Off-resonance metabolite magnetization transfer measurements on rat brain in situ

High-field ex vivo and in vivo two-dimensional nuclear magnetic resonance spectroscopy in murine brain: Resolving and exploring the molecular environment

The structural and chemical complexities within the brain pose a challenge that few noninvasive techniques can tackle with the dexterity of nuclear magnetic resonance (NMR) spectroscopy. Still, even with the advent of ultrahigh fields and of cryogenically cooled coils for in vivo research, the superposition of metabolic resonances arising from the brain remains a challenge. The present study explores the potential to tackle this milieu using a combination of two-dimensional (2D) NMR techniques, implemented on murine brains in vivo at 15.2 T and ex vivo at 14.1 T. While both experiments were affected by substantial inhomogeneous broadenings conveying distinct elongated lineshapes to the cross-peaks, the ability of increased fields to resolve off-diagonal resonances was clear. A comparison between the corresponding conventional and double quantum-filtered correlated spectroscopy traces enabled an improved assignment of in vivo resonances on the basis of more sensitive ex vivo 2D acquisitions, foremost on the basis of homonuclear cross-relaxation-driven correlations for peaks resonating downfield from water, and of heteronuclear correlations at natural abundance for the upfield protons. With the aid of such 2D correlations approximately 29 metabolites could be resolved and identified. This enhanced resolution was used to explore features related to the metabolites' diffusivities, their exposure to water, and their facility to undergo magnetization transfers to amide/amine/hydroxyl resonances. Cross-peaks from main murine brain biomolecules, including choline, creatine, γ-aminobutyric acid, N-acetyl aspartate, glutamine, and glutamate, showed enhancements in several of these various features, opening interesting vistas about metabolite compartmentalization as viewed by these 2D NMR experiments.

Detection of electrostatic molecular binding using the water proton signal

PURPOSE

Saturation transfer MRI has previously been used to probe molecular binding interactions with signal enhancement via the water signal. Here, we detail the relayed nuclear overhauser effect (rNOE) based mechanisms of this signal enhancement, develop a strategy of quantifying molecular binding affinity, i.e., the dissociation constant ( K D $$ {K}_D $$ ), and apply the method to detect electrostatic binding of several charged small biomolecules. Another goal was to estimate the detection limit for transient receptor-substrate binding.

THEORY AND METHODS

The signal enhancement mechanism was quantitatively described by a three-step magnetization transfer model, and numerical simulations were performed to verify this theory. The binding equilibria of arginine, choline, and acetyl-choline to anionic resin were studied as a function of ligand concentration, pH, and salt content. Equilibrium dissociation constants ( K D $$ {K}_D $$ ) were determined by fitting the multiple concentration data.

RESULTS

The numerical simulations indicate that the signal enhancement is sufficient to detect the molecular binding of sub-millimolar (∼100 μM) concentration ligands to low micromolar levels of molecular targets. The measured rNOE signals from arginine, choline, and acetyl-choline binding experiments show that several magnetization transfer pathways (intra-ligand rNOEs and intermolecular rNOEs) can contribute. The rNOEs that arise from molecular ionic binding were influenced by pH and salt concentration. The molecular binding strengths in terms of K D $$ {K}_{\mathrm{D}} $$ ranged from 70-160 mM for the three cations studied.

CONCLUSION

The capability to use MRI to detect the transient binding of small substrates paves a pathway towards the detection of micromolar level receptor-substrate binding in vivo.

Water and lipid suppression techniques for advanced 1 H MRS and MRSI of the human brain: Experts' consensus recommendations

The neurochemical information provided by proton magnetic resonance spectroscopy (MRS) or MR spectroscopic imaging (MRSI) can be severely compromised if strong signals originating from brain water and extracranial lipids are not properly suppressed. The authors of this paper present an overview of advanced water/lipid-suppression techniques and describe their advantages and disadvantages. Moreover, they provide recommendations for choosing the most appropriate techniques for proper use. Methods of water signal handling are primarily focused on the VAPOR technique and on MRS without water suppression (metabolite cycling). The section on lipid-suppression methods in MRSI is divided into three parts. First, lipid-suppression techniques that can be implemented on most clinical MR scanners (volume preselection, outer-volume suppression, selective lipid suppression) are described. Second, lipid-suppression techniques utilizing the combination of k-space filtering, high spatial resolutions and lipid regularization are presented. Finally, three promising new lipid-suppression techniques, which require special hardware (a multi-channel transmit system for dynamic B₁⁺ shimming, a dedicated second-order gradient system or an outer volume crusher coil) are introduced.

Combining chemical exchange saturation transfer and 1 H magnetic resonance spectroscopy for simultaneous determination of metabolite concentrations and effects of magnetization exchange

Purpose

A new sequence combining chemical‐exchange saturation‐transfer (CEST) with traditional MRS is used to simultaneously determine metabolite content and effects of magnetization exchange.

Methods

A CEST saturation block consisting of a train of RF‐pulses is placed before a metabolite‐cycled semi‐LASER single‐voxel spectroscopy sequence. The saturation parameters are adjustable to allow optimization of the saturation for a specific target. Data were collected in brain from 20 subjects in experiments with different B₁‐settings (0.4‐2.0 µT) on a 3T MR scanner. CEST Z‐spectra were calculated from water intensities and fitted with a multi‐pool Lorentzian model. Interrelated metabolite spectra were fitted in fitting tool for arrays of interrelated datasets (FiTAID).

Results

Evaluation of traditional Z‐spectra from water revealed exchange effects from amides, amines, and hydroxyls as well as an upfield nuclear Overhauser effect. The magnetization transfer effect was evaluated on metabolites and macromolecules for the whole spectral range and for the different B₁ levels. A correction scheme for direct saturation on metabolites is proposed. Both magnetization‐transfer and direct saturation proved to differ for individual metabolites.

Conclusion

Using non‐water‐suppressed spectroscopy offers time‐saving simultaneous recording of the traditional CEST Z‐spectrum from water and the metabolite spectrum under frequency‐selective saturation. In addition, exchange and magnetization‐transfer effects on metabolites and macromolecules can be detected, which might offer additional possibilities for quantification or give further insight into the composition of the traditional CEST Z‐spectrum. Apparent magnetization‐transfer effects on macromolecular signals in the ¹H‐MR spectrum have been found. Detailed knowledge of magnetization‐transfer effects is also relevant for judging the influence of water‐suppression on the quantification of metabolite signals.

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.

Creatine in the central nervous system: From magnetic resonance spectroscopy to creatine deficiencies

Creatine (Cr) is an important organic compound acting as intracellular high-energy phosphate shuttle and in energy storage. While located in most cells where it plays its main roles in energy metabolism and cytoprotection, Cr is highly concentrated in muscle and brain tissues, in which Cr also appears to act in osmoregulation and neurotransmission. This review discusses the basis of Cr metabolism, synthesis and transport within brain cells. The importance of Cr in brain function and the consequences of its impaired metabolism in primary and secondary Cr deficiencies are also discussed. Cr and phosphocreatine (PCr) in living systems can be well characterized using in vivo magnetic resonance spectroscopy (MRS). This review describes how ¹H MRS allows the measurement of Cr and PCr, and how ³¹P MRS makes it possible to estimate the creatine kinase (CK) rate constant and so detect dynamic changes in the Cr/PCr/CK system. Absolute quantification by MRS using creatine as internal reference is also debated. The use of in vivo MRS to study brain Cr in a non-invasive way is presented, as well as its use in clinical and preclinical studies, including diagnosis and treatment follow-up in patients.

MR imaging of a novel NOE-mediated magnetization transfer with water in rat brain at 9.4 T

PURPOSE

To detect, map, and quantify a novel nuclear Overhauser enhancement (NOE)-mediated magnetization transfer (MT) with water at approximately -1.6 ppm [NOE(-1.6)] in rat brain using MRI.

METHODS

Continuous wave MT sequences with a variety of radiofrequency irradiation powers were optimized to achieve the maximum contrast of this NOE(-1.6) effect at 9.4 T. The distribution of effect magnitudes, resonance frequency offsets, and line widths in healthy rat brains and the differences of the effect between tumors and contralateral normal brains were imaged and quantified using a multi-Lorentzian fitting method. MR measurements on reconstituted model phospholipids as well as two cell lines (HEK293 and 9L) were also performed to investigate the possible molecular origin of this NOE.

RESULTS

Our results suggest that the NOE(-1.6) effect can be detected reliably in rat brain. Pixel-wise fittings demonstrated the regional variations of the effect. Measurements in a rodent tumor model showed that the amplitude of NOE(-1.6) in brain tumor was significantly diminished compared with that in normal brain tissue. Measurements of reconstituted phospholipids suggest that this effect may originate from choline phospholipids.

CONCLUSION

NOE(-1.6) could be used as a new biomarker for the detection of brain tumor. Magn Reson Med 78:588-597, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Influence of muscle fiber orientation on water and metabolite relaxation times, magnetization transfer, and visibility in human skeletal muscle

PURPOSE

To gain a deeper understanding of the influence of skeletal muscle fiber orientation on metabolite visibility, magnetization transfer from water, and water proton relaxation rates in (1)H MR spectra.

METHODS

Non-water-suppressed MR spectroscopy was performed in tibialis anterior muscle (TA) of 10 healthy adults, with the TA oriented either parallel or at the magic angle to the 3T field. Spectra were acquired with metabolite-cycled PRESS, and water inversion from 50 to 2510 ms before excitation. Water proton T2 relaxation was sampled with STEAM with echo times from 12 to 272 ms.

RESULTS

Apparent concentrations of total creatine (tCr), taurine, and trimethylammonium compounds were reduced by 29% to 67% when TA was parallel to B0. Both tCr peak areas were strongly correlated to the methylene peak splitting. Magnetization transfer rates from water to tCr CH3 were not significantly different between orientations. Water T1s were similar between orientations, but T2s were statistically significantly shorter by 1 ms in the parallel orientation (P = 0.002).

CONCLUSION

Muscle metabolite visibilities in MR spectroscopy and water T2 times depend substantially on muscle fiber orientation relative to B0 . In contrast, magnetization transfer rates appear to depend on muscle composition, rather than fiber orientation.

Detection of cerebral NAD(+) by in vivo (1)H NMR spectroscopy

Nicotinamide adenine dinucleotide (NAD(+)) plays a central role in cellular metabolism both as a coenzyme for electron-transfer enzymes as well as a substrate for a wide range of metabolic pathways. In the current study NAD(+) was detected on rat brain in vivo at 11.7T by 3D localized (1)H MRS of the NAD(+) nicotinamide protons in the 8.7-9.5 ppm spectral region. Avoiding water perturbation was critical to the detection of NAD(+) as strong, possibly indirect cross-relaxation between NAD(+) and water would lead to a several-fold reduction of the NAD(+) intensity in the presence of water suppression. Water perturbation was minimized through the use of localization by adiabatic spin-echo refocusing (LASER) in combination with frequency-selective excitation. The NAD(+) concentration in the rat cerebral cortex was determined at 296 ± 28 μm, which is in good agreement with recently published (31) P NMR-based results as well as results from brain extracts in vitro (355 ± 34 μm). The T1 relaxation time constants of the NAD(+) nicotinamide protons as measured by inversion recovery were 280 ± 65 and 1136 ± 122 ms in the absence and presence of water inversion, respectively. This confirms the strong interaction between NAD(+) nicotinamide and water protons as observed during water suppression. The T2 relaxation time constants of the NAD(+) nicotinamide protons were determined at 60 ± 13 ms after confounding effects of scalar coupling evolution were taken into account. The simplicity of the MR sequence together with the robustness of NAD(+) signal detection and quantification makes the presented method a convenient choice for studies on NAD(+) metabolism and function. As the method does not critically rely on magnetic field homogeneity and spectral resolution it should find immediate applications in rodents and humans even at lower magnetic fields.

Hemi-spectrum substitution after water signal fitting (HESWAF): an improvement of the modulus post-processing of MR spectra

OBJECTIVE

In a previous study, we have shown that modulus post-processing is a simple and efficient tool to both phase correct and frequency align magnetic resonance (MR) spectra automatically. Furthermore, this technique also eliminates sidebands and phase distortions. The advantages of the modulus technique have been illustrated in several applications to brain proton MR spectroscopy. Two possible drawbacks have also been pointed out. The first one is the theoretical decrease in signal-to-noise ratio (SNR) by a factor up to √2 when comparing the spectrum obtained after modulus versus conventional post-processing. The second pitfall results from the symmetrization of the spectrum induced by modulus post-processing, since any resonance or artifact located at the left of the water resonance is duplicated at the right of the water resonance, thus contaminating the region of the spectrum containing the resonances of interest. Herein, we propose a strategy in order to eliminate these two limitations.

MATERIALS AND METHODS

Concerning the SNR issue, two complementary approaches are presented here. The first is based on the application of modulus post-processing before spatial apodization, and the second consists in substituting the left half of the spectrum by the fit of the water resonance before applying modulus post-processing. The symmetrization induced by modulus post-processing then combines the right half of the original spectrum containing the resonances of interest with the left half of the water fit, free of noise and artifacts. Consequently, the SNR is improved when compared to modulus post-processing alone. As a bonus, any artifact or resonance present in the left half of the original spectrum is removed. This solves the second limitation.

RESULTS

After validation of the technique on simulations, we demonstrated that this improvement of the modulus technique is significantly advantageous for both in vitro and in vivo applications.

CONCLUSION

By improving the SNR of the spectra and eliminating eventual contaminations, the new strategies proposed here confer an additional competitive advantage to the modulus post-processing technique.

Longitudinal relaxation enhancement in 1H NMR spectroscopy of tissue metabolites via spectrally selective excitation

Nuclear magnetic resonance spectroscopy is governed by longitudinal (T1) relaxation. For protein and nucleic acid experiments in solutions, it is well established that apparent T1 values can be enhanced by selective excitation of targeted resonances. The present study explores such longitudinal relaxation enhancement (LRE) effects for molecules residing in biological tissues. The longitudinal relaxation recovery of tissue resonances positioned both down- and upfield of the water peak were measured by spectrally selective excitation/refocusing pulses, and compared with conventional water-suppressed, broadband-excited counterparts at 9.4 T. Marked LRE effects with up to threefold reductions in apparent T1 values were observed as expected for resonances in the 6-9 ppm region; remarkably, statistically significant LRE effects were also found for several non-exchanging metabolite resonances in the 1-4 ppm region, encompassing 30-50 % decreases in apparent T1 values. These LRE effects suggest a novel means of increasing the sensitivity of tissue-oriented experiments, and open new vistas to investigate the nature of interactions among metabolites, water and macromolecules at a molecular level.

APT-weighted and NOE-weighted image contrasts in glioma with different RF saturation powers based on magnetization transfer ratio asymmetry analyses

PURPOSE

To investigate the saturation-power dependence of amide proton transfer (APT)-weighted and nuclear Overhauser enhancement-weighted image contrasts in a rat glioma model at 4.7 T.

METHODS

The 9L tumor-bearing rats (n = 8) and fresh chicken eggs (n = 4) were scanned on a 4.7-T animal magnetic resonance imaging scanner. Z-spectra over an offset range of ±6 ppm were acquired with different saturation powers, followed by the magnetization transfer-ratio asymmetry analyses around the water resonance.

RESULTS

The nuclear Overhauser enhancement signal upfield from the water resonance (-2.5 to -5 ppm) was clearly visible at lower saturation powers (e.g., 0.6 µT) and was larger in the contralateral normal brain tissue than in the tumor. Conversely, the APT effect downfield from the water resonance was maximized at relatively higher saturation powers (e.g., 2.1 µT) and was larger in the tumor than in the contralateral normal brain tissue. The nuclear Overhauser enhancement decreased the APT-weighted image signal, based on the magnetization transfer-ratio asymmetry analysis, but increased the APT-weighted image contrast between the tumor and contralateral normal brain tissue.

CONCLUSION

The APT and nuclear Overhauser enhancement image signals in tumor are maximized at different saturation powers. The saturation power of roughly 2 μT is ideal for APT-weighted imaging at clinical B0 field strengths.

Magnetization exchange observed in human skeletal muscle by non-water-suppressed proton magnetic resonance spectroscopy

Many metabolites in the proton magnetic resonance spectrum undergo magnetization exchange with water, such as those in the downfield region (6.0-8.5 ppm) and the upfield peaks of creatine, which can be measured to reveal additional information about the molecular environment. In addition, these resonances are attenuated by conventional water suppression techniques complicating detection and quantification. To characterize these metabolites in human skeletal muscle in vivo at 3 T, metabolite cycled non-water-suppressed spectroscopy was used to conduct a water inversion transfer experiment in both the soleus and tibialis anterior muscles. Resulting median exchange-independent T1 times for the creatine methylene resonances were 1.26 and 1.15 s, and for the methyl resonances were 1.57 and 1.74 s, for soleus and tibialis anterior muscles, respectively. Magnetization transfer rates from water to the creatine methylene resonances were 0.56 and 0.28 s(-1), and for the methyl resonances were 0.39 and 0.30 s(-1), with the soleus exhibiting faster transfer rates for both resonances, allowing speculation about possible influences of either muscle fibre orientation or muscle composition on the magnetization transfer process. These water magnetization transfer rates observed without water suppression are in good agreement with earlier reports that used either postexcitation water suppression in rats, or short CHESS sequences in human brain and skeletal muscle.

J-refocused coherence transfer spectroscopic imaging at 7 T in human brain

Short echo spectroscopy is commonly used to minimize signal modulation due to J-evolution of the cerebral amino acids. However, short echo acquisitions suffer from high sensitivity to macromolecules which make accurate baseline determination difficult. In this report, we describe implementation at 7 T of a double echo J-refocused coherence transfer sequence at echo time (TE) of 34 msec to minimize J-modulation of amino acids while also decreasing interfering macromolecule signals. Simulation of the pulse sequence at 7 T shows excellent resolution of glutamate, glutamine, and N-acetyl aspartate. B(1) sufficiency at 7 T for the double echo acquisition is achieved using a transceiver array with radiofrequency (RF) shimming. Using an alternate RF distribution to minimize receiver phase cancellation in the transceiver, accurate phase determination for the coherence transfer is achieved with rapid single scan calibration. This method is demonstrated in spectroscopic imaging mode with n = 5 healthy volunteers resulting in metabolite values consistent with literature and in a patient with epilepsy.

MRS signal quantitation: a review of time- and frequency-domain methods

In this paper an overview of time-domain and frequency-domain quantitation methods is given. Advantages and drawbacks of these two families of quantitation methods are discussed. An overview of preprocessing methods, such as lineshape correction methods or unwanted component removal methods, is also given. The choice of the quantitation method depends on the data under investigation and the pursued objectives.

An assessment of the effects of sample ischaemia and spinning time on the metabolic profile of brain tumour biopsy specimens as determined by high-resolution magic angle spinning (1)H NMR

High-resolution magic angle spinning (HRMAS) (1)H NMR of biopsy tissue provides a biochemical profile that has potential diagnostic and prognostic value, and can aid interpretation of the lower-resolution (1)H-NMR spectra obtained in vivo. However, the biochemical profile obtained may be affected by experimental factors such as a period of ischaemia before snap-freezing of the biopsy tissue for subsequent analysis and the mechanical stress of the spinning procedure of HRMAS itself. We have used normal rat brain cortex as a 'gold standard', either funnel-frozen or deliberately allowed to become ischaemic for set periods of time before snap-freezing, to quantitatively investigate these two effects. In addition, we have compared biochemical changes that occur in normal rat brain during HRMAS (spun continuously at 5 kHz for 4 h at 4 degrees C as could be required for a two-dimensional acquisition) with those that occur in biopsy samples from low-grade and high-grade adult human astrocytomas, during the same HRMAS procedure. Significant changes due to delayed initial sample freezing were noted in metabolites associated with glycolysis (alanine, glucose and lactate), as expected. However, for the funnel-frozen rat tissue at 4 degrees C, there were even more significant changes, which appear to be the result of extended spinning at 5 kHz. In particular, the 18% total creatine increase observed is unlikely to be de novo synthesis of creatine. More likely, the asymptotic exponential increase in creatine suggests an exponential release of an NMR-invisible bound creatine store as a result of tissue damage from mechanical stress of sample spinning. Overall, it appears that tissue ischaemia during biopsy excision and delays in snap-freezing may have less significant effects on metabolite profile than the prolonged spinning times required for two-dimensional HRMAS, and this must be accounted for when results are being interpreted.

Quantitative description of the asymmetry in magnetization transfer effects around the water resonance in the human brain

Magnetization transfer (MT) imaging provides a unique method of tissue characterization by capitalizing on the interaction between solid-like tissue components and bulk water. We used a continuous-wave (CW) MT pulse sequence with low irradiation power to study healthy human brains in vivo at 3 T and quantified the asymmetry of the MT effects with respect to the water proton frequency. This asymmetry was found to be a difference of approximately a few percent from the water signal intensity, depending on both the RF irradiation power and the frequency offset. The experimental results could be quantitatively described by a modified two-pool MT model extended with a shift contribution for the semisolid pool with respect to water. For white matter, this shift was fitted to be 2.34 +/- 0.17 ppm (N = 5) upfield from the water signal.

Inverse polarization transfer for detecting in vivo 13C magnetization transfer effect of specific enzyme reactions in 1H spectra

The wide chemical shift dispersion and long T(1) of (13)C have allowed determination of in vivo magnetization transfer effects caused by aspartate aminotransferase and lactate dehydrogenase reactions using (13)C magnetic resonance spectroscopy. In this report, we demonstrate that these effects can be observed in the proton spectra by transferring the equilibrium magnetization of (13)C via the one-bond scalar coupling between (13)C and (1)H using an inverse insensitive nuclei enhanced by polarization transfer-based heteronuclear polarization transfer method. This inverse method allows a combination of the advantages of the long (13)C T(1) for maximum magnetization transfer and the high sensitivity of proton detection. The feasibility of this in vivo inverse polarization transfer approach was evaluated for detecting the (13)C magnetization transfer effect of aspartate aminotransferase and lactate dehydrogenase reactions from a 72.5-microl voxel in the rat brain at 11.7 T.

In vivo 13C saturation transfer effect of the lactate dehydrogenase reaction

Lactate dehydrogenase (LDH, EC 1.1.1.27) catalyzes an exchange reaction between pyruvate and lactate. It is demonstrated here that this reaction is sufficiently fast to cause a significant magnetization (saturation) transfer effect when the 13C resonance of pyruvate is saturated by a continuous-wave (CW) RF pulse. Infusion of [2-(13)C]glucose was used to allow labeling of pyruvate C2 at 207.9 ppm to determine the pseudo first-order rate constant of the unidirectional lactate-->pyruvate flux in vivo. During systemic administration of GABAA receptor antagonist bicuculline, this pseudo first-order rate constant was determined to be 0.08+/-0.01 s-1 (mean+/-SD, N=4) in halothane-anesthetized adult rat brains. In 9L and C6 rat glioma models, the 13C saturation transfer effect of the LDH reaction was also detected in vivo. Our results demonstrate that the 13C magnetization transfer effect of the LDH reaction may be useful as a novel marker for utilizing noninvasive in vivo MRS to study many physiological and pathological conditions, such as cancer.

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

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

Dynamic MRS and MRI of skeletal muscle function and biomechanics

MR is a powerful technique for studying the biomechanical and functional properties of skeletal muscle in vivo in health and disease. This review focuses on 31P, 1H and 13C MR spectroscopy for assessment of the dynamics of muscle metabolism and on dynamic 1H MRI methods for non-invasive measurement of the biomechanical and functional properties of skeletal muscle. The information thus obtained ranges from the microscopic level of the metabolism of the myocyte to the macroscopic level of the contractile function of muscle complexes. The MR technology presented plays a vital role in achieving a better understanding of many basic aspects of muscle function, including the regulation of mitochondrial activity and the intricate interplay between muscle fiber organization and contractile function. In addition, these tools are increasingly being employed to establish novel diagnostic procedures as well as to monitor the effects of therapeutic and lifestyle interventions for muscle disorders that have an increasing impact in modern society.

Magnetisation transfer ratio of choline is reduced following epileptic seizures

The purpose of this study was to characterise the concentration and magnetisation transfer ratio (MTR) of brain metabolites following epileptic seizures. Magnetic resonance spectroscopy was performed in 10 patients with temporal or extra-temporal lobe epilepsy as soon as possible after a seizure, with a second interictal scan between 1 and 3 days after the postictal scan and 10 healthy controls were scanned twice. Voxels (26 +/- 2 mL) were placed in the frontal lobe in all patients and controls, on the side of seizure focus in the patient group. Spectra were obtained using a modified PRESS sequence (TE 30 ms, TR 3 s, with three hard pulses offset from the water frequency by 2,500 Hz for MT presaturation). Mean metabolite concentrations and median metabolite MTRs of N-acetylaspartate (NAA), creatine, choline (Cho), myo-inositol (Ins) and glutamate plus glutamine were compared between the first and second scans in each group. A significant decrease in the MTR of Cho was seen postictally in the patient group, but the metabolite concentrations showed no significant difference between interictal and postictal scans and in the control group there was no difference between the two scans. Inter-group comparison showed significantly reduced concentrations of NAA and Ins in the patients. Reduced MTR of Cho indicates a shift from a bound to a more mobile fraction. These changes might indicate membrane perturbation in areas of seizure spread.

Magnetization transfer effect on human brain metabolites and macromolecules

A pulse sequence was implemented to observe the magnetization transfer (MT) effect on metabolites, water, and macromolecules in human frontal lobes in vivo at 1.5 Tesla. Signals were compared following the application of three hard pulses of 0.745 muT amplitude, applied at frequency offsets of either 2500 Hz or 30 kHz, preceding a conventional point-resolved spectroscopy (PRESS)-localized acquisition with an echo time (TE) of 30 ms and repetition time (TR) of 3 s. This gave an MT effect on water in vivo of 46%, while direct saturation by the MT pulses at 2.5 kHz offset was confirmed to be under 4% for all metabolites. We observed significant MT saturation in vivo for N-acetylated compounds, choline (Cho), myo-inositol, and lactate (Lac); a trend of an effect on glutamate + glutamine (Glx); and the typically observed effect on creatine (Cr). No significant MT effect was seen on the macromolecule signal, which was observed using metabolite nulling.

New method for the simultaneous detection of metabolites and water in localized in vivo 1H nuclear magnetic resonance spectroscopy

A new two-scan method for localized 1H in vivo NMR spectroscopy (MRS) without water suppression (WS) is described. In one of the scans, two chemical shift selective 180 degrees pulses are applied prior to a standard localization sequence to invert all metabolite signals upfield and downfield from water, which remains unaffected. The difference spectrum records the metabolites whereas water and accompanying gradient induced artifacts are widely suppressed. The method was implemented on a 4.7-T system using point resolved spectroscopy with a short echo time of 18 ms. Phantom measurements proved the feasibility of absolute quantification using water as an internal reference. Measurements on healthy rat brain yielded comparable spectrum quality as measurements with water presaturation. The method does not require additional adjustments or sophisticated data postprocessing and scales favorably with increasing B(0) field. Therefore, the method should be useful for 1H MRS without WS. Although the two-step method doubles the minimum total measurement time, it may also be of interest for spectroscopic imaging (SI) without WS, in particular if fast SI techniques are applied.

An emerging concept of biomolecular dynamics and function: applications of NMR & MRI

A new concept of protein dynamics has emerged quite recently, and a crucial link between protein dynamics and function has been largely established using recent NMR techniques in the solution state. Protein structure is governed by the thermodynamic principle and may not necessarily be unique in the solution state. Enzyme catalysis, protein folding or allosteric transition occurs on the microsecond to millisecond time scale, implying that in order to prepare the specific nuclear coordinate for the electronic state transition, a protein must rearrange its nuclear coordinates substantially, and this process may generally take a long period of time almost comparable to that of protein folding. Protein coordinates optimized for the electronic reaction may form an energy state--which may be called an "excited state"--that is thermodynamically distinct from the native state. In contrast, the native state is called a "ground state." Relevant NMR techniques developed recently may also have useful application to MRI, since the critical time scale of various reactions in a living system is also around micro- to milliseconds.

Quantitative T1rho NMR spectroscopy of rat cerebral metabolites in vivo: effects of global ischemia

The NMR relaxation times (T(1rho), T(2), and T(1)) of water, N-acetylaspartate (NAA), creatine (Cr), choline-containing compounds (Cho), and lactate (Lac) were quantified in rat brain at 4.7 T. In control animals, the cerebral T(1rho) figures, as determined with a spin-lock field of 1.0 G, were 575 +/- 30 ms, 380 +/- 19 ms, 705 +/- 53 ms, and 90 +/- 1 ms for NAA, Cr, Cho, and water, respectively. The T(1rho) figures were 62-103% longer than their respective T(2) values determined by a multiecho method. In global (ischemic) ischemia, T(1rho) of NAA declined by 34%, that of Cr and Cho did not change, and that of water increased by 10%. The T(1rho) of lactate in ischemic brain was 367 +/- 44 ms. Similar patterns of changes were observed in the multiecho T(2) of these cerebral metabolites. The T(1) of water and NAA changed in a fashion similar to that of T(1rho) and T(2). These results show differential responses in metabolite and water T(1rho) relaxation times following ischemia, and indicate that metabolite T(1rho) and T(2) relaxation times behave similarly in the ischemic brain. The contributions of dipolar and nondipolar effects on T(1rho) relaxation in vivo are discussed in this work.

Magnetization transfer effect on the creatine methyl resonance studied by CW off-resonance irradiation in human skeletal muscle on a clinical MR system

Magnetization transfer (MT) between the mobile (MR-visible) spin pool and immobile (MR-invisible) spin pool of creatine (Cr) was studied on a clinical 1.5 T MR scanner in human skeletal muscle using continuous wave (CW) pre-irradiation as the saturation method for the immobile pool. For this purpose, only slight modifications to the MR system were made. A specially designed electronic circuit was used to couple a CW amplifier to the RF channel of the scanner. The CW pulse power (gammaB(2)/2pi) and pulse length were determined to be approximately 550 Hz and 3 s, respectively, for optimal signal attenuation of the Cr methyl signal. The bound Cr fraction in human gastrocnemius muscle was determined to be 0.4-1.3% using a two-pool exchange model function to describe the MT effect.

Quantitative 1H NMR spectroscopy of blood plasma metabolites

The absolute quantification of blood plasma metabolites by proton NMR spectroscopy is complicated by the presence of a baseline and broad resonances originating from serum macromolecules and lipoproteins. A method for spectral simplification of proton NMR spectra of blood plasma is presented. Serum macromolecules and metabolites are completely separated by utilizing the large difference in translational diffusion coefficients in combination with diffusion-sensitized proton NMR spectroscopy. The concentration of blood plasma metabolites can be quantified by using formate as an internal concentration reference. The results are compared with those obtained with ultrafiltration, a traditional method for separating macromolecules and metabolites, and demonstrate an excellent correlation between the two methods. The general nature of diffusion-sensitized NMR spectroscopy allows application on a wide range of biological fluids.

Indirect imaging of ethanol via magnetization transfer at high and low magnetic fields

Ethanol (EtOH) is believed to exert its neurochemical effects through interactions with brain cellular components, which causes a fraction of brain EtOH to have a lower molecular mobility. This facilitates magnetization transfer to other molecules similarly associated with macromolecules, such as water. It was hypothesized that this effect can be used in vivo to image EtOH indirectly via the much stronger brain tissue water resonance. EtOH-containing bovine serum albumin samples were used to demonstrate magnetic coupling between EtOH and water at 7 T and 1.5 T. Spectroscopy and imaging experiments demonstrated that EtOH signal saturation yielded greater water signal reduction than inversion and that this reduction scaled with EtOH concentration in the BSA samples. In human brain at physiologically relevant brain EtOH concentrations, water signal reductions were measurable when saturating the EtOH resonance. Strengths and limitations of indirectly imaging brain EtOH are discussed.

Mechanism of magnetization transfer during on-resonance water saturation. A new approach to detect mobile proteins, peptides, and lipids

The mechanism of magnetization transfer (MT) between water and components of the proton spectrum was studied ex vivo in a perfused cell system and in vivo in the rat brain (n = 5). Water was selectively labeled and spectral buildup consequential to transfer of longitudinal magnetization was followed as a function of time. At short mixing time (T(m)), nitrogen-bound solvent-exchangeable protons were observed, predominantly assigned to amide groups of proteins and peptides. At longer T(m), intramolecular nuclear Overhauser enhancement (NOE) was observed in the aliphatic proton region, leading to a mobile-macromolecule-weighted spectrum that resembles typical protein spectra described in the literature. This effect on the proton spectrum is distinct from that of classical off-resonance MT, which has been shown to be due to the immobile solid-like proton pool. When studying a solution of major brain metabolites under physiological concentrations and conditions (pH), no transfer effects were observed, in line with expectations based on reduced NOE effects in rapidly tumbling molecules and the fast proton exchange rates of amino, amine, SH, and OH groups. The spectral intensities of the amide protons may serve as indicators for pH and cellular levels of mobile proteins and peptides, while the aliphatic components are representative of several types of mobile macromolecules, including proteins, peptides, and lipids.

Proton magnetization transfer effect in rat liver lactate

Off-resonance lactate magnetization transfer (MT) experiments were performed on the in situ rat liver under perfused and ischemic conditions. A significant MT effect for lactate methyl protons was observed. The effect was larger for the ischemic condition than for the perfused condition, and was largest in the blood-filled ischemic livers. The size of the motionally restricted lactate pool, determined using a two-pool model fit, was estimated to be about 1% in perfused livers and about 1.8-2.5% after more than 1 hr of onset of ischemia, suggesting that lactate in liver is almost fully NMR-visible. The MT data for both the perfused and the ischemic condition appeared to be better approximated when assuming a superLorentzian lineshape for the immobile pool rather than a Gaussian lineshape. Finally, the experiments demonstrated a coupling between the lactate methyl and water protons, which may be mediated by macromolecules.

Localized two-dimensional shift correlated MR spectroscopy of human brain

A two-dimensional (2D) chemical shift correlated MR spectroscopic (COSY) sequence integrated into a new volume localization technique (90 degrees -180 degrees -90 degrees ) is proposed for whole-body MR spectroscopy (MRS). Using the product operator formalism, a theoretical calculation of the volume localization as well as the coherence transfer efficiencies in 2D MRS is presented. Phantom model solutions were used to test and optimize the efficiency of the proposed sequence. A combination of different MRI transmit/receive RF coils was used: a head MRI coil and a 3" surface coil receive combined with a body coil transmit. The J cross-peaks due to N-acetyl aspartate (NAA), glutamate/glutamine (Glx), myo-inositol (mI), creatine (Cr), choline (Ch), aspartate (Asp), gamma-aminobutyrate (GABA), taurine (Tau), glutathione (GSH), threonine (Thr), and macromolecules (MM) were identified. The cross-peak intensities excited by the proposed 2D sequence were asymmetric with respect to the diagonal peaks. Localized COSY (L-COSY) spectra of cerebral prefrontal and occipital gray/white matter regions in 15 healthy controls are presented. Magn Reson Med 46:58-67, 2001.

Magnetization transfer of fluoxetine in the human brain using fluorine magnetic resonance spectroscopy

Fluorine magnetic resonance spectroscopy ((19)F MRS) measurements of fluoxetine and metabolite concentration in the human brain underestimate true drug levels because of a bound, MRS-"invisible" pool of drug molecules. Magnetization transfer (MT) spectroscopy may be a useful technique for characterizing this bound pool of fluoxetine in the brain. Six subjects on consistent daily doses of fluoxetine underwent (19)F MT spectroscopy on a 1.5-T scanner using a train of three preparation pulses at -3000 Hz off resonance with 0.5 W of peak power deposition in tissue. One subject was scanned at multiple time points after initiation of drug therapy. Magnetization transfer signal contrast was quantified using VARPRO-based time domain fitting software. Magnetization transfer signal contrast was quantifiable with mean MT signal depression of 12.5% (SD = 5.0, n = 6). An inverse relationship between brain concentration and the MT signal contrast of fluoxetine was found (r = -.82, Spearman coefficient =.007). This study is the first in vivo application of (19)F MT spectroscopy and the first to demonstrate a quantifiable MT effect for a psychotropic medication in the human brain. Findings suggest that fluoxetine is substantially bound in the brain and that individual differences, inversely related to brain concentration, can be detected in the magnitude of MT contrast.

Dipolar coupling and ordering effects observed in magnetic resonance spectra of skeletal muscle

Skeletal muscle is a biological structure with a high degree of organization at different spatial levels. This order influences magnetic resonance (MR) in vivo-in particular 1H-spectra-by a series of effects that have very distinct physical sources and biomedical applications: (a) bulk fat (extramyocellular lipids, EMCL) along fasciae forms macroscopic plates, changing the susceptibility within these structures compared to the spherical droplets that contain intra-myocellular lipids (IMCL); this effect leads to a separation of the signals from EMCL and IMCL; (b) dipolar coupling effects due to anisotropic motional averaging have been shown for 1H-resonances of creatine, taurine, and lactate; (c) aromatic protons of carnosine show orientation-dependent effects that can be explained by dipolar coupling, chemical shift anisotropy or by relaxation anisotropy; (d) limited rotational freedom and/or compartmentation may explain differences of 1H-MR-visibility of the creatine/phosphocreatine resonances; (e) lactate 1H-MR resonances are reported to reveal information on tissue compartmentation; (f) transverse relaxation of water and metabolites show multiple components, indicative of intra-, extracellular and/or macromolecular-bound pools, and in addition dipolar or J-coupling lead to a modulation of the signal decay, hindering straightforward interpretation; (g) diffusion weighted 31P-MRS has shown restricted diffusion of phosphocreatine; (h) magnetization transfer (MT) indicates that there is a motionally restricted proton pool in spin-exchange with free creatine; reduced availability or restricted motion of creatine is particularly important for an estimation of ADP from 31P-MR spectra, and in addition MT effects may alter the signal intensity of creatine 1H-resonances following water-suppression pulses; (i) transcytolemmal water-exchange can be studied in 1H-MRS by contrast-agents applied to the extracellular space; (k) transport of glucose across the cell membrane has been studied in diabetes patients using a combination of 13C- and 31P-MRS; and l residual quadrupolar interaction in 23Na MR spectra from human skeletal muscle suggest that sodium ions are bound to ordered muscular structures.

Magnetization transfer MRS

This review deals with magnetization transfer (MT) effects observed in in vivo NMR spectroscopy. The basic experimental methods of MT experiments, the underlying kinetic mechanisms as well as the evaluation of measured data by fits to two- or three-pool models are described. Experimental results of both (31)P and (1)H in vivo MRS are reviewed showing the potential of MT experiments to characterize kinetic equilibrium reactions. This includes reactions where all involved components are MR visible, as well as situations where one indirectly measures pools of bound spins which cannot directly be observed in vivo. In particular, MT effects are described which have been observed in in vivo (1)H NMR spectra measured on the animal or human brain or on skeletal muscle. Possible mechanisms for the strong MT effects observed for the signals of creatine/phosphocreatine, lactate, alcohol and other metabolites are discussed. It is also emphasized that MT effects caused by water suppression techniques may lead to systematic errors in the quantification of in vivo (1)H NMR spectra.

Two-voxel localization sequence for in vivo two-dimensional homonuclear correlation spectroscopy

The combination of localized 2D 1H MR correlation spectroscopy and Hadamard encoding allows the simultaneous acquisition of multiple volumes of interest without an increase in the experimental duration, compared to single-voxel acquisition. In the present study, 2D correlation spectra were acquired simultaneously within 20 to 40 min in two voxels located in each hemisphere of the rat brain. An intervoxel distance of 20% of the voxel size was sufficient to limit spatial contamination. The following cerebral metabolites gave detectable crosspeaks: N-acetylaspartate, the glutamate/glutamine pool, aspartate, phosphoethanolamine, glucose, glutathione, taurine, myo-inositols, lactate, threonine, gamma-aminobutyric acid, and alanine. Most of the metabolites were measured without contamination of other resonances.

On the importance of exchangeable NH protons in creatine for the magnetic coupling of creatine methyl protons in skeletal muscle

The methyl protons of creatine in skeletal muscle exhibit a strong off-resonance magnetization transfer effect. The mechanism of this process is unknown. We previously hypothesized that the exchangeable amide/amino protons of creatine might be involved. To test this the characteristics of the creatine magnetization transfer effect were investigated in excised rat hindleg skeletal muscle that was equilibrated in either H2O or D2O solutions containing creatine. The efficiency of off-resonance magnetization transfer to the protons of mobile creatine in excised muscle was similar to that previously reported in intact muscle in vivo. Equilibrating the isolated muscle in D2O solution had no effect on the magnetic coupling to the immobile protons. It is concluded that exchangeable protons play a negligible role in the magnetic coupling of creatine methyl protons in muscle.

Magnetic coupling between water and creatine protons in human brain and skeletal muscle, as measured using inversion transfer (1)H-MRS

Using the inversion transfer technique, the possible magnetic coupling between water protons and the protons of low-molecular weight metabolites was investigated in human brain and skeletal muscle at 1.5 T. The localized (1)H-MR spectra were recorded at different times after selective inversion of the water resonance. Water inversion led to a significant transient reduction in the signal intensity of the methyl protons of creatine/phosphocreatine, in both tissues. This is indicative of magnetic coupling between the protons of water and those of creatine/phosphocreatine. Neither the choline and N-acetylaspartate protons in brain nor the protons of the trimethylammonium pool in skeletal muscle showed a significant magnetic coupling to mobile water.

1H and (31)P magnetization transfer studies of hindleg muscle in wild-type and creatine kinase-deficient mice

The authors addressed the hypothesis that interactions with creatine kinase (CK) play a role in the off-resonance magnetization transfer (MT) effect of creatine in skeletal muscle. Toward that aim, (1)H MT studies were done on hindleg muscle in wild-type mice and in transgenic mice, lacking cytoplasmic CK and/or mitochondrial CK. The (1)H MT effect was essentially identical in wild-type muscle and the two single CK knock-out muscles, while moderately decreased in tissue lacking both CK isoforms. (31)P-NMR showed no off-resonance (31)P MT effect in skeletal muscle for PCr in any of the mice, while the enzymatic CK reaction flux was circa 0.2-0.3 sec(-1) in the wild-type muscle and in muscle deficient in mitochondrial CK. The CK enzyme flux was negligible in the other two CK knock-outs. These data suggest that CK plays a minor role in the (1)H MT effect of creatine. Irrespective of the underlying mechanism the creatine MT phenomenon probably has no significant consequences for the thermodynamic availability of total creatine to the CK reaction.

Proton NMR chemical shifts and coupling constants for brain metabolites

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

Proton magnetization transfer of metabolites in human brain

Off-resonance or pulsed on-resonance saturation pulses were used together with localized proton magnetic resonance spectroscopy in three brain regions of 20 healthy individuals. Statistically significant signal attenuations were observed for creatine-containing metabolites in posterior-parietal brain (12%), basal ganglia (18%), and cerebellum (15%). N-acetyl- and choline-containing metabolites were not significantly attenuated upon application of saturation pulses in either brain region. The findings are interpreted to reflect possible magnetization transfer between pools of creatine-containing metabolites with different molecular mobility. Magn Reson Med 42:417-420, 1999.

Molecular aspects of magnetic resonance imaging and spectroscopy

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

Effect of exercise on the creatine resonances in 1H MR spectra of human skeletal muscle

1H MR spectra of human muscles were recorded before, during, and after fatiguing exercise. In contrast to expectations, it was found that the spectral contributions of creatine/phosphocreatine (Cr/PCr) were subject to change as a function of exercise. In particular, the dipolar-coupled methylene protons of Cr/PCr were found to be reduced in intensity in proportion to the co-registered PCr levels. Recovery after exercise and behavior under ischemic conditions provide further evidence to suggest that the contributions of the CH2 protons of Cr/PCr to 1H MR spectra of human muscle in vivo reflect PCr rather than Cr levels. Variation of experimental parameters showed that this effect is not due to a trivial change in relaxation times. At present it can only be speculated about why the Cr resonances have reduced NMR visibility. If temporary binding to macromolecules should be involved, the free Cr concentration-important for equilibrium calculations of the creatine kinase reaction-might be different from what was previously assumed.