral quality (median signal-to-noise ratios, >6.3 and linewidths, <0.082 ppm) and fitting quality. Cramér-Rao lower bounds were ranging from less than 22% for glutamine (highest CRLB in subcortical gray matter) to less than 9.5% for N-acetylaspartate for the 80 × 80 × 47 matrix (highest CRLB in the temporal lobe). This enabled reliable mapping of up to 8 metabolites (N-acetylaspartate, N-acetylaspartyl glutamate, total creatine, glutamine, glutamate, total choline, myo-inositol, glycine) and macromolecules for all resolutions. Coverage of the whole cerebrum allowed visualization of the full extent of diffuse and local multiple sclerosis-related neurochemical changes (eg, up to 100% increased myo-inositol). Three-dimensional brain tumor metabolic maps provided valuable information beyond that of single-slice MRSI, with up to 200% higher choline, up to 100% increased glutamine, and increased glycine in tumor tissue.

CONCLUSIONS

Seven Tesla FID-MRSI with time-efficient CRT readouts offers clinically attractive acquisition protocols tailored either for speed or for the investigation of small pathologic details and low-abundant metabolites. This can complement clinical MR studies of various brain disorders. Significant metabolic anomalies were demonstrated in a multiple sclerosis and a glioma patient for myo-inositol, glutamine, total choline, glycine, and N-acetylaspartate concentrations.

Simultaneous edited MRS of GABA, glutathione, and ethanol

The aim of this work was to develop simultaneous edited MRS of γ-aminobutyric acid (GABA), glutathione (GSH), and ethanol (EtOH) using Hadamard encoding and reconstruction of MEGA-edited spectroscopy (HERMES) at 3T. Density-matrix simulations of HERMES were carried out and compared with phantom experiments. In vivo experiments were performed in six healthy volunteers about 30 min after alcohol consumption. Simulations of HERMES showed GABA-, GSH-, and EtOH-edited spectra with low levels of crosstalk and excellent agreement with phantom spectra. In vivo experiments showed well edited GABA signals at 3.0 ppm, GSH at 2.95 ppm, and EtOH at 1.18 ppm in the respective Hadamard combination spectra. Measured integral ratios were 0.082 ± 0.012 for GABA/Cr, 0.037 ± 0.006 for GSH/Cr, and 0.305 ± 0.129 for EtOH/Cr. Simulated, phantom, and in vivo measurements of HERMES show excellent separation of GABA-, GSH-, and EtOH-edited signals with negligible levels of crosstalk. HERMES allows a threefold acceleration of editing while maintaining spectral quality compared with sequentially acquired MEGA-PRESS measurements.

Influence of labeling parameters and respiratory motion on velocity-selective arterial spin labeling for renal perfusion imaging

Purpose

Arterial transit time uncertainties and challenges during planning are potential issues for renal perfusion measurement using spatially selective arterial spin labeling techniques. To mitigate these potential issues, a spatially non‐selective technique, such as velocity‐selective arterial spin labeling (VSASL), could be an alternative. This article explores the influence of VSASL sequence parameters and respiratory induced motion on VS‐label generation.

Methods

VSASL data were acquired in human subjects (n = 15), with both single and dual labeling, during paced‐breathing, while essential sequence parameters were systematically varied; (1) cutoff velocity, (2) labeling gradient orientation and (3) post‐labeling delay (PLD). Pseudo‐continuous ASL was acquired as a spatially selective reference. In an additional free‐breathing single VSASL experiment (n = 9) we investigated respiratory motion influence on VS‐labeling. Absolute renal blood flow (RBF), perfusion weighted signal (PWS), and temporal signal‐to‐noise ratio (tSNR) were determined.

Results

(1) With decreasing cutoff velocity, tSNR and PWS increased. However, undesired tissue labeling occurred at low cutoff velocities (≤ 5.4 cm/s). (2) Labeling gradient orientation had little effect on tSNR and PWS. (3) For single VSASL high signal appeared in the kidney pedicle at PLD < 800 ms, and tSNR and PWS decreased with increasing PLD. For dual VSASL, maximum tSNR occurred at PLD = 1200 ms. Average cortical RBF measured with dual VSASL (264 ± 34 mL/min/100 g) at a cutoff velocity of 5.4 cm/s, and feet‐head labeling was slightly lower than with pseudo‐continuous ASL (283 ± 55 mL/min/100 g).

Conclusion

With well‐chosen sequence parameters, tissue labeling induced by respiratory motion can be minimized, allowing to obtain good quality RBF maps using planning‐free labeling with dual VSASL.

Translating pH-sensitive PROgressive saturation for QUantifying Exchange rates using Saturation Times (PRO-QUEST) MRI to a 3T clinical scanner

PURPOSE

To translate the recently developed PRO-QUEST (Progressive saturation for quantifying exchange rates using saturation times) sequence from preclinical 9.4T to 3T clinical magnetic field strength.

METHODS

Numerical simulations were performed to define the optimal saturation flip angles for PRO-QUEST saturation pulses at 3T and demonstrate the effect of a ∆T₂ error on the exchange rate (k_ex ) estimation at various field strengths. Exchange-dependent relaxation rate (R_ex ) was measured for glutamate solutions in various pH, healthy volunteers and patients with multiple sclerosis (MS). Additionally, concentration-independent ratiometric R_ex maps were produced to evaluate regional signal variations across the brain of human volunteers.

RESULTS

The calculated R_ex significantly correlates with pH in glutamate samples, however, k_ex values are underestimated as compared to those previously obtained at 9.4T. In the ratiometric R_ex map of healthy volunteers, no significant differences are found between grey matter, white matter, and basal ganglia. In patients with MS, white matter lesions are visible in single saturation power R_ex maps whereas only a periventricular lesion is apparent in the ratiometric R_ex map.

CONCLUSION

We demonstrate that quantification of pH sensitive indices using PRO-QUEST is feasible at 3T within clinically acceptable acquisition times. Our initial findings in patients with MS show that pH sensitive indices varied with the type of lesion examined whereas no significant difference was found in healthy volunteers between tissue types, suggesting that it would be worthwhile to apply PRO-QUEST in a larger cohort of patients to better understand its distinct imaging features relative to conventional techniques.

Ultrafast magnetic resonance spectroscopic imaging using SPICE with learned subspaces

PURPOSE

To develop a subspace learning method for the recently proposed subspace-based MRSI approach known as SPICE, and achieve ultrafast 1 H-MRSI of the brain.

THEORY AND METHODS

A novel strategy is formulated to learn a low-dimensional subspace representation of MR spectra from specially acquired training data and use the learned subspace for general MRSI experiments. Specifically, the subspace learning problem is formulated as learning "empirical" distributions of molecule-specific spectral parameters (e.g., concentrations, lineshapes, and frequency shifts) by integrating physics-based model and the training data. The learned spectral parameters and quantum mechanical simulation basis can then be combined to construct acquisition-specific subspace for spatiospectral encoding and processing. High-resolution MRSI acquisitions combining ultrashort-TE/short-TR excitation, sparse sampling, and the elimination of water suppression have been performed to evaluate the feasibility of the proposed method.

RESULTS

The accuracy of the learned subspace and the capability of the proposed method in producing high-resolution 3D 1 H metabolite maps and high-quality spatially resolved spectra (with a nominal resolution of ∼2.4 × 2.4 × 3 mm3 in 5 minutes) were demonstrated using phantom and in vivo studies. By eliminating water suppression, we are also able to extract valuable information from the water signals for data processing ( B 0 map, frequency drift, and coil sensitivity) as well as for mapping tissue susceptibility and relaxation parameters.

CONCLUSIONS

The proposed method enables ultrafast 1 H-MRSI of the brain using a learned subspace, eliminating the need of acquiring subject-dependent navigator data (known as D 1 ) in the original SPICE technique. It represents a new way to perform MRSI experiments and an important step toward practical applications of high-resolution MRSI.

Comparison of multi-delay FAIR and pCASL labeling approaches for renal perfusion quantification at 3T MRI

OBJECTIVE

To compare the most commonly used labeling approaches, flow-sensitive alternating inversion recovery (FAIR) and pseudocontinuous arterial spin labeling (pCASL), for renal perfusion measurement using arterial spin labeling (ASL) MRI.

METHODS

Multi-delay FAIR and pCASL were performed in 16 middle-aged healthy volunteers on two different occasions at 3T. Relative perfusion-weighted signal (PWS), temporal SNR (tSNR), renal blood flow (RBF), and arterial transit time (ATT) were calculated for the cortex and medulla in both kidneys. Bland-Altman plots, intra-class correlation coefficient, and within-subject coefficient of variation were used to assess reliability and agreement between measurements.

RESULTS

For the first visit, RBF was 362 ± 57 and 140 ± 47 mL/min/100 g, and ATT was 0.47 ± 0.13 and 0.70 ± 0.10 s in cortex and medulla, respectively, using FAIR; RBF was 201 ± 72 and 84 ± 27 mL/min/100 g, and ATT was 0.71 ± 0.25 and 0.86 ± 0.12 s in cortex and medulla, respectively, using pCASL. For both labeling approaches, RBF and ATT values were not significantly different between visits. Overall, FAIR showed higher PWS and tSNR. Moreover, repeatability of perfusion parameters was better using FAIR.

DISCUSSION

This study showed that compared to (balanced) pCASL, FAIR perfusion values were significantly higher and more comparable between visits.

In vivo MRS measurement of 2-hydroxyglutarate in patient-derived IDH-mutant xenograft mouse models versus glioma patients

PURPOSE

To generate a preclinical model of isocitrate dehydrogenase (IDH) mutant gliomas from glioma patients and design a MRS method to test the compatibility of 2-hydroxyglutarate (2HG) production between the preclinical model and patients.

METHODS

Five patient-derived xenograft (PDX) mice were generated from two glioma patients with IDH1 R132H mutation. A PRESS sequence was tailored at 9.4 T, with computer simulation and phantom analyses, for improving 2HG detection in mice. 2HG and other metabolites in the PDX mice were measured using the optimized MRS at 9.4 T and compared with 3 T MRS measurements of the metabolites in the parental-tumor patients. Spectral fitting was performed with LCModel using in-house basis spectra. Metabolite levels were quantified with reference to water.

RESULTS

The PRESS TE was optimized to be 96 ms, at which the 2HG 2.25 ppm signal was narrow and inverted, thereby leading to unequivocal separation of the 2HG resonance from adjacent signals from other metabolites. The optimized MRS provided precise detection of 2HG in mice compared to short-TE MRS at 9.4 T. The 2HG estimates in PDX mice were in excellent agreement with the 2HG measurements in the patients.

CONCLUSION

The similarity of 2HG production between PDX models and parental-tumor patients indicates that PDX tumors retain the parental IDH metabolic fingerprint and can serve as a preclinical model for improving our understanding of the IDH-mutation associated metabolic reprogramming.

Improving PCASL at ultra-high field using a VERSE-guided parallel transmission strategy

Purpose

To improve the labeling efficiency of pseudo‐continuous arterial spin labeling (PCASL) at 7T using parallel transmission (pTx).

Methods

Five healthy subjects were scanned on an 8‐channel‐transmit 7T human MRI scanner. Time‐of‐flight (TOF) angiography was acquired to identify regions of interest (ROIs) around the 4 major feeding arteries to the brain, and B1+ and B₀ maps were acquired in the labeling plane for tagging pulse design. Complex weights of the labeling pulses for each of the 8 transmit channels were calculated to produce a homogenous radiofrequency (RF) ‐shimmed labeling across the ROIs. Variable‐Rate Selective Excitation (VERSE) pulses were also implemented as a part of the labeling pulse train. Whole‐brain perfusion‐weighted images were acquired under conditions of RF shimming, VERSE with RF shimming, and standard circularly polarized (CP) mode. The same subjects were scanned on a 3T scanner for comparison.

Results

In simulation, VERSE with RF shimming improved the flip‐angles across the ROIs in the labeling plane by 90% compared with CP mode. VERSE with RF shimming improved the temporal signal‐to‐noise ratio by 375% compared with CP mode, but did not outperform a matched 3T sequence with a matched flip‐angle.

Conclusion

We have demonstrated improved PCASL tagging at 7T using VERSE with RF shimming on a commercial head coil under conservative SAR limits at 7T. However, improvements of 7T over 3T may require strategies with less conservative SAR restrictions.

Nuclear magnetic resonance spectroscopy of human body fluids and in vivo magnetic resonance spectroscopy: Potential role in the diagnosis and management of prostate cancer

Prostate cancer is the most common solid organ cancer in men, and the second most common cause of male cancer-related mortality. It has few effective therapies, and is difficult to diagnose accurately. Prostate-specific antigen (PSA), which is currently the most effective diagnostic tool available, cannot reliably discriminate between different pathologies, and in fact only around 30% of patients found to have elevated levels of PSA are subsequently confirmed to actually have prostate cancer. As such, there is a desperate need for more reliable diagnostic tools that will allow the early detection of prostate cancer so that the appropriate interventions can be applied. Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance spectroscopy (MRS) are 2 high throughput, noninvasive analytical procedures that have the potential to enable differentiation of prostate cancer from other pathologies using metabolomics, by focusing specifically on certain metabolites which are associated with the development of prostate cancer cells and its progression. The value that this type of approach has for the early detection, diagnosis, prognosis, and personalized treatment of prostate cancer is becoming increasingly apparent. Recent years have seen many promising developments in the fields of NMR spectroscopy and MRS, with improvements having been made to hardware as well as to techniques associated with the acquisition, processing, and analysis of related data. This review focuses firstly on proton NMR spectroscopy of blood serum, urine, and expressed prostatic secretions in vitro, and then on 1- and 2-dimensional proton MRS of the prostate in vivo. Major advances in these fields and methodological principles of data collection, acquisition, processing, and analysis are described along with some discussion of related challenges, before prospects that proton MRS has for future improvements to the clinical management of prostate cancer are considered.

Online reaction monitoring by single-scan 2D NMR under flow conditions

Single-scan 2D NMR based on spatial encoding can be used to monitor chemical reactions with a flow unit in realistic reaction conditions.

Robust outer volume suppression utilizing elliptical pulsed second order fields (ECLIPSE) for human brain proton MRSI

PURPOSE

The robust and reliable utilization of proton magnetic resonance spectroscopic imaging (MRSI) at high fields is hampered by several key technical difficulties, including contamination from extracranial lipids. To that end, this work presents novel lipid suppression sequences for proton MRSI in the human brain utilizing elliptical localization with pulsed second-order fields (ECLIPSE).

METHODS

Two lipid suppression methods were implemented with the ECLIPSE gradient insert. One method is a variable power, 4-pulse sequence optimized to achieve outer volume suppression (OVS) and compared against a standard, 8-slice OVS method. The second ECLIPSE method is implemented as an inversion recovery (IR) sequence with elliptical inner volume selection (IVS) and compared against a global IR method.

RESULTS

The ECLIPSE-OVS sequence provided a 116-fold mean lipid suppression (range, 104-134), whereas an optimized 8-slice OVS sequence achieved 15-fold suppression (range, 13-18). Furthermore, the superior ECLIPSE-OVS suppression was achieved at 30% of the radiofrequency (RF) power required by 8-slice OVS. The ECLIPSE-based IR sequence suppressed skull lipids by 155-fold (range, 122-257), compared to 16-fold suppression (range, 14-19) achieved with IR.

CONCLUSION

OVS and IVS executed with ECLIPSE provide robust and effective lipid suppression at reduced RF power with high immunity to variations in B₁ and T₁ .

Retrospective motion compensation for edited MR spectroscopic imaging

Edited magnetic resonance spectroscopic imaging (MRSI) is capable of mapping the distribution of low concentration metabolites such as gamma-aminobutyric acid (GABA) or and glutathione (GSH), but is prone to subtraction artifacts due to head motion or other instabilities. In this study, a retrospective motion compensation algorithm for edited MRSI is proposed. The algorithm identifies movement-affected signals by comparing residual water and lipid peaks between different transients recorded at the same point in k-space, and either phase corrects, replaces or removes affected spectra prior to spatial Fourier transformation. The method was tested on macromolecule-unsuppressed GABA-edited spin-echo MR spectroscopic imaging data acquired from 8 healthy adults scanned at 3T. Relative to non-motion compensated data sets, the motion compensated data had significantly less subtraction artifacts across subjects. The residual choline (Cho) peak in the spectrum (which is well resolved from as a different chemical shift from GABA and is completely absent in a spectrum without subtraction artifact) was used as a metric of motion artifact severity. The normalized Cho area was 5.14 times lower with motion compensation than without motion compensation. A 'removal-only' version of the technique is also shown to be promising in removing motion-corrupted artifacts in a GSH-edited MRSI acquisition acquired in 1 healthy subject. This study introduces a motion compensation technique and demonstrates that retrospective compensation in k-space is possible and significantly reduces the amount of subtraction artifacts in the resulting edited spectra.

GABAergic changes in the thalamocortical circuit in Parkinson's disease

Parkinson's disease is characterized by bradykinesia, rigidity, and tremor. These symptoms have been related to an increased gamma‐aminobutyric acid (GABA)ergic inhibitory drive from globus pallidus onto the thalamus. However, in vivo empirical evidence for the role of GABA in Parkinson's disease is limited. Some discrepancies in the literature may be explained by the presence or absence of tremor. Specifically, recent functional magnetic resonance imaging (fMRI) findings suggest that Parkinson's tremor is associated with reduced, dopamine‐dependent thalamic inhibition. Here, we tested the hypothesis that GABA in the thalamocortical motor circuit is increased in Parkinson's disease, and we explored differences between clinical phenotypes. We included 60 Parkinson patients with dopamine‐resistant tremor (n = 17), dopamine‐responsive tremor (n = 23), or no tremor (n = 20), and healthy controls (n = 22). Using magnetic resonance spectroscopy, we measured GABA‐to‐total‐creatine ratio in motor cortex, thalamus, and a control region (visual cortex) on two separate days (ON and OFF dopaminergic medication). GABA levels were unaltered by Parkinson's disease, clinical phenotype, or medication. However, motor cortex GABA levels were inversely correlated with disease severity, particularly rigidity and tremor, both ON and OFF medication. We conclude that cortical GABA plays a beneficial rather than a detrimental role in Parkinson's disease, and that GABA depletion may contribute to increased motor symptom expression.

Conformational analysis and in vitro immunomodulatory and insulinotropic properties of the frog skin host-defense peptide rhinophrynin-27 and selected analogs

The study investigates conformational analysis and the in vitro cytokine-mediated immunomodulatory and insulin-releasing activities of rhinophrynin-27 (ELRLPEIARPVPEVLPARLPLPALPRN; RP-27), a proline-arginine-rich peptide first isolated from skin secretions of the Mexican burrowing toad Rhinophrynus dorsalis (Rhinophrynidae). In both water and 50% trifluoroethanol-water, the peptide adopts a polyproline type II helical conformation with a high degree of deviation from the canonical collagen-like folding and a pronounced bend in the molecule at the Glu¹³ residue. Incubation of mouse peritoneal cells with RP-27 significantly (P < 0.05) inhibited production of the pro-inflammatory cytokines TNF-α and IL-1β and stimulated production of the anti-inflammatory cytokine IL-10. The peptide significantly (P < 0.01) stimulated release of insulin from BRIN-BD11 rat clonal β-cells at concentrations ≥ 1 nM while maintaining the integrity of the plasma membrane and also stimulated insulin release from isolated mouse islets at a concentration of 10^-6 M. Increasing the cationicity of RP-27 by substituting glutamic acid residues in the peptide by arginine and increasing hydrophobicity by substituting alanine residues by tryptophan did not result in analogues with increased activity with respect to cytokine production and insulin release. The combination of immunosuppressive and insulinotropic activities together with very low cytotoxicity suggests that RP-27 may represent a template for the development of an agent for use in anti-inflammatory and Type 2 diabetes therapies.

A comparison of sLASER and MEGA-sLASER using simultaneous interleaved acquisition for measuring GABA in the human brain at 7T

γ-Aminobutyric acid (GABA), the major inhibitory neurotransmitter, is challenging to measure using proton spectroscopy due to its relatively low concentration, J-coupling and overlapping signals from other metabolites. Currently, the prevalent methods for detecting GABA at ultrahigh field strengths (≥ 7 T) are GABA-editing and model fitting of non-editing single voxel spectra. These two acquisition approaches have their own advantages: the GABA editing approach directly measures the GABA resonance at 3 ppm, whereas the fitting approach on the non-editing spectrum allows the detection of multiple metabolites, and has an SNR advantage over longer echo time (TE) acquisitions. This study aims to compare these approaches for estimating GABA at 7 T. We use an interleaved sequence of semi-LASER (sLASER: TE = 38 ms) and MEGA-sLASER (TE = 80 ms). This simultaneous interleaved acquisition minimizes the differential effect of extraneous factors, and enables an accurate comparison of the two acquisition methods. Spectra were acquired with an 8 ml isotropic voxel at six different brain regions: anterior-cingulate cortex, dorsolateral-prefrontal cortex, motor cortex, occipital cortex, posterior cingulate cortex, and precuneus. Spectral fitting with LCModel quantified the GABA to total Cr (tCr: Creatine + Phosphocreatine) concentration ratio. After correcting the T2 relaxation time variation, GABA/tCr ratios were similar between the two acquisition approaches. GABA editing showed smaller spectral fitting error according to Cramér-Rao lower bound than the sLASER approach for all regions examined. We conclude that both acquisition methods show similar accuracy but the precision of the MEGA-editing approach is higher for GABA measurement. In addition, the 2.28 ppm GABA resonance was found to be important for estimating GABA concentration without macromolecule contamination in the GABA-edited acquisition, when utilizing spectral fitting with LCModel.

Background suppressed magnetization transfer MRI

PURPOSE

Up to 30% of the hydrogen atoms in brain tissue are part of molecules ("semisolids") other than water. In MRI, their magnetization is typically not observed directly, but can influence the water magnetization through magnetization transfer (MT). Comparison of MRI scans differentially sensitized to MT allows estimation of the semisolid fraction and potential changes with disease. Here, we present an approach designed to improve this estimate by measuring the size of the MT effect in a single scan.

METHODS

A stimulated echo sequence was used to generate a spatial pattern in the longitudinal water magnetization, which was then given time to exchange with semisolids. After saturating the remaining water magnetization, reverse exchange was allowed to partly re-establish the original water magnetization pattern. The third excitation pulse then formed a stimulated echo out of this pattern.

RESULTS

MT data were obtained on 10 human subjects at 7 T with varying exchange times. The images showed the expected time dependence of signal associated with the forward and reverse exchange processes. Excellent suppression of non-exchanging background signal was achieved. As expected, this suppression came at the price of a substantial reduction in exchange-related signal (by ~75% compared to the signal in saturation recovery MT), in part because of the reliance on a 2-step exchange process.

CONCLUSION

The results demonstrate an MT signal can be observed in a single acquisition without subtraction. This may be advantageous for MT measurements when signal instabilities related to motion and physiological variations exceed thermal noise sources.

Single-Voxel 1 H MR spectroscopy of cerebral nicotinamide adenine dinucleotide (NAD+ ) in humans at 7T using a 32-channel volume coil

PURPOSE

Reliable monitoring of tissue nicotinamide adenine dinucleotide (NAD⁺ ) concentration may provide insights on its roles in normal and pathological aging. In the present study, we report a ¹ H MRS pulse sequence for the in vivo, localized ¹ H MRS detection of NAD⁺ from the human brain.

METHODS

Studies were carried out on a 7T Siemens MRI scanner using a 32-channel product volume coil. The pulse sequence consisted of a spectrally selective low bandwidth E-BURP-1 90° pulse. PRESS localization was achieved using optimized Shinnar-Le Roux 180° pulses and overlapping gradients were used to minimize the TE. The reproducibility of NAD⁺ quantification was measured in 11 healthy volunteers. The association of cerebral NAD⁺ with age was assessed in 16 healthy subjects 26-78 years old.

RESULTS

Spectra acquired from a voxel placed in subjects' occipital lobe consisted of downfield peaks from the H₂ , H₄ , and H₆ protons of the nicotinamide moiety of NAD⁺ between 8.9-9.35 ppm. The mean ± SD within-session and between-session coefficients of variation were found to be 6.14 ± 2.03% and 6.09 ± 3.20%, respectively. In healthy volunteers, an age-dependent decline of the NAD⁺ levels in the brain was also observed (β = -1.24 μM/y, SE = 0.21, P < 0.001).

CONCLUSION

We demonstrated the feasibility and robustness of a newly developed ¹ H MRS technique to measure localized cerebral NAD⁺ at 7T MRI using a commercially available RF head coil. This technique may be further applied to detect and quantify NAD⁺ from different regions of the brain as well as from other tissues.

Navigator-free metabolite-cycled proton spectroscopy of the heart

PURPOSE

Respiratory gating in cardiac water-suppressed (WS) proton spectroscopy leads to long and unpredictable scan times. Metabolite cycling allows to perform frequency and phase correction on the water signal and, hence, offers an approach to navigator-free cardiac spectroscopy with fixed scan time. The objective of the present study was to develop and implement navigator-free metabolite-cycled cardiac proton spectroscopy (MC nonav) and compare it with standard navigator-gated WS (WS nav) and navigator-free WS (WS nonav) measurements for the assessment of triglyceride-to-water ratios (TG/W) and creatine-to-water ratios (CR/W) in the intraventricular septum of the in vivo heart.

METHODS

Navigator-free metabolite-cycled spectroscopy was implemented on a clinical 1.5T system. In vivo measurements were performed on 10 young and 5 older healthy volunteers to assess signal-to-noise ratio efficiency as well as TG/W and CR/W and the relative Cramér-Rao lower bounds for CR. The performance of the metabolite-cycled sequence was verified using simulations.

RESULTS

On average, scan times of MC nonav were 3.4 times shorter compared with WS nav, while no significant bias for TG/W was observed (coefficient of variation = 14.0%). signal-to-noise ratio efficiency of both TG and CR increased for MC nonav compared with WS nav. Relative Cramér-Rao lower bounds of CR decreased for MC nonav. Overall spectral quality was found comparable between MC nonav and WS nav, while it was inferior for WS nonav.

CONCLUSION

Navigator-free metabolite-cycled cardiac proton spectroscopy offers 3.4-fold accelerated assessment of TG/W and CR/W in the heart with preserved spectral quality when compared with navigator-gated WS scans.

Methodological consensus on clinical proton MRS of the brain: Review and recommendations

Proton MRS (1 H MRS) provides noninvasive, quantitative metabolite profiles of tissue and has been shown to aid the clinical management of several brain diseases. Although most modern clinical MR scanners support MRS capabilities, routine use is largely restricted to specialized centers with good access to MR research support. Widespread adoption has been slow for several reasons, and technical challenges toward obtaining reliable good-quality results have been identified as a contributing factor. Considerable progress has been made by the research community to address many of these challenges, and in this paper a consensus is presented on deficiencies in widely available MRS methodology and validated improvements that are currently in routine use at several clinical research institutions. In particular, the localization error for the PRESS localization sequence was found to be unacceptably high at 3 T, and use of the semi-adiabatic localization by adiabatic selective refocusing sequence is a recommended solution. Incorporation of simulated metabolite basis sets into analysis routines is recommended for reliably capturing the full spectral detail available from short TE acquisitions. In addition, the importance of achieving a highly homogenous static magnetic field (B0 ) in the acquisition region is emphasized, and the limitations of current methods and hardware are discussed. Most recommendations require only software improvements, greatly enhancing the capabilities of clinical MRS on existing hardware. Implementation of these recommendations should strengthen current clinical applications and advance progress toward developing and validating new MRS biomarkers for clinical use.

Lamotrigine Therapy and Biomarkers of Cerebral Energy Metabolism in Older Age Bipolar Depression

OBJECTIVE

This study compared brain energy metabolism, as measured by cerebral concentrations of glutamate (Glu), glutamine (Gln), and N-acetyl aspartate (NAA), in older age bipolar depression (OABD) to that of psychiatrically healthy comparison subjects using proton (¹H) magnetic resonance spectroscopy imaging at 4-Tesla. Metabolite levels were assessed in OABD subjects before and after 8 weeks of lamotrigine therapy with the goal of determining relationships between cerebral energy metabolism, depression symptom severity, and changes in depression symptom response.

METHODS

Individuals (n = 21, mean age: 62.0 ± 5.9 years) with bipolar disorder, current episode depressed, and a healthy comparison group (n = 14, mean age: 67.5 ± 8.8 years) were selected. Participants with bipolar disorder, current episode depressed, were treated in open label fashion with lamotrigine monotherapy for 8 weeks. All subjects were scanned with ¹H magnetic resonance spectroscopy at 4T at baseline and again after 8 weeks to assess levels of cerebral metabolites in the anterior cingulate cortex and parieto-occipital cortex. Metabolite levels were examined as ratios relative to creatine (Cr). Response to 8 weeks of lamotrigine treatment in the bipolar disorder, current episode depressed group, was assessed as a continuous measure on the Montgomery-Asberg Depression Rating Scale.

RESULTS

NAA/Cr ratio in OABD was significantly lower by 14% (95% confidence interval: [1%, 26%]) than in comparison subjects at baseline. However, there were no associations between NAA/Cr, Glu/Cr, or Gln/Cr and either depression severity or lamotrigine treatment.

CONCLUSION

Group differences in NAA suggest evidence for a deficit in cerebral energy metabolism in OABD.

Benchtop NMR for the monitoring of bioprocesses

This mini-review highlights the potential of benchtop nuclear magnetic resonance (NMR) for the monitoring of bioprocesses. It describes recent perspectives opened by the reduced size of devices in relaxometry, magnetic resonance imaging and NMR spectroscopy. In particular, the recent emergence of the benchtop NMR spectroscopy gives access to many applications thanks to the implementation of advanced experiments.

Can Galactose Be Converted to Glucose in HepG2 Cells? Improving the in Vitro Mitochondrial Toxicity Assay for the Assessment of Drug Induced Liver Injury

Human hepatocellular carcinoma cells, HepG2, are often used for drug mediated mitochondrial toxicity assessments. Glucose in HepG2 culture media is replaced by galactose to reveal drug-induced mitochondrial toxicity as a marked shift of drug IC50 values for the reduction of cellular ATP. It has been postulated that galactose sensitizes HepG2 mitochondria by the additional ATP consumption demand in the Leloir pathway. However, our NMR metabolomics analysis of HepG2 cells and culture media showed very limited galactose metabolism. To clarify the role of galactose in HepG2 cellular metabolism, U-¹³C₆-galactose or U-¹³C₆-glucose was added to HepG2 culture media to help specifically track the metabolism of those two sugars. Conversion to U-¹³C₃-lactate was hardly detected when HepG2 cells were incubated with U-¹³C₆-galactose, while an abundance of U-¹³C₃-lactate was produced when HepG2 cells were incubated with U-¹³C₆-glucose. In the absence of glucose, HepG2 cells increased glutamine consumption as a bioenergetics source. The requirement of additional glutamine almost matched the amount of glucose needed to maintain a similar level of cellular ATP in HepG2 cells. This improved understanding of galactose and glutamine metabolism in HepG2 cells helped optimize the ATP-based mitochondrial toxicity assay. The modified assay showed 96% sensitivity and 97% specificity in correctly discriminating compounds known to cause mitochondrial toxicity from those with prior evidence of not being mitochondrial toxicants. The greatest significance of the modified assay was its improved sensitivity in detecting the inhibition of mitochondrial fatty acid β-oxidation (FAO) when glutamine was withheld. Use of this improved assay for an empirical prediction of the likely contribution of mitochondrial toxicity to human DILI (drug induced liver injury) was attempted. According to testing of 65 DILI positive compounds representing numerous mechanisms of DILI together with 55 DILI negative compounds, the overall prediction of mitochondrial mechanism-related DILI showed 25% sensitivity and 95% specificity.

Ultralong TE In Vivo 1 H MR Spectroscopy of Omega-3 Fatty Acids in Subcutaneous Adipose Tissue at 7 T

BACKGROUND

Omega-3 (n-3) fatty acids (FA) play and important role in neural development and other metabolic diseases such as obesity and diabetes. The knowledge about the in vivo content and distribution of n-3 FA in human body tissues is not well established and the standard quantification of FA is invasive and costly.

PURPOSE

To detect omega-3 (n-3 CH3 ) and non-omega-3 (CH3 ) methyl group resonance lines with echo times up to 1200 msec, in oils, for the assessment of n-3 FA content, and the n-3 FA fraction in adipose tissue in vivo.

STUDY TYPE

Prospective technical development.

POPULATION

Three oils with different n-3 FA content and 24 healthy subjects.

FIELD STRENGTH/SEQUENCE

Single-voxel MR spectroscopy (SVS) with a point-resolved spectroscopy (PRESS) sequence with an echo time (TE) of 1000 msec at 7 T.

ASSESSMENT

Knowledge about the J-coupling evolution of both CH3 resonances was used for the optimal detection of the n-3 CH3 resonance line at a TE of 1000 msec. The accuracy of the method in oils and in vivo was validated from a biopsy sample with gas chromatography analysis.

STATISTICAL TESTS

SVS data were compared to gas chromatography with the Pearson correlation coefficient.

RESULTS

T2 relaxation times in oils were assessed as follows: CH2 , 65 ± 22 msec; CH3 , 325 ± 7 msec; and n-3 CH3 , 628 ± 34 msec. The n-3 FA fractions from oil phantom experiments (n = 3) were in agreement with chromatography analysis and the comparison of in vivo obtained data with the results of chromatography analysis (n = 5) yielded a significant correlation (P = 0.029).

DATA CONCLUSION

PRESS with ultralong-TE can detect and quantify the n-3 CH3 signal in vivo at 7 T.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;50:71-82.

Elevated brain glutamate levels in type 1 diabetes: correlations with glycaemic control and age of disease onset but not with hypoglycaemia awareness status

AIMS/HYPOTHESIS

Chronic hyperglycaemia in type 1 diabetes affects the structure and functioning of the brain, but the impact of recurrent hypoglycaemia is unclear. Changes in the neurochemical profile have been linked to loss of neuronal function. We therefore aimed to investigate the impact of type 1 diabetes and burden of hypoglycaemia on brain metabolite levels, in which we assumed the burden to be high in individuals with impaired awareness of hypoglycaemia (IAH) and low in those with normal awareness of hypoglycaemia (NAH).

METHODS

We investigated 13 non-diabetic control participants, 18 individuals with type 1 diabetes and NAH and 13 individuals with type 1 diabetes and IAH. Brain metabolite levels were determined by analysing previously obtained ¹H magnetic resonance spectroscopy data, measured under hyperinsulinaemic-euglycaemic conditions.

RESULTS

Brain glutamate levels were higher in participants with diabetes, both with NAH (+15%, p = 0.013) and with IAH (+19%, p = 0.003), compared with control participants. Cerebral glutamate levels correlated with HbA_1c levels (r = 0.40; p = 0.03) and correlated inversely (r = -0.36; p = 0.04) with the age at diagnosis of diabetes. Other metabolite levels did not differ between groups, apart from an increase in aspartate in IAH.

CONCLUSIONS/INTERPRETATION

In conclusion, brain glutamate levels are elevated in people with type 1 diabetes and correlate with glycaemic control and age of disease diagnosis, but not with burden of hypoglycaemia as reflected by IAH. This suggests a potential role for glutamate as an early marker of hyperglycaemia-induced cerebral complications of type 1 diabetes. ClinicalTrials.gov NCT03286816; NCT02146404; NCT02308293.

Water removal in MR spectroscopic imaging with L2 regularization

PURPOSE

An L2-regularization based postprocessing method is proposed and tested for removal of residual or unsuppressed water signals in proton MR spectroscopic imaging (MRSI) data recorded from the human brain at 3T.

METHODS

Water signals are removed by implementation of the L2 regularization using a synthesized water-basis matrix that is orthogonal to metabolite signals of interest in the spectral dimension. Simulated spectra with variable water amplitude and in vivo brain MRSI datasets were used to demonstrate the proposed method. Results were compared with two commonly-used postprocessing methods for removing water signals.

RESULTS

The L2 method yielded metabolite signals that were close to true values for the simulated spectra. Residual/unsuppressed water signals in human brain short- and long-echo time MRSI datasets were efficiently removed by the proposed method allowing good quality metabolite maps to be reconstructed with minimized contamination from water signals. Significant differences of the creatine signal were observed between brain long-echo time MRSI without and with water saturation, attributable to the previously described magnetization transfer effect.

CONCLUSIONS

With usage of a synthesized water matrix generated based on reasonable prior knowledge about water and metabolite resonances, the L2 method is shown to be an effective way to remove water signals from MRSI of the human brain.

Volume-localized measurement of oxygen extraction fraction in the brain using MRI

Purpose

T₂‐relaxation‐under‐spin‐tagging (TRUST) is an MR technique for the non‐invasive assessment of whole‐brain cerebral oxygen extraction fraction (OEF), through measurement of the venous blood T₂ relaxation time in the sagittal sinus. A key limitation of TRUST, however, is the lack of spatial specificity of the measurement. We sought to develop a modified TRUST sequence, selective localized TRUST (SL‐TRUST), having sensitivity to venous blood T₂ within a targeted brain region, and therefore achieving spatially localized measurements of cerebral tissue OEF, while still retaining acquisition in the sagittal sinus.

Methods

A method for selective localization of TRUST sequence was developed, and the reproducibility of the technique was evaluated in healthy participants. Regional measurements were achieved for a single hemisphere and for a 3D‐localized 70 × 70 × 80 mm³ tissue region using SL‐TRUST and compared to a global TRUST measure. An additional measure of venous blood T₁ in the sagittal sinus was used to estimate subject‐specific hematocrit. Six subjects were scanned over 4 sessions, including intra‐session repeat measurements.

Results

The average T₂ in the sagittal sinus was found to be 60.8 ± 8.9, 62.7 ± 7.9, 64.6 ± 8.4, and 66.3 ± 10.3 ms (mean ± SD) for conventional TRUST, global SL‐TRUST, hemispheric SL‐TRUST, and 3D‐localized SL‐TRUST, respectively. Intra‐, inter‐session, and inter‐subject coefficients of variation for OEF using SL‐TRUST were found to be comparable and in some cases superior to those obtained using TRUST.

Conclusion

OEF comparison of 2 contralateral regions was achievable in under 5 min suggesting SL‐TRUST offers potential for quantifying regional OEF differences in both healthy and clinical populations.

Central IGF-1 protects against features of cognitive and sensorimotor decline with aging in male mice

Disruptions in growth hormone/insulin-like growth factor-1 (GH/IGF-1) signaling have been linked to improved longevity in mice and humans. Nevertheless, while IGF-1 levels are associated with increased cancer risk, they have been paradoxically implicated with protection from other age-related conditions, particularly in the brain, suggesting that strategies aimed at selectively increasing central IGF-1 action may have favorable effects on aging. To test this hypothesis, we generated inducible, brain-specific (TRE-IGF-1 × Camk2a-tTA) IGF-1 (bIGF-1) overexpression mice and studied effects on healthspan. Doxycycline was removed from the diet at 12 weeks old to permit post-development brain IGF-1 overexpression, and animals were monitored up to 24 months. Brain IGF-1 levels were increased approximately twofold in bIGF-1 mice, along with greater brain weights, volume, and myelin density (P < 0.05). Age-related changes in rotarod performance, exercise capacity, depressive-like behavior, and hippocampal gliosis were all attenuated specifically in bIGF-1 male mice (P < 0.05). However, chronic brain IGF-1 failed to prevent declines in cognitive function or neurovascular coupling. Therefore, we performed a short-term intranasal (IN) treatment of either IGF-1 or saline in 24-month-old male C57BL/6 mice and found that IN IGF-1 treatment tended to reduce depressive (P = 0.09) and anxiety-like behavior (P = 0.08) and improve motor coordination (P = 0.07) and unlike transgenic mice improved motor learning (P < 0.05) and visuospatial and working memory (P < 0.05). These data highlight important sex differences in how brain IGF-1 action impacts healthspan and suggest that translational approaches that target IGF-1 centrally can restore cognitive function, a possibility that should be explored as a strategy to combat age-related cognitive decline.

Age-dependent changes in metabolite profile and lipid saturation in dystrophic mice

Duchenne Muscular Dystrophy (DMD) is a fatal X-linked genetic disorder. In DMD, the absence of the dystrophin protein causes decreased sarcolemmal integrity resulting in progressive replacement of muscle with fibrofatty tissue. The effects of lacking dystrophin on muscle and systemic metabolism are still unclear. Therefore, to determine the impact of the absence of dystrophin on metabolism, we investigated the metabolic and lipid profile at two different, well-defined stages of muscle damage and stabilization in mdx mice. We measured NMR-detectable metabolite and lipid profiles in the serum and muscles of mdx mice at 6 and 24 weeks of age. Metabolites were determined in muscle in vivo using ¹ H MRI/MRS, in isolated muscles using ¹ H-HR-MAS NMR, and in serum using high resolution ¹ H/¹³ C NMR. Dystrophic mice were found to have a unique lipid saturation profile compared with control mice, revealing an age-related metabolic change. In the 6-week-old mdx mice, serum lipids were increased and the degree of lipid saturation changed between 6 and 24 weeks. The serum taurine-creatine ratio increased over the life span of mdx, but not in control mice. Furthermore, the saturation index of lipids increased in the serum but decreased in the tissue over time. Finally, we demonstrated associations between MRI-T₂ , a strong indicator of inflammation/edema, with tissue and serum lipid profiles. These results indicate the complex temporal changes of metabolites in the tissue and serum during repetitive bouts of muscle damage and regeneration that occur in dystrophic muscle.

Reduced gamma-amino butyric acid (GABA) and glutamine in the anterior cingulate cortex (ACC) of veterans exposed to trauma

BACKGROUND

Trauma-related diagnoses such as posttraumatic stress disorder (PTSD) are prevalent in veterans. The identification of mechanisms related to stress vulnerability and development of PTSD specifically in a veteran population may aid in the prevention of PTSD and identification of novel treatment targets.

METHODS

Veterans with PTSD (n = 27), trauma-exposed veterans with no PTSD (TEC, n = 18) and non-trauma-exposed controls (NTEC, n = 28) underwent single-voxel proton (¹H) magnetic resonance spectroscopy (MRS) at 3 Tesla in the dorsal anterior cingulate cortex (dACC) using a two-dimensional (2D) J-resolved point spectroscopy sequence in addition to completing a clinical battery.

RESULTS

The PTSD and TEC groups demonstrated lower gamma-amino butyric acid (GABA)/H₂O (p = 0.02) and glutamine (Gln)/H₂O (p = 0.02) in the dACC as compared to the NTEC group. The PTSD group showed a trend towards higher Glu/GABA (p = 0.053) than the NTEC group. Further, GABA/H₂O in the dACC correlated negatively with sleep symptoms in the PTSD group (p = 0.03) but not in the TEC and NTEC groups.

LIMITATIONS

Cross-sectional study design, concomitant medications, single voxel measurement as opposed to global changes, absence of measure of childhood or severity of trauma and objective sleep measures, female participants not matched for menstrual cycle phase.

CONCLUSIONS

Exposure to trauma in veterans may be associated with lower GABA/H₂O and Gln/H₂O in the dACC, suggesting disruption in the GABA-Gln-glutamate cycle. Further, altered Glu/GABA in the dACC in the PTSD group may indicate an excitatory-inhibitory imbalance. Further, lower GABA/H₂O in the ACC was associated with poor sleep in the PTSD group. Treatments that restore GABAergic balance may be particularly effective in reducing sleep symptoms in PTSD.

A framework for motion correction of background suppressed arterial spin labeling perfusion images acquired with simultaneous multi-slice EPI

PURPOSE

When using simultaneous multi-slice (SMS) EPI for background suppressed (BGS) arterial spin labeling (ASL), correction of through-plane motion could introduce artefacts, because the slices with most effective BGS are adjacent to slices with the least BGS. In this study, a new framework is presented to correct for such artefacts.

METHODS

The proposed framework consists of 3 steps: (1) homogenization of the static tissue signal over the different slices to eliminate most inter-slice differences because of different levels of BGS, (2) application of motion correction, and (3) extraction of a perfusion-weighted signal using a general linear model. The proposed framework was evaluated by simulations and a functional ASL study with intentional head motion.

RESULTS

Simulation studies demonstrated that the strong signal differences between slices with the most and least effective BGS caused sub-optimal estimation of motion parameters when through-plane motion was present. Although use of the M0 image as the reference for registration allowed 82% improvement of motion estimation for through-plane motion, it still led to residual subtraction errors caused by different static tissue signal between control and label because of different BGS levels. By using our proposed framework, those problems were minimized, and the accuracy of CBF estimation was improved. Moreover, the functional ASL study showed improved detection of visual and motor activation when applying the framework as compared to conventional motion correction, as well as when motion correction was completely omitted.

CONCLUSION

When combining BGS-ASL with SMS-EPI, particular attention is needed to avoid artefacts introduced by motion correction. With the proposed framework, these issues are minimized.

High-resolution metabolic mapping of gliomas via patch-based super-resolution magnetic resonance spectroscopic imaging at 7T

OBJECTIVES

To demonstrate the feasibility of 7 T magnetic resonance spectroscopic imaging (MRSI), combined with patch-based super-resolution (PBSR) reconstruction, for high-resolution multi-metabolite mapping of gliomas.

MATERIALS AND METHODS

Ten patients with WHO grade II, III and IV gliomas (6/4, male/female; 45 ± 9 years old) were prospectively measured between 2014 and 2018 on a 7 T whole-body MR imager after routine 3 T magnetic resonance imaging (MRI) and positron emission tomography (PET). Free induction decay MRSI with a 64 × 64-matrix and a nominal voxel size of 3.4 × 3.4 × 8 mm³ was acquired in six minutes, along with standard T1/T2-weighted MRI. Metabolic maps were obtained via spectral LCmodel processing and reconstructed to 0.9 × 0.9 × 8 mm³ resolutions via PBSR.

RESULTS

Metabolite maps obtained from combined 7 T MRSI and PBSR resolved the density of metabolic activity in the gliomas in unprecedented detail. Particularly in the more heterogeneous cases (e.g. post resection), metabolite maps enabled the identification of complex metabolic activities, which were in topographic agreement with PET enhancement.

CONCLUSIONS

PBSR-MRSI combines the benefits of ultra-high-field MR systems, cutting-edge MRSI, and advanced postprocessing to allow millimetric resolution molecular imaging of glioma tissue beyond standard methods. An ideal example is the accurate imaging of glutamine, which is a prime target of modern therapeutic approaches, made possible due to the higher spectral resolution of 7 T systems.

Quantifying cardiac-induced brain tissue expansion using DENSE

Brain tissue undergoes viscoelastic deformation and volumetric strain as it expands over the cardiac cycle due to blood volume changes within the underlying microvasculature. Volumetric strain measurements may therefore provide insights into small vessel function and tissue viscoelastic properties. Displacement encoding via stimulated echoes (DENSE) is an MRI technique that can quantify the submillimetre displacements associated with brain tissue motion. Despite previous studies reporting brain tissue displacements using DENSE and other MRI techniques, a complete picture of brain tissue volumetric strain over the cardiac cycle has not yet been obtained. To address this need we implemented 3D cine-DENSE at 7 T and 3 T to investigate the feasibility of measuring cardiac-induced volumetric strain as a marker for small vessel blood volume changes. Volumetric strain over the entire cardiac cycle was computed for the whole brain and for grey and white matter tissue separately in six healthy human subjects. Signal-to-noise ratio (SNR) measurements were used to determine the voxel-wise volumetric strain noise. Mean peak whole brain volumetric strain at 7 T (mean ± SD) was (4.5 ± 1.0) × 10^-4 (corresponding to a volume expansion of 0.48 ± 0.1 mL), which is in agreement with literature values for cerebrospinal fluid that is displaced into the spinal canal to maintain a stable intracranial pressure. The peak volumetric strain ratio of grey to white matter was 4.4 ± 2.8, reflecting blood volume and tissue stiffness differences between these tissue types. The mean peak volumetric strains of grey and white matter tissue were found to be significantly different (p < 0.001). The mean SNR at 7 T and 3 T of the DENSE measurements was 22.0 ± 7.3 and 7.0 ± 2.8 respectively, which currently limits a voxel-wise strain analysis at both field strengths. We demonstrate that tissue specific quantification of volumetric strain is feasible with DENSE. This metric holds potential for studying blood volume pulsations in the ageing brain in healthy and diseased states.

Multi-vendor standardized sequence for edited magnetic resonance spectroscopy

Spectral editing allows direct measurement of low-concentration metabolites, such as GABA, glutathione (GSH) and lactate (Lac), relevant for understanding brain (patho)physiology. The most widely used spectral editing technique is MEGA-PRESS, which has been diversely implemented across research sites and vendors, resulting in variations in the final resolved edited signal. In this paper, we describe an effort to develop a new universal MEGA-PRESS sequence with HERMES functionality for the major MR vendor platforms with standardized RF pulse shapes, durations, amplitudes and timings. New RF pulses were generated for the universal sequence. Phantom experiments were conducted on Philips, Siemens, GE and Canon 3 T MRI scanners using 32-channel head coils. In vivo experiments were performed on the same six subjects on Philips and Siemens scanners, and on two additional subjects, one on GE and one on Canon scanners. On each platform, edited MRS experiments were conducted with the vendor-native and universal MEGA-PRESS sequences for GABA (TE = 68 ms) and Lac editing (TE = 140 ms). Additionally, HERMES for GABA and GSH was performed using the universal sequence at TE = 80 ms. The universal sequence improves inter-vendor similarity of GABA-edited and Lac-edited MEGA-PRESS spectra. The universal HERMES sequence yields both GABA- and GSH-edited spectra with negligible levels of crosstalk on all four platforms, and with strong agreement among vendors for both edited spectra. In vivo GABA+/Cr, Lac/Cr and GSH/Cr ratios showed relatively low variation between scanners using the universal sequence. In conclusion, phantom and in vivo experiments demonstrate successful implementation of the universal sequence across all four major vendors, allowing editing of several metabolites across a range of TEs.