In vivo (31)P magnetic resonance spectroscopy of the human liver at 7 T: an initial experience

In vivo phosphorus magnetic resonance spectroscopic imaging of the whole human liver at 7 T using a phosphorus whole-body transmit coil and 16-channel receive array: Repeatability and effects of principal component analysis-based denoising

Quantitative three-dimensional (3D) imaging of phosphorus (³¹ P) metabolites is potentially a promising technique with which to assess the progression of liver disease and monitor therapy response. However, ³¹ P magnetic resonance spectroscopy has a low sensitivity and commonly used ³¹ P surface coils do not provide full coverage of the liver. This study aimed to overcome these limitations by using a ³¹ P whole-body transmit coil in combination with a 16-channel ³¹ P receive array at 7 T. Using this setup, we determined the repeatability of whole-liver ³¹ P magnetic resonance spectroscopic imaging (³¹ P MRSI) in healthy subjects and assessed the effects of principal component analysis (PCA)-based denoising on the repeatability parameters. In addition, spatial variations of ³¹ P metabolites within the liver were analyzed. 3D ³¹ P MRSI data of the liver were acquired with a nominal voxel size of 20 mm isotropic in 10 healthy volunteers twice on the same day. Data were reconstructed without denoising, and with PCA-based denoising before or after channel combination. From the test-retest data, repeatability parameters for metabolite level quantification were determined for 12 ³¹ P metabolite signals. On average, ³¹ P MR spectra from 100 ± 25 voxels in the liver were analyzed. Only voxels with contamination from skeletal muscle or the gall bladder were excluded and no voxels were discarded based on (low) signal-to-noise ratio (SNR). Repeatability for most quantified ³¹ P metabolite levels in the liver was good to excellent, with an intrasubject variability below 10%. PCA-based denoising increased the SNR ~ 3-fold, but did not improve the repeatability for mean liver ³¹ P metabolite quantification with the fitting constraints used. Significant spatial heterogeneity of various ³¹ P metabolite levels within the liver was observed, with marked differences for the phosphomonoester and phosphodiester metabolites between the left and right lobe. In conclusion, using a ³¹ P whole-body transmit coil in combination with a 16-channel ³¹ P receive array at 7 T allowed ³¹ P MRSI acquisitions with full liver coverage and good to excellent repeatability.

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.

Non-Invasive Analysis of Human Liver Metabolism by Magnetic Resonance Spectroscopy

The liver is a key node of whole-body nutrient and fuel metabolism and is also the principal site for detoxification of xenobiotic compounds. As such, hepatic metabolite concentrations and/or turnover rates inform on the status of both hepatic and systemic metabolic diseases as well as the disposition of medications. As a tool to better understand liver metabolism in these settings, in vivo magnetic resonance spectroscopy (MRS) offers a non-invasive means of monitoring hepatic metabolic activity in real time both by direct observation of concentrations and dynamics of specific metabolites as well as by observation of their enrichment by stable isotope tracers. This review summarizes the applications and advances in human liver metabolic studies by in vivo MRS over the past 35 years and discusses future directions and opportunities that will be opened by the development of ultra-high field MR systems and by hyperpolarized stable isotope tracers.

Concentration of Gallbladder Phosphatidylcholine in Cholangiopathies: A Phosphorus-31 Magnetic Resonance Spectroscopy Pilot Study

BACKGROUND

Biliary phosphatidylcholine (PtdC) concentration plays a role in the pathogenesis of bile duct diseases. In vivo phosphorus-31 magnetic resonance spectroscopy (³¹ P-MRS) at 7 T offers the possibility to assess this concentration noninvasively with high spectral resolution and signal intensity.

PURPOSE

Comparison of PtdC levels of cholangiopathic patient groups to a control group using a measured T₁ relaxation time of PtdC in healthy subjects.

STUDY TYPE

Case control.

SUBJECTS

Two patient groups with primary sclerosing cholangitis (PSC, 2f/3 m; age: 43 ± 7 years) and primary biliary cholangitis (PBC, 4f/2 m; age: 57 ± 6 years), and a healthy control group (CON, 2f/3 m; age: 38 ± 7 years). Ten healthy subjects for the assessment of the T₁ relaxation time of PtdC.

FIELD STRENGTH/SEQUENCE

A 3D phase-encoded pulse-acquire ³¹ P-MRSI sequence for PtdC quantification and a 1D image-selected in vivo ³¹ P spectroscopy for T₁ estimation at 7 T, and a T2-weighted half-Fourier single-shot turbo spin echo MRI sequence for volumetry at 3 T.

ASSESSMENT

Calculation of gallbladder volumes and PtdC concentration in groups using hepatic gamma-adenosine triphosphate signal as an internal reference and correction for insufficient relaxation of PtdC with a T₁ value assessed in healthy subjects.

STATISTICAL TESTS

Group comparison of PtdC content and gallbladder volumes of the PSC/PBC and CON group using Student's t-tests with a significance level of 5%.

RESULTS

PtdC T₁ value of 357 ± 85 msec in the gallbladder. Significant lower PtdC content for the PSC group, and for the female subgroup of the PBC group compared to the CON group (PSC/CON: 5.74 ± 0.73 mM vs. 9.64 ± 0.97 mM, PBC(f)/CON: 5.77 ± 1.44 mM vs. 9.64 ± 0.97 mM). Significant higher gallbladder volumes of the patient groups compared to the CON group (PSC/CON: 66.3 ± 15.8 mL vs. 20.9 ± 2.2 mL, PBC/CON: 49.8 ± 18.2 mL vs. 20.9 ± 2.2 mL).

DATA CONCLUSION

This study demonstrated the application of a ³¹ P-MRSI protocol for the quantification of PtdC in the human gallbladder at 7 T. Observed differences in PtdC concentration suggest that this metabolite could serve as a biomarker for specific hepatobiliary disorders.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 3.

Accelerated MR spectroscopic imaging-a review of current and emerging techniques

Over more than 30 years in vivo MR spectroscopic imaging (MRSI) has undergone an enormous evolution from theoretical concepts in the early 1980s to the robust imaging technique that it is today. The development of both fast and efficient sampling and reconstruction techniques has played a fundamental role in this process. State-of-the-art MRSI has grown from a slow purely phase-encoded acquisition technique to a method that today combines the benefits of different acceleration techniques. These include shortening of repetition times, spatial-spectral encoding, undersampling of k-space and time domain, and use of spatial-spectral prior knowledge in the reconstruction. In this way in vivo MRSI has considerably advanced in terms of spatial coverage, spatial resolution, acquisition speed, artifact suppression, number of detectable metabolites and quantification precision. Acceleration not only has been the enabling factor in high-resolution whole-brain 1 H-MRSI, but today is also common in non-proton MRSI (31 P, 2 H and 13 C) and applied in many different organs. In this process, MRSI techniques had to constantly adapt, but have also benefitted from the significant increase of magnetic field strength boosting the signal-to-noise ratio along with high gradient fidelity and high-density receive arrays. In combination with recent trends in image reconstruction and much improved computation power, these advances led to a number of novel developments with respect to MRSI acceleration. Today MRSI allows for non-invasive and non-ionizing mapping of the spatial distribution of various metabolites' tissue concentrations in animals or humans, is applied for clinical diagnostics and has been established as an important tool for neuro-scientific and metabolism research. This review highlights the developments of the last five years and puts them into the context of earlier MRSI acceleration techniques. In addition to 1 H-MRSI it also includes other relevant nuclei and is not limited to certain body regions or specific applications.

Feasibility of 31 P spectroscopic imaging at 7 T in lung carcinoma patients

Currently, it is difficult to predict effective therapy response to molecular therapies for the treatment of lung cancer based solely on anatomical images. ³¹ P MR spectroscopic imaging could provide as a non-invasive method to monitor potential biomarkers for early therapy evaluation, a necessity to improve personalized care and reduce cost. However, surface coils limit the imaging volume in conventional ³¹ P MRSI. High-energetic adiabatic RF pulses are required to achieve flip angle homogeneity but lead to high SAR. Birdcage coils permit use of conventional amplitude modulated pulses, even over large FOV, potentially decreasing overall SAR massively. Here, we investigate the feasibility of 3D ³¹ P MRSI at 7 T in lung carcinoma patients using an integrated ³¹ P birdcage body coil in combination with either a dual-coil or a 16-channel receiver. Simulations showed a maximum decrease in SNR per unit of time of 8% for flip angle deviations in short TR low flip-angle excitation 3D CSI. The minimal SNR loss allowed for fast 3D CSI without time-consuming calibration steps (>10:00 min.). ³¹ P spectra from four lung carcinoma patients were acquired within 29:00 minutes and with high SNR using both receivers. The latter allowed discrimination of individual phosphodiesters, inorganic phosphate, phosphocreatine and ATP. The receiver array allowed for an increased FOV compared to the dual-coil receiver. 3D ³¹ P-CSI were acquired successfully in four lung carcinoma patients using the integrated ³¹ P body coil at ultra-high field. The increased spectral resolution at 7 T allowed differentiation of multiple ³¹ P metabolites related to phospholipid and energy metabolism. Simulations provide motivation to exclude ³¹ P B₁ calibrations, potentially decreasing total scan duration. Employing large receiver arrays improves the field of view allowing for full organ coverage. ³¹ P MRSI is feasible in lung carcinoma patients and has potential as a non-invasive method for monitoring personalized therapy response in lung tumors.

In vivo absolute quantification of hepatic γ-ATP concentration in mice using 31 P MRS at 11.7 T

Measurement of ATP concentrations and synthesis in humans indicated abnormal hepatic energy metabolism in obesity, non-alcoholic fatty liver disease (NAFLD) and Type 2 diabetes. Further mechanistic studies on energy metabolism require the detailed phenotyping of specific mouse models. Thus, this study aimed to establish and evaluate a robust and fast single voxel ³¹ P MRS method to quantify hepatic γ-ATP concentrations at 11.7 T in three mouse models with different insulin sensitivities and liver fat contents (72-week-old C57BL/6 control mice, 72-week-old insulin resistant sterol regulatory-element binding protein-1c overexpressing (SREBP-1c⁺ ) mice and 10-12-week-old prediabetic non-obese diabetic (NOD) mice). Absolute quantification was performed by employing an external reference and a matching replacement ATP phantom with 3D image selected in vivo spectroscopy ³¹ P MRS. This single voxel ³¹ P MRS method non-invasively quantified hepatic γ-ATP within 17 min and the repeatability tests provided a coefficient of variation of 7.8 ± 1.1%. The mean hepatic γ-ATP concentrations were markedly lower in SREBP-1c⁺ mice (1.14 ± 0.10 mM) than in C57BL/6 mice (2.15 ± 0.13 mM; p < 0.0002) and NOD mice (1.78 ± 0.13 mM; p < 0.006, one-way ANOVA test). In conclusion, this method allows us to rapidly and precisely measure hepatic γ-ATP concentrations, and thereby to non-invasively detect abnormal hepatic energy metabolism in mice with different degrees of insulin resistance and NAFLD. Thus, this ³¹ P MRS will also be useful for future mechanistic as well as therapeutic translational studies in other murine models.

PCA denoising and Wiener deconvolution of 31 P 3D CSI data to enhance effective SNR and improve point spread function

Purpose

This study evaluates the performance of 2 processing methods, that is, principal component analysis‐based denoising and Wiener deconvolution, to enhance the quality of phosphorus 3D chemical shift imaging data.

Methods

Principal component analysis‐based denoising increases the SNR while maintaining spectral information. Wiener deconvolution reduces the FWHM of the voxel point spread function, which is increased by Hamming filtering or Hamming‐weighted acquisition. The proposed methods are evaluated using simulated and in vivo 3D phosphorus chemical shift imaging data by 1) visual inspection of the spatial signal distribution; 2) SNR calculation of the PCr peak; and 3) fitting of metabolite basis functions.

Results

With the optimal order of processing steps, we show that the effective SNR of in vivo phosphorus 3D chemical shift imaging data can be increased. In simulations, we show we can preserve phosphorus‐containing metabolite peaks that had an SNR < 1 before denoising. Furthermore, using Wiener deconvolution, we were able to reduce the FWHM of the voxel point spread function with only partially reintroducing Gibb‐ringing artifacts while maintaining the SNR. After data processing, fitting of the phosphorus‐containing metabolite signals improved.

Conclusion

In this study, we have shown that principal component analysis‐based denoising in combination with regularized Wiener deconvolution allows increasing the effective spectral SNR of in vivo phosphorus 3D chemical shift imaging data, with reduction of the FWHM of the voxel point spread function. Processing increased the effective SNR by at least threefold compared to Hamming weighted acquired data and minimized voxel bleeding. With these methods, fitting of metabolite amplitudes became more robust with decreased fitting residuals.

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.

Open-label phase II study evaluating safety and efficacy of the non-steroidal farnesoid X receptor agonist PX-104 in non-alcoholic fatty liver disease

Background

The PX-104 is an oral non-steroidal agonist for the farnesoid X receptor (FXR), a key regulator of bile acid (BA), glucose and lipid homeostasis.

Aims and methods

This single center, proof of concept study evaluated the efficacy, safety and tolerability of PX-104 in non-diabetic NAFLD patients. 12 individuals were treated daily with 5 mg of PX-104 orally for 4 weeks. Serum liver enzymes, insulin sensitivity by clamp like index (CLIX) and hepatic fat by proton ¹H‑MRS, MRI-PDFF and CAP were assessed. Hepatic energy metabolism and Kupffer cell function were evaluated by phosphorus ³¹P‑MRS and superparamagnetic iron oxide MRI (SPIO-MRI). Other readouts included serum lipids and markers of BA metabolism/signaling besides fecal microbiome and BA analysis.

Results

A significant decrease in ALT (p = 0.027; 1‑tailed) and GGT (p = 0.019) was observed, without changes in serum alkaline phosphatase or serum lipids. Insulin sensitivity improved in 92% of patients (p = 0.02). However, hepatic steatosis measured by PDFF-MRI, ¹H‑MRS and CAP besides extended serum lipoprotein and BA profiles did not change. NADPH/γATP ratios at ³¹P‑MRS significantly decreased (p = 0.022) possibly reflecting reduced hepatic inflammatory stress, but SPIO-MRI remained unchanged. Reduced preponderance of Coriobacteriaceae (p = 0.036) correlated with a relative reduction of total fecal BAs. There were no serious adverse events but short intervals of cardiac arrhythmia recorded in 2 patients led to termination of the study.

Conclusion

The non-steroidal FXR agonist PX-104 improved insulin sensitivity and liver enzymes after 4 weeks of treatment in non-diabetic NAFLD patients. Changes in fecal BAs and gut microbiota deserve more extensive investigations.

Electronic supplementary material

The online version of this article (10.1007/s00508-020-01735-5) contains supplementary material, which is available to authorized users.

Inherently decoupled 1 H antennas and 31 P loops for metabolic imaging of liver metastasis at 7 T

High field ³¹ P spectroscopy has thus far been limited to diffuse liver disease. Unlike lower field-strength scanners, there is no body coil in the bore of the 7 T and despite inadequate penetration depth (<10 cm), surface coils are the current state-of-the-art for acquiring anatomical images to support multinuclear studies. We present a system of proton antennas and phosphorus loops for ³¹ P spectroscopy and provide the first ultrahigh-field phosphorus metabolic imaging of a tumor in the abdomen. Herein we characterize the degree to which antennas are isolated from underlying loops. Next, we evaluate the penetration depth of the two antennas available during multinuclear examinations. Finally, we combine phosphorus spectroscopy (two loops) with parallel transmit imaging (eight antennas) in a patient. The loops and antennas are inherently decoupled (no added circuitry, <0.1% power coupling). The penetration depth of two antennas gives twice that of conventional loops. The liver and full axial slice of the abdomen were imaged with eight transmit/receive antennas using parallel transmit B1-shimming to overcome image voids. Phosphorus spectroscopy from a liver metastasis resolved individual peaks for phosphocholine and phosphoethenalomine. Proton antennas are inherently decoupled from phosphorus loops. By using two proton antennas it is possible to perform region-of-interest image-based shimming in over 80% of the liver volume, thereby enabling phosphorus spectroscopy of localized disease. Shimming of the full extent of the abdominal cross-section is feasible using a parallel transmit array of eight antennas. A system architecture capable of supporting eight-channel parallel transmit and multinuclear spectroscopy is optimal for supporting multiparametric body imaging, including metabolic imaging, for monitoring the response of patients with liver metastases to cancer treatments and for patient risk stratification. In the meantime, the existing infrastructure using two antennas is sufficient for preliminary studies in metabolic imaging of tumors in the liver.

Low SAR 31 P (multi-echo) spectroscopic imaging using an integrated whole-body transmit coil at 7T

Phosphorus (³¹ P) MRSI provides opportunities to monitor potential biomarkers. However, current applications of ³¹ P MRS are generally restricted to relatively small volumes as small coils are used. Conventional surface coils require high energy adiabatic RF pulses to achieve flip angle homogeneity, leading to high specific absorption rates (SARs), and occupy space within the MRI bore. A birdcage coil behind the bore cover can potentially reduce the SAR constraints massively by use of conventional amplitude modulated pulses without sacrificing patient space. Here, we demonstrate that the integrated ³¹ P birdcage coil setup with a high power RF amplifier at 7 T allows for low flip angle excitations with short repetition time (T_R ) for fast 3D chemical shift imaging (CSI) and 3D T₁ -weighted CSI as well as high flip angle multi-refocusing pulses, enabling multi-echo CSI that can measure metabolite T₂ , over a large field of view in the body. B₁⁺ calibration showed a variation of only 30% in maximum B₁ in four volunteers. High signal-to-noise ratio (SNR) MRSI was obtained in the gluteal muscle using two fast in vivo 3D spectroscopic imaging protocols, with low and high flip angles, and with multi-echo MRSI without exceeding SAR levels. In addition, full liver MRSI was achieved within SAR constraints. The integrated ³¹ P body coil allowed for fast spectroscopic imaging and successful implementation of the multi-echo method in the body at 7 T. Moreover, no additional enclosing hardware was needed for ³¹ P excitation, paving the way to include larger subjects and more space for receiver arrays. The increase in possible number of RF excitations per scan time, due to the improved B₁⁺ homogeneity and low SAR, allows SNR to be exchanged for spatial resolution in CSI and/or T₁ weighting by simply manipulating T_R and/or flip angle to detect and quantify ratios from different molecular species.

Pros and cons of ultra-high-field MRI/MRS for human application

Magnetic resonance imaging and spectroscopic techniques are widely used in humans both for clinical diagnostic applications and in basic research areas such as cognitive neuroimaging. In recent years, new human MR systems have become available operating at static magnetic fields of 7 T or higher (≥300 MHz proton frequency). Imaging human-sized objects at such high frequencies presents several challenges including non-uniform radiofrequency fields, enhanced susceptibility artifacts, and higher radiofrequency energy deposition in the tissue. On the other side of the scale are gains in signal-to-noise or contrast-to-noise ratio that allow finer structures to be visualized and smaller physiological effects to be detected. This review presents an overview of some of the latest methodological developments in human ultra-high field MRI/MRS as well as associated clinical and scientific applications. Emphasis is given to techniques that particularly benefit from the changing physical characteristics at high magnetic fields, including susceptibility-weighted imaging and phase-contrast techniques, imaging with X-nuclei, MR spectroscopy, CEST imaging, as well as functional MRI. In addition, more general methodological developments such as parallel transmission and motion correction will be discussed that are required to leverage the full potential of higher magnetic fields, and an overview of relevant physiological considerations of human high magnetic field exposure is provided.

Absolute Quantification of Phosphor-Containing Metabolites in the Liver Using 31 P MRSI and Hepatic Lipid Volume Correction at 7T Suggests No Dependence on Body Mass Index or Age

Background

Hepatic disorders are often associated with changes in the concentration of phosphorus‐31 (³¹P) metabolites. Absolute quantification offers a way to assess those metabolites directly but introduces obstacles, especially at higher field strengths (B₀ ≥ 7T).

Purpose

To introduce a feasible method for in vivo absolute quantification of hepatic ³¹P metabolites and assess its clinical value by probing differences related to volunteers' age and body mass index (BMI).

Study Type

Prospective cohort.

Subjects/Phantoms

Four healthy volunteers included in the reproducibility study and 19 healthy subjects arranged into three subgroups according to BMI and age. Phantoms containing ³¹P solution for correction and validation.

Field Strength/Sequence

Phase‐encoded 3D pulse‐acquire chemical shift imaging for ³¹P and single‐volume ¹H spectroscopy to assess the hepatocellular lipid content at 7T.

Assessment

A phantom replacement method was used. Spectra located in the liver with sufficient signal‐to‐noise ratio and no contamination from muscle tissue, were used to calculate following metabolite concentrations: adenosine triphosphates (γ‐ and α‐ATP); glycerophosphocholine (GPC); glycerophosphoethanolamine (GPE); inorganic phosphate (P_i); phosphocholine (PC); phosphoethanolamine (PE); uridine diphosphate‐glucose (UDPG); nicotinamide adenine dinucleotide‐phosphate (NADH); and phosphatidylcholine (PtdC). Correction for hepatic lipid volume fraction (HLVF) was performed.

Statistical Tests

Differences assessed by analysis of variance with Bonferroni correction for multiple comparison and with a Student's t‐test when appropriate.

Results

The concentrations for the young lean group corrected for HLVF were 2.56 ± 0.10 mM for γ‐ATP (mean ± standard deviation), α‐ATP: 2.42 ± 0.15 mM, GPC: 3.31 ± 0.27 mM, GPE: 3.38 ± 0.87 mM, P_i: 1.42 ± 0.20 mM, PC: 1.47 ± 0.24 mM, PE: 1.61 ± 0.20 mM, UDPG: 0.74 ± 0.17 mM, NADH: 1.21 ± 0.38 mM, and PtdC: 0.43 ± 0.10 mM. Differences found in ATP levels between lean and overweight volunteers vanished after HLVF correction.

Data Conclusion

Exploiting the excellent spectral resolution at 7T and using the phantom replacement method, we were able to quantify up to 10 ³¹P‐containing hepatic metabolites. The combination of ³¹P magnetic resonance spectroscopy imaging data acquisition and HLVF correction was not able to show a possible dependence of ³¹P metabolite concentrations on BMI or age, in the small healthy population used in this study.

Level of Evidence: 2

Technical Efficacy: Stage 1

J. Magn. Reson. Imaging 2019;49:597–607.

Assessing tissue metabolism by phosphorous-31 magnetic resonance spectroscopy and imaging: a methodology review

Many human diseases are caused by an imbalance between energy production and demand. Magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) provide the unique opportunity for in vivo assessment of several fundamental events in tissue metabolism without the use of ionizing radiation. Of particular interest, phosphate metabolites that are involved in ATP generation and utilization can be quantified noninvasively by phosphorous-31 (31P) MRS/MRI. Furthermore, 31P magnetization transfer (MT) techniques allow in vivo measurement of metabolic fluxes via creatine kinase (CK) and ATP synthase. However, a major impediment for the clinical applications of 31P-MRS/MRI is the prohibitively long acquisition time and/or the low spatial resolution that are necessary to achieve adequate signal-to-noise ratio. In this review, current 31P-MRS/MRI techniques used in basic science and clinical research are presented. Recent advances in the development of fast 31P-MRS/MRI methods are also discussed.

OXSA: An open-source magnetic resonance spectroscopy analysis toolbox in MATLAB

In vivo magnetic resonance spectroscopy provides insight into metabolism in the human body. New acquisition protocols are often proposed to improve the quality or efficiency of data collection. Processing pipelines must also be developed to use these data optimally. Current fitting software is either targeted at general spectroscopy fitting, or for specific protocols. We therefore introduce the MATLAB-based OXford Spectroscopy Analysis (OXSA) toolbox to allow researchers to rapidly develop their own customised processing pipelines. The toolbox aims to simplify development by: being easy to install and use; seamlessly importing Siemens Digital Imaging and Communications in Medicine (DICOM) standard data; allowing visualisation of spectroscopy data; offering a robust fitting routine; flexibly specifying prior knowledge when fitting; and allowing batch processing of spectra. This article demonstrates how each of these criteria have been fulfilled, and gives technical details about the implementation in MATLAB. The code is freely available to download from https://github.com/oxsatoolbox/oxsa.

High-resolution 31 P echo-planar spectroscopic imaging in vivo at 7T

PURPOSE

Conventional ³¹ P chemical shift imaging is time-consuming and yields only limited spatial resolution. The purpose of this study was to demonstrate feasibility of ³¹ P echo-planar spectroscopic imaging (EPSI) in vivo at 7T.

METHODS

A 3D ³¹ P EPSI sequence with trapezoidal-shaped gradient pulses was implemented on a 7T MR scanner. To increase spectral width with reduced demand on gradient performance, a multishot approach was chosen. Acquisition weighting and ³¹ P-{¹ H} double resonance for nuclear Overhauser signal enhancement were applied to increase sensitivity.

RESULTS

3D ³¹ P-{¹ H} EPSI data from model solution and from human calf muscle and brain were obtained from voxels with effective sizes of 4.1 to 16.2 cm³ in measurement times of approximately 10 min. Individual spectra showed well-resolved resonances of endogenous ³¹ P-metabolites without artifacts. Volumetric high-resolution ³¹ P-metabolite maps in vivo showed metabolic heterogeneity of different tissues.

CONCLUSION

In vivo ³¹ P EPSI at 7T yields high-quality metabolic images. The proposed multishot EPSI technique reduces the measurement times for acquisition of volumetric high-resolution maps of ³¹ P-metabolites or intracellular pH in human studies. Magn Reson Med 79:1251-1259, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Phosphodiester content measured in human liver by in vivo 31 P MR spectroscopy at 7 tesla

Purpose

Phosphorus (³¹P) metabolites are emerging liver disease biomarkers. Of particular interest are phosphomonoester and phosphodiester (PDE) “peaks” that comprise multiple overlapping resonances in ³¹P spectra. This study investigates the effect of improved spectral resolution at 7 Tesla (T) on quantifying hepatic metabolites in cirrhosis.

Methods

Five volunteers were scanned to determine metabolite T₁s. Ten volunteers and 11 patients with liver cirrhosis were scanned at 7T. Liver spectra were acquired in 28 min using a 16‐channel ³¹P array and 3D chemical shift imaging. Concentrations were calculated using γ‐adenosine‐triphosphate (γ‐ATP) = 2.65 mmol/L wet tissue.

Results

T₁ means ± standard deviations: phosphatidylcholine 1.05 ± 0.28 s, nicotinamide‐adenine‐dinucleotide (NAD⁺) 2.0 ± 1.0 s, uridine‐diphosphoglucose (UDPG) 3.3 ± 1.4 s. Concentrations in healthy volunteers: α‐ATP 2.74 ± 0.11 mmol/L wet tissue, inorganic phosphate 2.23 ± 0.20 mmol/L wet tissue, glycerophosphocholine 2.34 ± 0.46 mmol/L wet tissue, glycerophosphoethanolamine 1.50 ± 0.28 mmol/L wet tissue, phosphocholine 1.06 ± 0.16 mmol/L wet tissue, phosphoethanolamine 0.77 ± 0.14 mmol/L wet tissue, NAD⁺ 2.37 ± 0.14 mmol/L wet tissue, UDPG 2.00 ± 0.22 mmol/L wet tissue, phosphatidylcholine 1.38 ± 0.31 mmol/L wet tissue. Inorganic phosphate and phosphatidylcholine concentrations were significantly lower in patients; glycerophosphoethanolamine concentrations were significantly higher (P < 0.05).

Conclusion

We report human in vivo hepatic T₁s for phosphatidylcholine, NAD⁺, and UDPG for the first time at 7T. Our protocol allows high signal‐to‐noise, repeatable measurement of metabolite concentrations in human liver. The splitting of PDE into its constituent peaks at 7T may allow more insight into changes in metabolism. Magn Reson Med 78:2095–2105, 2017. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

In-vivo31P-MRS of skeletal muscle and liver: A way for non-invasive assessment of their metabolism

In addition to direct assessment of high energy phosphorus containing metabolite content within tissues, phosphorus magnetic resonance spectroscopy (³¹P-MRS) provides options to measure phospholipid metabolites and cellular pH, as well as the kinetics of chemical reactions of energy metabolism in vivo. Even though the great potential of ³¹P-MR was recognized over 30 years ago, modern MR systems, as well as new, dedicated hardware and measurement techniques provide further opportunities for research of human biochemistry. This paper presents a methodological overview of the ³¹P-MR techniques that can be used for basic, physiological, or clinical research of human skeletal muscle and liver in vivo. Practical issues of ³¹P-MRS experiments and examples of potential applications are also provided. As signal localization is essential for liver ³¹P-MRS and is important for dynamic muscle examinations as well, typical localization strategies for ³¹P-MR are also described.

Metabolic profile of liver damage in non-cirrhotic virus C and autoimmune hepatitis: A proton decoupled 31P-MRS study

PURPOSE

To study liver ³¹P MRS, histology, transient elastography, and liver function tests in patients with virus C hepatitis (HCV) or autoimmune hepatitis (AIH) to test the hypothesis that ³¹P MR metabolic profile of these diseases differ.

MATERIALS AND METHODS

25 patients with HCV (n=12) or AIH (n=13) underwent proton decoupled ³¹P MRS spectroscopy performed on a 3.0T MR imager. Intensities of phosphomonoesters (PME) of phosphoethanolamine (PE) and phosphocholine (PC), phosphodiesters (PDE) of glycerophosphoethanolamine (GPE) and glycerophosphocholine (GPC), and γ, α and β resonances of adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide phosphate (NADPH) were determined. Liver stiffness was measured by transient elastography. Inflammation and fibrosis were staged according to METAVIR from biopsy samples. Activities of alanine transaminase (ALT), aspartate transaminase (AST), alkaline phosphatase (ALT) and thromboplastin time (TT) were determined from serum samples.

RESULTS

PME had a stronger correlation with AST (z=1.73, p=0.04) and ALT (z=1.77, p=0.04) in HCV than in AIH patients. PME, PME/PDE, PE/GPE correlated positively and PDE negatively with inflammatory activity. PE, PC and PME correlated positively with liver function tests.

CONCLUSION

³¹P-MRS suggests a more serious liver damage in HCV than in AIH with similar histopathological findings. ³¹P-MRS is more sensitive in detecting inflammation than fibrosis in the liver.

Clinical applications at ultrahigh field (7 T). Where does it make the difference?

Presently, three major MR vendors provide commercial 7-T units for clinical research under ethical permission, with the number of operating 7-T systems having increased to over 50. This rapid increase indicates the growing interest in ultrahigh-field MRI because of improved clinical results with regard to morphological as well as functional and metabolic capabilities. As the signal-to-noise ratio scales linearly with the field strength (B0 ) of the scanner, the most obvious application at 7 T is to obtain higher spatial resolution in the brain, musculoskeletal system and breast. Of specific clinical interest for neuro-applications is the cerebral cortex at 7 T, for the detection of changes in cortical structure as a sign of early dementia, as well as for the visualization of cortical microinfarcts and cortical plaques in multiple sclerosis. In the imaging of the hippocampus, even subfields of the internal hippocampal anatomy and pathology can be visualized with excellent resolution. The dynamic and static blood oxygenation level-dependent contrast increases linearly with the field strength, which significantly improves the pre-surgical evaluation of eloquent areas before tumor removal. Using susceptibility-weighted imaging, the plaque-vessel relationship and iron accumulation in multiple sclerosis can be visualized for the first time. Multi-nuclear clinical applications, such as sodium imaging for the evaluation of repair tissue quality after cartilage transplantation and (31) P spectroscopy for the differentiation between non-alcoholic benign liver disease and potentially progressive steatohepatitis, are only possible at ultrahigh fields. Although neuro- and musculoskeletal imaging have already demonstrated the clinical superiority of ultrahigh fields, whole-body clinical applications at 7 T are still limited, mainly because of the lack of suitable coils. The purpose of this article was therefore to review the clinical studies that have been performed thus far at 7 T, compared with 3 T, as well as those studies performed at 7 T that cannot be routinely performed at 3 T. Copyright © 2015 John Wiley & Sons, Ltd.

Whole-body radiofrequency coil for (31) P MRSI at 7 T

Widespread use of ultrahigh-field (31) P MRSI in clinical studies is hindered by the limited field of view and non-uniform radiofrequency (RF) field obtained from surface transceivers. The non-uniform RF field necessitates the use of high specific absorption rate (SAR)-demanding adiabatic RF pulses, limiting the signal-to-noise ratio (SNR) per unit of time. Here, we demonstrate the feasibility of using a body-sized volume RF coil at 7 T, which enables uniform excitation and ultrafast power calibration by pick-up probes. The performance of the body coil is examined by bench tests, and phantom and in vivo measurements in a 7-T MRI scanner. The accuracy of power calibration with pick-up probes is analyzed at a clinical 3-T MR system with a close to identical (1) H body coil integrated at the MR system. Finally, we demonstrate high-quality three-dimensional (31) P MRSI of the human body at 7 T within 5 min of data acquisition that includes RF power calibration. Copyright © 2016 John Wiley & Sons, Ltd.

(31)P CSI of the human brain in healthy subjects and tumor patients at 9.4 T with a three-layered multi-nuclear coil: initial results

OBJECTIVE

Investigation of the feasibility and performance of phosphorus ((31)P) magnetic resonance spectroscopic imaging (MRSI) at 9.4 T with a three-layered phosphorus/proton coil in human normal brain tissue and tumor.

MATERIALS AND METHODS

A multi-channel (31)P coil was designed to enable MRSI of the entire human brain. The performance of the coil was evaluated by means of electromagnetic field simulations and actual measurements. A 3D chemical shift imaging approach with a variable repetition time and flip angle was used to increase the achievable signal-to-noise ratio of the acquired (31)P spectra. The impact of the resulting k-space modulation was investigated by simulations. Three tumor patients and three healthy volunteers were scanned and differences between spectra from healthy and cancerous tissue were evaluated qualitatively.

RESULTS

The high sensitivity provided by the 27-channel (31)P coil allowed acquiring CSI data in 22 min with a nominal voxel size of 15 × 15 × 15 mm(3). Shimming and anatomical localization could be performed with the integrated four-channel proton dipole array. The amplitudes of the phosphodiesters and phosphoethanolamine appeared reduced in tumorous tissue for all three patients. A neutral or slightly alkaline pH was measured within the brain lesions.

CONCLUSION

These initial results demonstrate that (31)P 3D CSI is feasible at 9.4 T and could be performed successfully in healthy subjects and tumor patients in under 30 min.

In vivo (1)H MRS and (31)P MRSI of the response to cyclocreatine in transgenic mouse liver expressing creatine kinase

Hepatocyte transplantation has been explored as a therapeutic alternative to liver transplantation, but a means to monitor the success of the procedure is lacking. Published findings support the use of in vivo (31)P MRSI of creatine kinase (CK)-expressing hepatocytes to monitor proliferation of implanted hepatocytes. Phosphocreatine tissue level depends upon creatine (Cr) input to the CK enzyme reaction, but Cr measurement by (1)H MRS suffers from low signal-to-noise ratio (SNR). We examine the possibility of using the Cr analog cyclocreatine (CCr, a substrate for CK), which is quickly phosphorylated to phosphocyclocreatine (PCCr), as a higher SNR alternative to Cr. (1)H MRS and (31)P MRSI were employed to measure the effect of incremental supplementation of CCr upon PCCr, γ-ATP, pH and Pi /ATP in the liver of transgenic mice expressing the BB isoform of CK (CKBB) in hepatocytes. Water supplementation with 0.1% CCr led to a peak total PCCr level of 17.15 ± 1.07 mmol/kg wet weight by 6 weeks, while adding 1.0% CCr led to a stable PCCr liver level of 18.12 ± 3.91 mmol/kg by the fourth day of feeding. PCCr was positively correlated with CCr, and ATP concentration and pH declined with increasing PCCr. Feeding with 1% CCr in water induced an apparent saturated level of PCCr, suggesting that CCr quantization may not be necessary for quantifying expression of CK in mice. These findings support the possibility of using (31)P MRS to noninvasively monitor hepatocyte transplant success with CK-expressing hepatocytes.

2D AMESING multi-echo (31)P-MRSI of the liver at 7T allows transverse relaxation assessment and T2-weighted averaging for improved SNR

PURPOSE

Liver diseases are a major global health concern often requiring invasive assessment by needle biopsy. (31)P magnetic resonance spectroscopic imaging (MRSI) allows non-invasive probing of important liver metabolites. Recently, the adiabatic multi-echo spectroscopic imaging sequence with spherical k-space sampling (AMESING) was introduced at 7T, enabling acquisition of T2 information. T2-weighed averaging of the multiple echoes improves signal-to-noise ratio (SNR). The purpose of our study was to implement AMESING MRSI of the liver at 3T and 7T, derive localized T2 information and compare T2-weighted average spectra in terms of SNR.

METHODS

Ten male volunteers underwent 2D AMESING MRSI at 3T and 7T after a minimum four-hour fast. SNR was calculated for PC, PE, Pi, GPE, GPC and α-ATP using maximum peak amplitudes and the SD of the noise. Metabolite peak ratios were calculated after fitting in jMRUI. SNR values and peak ratios were compared with the Wilcoxon signed-rank test.

RESULTS

For the first time liver metabolites' T2 values at 7T were measured: PE (55.6±3.5 ms), PC (51.2±2.3 ms), Pi (46.4±1.1 ms), GPE (44.0±0.8 ms), GPC (50.4±0.8 ms) and α-ATP (18.2±0.4 ms). SNR gain using T2-weighted averaging at 7T resulted in a 1.2× SNR gain. In conjunction with higher field strength and improved coil set-up T2-weighted averaging at 7T allowed a total 3.2× SNR gain compared to 3T FID-only.

CONCLUSION

AMESING 2D MRSI of the liver at 7T provides T2 values that allow T2-weighted averaging of data from multiple echoes resulting in improved SNR.

Characterization of the peak at 2.06 ppm in (31) P magnetic resonance spectroscopy of human liver: phosphoenolpyruvate or phosphatidylcholine?

High field MR scanners can resolve a metabolite resonating at 2.06 ppm in the in vivo proton-decoupled liver (31) P MR spectrum. Traditionally this peak has been assigned to phosphoenolpyruvate (PEP), the key metabolite for gluconeogenesis. However, recent evidence supported the assignment to biliary phosphatidylcholine (PtdCh), which is produced in the liver and stored in the gall bladder. To elucidate the respective contributions of PtdCh and PEP to the in vivo resonance at 2.06 ppm (PEP-PtdCh), we made phantom measurements that confirmed that both biliary PtdCh and PEP resonate approximately at 2 ppm. The absolute quantification of PEP-PtdCh yielded concentrations ranging from 0.6 to 2.0 mmol/l, with mean coefficients of variation of 4.8% for intraday and 7.2% for interday reproducibility in healthy volunteers. The T1 relaxation time of PEP-PtdCh was 0.97 ± 0.30 s in the liver and 0.44 ± 0.11 s in the gallbladder. Ingestion of a mixed meal decreased the concentration of PtdCh-PEP by approximately 12%. In the retrospective analysis, PEP-PtdCh was 68% higher in the liver of subjects with gallbladder infiltration of the volume of interest (VOI) compared with those without gallbladder infiltration. PEP-PtdCh was also significantly higher in the liver of cholecystectomy patients compared with volunteers without gallbladder infiltration, which suggests increased intrahepatic bile fluid as a compensation for gall bladder removal. These results show that liver PtdCh is the major component of the resonance at 2.06 ppm and that careful VOI positioning is mandatory to avoid interference from the gallbladder.

Triptolide improves systolic function and myocardial energy metabolism of diabetic cardiomyopathy in streptozotocin-induced diabetic rats

BACKGROUND

Triptolide treatment leads to an improvement in Diabetic Cardiomyopathy (DCM) in streptozotocin-induced diabetic rat model. DCM is characterized by abnormal cardiac energy metabolism. We hypothesized that triptolide ameliorated cardiac metabolic abnormalities in DCM. We proposed (31)P nuclear magnetic resonance ((31)P NMR) spectrometry method for assessing cardiac energy metabolism in vivo and evaluating the effect of triptolide treatment in DCM rats.

METHODS

Six weeks triptolide treatment was conducted on streptozotocin-induced diabetic rats with dose of 100, 200 or 400 μg/kg/day respectively. Sex- and age-matched non-diabetic rats were used as control group. Cardiac chamber dimension and function were determined with echocardiography. Whole heart preparations were perfused with Krebs-Henseleit buffer and (31)P NMR spectroscopy was performed. Cardiac p38 Mitogen Activating Protein Kinase (MAPK) was measured using real time PCR and western blot analysis.

RESULTS

In diabetic rats, cardiac mass index was significantly higher, where as cardiac EF was lower than control group. (31)P NMR spectroscopy showed that ATP and pCr concentrations in diabetic groups were also remarkably lower than control group. Compared to non-treated diabetic rats, triptolide-treated diabetic groups showed remarkable lower cardiac mass index and higher EF, ATP, pCr concentrations, and P38 MAPK expressions. Best improvement was seen in group treated with Triptolide with dose 200 μg/kg/day.

CONCLUSIONS

(31)P NMR spectroscopy enables assessment of cardiac energy metabolism in whole heart preparations. It detects energy metabolic abnormalities in DCM hearts. Triptolide therapy improves cardiac function and increases cardiac energy metabolism at least partly through upregulation of MAPK signaling transduction.

Suppression of skeletal muscle signal using a crusher coil: A human cardiac (31) p-MR spectroscopy study at 7 tesla

Purpose

The translation of sophisticated phosphorus MR spectroscopy (³¹P‐MRS) protocols to 7 Tesla (T) is particularly challenged by the issue of radiofrequency (RF) heating. Legal limits on RF heating make it hard to reliably suppress signals from skeletal muscle that can contaminate human cardiac ³¹P spectra at 7T. We introduce the first surface‐spoiling crusher coil for human cardiac ³¹P‐MRS at 7T.

Methods

A planar crusher coil design was optimized with simulations and its performance was validated in phantoms. Crusher gradient pulses (100 μs) were then applied during human cardiac ³¹P‐MRS at 7T.

Results

In a phantom, residual signals were 50 ± 10% with BISTRO (B₁‐insensitive train to obliterate signal), and 34 ± 8% with the crusher coil. In vivo, residual signals in skeletal muscle were 49 ± 4% using BISTRO, and 24 ± 5% using the crusher coil. Meanwhile, in the interventricular septum, spectral quality and metabolite quantification did not differ significantly between BISTRO (phosphocreatine/adenosine triphosphate [PCr/ATP] = 2.1 ± 0.4) and the crusher coil (PCr/ATP = 1.8 ± 0.4). However, the specific absorption rate (SAR) decreased from 96 ± 1% of the limit (BISTRO) to 16 ± 1% (crusher coil).

Conclusion

A crusher coil is an SAR‐efficient alternative for selectively suppressing skeletal muscle during cardiac ³¹P‐MRS at 7T. A crusher coil allows the use of sequence modules that would have been SAR‐prohibitive, without compromising skeletal muscle suppression. Magn Reson Med 75:962–972, 2016. © 2015 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance.

Improving the hyperpolarization of (31)P nuclei by synthetic design

Traditional ³¹P NMR or MRI measurements suffer from low sensitivity relative to ¹H detection and consequently require longer scan times. We show here that hyperpolarization of ³¹P nuclei through reversible interactions with parahydrogen can deliver substantial signal enhancements in a range of regioisomeric phosphonate esters containing a heteroaromatic motif which were synthesized in order to identify the optimum molecular scaffold for polarization transfer. A 3588-fold ³¹P signal enhancement (2.34% polarization) was returned for a partially deuterated pyridyl substituted phosphonate ester. This hyperpolarization level is sufficient to allow single scan ³¹P MR images of a phantom to be recorded at a 9.4 T observation field in seconds that have signal-to-noise ratios of up to 94.4 when the analyte concentration is 10 mM. In contrast, a 12 h 2048 scan measurement under standard conditions yields a signal-to-noise ratio of just 11.4. ³¹P-hyperpolarized images are also reported from a 7 T preclinical scanner.

Phosphatidylcholine contributes to in vivo (31)P MRS signal from the human liver

OBJECTIVES

To demonstrate the overlap of the hepatic and bile phosphorus ((31)P) magnetic resonance (MR) spectra and provide evidence of phosphatidylcholine (PtdC) contribution to the in vivo hepatic (31)P MRS phosphodiester (PDE) signal, suggested in previous reports to be phosphoenolpyruvate (PEP).

METHODS

Phantom measurements to assess the chemical shifts of PEP and PtdC signals were performed at 7 T. A retrospective analysis of hepatic 3D (31)P MR spectroscopic imaging (MRSI) data from 18 and five volunteers at 3 T and 7 T, respectively, was performed. Axial images were inspected for the presence of gallbladder, and PDE signals in representative spectra were quantified.

RESULTS

Phantom experiments demonstrated the strong pH-dependence of the PEP chemical shift and proved the overlap of PtdC and PEP (~2 ppm relative to phosphocreatine) at hepatic pH. Gallbladder was covered in seven of 23 in vivo 3D-MRSI datasets. The PDE(gall)/γ-ATP(liver) ratio was 4.8-fold higher (p = 0.001) in the gallbladder (PDE(gall)/γ-ATP(liver) = 3.61 ± 0.79) than in the liver (PDE(liver)/γ-ATP(liver) = 0.75 ± 0.15). In vivo 7 T (31)P MRSI allowed good separation of PDE components. The gallbladder is a strong source of contamination in adjacent (31)P MR hepatic spectra due to biliary phosphatidylcholine.

CONCLUSIONS

In vivo (31)P MR hepatic signal at 2.06 ppm may represent both phosphatidylcholine and phosphoenolpyruvate, with a higher phosphatidylcholine contribution due to its higher concentration.

KEY POINTS

• In vivo (31)P MRS from the gallbladder shows a dominant biliary phosphatidylcholine signal at 2.06 ppm. • Intrahepatic (31)P MRS signal at 2.06 ppm may represent both intrahepatic phosphatidylcholine and phosphoenolpyruvate. • In vivo (31)P MRS has the potential to monitor hepatic phosphatidylcholine.

Depth-resolved surface coil MRS (DRESS)-localized dynamic (31) P-MRS of the exercising human gastrocnemius muscle at 7 T

Dynamic (31) P-MRS with sufficiently high temporal resolution enables the non-invasive evaluation of oxidative muscle metabolism through the measurement of phosphocreatine (PCr) recovery after exercise. Recently, single-voxel localized (31) P-MRS was compared with surface coil localization in a dynamic fashion, and was shown to provide higher anatomical and physiological specificity. However, the relatively long TE needed for the single-voxel localization scheme with adiabatic pulses limits the quantification of J-coupled spin systems [e.g. adenosine triphosphate (ATP)]. Therefore, the aim of this study was to evaluate depth-resolved surface coil MRS (DRESS) as an alternative localization method capable of free induction decay (FID) acquisition for dynamic (31) P-MRS at 7 T. The localization performance of the DRESS sequence was tested in a phantom. Subsequently, two dynamic examinations of plantar flexions at 25% of maximum voluntary contraction were conducted in 10 volunteers, one examination with and one without spatial localization. The DRESS slab was positioned obliquely over the gastrocnemius medialis muscle, avoiding other calf muscles. Under the same load, significant differences in PCr signal drop (31.2 ± 16.0% versus 43.3 ± 23.4%), end exercise pH (7.06 ± 0.02 versus 6.96 ± 0.11), initial recovery rate (0.24 ± 0.13 mm/s versus 0.35 ± 0.18 mm/s) and maximum oxidative flux (0.41 ± 0.14 mm/s versus 0.54 ± 0.16 mm/s) were found between the non-localized and DRESS-localized data, respectively. Splitting of the inorganic phosphate (Pi) signal was observed in several non-localized datasets, but in none of the DRESS-localized datasets. Our results suggest that the application of the DRESS localization scheme yielded good spatial selection, and provided muscle-specific insight into oxidative metabolism, even at a relatively low exercise load. In addition, the non-echo-based FID acquisition allowed for reliable detection of ATP resonances, and therefore calculation of the specific maximum oxidative flux, in the gastrocnemius medialis using standard assumptions about resting ATP concentration in skeletal muscle.

Measuring short-term liver metabolism non-invasively: postprandial and post-exercise ¹H and ³¹P MR spectroscopy

OBJECT

The objective of this study was to determine the effects of a standardized fat rich meal and subsequent exercise on liver fat content by ¹H MRS and on liver adenosine triphosphate (ATP) content by ³¹P MRS in healthy subjects.

MATERIALS AND METHODS

Hepatic ¹H and proton decoupled ³¹P MRS were performed on nine healthy subjects on a clinical 3.0 T MR imager three times during a day: after (1) an overnight fast, (2) a following standardized fat rich meal and (3) a subsequent exercise session. Blood parameters were followed during the day to serve as a reference to MRS.

RESULTS

Liver fat content increased gradually over the day (p = 0.0001) with an overall increase of 30 %. Also γ-NTP changed significantly over the day (p = 0.005). γ-NTP/tP decreased by 9 % (p = 0.019, post hoc) from the postprandial to the post-exercise state.

CONCLUSION

Our study shows that in vivo MRS can depict short lived physiological changes; entering of fat into liver cells and consumption of ATP during exercise can be measured non-invasively in healthy subjects. The physiological state may have an impact on fat and energy metabolite levels. Hepatic ¹H and ³¹P MRS studies should be performed under standardized conditions.

Application of localized ³¹P MRS saturation transfer at 7 T for measurement of ATP metabolism in the liver: reproducibility and initial clinical application in patients with non-alcoholic fatty liver disease

OBJECTIVES

Saturation transfer (ST) phosphorus MR spectroscopy ((31)P MRS) enables in vivo insight into energy metabolism and thus could identify liver conditions currently diagnosed only by biopsy. This study assesses the reproducibility of the localized (31)P MRS ST in liver at 7 T and tests its potential for noninvasive differentiation of non-alcoholic fatty liver (NAFL) and steatohepatitis (NASH).

METHODS

After the ethics committee approval, reproducibility of the localized (31)P MRS ST at 7 T and the biological variation of acquired hepato-metabolic parameters were assessed in healthy volunteers. Subsequently, 16 suspected NAFL/NASH patients underwent MRS measurements and diagnostic liver biopsy. The Pi-to-ATP exchange parameters were compared between the groups by a Mann-Whitney U test and related to the liver fat content estimated by a single-voxel proton ((1)H) MRS, measured at 3 T.

RESULTS

The mean exchange rate constant (k) in healthy volunteers was 0.31 ± 0.03 s(-1) with a coefficient of variation of 9.0 %. Significantly lower exchange rates (p < 0.01) were found in NASH patients (k = 0.17 ± 0.04 s(-1)) when compared to healthy volunteers, and NAFL patients (k = 0.30 ± 0.05 s(-1)). Significant correlation was found between the k value and the liver fat content (r = 0.824, p < 0.01).

CONCLUSIONS

Our data suggest that the (31)P MRS ST technique provides a tool for gaining insight into hepatic ATP metabolism and could contribute to the differentiation of NAFL and NASH.

KEY POINTS

• 1D localized (31) P MRS saturation transfer in the liver is reproducible at 7 T • NASH patients have decreased hepatic Pi-to-ATP exchange rate • In this study, hepatic metabolic activity correlates with liver fat content.