Double quantum filtered (1)H NMR spectroscopy enables quantification of lactate in muscle

Interleaved and simultaneous multi-nuclear magnetic resonance in vivo. Review of principles, applications and potential

Magnetic resonance signals from different nuclei can be excited or received at the same time,rendering simultaneous or rapidly interleaved multi-nuclear acquisitions feasible. The advan-tages are a reduction of total scan time compared to sequential multi-nuclear acquisitions or that additional information from heteronuclear data is obtained at thesame time and anatomical position. Information content can be qualitatively increased by delivering a more comprehensive MR-based picture of a transient state (such as an exercise bout). Also, combiningnon-proton MR acquisitions with ¹ Hinformation (e.g., dynamic shim updates and motion correction) can be used to improve data quality during long scans and benefits image coregistration. This work reviews the literature on interleaved and simultaneous multi-nuclear MRI and MRS in vivo. Prominent use cases for this methodology in clinical and research applications are brain and muscle, but studies have also been carried out in other targets, including the lung, knee, breast and heart. Simultaneous multi-nuclear measurements in the liver and kidney have also been performed, but exclusively in rodents. In this review, a consistent nomenclature is proposed, to help clarify the terminology used for this principle throughout the literature on in-vivo MR. An overview covers the basic principles, the technical requirements on the MR scanner and the implementations realised either by MR system vendors or research groups, from the early days until today. Considerations regarding the multi-tuned RF coils required and heteronuclear polarisation interactions are briefly discussed, and fields for future in-vivo applications for interleaved multi-nuclear MR pulse sequences are identified.

3D localized lactate detection in muscle tissue using double-quantum filtered 1 H MRS with adiabatic refocusing pulses at 7 T

PURPOSE

Lactate is a key metabolite in skeletal muscle and whole-body physiology. Its MR visibility in muscle is affected by overlapping lipid signals and fiber orientation. Double-quantum filtered (DQF) ¹ H MRS selectively detects lactate at 1.3 ppm, but at ultra-high field the efficiency of slice-selective 3D-localization with conventional RF pulses is limited by bandwidth. This novel 3D-localized ¹ H DQF MRS sequence uses adiabatic refocusing pulses to unambiguously detect lactate in skeletal muscle at 7 T.

METHODS

Lactate double-quantum coherences were 3D-localized using slice-selective Shinnar-Le Roux optimized excitation and adiabatic refocusing pulses (similar to semi-LASER). DQF MR spectra were acquired at 7 T from lactate phantoms, meat specimens with injected lactate (exploring multiple TEs and fiber orientations), and human gastrocnemius in vivo during and after exercise (without cuff ischemia).

RESULTS

Lactate was readily detected, achieving the full potential of 50% signal with a DQF, in solution. The effects of fiber orientation and TE on the lactate doublet (peak splitting, amplitude, and phase) were in good agreement with theory and literature. Exercise-induced lactate accumulation was detected with 30 s time resolution.

CONCLUSION

This novel 3D-localized ¹ H DQF MRS sequence can dynamically detect glycolytically generated lactate in muscle during exercise and recovery at 7 T.

Proton magnetic resonance spectroscopy in skeletal muscle: Experts' consensus recommendations

1 H-MR spectroscopy of skeletal muscle provides insight into metabolism that is not available noninvasively by other methods. The recommendations given in this article are intended to guide those who have basic experience in general MRS to the special application of 1 H-MRS in skeletal muscle. The highly organized structure of skeletal muscle leads to effects that change spectral features far beyond simple peak heights, depending on the type and orientation of the muscle. Specific recommendations are given for the acquisition of three particular metabolites (intramyocellular lipids, carnosine and acetylcarnitine) and for preconditioning of experiments and instructions to study volunteers.

Noninvasive monitoring of lactate dynamics in human forearm muscle after exhaustive exercise by (1)H-magnetic resonance spectroscopy at 7 tesla

Despite its importance in energy metabolism, lactate in human skeletal muscle has been difficult to detect by noninvasive (1)H-magnetic resonance spectroscopy mainly due to interference from large water and lipid signals. Long echo-time acquisitions at 7 T effectively attenuates the water and lipid signals in forearm muscle allowing direct observation of both lactate resonances, the methine at 4.09 ppm and the methyl at 1.31 ppm. Using this approach, we were able to monitor lactate dynamics at a temporal resolution of 32 s. While lactate was not detectable at rest, immediately after an acute period of exercise to fatigue the forearm muscle, lactate rose to a level comparable to that of creatine (∼30 mmol/kg wet weight). In a typical (1)H-magnetic resonance spectrum collected using a echo-time of 140 ms, the lactate methine and methyl resonances both appear as doublets with an unusually large splitting of ∼20 Hz due to residual dipolar coupling. During muscle recovery following exercise, the lactate signals decay rapidly with a time constant of t½ = 2.0 ± 0.6 min (n = 12 subjects). This fast and simple lactate detection method may prove valuable for monitoring lactate metabolism in cancer and in sports medicine applications.

Direct noninvasive quantification of lactate and high energy phosphates simultaneously in exercising human skeletal muscle by localized magnetic resonance spectroscopy

A novel method based on interleaved localized 31P- and 1H MRS is presented, by which lactate accumulation and the accompanying changes in high energy phosphates in human skeletal muscle can be monitored simultaneously during exercise and recovery. Lactate is quantified using a localized double quantum filter suppressing the abundant lipid signals while taking into account orientation dependent signal modulations. Lactate concentration after ischemic exercise directly quantified by DQF 1H spectroscopy was 24 +/- 3 mmol/L cell water, while 22 +/- 3 mmol/L was expected on the basis of 31P MRS acquired simultaneously. Lactate concentration in a sample of porcine meat was estimated to be 40 +/- 7 mmol/L by means of DQF quantitation, versus 39 +/- 5 mmol/L by biochemical methods. Excellent agreement is shown between lactate concentrations measured noninvasively by 1H MRS, measured biochemically ex vivo, and inferred indirectly in vivo from changes in pH, P(i), and PCr as obtained from 31P MRS data.

Musculoskeletal spectroscopy

Magnetic resonance spectroscopy (MRS) of skeletal muscle has been successfully applied by physiologists over several decades, particularly for studies of high-energy phosphates (by (31)P-MRS) and glycogen (by (13)C-MRS). Unfortunately, the observation of these heteronuclei requires equipment that is typically not available on clinical MR scanners, such as broadband capability and a second channel for decoupling and nuclear Overhauser enhancement (NOE). On the other hand, (1)H-MR spectra of skeletal muscle can be acquired on many routine MR systems and also provide a wealth of physiological information. In particular, studies of intramyocellular lipids (IMCL) attract physiologists and endocrinologists because IMCL levels are related to insulin resistance and thus can lead to a better understanding of major health problems in industrial countries. The combination of (1)H-, (13)C-, and (31)P-MRS gives access to the major long- and short-term energy sources of skeletal muscle. This review summarizes the technical aspects and unique MR-methodological features of the different nuclei. It reviews clinical studies that employed MRS of one or more nuclei, or combinations of MRS with other MR modalities. It also illustrates that MR spectra contain additional physiological information that is not yet used in routine clinical applications.

Non-invasive quantification of lactate by proton MR spectroscopy and its clinical applications

Lactate is an important metabolite in clinical cases indicating the status of metabolic impairment. We applied a clinically relevant simple method for lactate quantification using magnetic resonance spectroscopy (MRS). We used two long in-phase echo time (TE=272,544 ms) to calculate T2 relaxation time and the absolute concentration of lactate. This method was optimized using phantom study and applied to clinical cases. This technique does not require complicated processing and could be applied in daily clinical practice. Moreover, this technique enables lactate quantification in cases (e.g. tumor) where lipid peak is overlapped with the lactate peak at short echo times.

Effects of intramolecular dipolar coupling on the isotropic-nematic phase transition of a hard spherocylinder fluid

The thermodynamics of a simple model, containing the minimum set of features required to provide liquid crystal-like phase behavior and the dipolar coupling observable in the NMR spectrum of orientationally ordered fluids, are presented within the framework of Onsager theory. The model comprises a fluid of hard spherocylinders with a pair of embedded freely rotating magnetic dipoles. The behavior of the isotropic-nematic phase transition is explored as a function magnetic field strength and of the relative orientation between the nematic director and the external magnetic field. When the field and director are aligned the phase diagram is similar to those predicted for a hard rod fluid in flow fields, electric fields, and magnetic fields, with the field promoting orientational order in the fluid and the isotropic-nematic phase transition being replaced by a paranematic-nematic phase transition. In contrast, when the field and director are perpendicular, the field destabilizes the nematic phase and the phase transition is shifted to higher densities. The variation of the mean magnetic moment and the dipolar coupling are examined as a function of the orientational structure of the fluid. The model is used to support the hypothesis that dipolar couplings observed in the spectra of human leg muscle originate from nematic-like liquid crystal phases in relatively small metabolite molecules. The fitted theoretical predictions of the dependence of the dipolar coupling on the orientation of the field with respect to the nematic director are shown to provide a good description of the experimental data.

Effects of pH and molecular charge on dipolar coupling interactions of solutes in skeletal muscle observed by DQF, 1H NMR spectroscopy

In this study we tested the effect of molecular charge and chirality as well as tissue pH on dipolar coupling interaction in skeletal muscle. These results were demonstrated by double quantum filtered, DQF, 1H NMR spectra acquired on permeable skeletal muscle samples dialyzed against buffered solutions containing three classes of solutes-electrolytes (lactate and Tris), zwitterions (alanine and glycine), and non-electrolytes (dioxane and ethanol)-as a function of pH ranging from 5.0 to 8.5. The results show that charge density on the protein filaments strongly influences dipolar coupling of solutes in muscle whereas charge on the solutes themselves has only a small effect. The frequency splitting of the dipolar coupled peaks for all the molecules tested was strongly affected by muscle pH. Higher pH increased negative charge density on the filaments and resulted in weaker dipolar coupling for anions and zwitterions but stronger coupling for the cation TRIS. Molecular charge per se or chirality did not affect the frequency splitting of the dipolar coupled peaks. The molecules, lactate, ethanol, and alanine, have scalar coupled spins and consequently a double quantum signal in solution. However, spectra acquired from these molecules in muscle showed an additional frequency splitting due to additional dipolar coupling interactions. Due to lack of scalar coupling, spectra from Tris, glycine, and dioxane showed no double quantum signal in solution but did when in muscle. All these observations can be explained by the fact that the net charge on protein filaments dominates the mechanism of dipolar coupling interactions in the highly anisotropic structures in muscle.