Efficient 31 P band inversion transfer approach for measuring creatine kinase activity, ATP synthesis, and molecular dynamics in the human brain at 7 T

Fast in vivo assay of creatine kinase activity in the human brain by 31 P magnetic resonance fingerprinting

A new and efficient magnetisation transfer 31 P magnetic resonance fingerprinting (MT-31 P-MRF) approach is introduced to measure the creatine kinase metabolic ratek CK between phosphocreatine (PCr) and adenosine triphosphate (ATP) in human brain. The MRF framework is extended to overcome challenges in conventional 31 P measurement methods in the human brain, enabling reduced acquisition time and specific absorption rate (SAR). To address the challenge of creating and matching large multiparametric dictionaries in an MRF scheme, a nested iteration interpolation method (NIIM) is introduced. As the number of parameters to estimate increases, the size of the dictionary grows exponentially. NIIM can reduce the computational load by breaking dictionary matching into subsolutions of linear computational order. MT-31 P-MRF combined with NIIM providesT 1 PCr ,T 1 ATP andk CK estimates in good agreement with those obtained by the exchange kinetics by band inversion transfer (EBIT) method and literature values. Furthermore, the test-retest reproducibility results showed that MT-31 P-MRF achieves a similar or better coefficient of variation (<12%) forT 1 ATP andk CK measurements in 4 min 15 s, than EBIT with 17 min 4 s scan time, enabling a fourfold reduction in scan time. We conclude that MT-31 P-MRF in combination with NIIM is a fast, accurate, and reproducible approach for in vivok CK assays in the human brain, which enables the potential to investigate energy metabolism in a clinical setting.

Multinuclear Magnetic Resonance Spectroscopy at Ultra-High-Field: Assessing Human Cerebral Metabolism in Healthy and Diseased States

The brain is a highly energetic organ. Although the brain can consume metabolic substrates, such as lactate, glycogen, and ketone bodies, the energy metabolism in a healthy adult brain mainly relies on glucose provided via blood. The cerebral metabolism of glucose produces energy and a wide variety of intermediate metabolites. Since cerebral metabolic alterations have been repeatedly implicated in several brain disorders, understanding changes in metabolite levels and corresponding cell-specific neurotransmitter fluxes through different substrate utilization may highlight the underlying mechanisms that can be exploited to diagnose or treat various brain disorders. Magnetic resonance spectroscopy (MRS) is a noninvasive tool to measure tissue metabolism in vivo. ¹H-MRS is widely applied in research at clinical field strengths (≤3T) to measure mostly high abundant metabolites. In addition, X-nuclei MRS including, ¹³C, ²H, ¹⁷O, and ³¹P, are also very promising. Exploiting the higher sensitivity at ultra-high-field (>4T; UHF) strengths enables obtaining unique insights into different aspects of the substrate metabolism towards measuring cell-specific metabolic fluxes in vivo. This review provides an overview about the potential role of multinuclear MRS (¹H, ¹³C, ²H, ¹⁷O, and ³¹P) at UHF to assess the cerebral metabolism and the metabolic insights obtained by applying these techniques in both healthy and diseased states.

MR-Nucleomics: The study of pathological cellular processes with multinuclear magnetic resonance spectroscopy and imaging in vivo

Clinical medicine has experienced a rapid development in recent decades, during which therapies targeting specific cellular signaling pathways, or specific cell surface receptors, have been increasingly adopted. While these developments in clinical medicine call for improved precision in diagnosis and treatment monitoring, modern medical imaging methods are restricted mainly to anatomical imaging, lagging behind the requirements of precision medicine. Although positron emission tomography and single photon emission computed tomography have been used clinically for studies of metabolism, their applications have been limited by the exposure risk to ionizing radiation, the subsequent limitation in repeated and longitudinal studies, and the incapability in assessing downstream metabolism. Magnetic resonance spectroscopy (MRS) or spectroscopic imaging (MRSI) are, in theory, capable of assessing molecular activities in vivo, although they are often limited by sensitivity. Here, we review some recent developments in MRS and MRSI of multiple nuclei that have potential as molecular imaging tools in the clinic.

ATP line splitting in association with reduced intracellular magnesium and pH: a brain 31 P MR spectroscopic imaging (MRSI) study of pediatric patients with myelin oligodendrocyte glycoprotein antibody-associated disorders (MOGADs)

Over the past four decades, ATP, the obligatory energy molecule for keeping all cells alive and functioning, has been thought to contribute only one set of signals in brain ³¹ P MR spectra. Here we report for the first time the observation of two separate β-ATP peaks in brain spectra acquired from patients with myelin oligodendrocyte glycoprotein antibody-associated disorders (MOGADs) using 3D MRSI at 7 T. In voxel spectra with β-ATP line splitting, these two peaks are separated by 0.46 ± 0.18 ppm (n = 6). Spectral lineshape analysis indicates that the upper field β-ATP peak is smaller in relative intensity (24 ± 11% versus 76 ± 11%), and narrower in linewidth (56.8 ± 10.3 versus 41.2 ± 10.3 Hz) than the downfield one. Data analysis also reveals a similar line splitting for the intracellular inorganic phosphate (P_i ) signal, which is characterized by two components with a smaller separation (0.16 ± 0.09 ppm) and an intensity ratio (26 ± 7%:74 ± 7%) comparable to that of β-ATP. While the major components of P_i and β-ATP correspond to a neutral intracellular pH (6.99 ± 0.01) and a free Mg²⁺ level (0.18 ± 0.02 mM, by Iotti's conversion formula) as found in healthy subjects, their minor counterparts relate to a slightly acidic pH (6.86 ± 0.07) and a 50% lower [Mg²⁺ ] (0.09 ± 0.02 mM), respectively. Data correlation between β-ATP and P_i signals appears to suggest an association between an increased [H⁺ ] and a reduced [Mg²⁺ ] in MOGAD patients.

Multinuclear MRI in Drug Discovery

The continuous development of magnetic resonance imaging broadens the range of applications to newer areas. Using MRI, we can not only visualize, but also track pharmaceutical substances and labeled cells in both in vivo and in vitro tests. ¹H is widely used in the MRI method, which is determined by its high content in the human body. The potential of the MRI method makes it an excellent tool for imaging the morphology of the examined objects, and also enables registration of changes at the level of metabolism. There are several reports in the scientific publications on the use of clinical MRI for in vitro tracking. The use of multinuclear MRI has great potential for scientific research and clinical studies. Tuning MRI scanners to the Larmor frequency of a given nucleus, allows imaging without tissue background. Heavy nuclei are components of both drugs and contrast agents and molecular complexes. The implementation of hyperpolarization techniques allows for better MRI sensitivity. The aim of this review is to present the use of multinuclear MRI for investigations in drug delivery.

Phosphorus transversal relaxation times and metabolite concentrations in the human brain at 9.4 T

A method to estimate phosphorus (³¹ P) transversal relaxation times (T₂ s) of coupled spin systems is demonstrated. Additionally, intracellular and extracellular pH and relaxation-corrected metabolite concentrations are reported. Echo time (TE) series of ³¹ P metabolite spectra were acquired using stimulated echo acquisition mode (STEAM) localization. Spectra were fitted using LCModel with accurately modeled Versatile Simulation, Pulses and Analysis (VeSPA) basis sets accounting for J-evolution of the coupled spin systems. T₂ s were estimated by fitting a single exponential two-parameter model across the TE series. Fitted inorganic phosphate frequencies were used to calculate pH, and estimated relaxation times were used to determine the relaxation-corrected brain metabolite concentrations on an assumption of 3 mM γ-ATP. The method was demonstrated in healthy human brain at a field strength of 9.4 T. T₂ times of ATP and nicotinamide adenine dinucleotide (NAD) were shortest between 8 and 20 ms, followed by T₂ s of inorganic phosphate between 25 and 50 ms, and phosphocreatine with a T₂ of 100 ms. Phosphomonoesters and phosphodiesters had the longest T₂ s of about 130 ms. The measured T₂ s are comparable with literature values and fit in a decreasing trend with increasing field strengths. Calculated pHs and metabolite concentrations are also comparable with literature values.

7 Tesla and Beyond: Advanced Methods and Clinical Applications in Magnetic Resonance Imaging

ABSTRACT

Ultrahigh magnetic fields offer significantly higher signal-to-noise ratio, and several magnetic resonance applications additionally benefit from a higher contrast-to-noise ratio, with static magnetic field strengths of B0 ≥ 7 T currently being referred to as ultrahigh fields (UHFs). The advantages of UHF can be used to resolve structures more precisely or to visualize physiological/pathophysiological effects that would be difficult or even impossible to detect at lower field strengths. However, with these advantages also come challenges, such as inhomogeneities applying standard radiofrequency excitation techniques, higher energy deposition in the human body, and enhanced B0 field inhomogeneities. The advantages but also the challenges of UHF as well as promising advanced methodological developments and clinical applications that particularly benefit from UHF are discussed in this review article.

31 P-MRS of the healthy human brain at 7 T detects multiple hexose derivatives of uridine diphosphate glucose

Nucleotide sugars are required for the synthesis of glycoproteins and glycolipids, which play crucial roles in many cellular functions such as cell communication and immune responses. Uridine diphosphate-glucose (UDP-Glc) was previously believed to be the only nucleotide sugar detectable in brain by ³¹ P-MRS. Using spectra of high SNR and high resolution acquired at 7 T, we showed that multiple nucleotide sugars are coexistent in brain and can be measured simultaneously. In addition to UDP-Glc, these also include UDP-galactose (UDP-Gal), -N-acetyl-glucosamine (UDP-GlcNAc) and -N-acetyl-galactosamine (UDP-GalNAc), collectively denoted as UDP(G). Coexistence of these UDP(G) species is evident from a quartet-like multiplet at -9.8 ppm (M_-9.8 ), which is a common feature seen across a wide age range (24-64 years). Lineshape fitting of M_-9.8 allows an evaluation of all four UDP(G) components, which further aids in analysis of a mixed signal at -8.2 ppm (M_-8.2 ) for deconvolution of NAD⁺ and NADH. For a group of seven young healthy volunteers, the concentrations of UDP(G) species were 0.04 ± 0.01 mM for UDP-Gal, 0.07 ± 0.03 mM for UDP-Glc, 0.06 ± 0.02 mM for UDP-GalNAc and 0.08 ± 0.03 mM for UDP-GlcNA, in reference to ATP (2.8 mM). The combined concentration of all UDP(G) species (average 0.26 ± 0.06 mM) was similar to the pooled concentration of NAD⁺ and NADH (average 0.27 ± 0.06 mM, with a NAD⁺ /NADH ratio of 6.7 ± 2.1), but slightly lower than previously found in an older cohort (0.31 mM). The in vivo NMR analysis of UDP-sugar composition is consistent with those from tissue extracts by other modalities in the literature. Given that glycosylation is dependent on the availability of nucleotide sugars, assaying multiple nucleotide sugars may provide valuable insights into potential aberrant glycosylation, which has been implicated in certain diseases such as cancer and Alzheimer's disease.

Comparison of four 31P single-voxel MRS sequences in the human brain at 9.4 T

PURPOSE

In this study, different single-voxel localization sequences were implemented and systematically compared for the first time for phosphorous MRS (³¹ P-MRS) in the human brain at 9.4 T.

METHODS

Two multishot sequences, image-selected in vivo spectroscopy (ISIS) and a conventional slice-selective excitation combined with localization by adiabatic selective refocusing (semiLASER) variant of the spin-echo full intensity-acquired localized spectroscopy (SPECIAL-semiLASER), and two single-shot sequences, semiLASER and stimulated echo acquisition mode (STEAM), were implemented and optimized for ³¹ P-MRS in the human brain at 9.4 T. Pulses and coil setup were optimized, localization accuracy was tested in phantom experiments, and absolute SNR of the sequences was compared in vivo. The SNR per unit time (SNR/t) was derived and compared for all four sequences and verified experimentally for ISIS in two different voxel sizes (3 × 3 × 3 cm³ , 5 × 5 × 5 cm³ , 10-minute measurement time). Metabolite signals obtained with ISIS were quantified. The possible spectral quality in vivo acquired in clinically feasible time (3:30 minutes, 3 × 3 × 3 cm³ ) was explored for two different coil setups.

RESULTS

All evaluated sequences performed with good localization accuracy in phantom experiments and provided well-resolved spectra in vivo. However, ISIS has the lowest chemical shift displacement error, the best localization accuracy, the highest SNR/t for most metabolites, provides metabolite concentrations comparable to literature values, and is the only one of the sequences that allows for the detection of the whole ³¹ P spectrum, including β-adenosine triphosphate, with the used setup. The SNR/t of STEAM is comparable to the SNR/t of ISIS. The semiLASER and SPECIAL-semiLASER sequences provide good results for metabolites with long T₂ .

CONCLUSION

At 9.4 T, high-quality single-voxel localized ³¹ P-MRS can be performed in the human brain with different localization methods, each with inherent characteristics suitable for different research issues.

Quantitative measurement of redox state in human brain by 31 P MRS at 7T with spectral simplification and inclusion of multiple nucleotide sugar components in data analysis

PURPOSE

To develop a simplified method for quantitative measurement of NAD⁺ /NADH (nicotinamide adenine dinucleotides) levels in human brain by ³¹ P MRS without interference from the α-ATP signal and with inclusion of multiple UDP-sugar components.

METHODS

Simple pulse-acquire ³¹ P MR spectra were collected at 7T with and without a frequency-selective inversion pulse to remove the dominant α-ATP signal from the underlying NAD(H) signal. Careful inspection of the ³¹ P signal at -9.8 ppm previously assigned to UDP-glucose revealed multiple UDP-sugar components that must also be considered when deconvoluting the NAD(H) signal to quantify NAD⁺ and NADH. Finally, the overlapping NAD(H) and UDP(G) resonances were deconvoluted into individual components using Voigt lineshape analysis and UDP(G) modeling.

RESULTS

The inversion-based spectral editing method enabled clean separation of the NAD(H) signal from the otherwise dominant α-ATP signal. In addition, the upfield signal near -9.8 ppm appears more "quartet-like" than a simple doublet consistent with contributions from other nucleotide sugars such as UDP-galactose, UDP-N-acetyl-galactosamine, and UDP-N-acetyl-glucosamine in addition to UDP-glucose. Deconvolution of the combined NAD(H) and UDP(G) signals showed that the measured NAD⁺ /NAD ratio was heavily influenced by UDP(G) modeling (7.5 ± 1.8 when the UDP(G) signal was fitted as multiple doublets versus 5.3 ± 0.6 when a simplified pseudo doublet model was used). In a test/re-test experiments separated by 2 weeks, consistent NAD⁺ /NADH ratios were measured in the brain of seven human subjects.

CONCLUSIONS

The NAD⁺ /NADH ratio in human brain can be measured using ³¹ P MR spectra simplified by spectral editing and with inclusion of multiple UDP-sugar components in the data analysis.

Shortening of apparent transverse relaxation time of inorganic phosphate as a breast cancer biomarker

Phosphorus MRS offers a non-invasive tool for monitoring cell energy and phospholipid metabolism and can be of additional value in diagnosing cancer and monitoring cancer therapy. In this study, we determined the transverse relaxation times of a number of phosphorous metabolites in a group of breast cancer patients by adiabatic multi-echo spectroscopic imaging at 7 T. The transverse relaxation times of phosphoethanolamine, phosphocholine, inorganic phosphate (P_i ), glycerophosphocholine and glycerophosphatidylcholine were 184 ± 8 ms, 203 ± 17 ms, 87 ± 8 ms, 240 ± 56 ms and 20 ± 10 ms, respectively. The transverse relaxation time of P_i in breast cancer tissue was less than half that of healthy fibroglandular tissue. This effect is most likely caused by an up-regulation of glycolysis in breast cancer tissue that leads to interaction of P_i with the GAPDH enzyme, which forms part of the reversible pathway of exchange of P_i with gamma-adenosine tri-phosphate, thus shortening its apparent transverse relaxation time. As healthy breast tissue shows very little glycolytic activity, the apparent T₂ shortening of P_i due to malignant transformation could possibly be used as a biomarker for cancer.

Modular 31 P wideband inversion transfer for integrative analysis of adenosine triphosphate metabolism, T1 relaxation and molecular dynamics in skeletal muscle at 7T

PURPOSE

For efficient and integrative analysis of de novo adenosine triphosphate (ATP) synthesis, creatine-kinase-mediated ATP synthesis, T₁ relaxation time, and ATP molecular motion dynamics in human skeletal muscle at rest.

METHODS

Four inversion-transfer modules differing in center inversion frequency were combined to generate amplified magnetization transfer (MT) effects in targeted MT pathways, including Pi ↔ γ-ATP, PCr ↔ γ-ATP, and ³¹ P_γ(α)ATP ↔ ³¹ P_βATP . MT effects from both forward and reverse exchange kinetic pathways were acquired to reduce potential bias and confounding factors in integrated data analysis.

RESULTS

Kinetic data collected using 4 wideband inversion modules (8 minutes each) yielded the forward exchange rate constants, k_{PCr →γ ATP} = 0.31 ± 0.05 s^-1 and k_{Pi →γ ATP} = 0.064 ± 0.012 s^-1 , and the reverse exchange rate constants, k_γATP→Pi = 0.034 ± 0.006 s^-1 and k_γATP→PCr = 1.37 ± 0.22 s^-1 , respectively. The cross-relaxation rate constant, σ_{γ(α) ↔ βATP} was -0.20 ± 0.03 s^-1 , corresponding to ATP rotational correlation time τ_c of 0.8 ± 0.1 × 10^-7 seconds. The intrinsic T₁ relaxation times were Pi (9.2 ± 1.4 seconds), PCr (6.2 ± 0.4 seconds), γ-ATP (1.8 ± 0.1 seconds), α-ATP (1.4 ± 0.1 seconds), and β-ATP (1.1 ± 0.1 seconds). Muscle ATP T₁ values were found to be significantly longer than those previously measured in the brain using a similar method.

CONCLUSION

A combination of multiple inversion transfer modules provides a comprehensive and integrated analysis of ATP metabolism and molecular motion dynamics. This relatively fast technique could be potentially useful for studying metabolic disorders in skeletal muscle.

In vivo X-Nuclear MRS Imaging Methods for Quantitative Assessment of Neuroenergetic Biomarkers in Studying Brain Function and Aging

Brain relies on glucose and oxygen metabolisms to generate biochemical energy in the form of adenosine triphosphate (ATP) for supporting electrophysiological activities and neural signaling under resting or working state. Aging is associated with declined mitochondrial functionality and decreased cerebral energy metabolism, and thus, is a major risk factor in developing neurodegenerative diseases including Alzheimer’s disease (AD). However, there is an unmet need in the development of novel neuroimaging tools and sensitive biomarkers for detecting abnormal energy metabolism and impaired mitochondrial function, especially in an early stage of the neurodegenerative diseases. Recent advancements in developing multimodal high-field in vivo X-nuclear (e.g., ²H, ¹⁷O and ³¹P) MRS imaging techniques have shown promise for quantitative and noninvasive measurement of fundamental cerebral metabolic rates of glucose and oxygen consumption, ATP production as well as nicotinamide adenine dinucleotide (NAD) redox state in preclinical animal and human brains. These metabolic neuroimaging measurements could provide new insights and quantitative bioenergetic markers associated with aging processing and neurodegeneration and can therefore be employed to monitor disease progression and/or determine effectiveness of therapeutic intervention.

Unveiling a hidden 31 P signal coresonating with extracellular inorganic phosphate by outer-volume-suppression and localized 31 P MRS in the human brain at 7T

PURPOSE

The study was undertaken to demonstrate that there is more than 1 component in the extracellular Pi31 P signal ( Piex) acquired from human head using nonlocalized 31 P MRS.

METHODS

Outer-volume-suppression (OVS) saturation and 1D/2D 31 P CSI were utilized to reveal the presence of an additional component in the Piex signal.

RESULTS

67% of the head extracellular Pi signal was attenuated upon OVS saturation of the peripheral meningeal tissues, likely reflecting elimination of the Pi signal in the meningeal fluids (the blood and CSF). Localized 1D/2D CSI data provided further support for this assignment. Upon correction for the meningeal contribution, the extracellular Pi concentration was 0.51 ± 0.07 mM, whereas the intracellular Pi was 0.85 ± 0.10 mM. The extracellular pH was measured as 7.32 ± 0.04 when using OVS, as compared to 7.39 ± 0.03 when measured without OVS (N = 7 subjects).

CONCLUSION

The extracellular Pi signal acquired from the human head using nonlocalized 31 P MRS contains a significant component likely contributed by peripheral blood and CSF in meninges that must be removed in order to use this signal as an endogenous probe for measuring extracellular pH and other properties in the brain.

Functional energetic responses and individual variance of the human brain revealed by quantitative imaging of adenosine triphosphate production rates

Cellular ATP energy metabolism and regulation are essential for brain function and health. Given the high ATP expenditure at resting-state, it is not yet clear how the human brain at working-state can effectively regulate ATP production to meet higher energy requirement. Through quantitative measurement of regional cerebral ATP production rates and associated neurophysiological parameters in human visual cortex at rest and during visual stimulation, we found significant stimulus-induced and highly correlated neuroenergetic changes, indicating distinctive and complementary roles of the ATP synthesis reactions in supporting evoked neuronal activity and maintaining ATP homeostasis. We also uncovered large individual variances in the neuroenergetic responses and significant reductions in intracellular [H+] and free [Mg2+] during the stimulation. These results provide new insights into the mechanism underlying the brain ATP energy regulation and present a sensitive and much-needed neuroimaging tool for quantitatively assessing neuroenergetic state in healthy and diseased human brain.