In vivo proton magnetic resonance spectroscopy of liver metabolites in non-alcoholic fatty liver disease in rats: T2 relaxation times in methylene protons

Feasibility of omega-3 fatty acid fraction mapping using chemical shift encoding-based imaging at 3 T

PURPOSE

The aim of this work is to develop an ω-3 fatty acid fraction mapping method at 3 T based on a chemical shift encoding model, to assess its performance in a phantom and in vitro study, and to further demonstrate its feasibility in vivo.

METHODS

A signal model was heuristically derived based on spectral appearance and theoretical considerations of the corresponding molecular structures to differentiate between ω-3 and non-ω-3 fatty acid substituents in triacylglycerols in addition to the number of double bonds (ndb), the number of methylene-interrupted double bonds (nmidb), and the mean fatty acid chain length (CL). First, the signal model was validated using single-voxel spectroscopy and a time-interleaved multi-echo gradient-echo (TIMGRE) sequence in gas chromatography-mass spectrometry (GC-MS)-calibrated oil phantoms. Second, the TIMGRE-based method was validated in vitro in 21 adipose tissue samples with corresponding GC-MS measurements. Third, an in vivo feasibility study was performed for the TIMGRE-based method in the gluteal region of two healthy volunteers. Phantom and in vitro data was analyzed using a Bland-Altman analysis.

RESULTS

Compared with GC-MS, MRS showed in the phantom study significant correlations in estimating the ω-3 fraction (p < 0.001), ndb (p < 0.001), nmidb (p < 0.001), and CL (p = 0.001); MRI showed in the phantom study significant correlations (all p < 0.001) for the ω-3 fraction, ndb, and nmidb, but no correlation for CL. Also in the in vitro study, significant correlations (all p < 0.001) between MRI and GC-MS were observed for the ω-3 fraction, ndb, and nmidb, but not for CL. An exemplary ROI measurement in vivo in the gluteal subcutaneous adipose tissue yielded (mean ± standard deviation) 0.8% ± 1.9% ω-3 fraction.

CONCLUSION

The present study demonstrated strong correlations between gradient-echo imaging-based ω-3 fatty acid fraction mapping and GC-MS in the phantom and in vitro study. Furthermore, feasibility was demonstrated for characterizing adipose tissue in vivo.

Detection and alterations of acetylcarnitine (AC) in human liver by 1 H MRS at 3T after supplementation with l-carnitine

PURPOSE

Acetylcarnitine can be assessed in vivo using proton MRS (1 H-MRS) with long TEs and this has been previously applied successfully in muscle. The aim of this study was to evaluate a 1 H-MRS technique for liver acetylcarnitine quantification in healthy humans before and after l-carnitine supplementation.

METHOD

Baseline acetylcarnitine levels were quantified using a STEAM sequence with prolonged TE in 15 healthy adults. Using STEAM with four different TEs was evaluated in phantoms. To assess reproducibility of the measurements, five of the participants had repeated 1 H-MRS without receiving l-carnitine supplementation. To determine if liver acetylcarnitine could be changed after l-carnitine supplementation, acetylcarnitine was quantified 2 h after intravenous l-carnitine supplementation (50 mg/kg body weight) in the other 10 participants. Hepatic lipids were also quantified from the 1 H-MRS spectra.

RESULTS

There was good separation between the acetylcarnitine and fat in the phantoms using TE = 100 ms. Hepatic acetylcarnitine levels were reproducible (coefficient of reproducibility = 0.049%) and there was a significant (p < 0.001) increase in the relative abundance after a single supplementation of l-carnitine. Hepatic allylic, methyl, and methylene peaks were not altered by l-carnitine supplementation in healthy volunteers.

CONCLUSION

Our results demonstrate that our 1 H-MRS technique could be used to measure acetylcarnitine in the liver and detect changes following intravenous supplementation in healthy adults despite the presence of lipids. Our techniques should be explored further in the study of fatty liver disease, where acetylcarnitine is suggested to be altered due to hepatic inflexibilities.

Single-voxel short-TR multi-TI multi-TE STEAM MRS for water-fat relaxometry

PURPOSE

To propose a short-TR multi-TI multi-TE (SHORTIE, ['shȯr-tē]) STEAM single-voxel MRS acquisition scheme for the simultaneous assessment of T₁ relaxation, T₂ relaxation, and the proton density fat fraction at reduced scan times when compared with conventional long-TR multi-TI STEAM and long-TR multi-TE STEAM single-voxel MRS.

METHODS

Theoretical analysis for multi-TI (TI = 10, 100, 500, 1500 ms; scan time = 2:43 minutes), multi-TE (TE = 12, 15, 20, 25 ms; scan time = 2:24 minutes), and SHORTIE STEAM (all TI and TE combinations; scan time = 2:52 minutes) was carried out including Cramér-Rao lower bound and parameter estimation efficiency analysis for T₁ (150-2000 ms) and T₂ (5-150 ms) relaxation. The SHORTIE STEAM acquisition was compared with multi-TI STEAM and multi-TE STEAM in water-fat phantoms and in a human in vivo study of the adipose tissue depot in the supraclavicular fossa in 7 volunteers at 3 T.

RESULTS

Cramér-Rao lower bound analysis revealed similar to increased variances for T₁ and T₂ estimators for SHORTIE STEAM. Parameter efficiency analysis demonstrated superior performance of SHORTIE, particularly for shorter T₁ and T₂ when compared with multi-TI STEAM and multi-TE STEAM. For the phantom data, linear regression and Bland-Altmann analysis yielded a slope/intercept/mean difference of 1.07/-15.40/-17.18 for T₁ (in ms; r = 0.999), 0.93/+1.32/+1.09 for T₂ (in ms; r = 0.995), and 0.98/-0.04/+0.78 for the fat fraction (in percent; r = 0.999); and for the in vivo data 1.08/+1.77/-62.2 for T₁ (r = 0.994), 0.88/+6.69/-1.55 for T₂ (r = 0.884), and 0.56/+34.40/-0.46 for the fat fraction (r = 0.673), respectively.

CONCLUSION

The SHORTIE STEAM acquisition allows shorter scan times for the simultaneous probing of relaxation properties and spectral content in the water-fat environment when compared with combined long-TR multi-TI, and long-TR multi-TE STEAM.

Study of lipid proton difference evaluation via 9.4T MRI analysis of fatty liver induced by exposure to methionine and choline-deficient (MCD) diet and high-fat diet (HFD) in an animal model

The selection of an animal model is based on the pathological mechanism appropriate for experimental investigation because the therapeutic effect was low depending on the pathological occurrence mechanism. The purpose of this study is to elucidate the changes in lipid proton concentration in two animal models of nonalcoholic fatty liver disease (NAFLD): methionine and choline-deficient (MCD) diet and high-fat diet (HFD). We calculated the T2 relaxation time of 7 lipid protons (LP) in the 9.4 T MRS phantom experiment. The concentrations of LPs were adjusted for T2 and T2* of MCD, HFD, and CCl₄ fatty liver animal models. Multivariate analysis and Pearson correlation were performed to analyze LP concentration, and the difference was investigated via Kendall correlation and independent t-test using LP composition ratio. The T2 relaxation time of each LP was accurately determined using phantom experiments. The in vivo magnetic resonance spectroscopy (MRS) data were obtained by quantifying the t2/t2* corrected LP concentration in the liver of the animal model. In case of MCD and HFD, there was an average difference in all LPs except 0.9 ppm LP, and the MCD and CCl₄ groups showed differences in the average of all LPs. However, there was no difference between LP of HFD and CCl₄ groups. A higher level of unsaturated fatty acids was found in the MCD fatty liver model than in HFD induced fatty liver.

Improved quantitative fatty acid values with correction of T2 relaxation time in terminal methyl group: In vivo proton magnetic resonance spectroscopy at ultra high field in hepatic steatosis

Proton magnetic resonance spectroscopy (MRS) with optimized relaxation time is an effective method to quantify hepatic fatty acid values and characterize steatosis. The aim of this study is to quantify the difference in hepatic lipid content with metabolic changes during the progression of steatosis by using localized MRS sequence with T₂ relaxation time determination. Fatty liver disease was induced in C57BL/6N mice through a high-fat diet (HFD) of pellets containing 60% fat, 20% protein, and 20% carbohydrates. We used stimulated echo acquisition mode (repetition time: 3500 ms; mixing time: 10 ms; echo time: 20 ms) sequence. Using enhanced and mono exponential curve-fitting methods, the lipid relaxation time in mice was estimated at a fixed repetition time of 5000 ms and echo time ranging from 20 to 70 ms. The calculated lipid contents with incorrect and correct relaxation times were as follows: total saturated fatty acid (4.00 ± 2.90 vs 6.74 ± 2.25, p < 0.05 at week 0; 15.23 ± 9.94 vs 25.53 ± 10.49, p < 0.05 at week 4); total unsaturated fatty acid (0.40 ± 0.49 vs 0.56 ± 0.47, p < 0.05 at week 4; 0.33 ± 0.26 vs 0.60 ± 0.21, p < 0.01 at week 7); total unsaturated bond (0.48 ± 0.52 vs 1.05 ± 0.58, p < 0.05 at week 10). Furthermore, we determined that the correct relaxation times of triglycerides between 0 and 10 weeks were significantly altered in the resonances (∼2.03 ppm: 31.07 ± 1.00 vs 27.62 ± 1.20, p < 0.01; ∼2.25 ppm: 29.10 ± 1.52 vs 26.39 ± 1.08, p < 0.05; ∼2.78 ppm: 37.67 ± 2.92 vs 29.37 ± 2.64, p < 0.001). The work presented focused on the significance of the J-coupling effect. The selection of an appropriate relaxation time considering the J-coupling effect provides an effective method for quantifying lipid contents and characterizing hepatic steatosis.

Time-course metabolic changes in high-fat diet-induced obesity rats: A pilot study using hyperpolarized (13)C dynamic MRS

The purpose of this study was to investigate the time-course metabolic changes based on hyperpolarized (13)C magnetic resonance spectroscopy (MRS) in high-fat diet (HFD)-induced obesity rats and the correlation between metabolic and serum enzyme levels. Sprague-Dawley rats were fed either HFD (60% fat) or normal diet (10% fat) for 6weeks. A HyperSense DNP was used to hyperpolarize [1-(13)C] pyruvic acid and the hyperpolarized (13)C MRS was examined every 2weeks in the course of 6weeks using a 3T GE MR750 scanner. The body weight of HFD-induced obese rats was significantly increased compared to normal rats at the 6th week after the onset of feeding (p=0.05). Simultaneously, the HFD-induced obese rats showed significantly increased levels of serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and low-density lipoprotein (LDL)-cholesterol compared to normal rats (p≤0.05). In the dynamic (13)C MR spectra acquired at the 6th week, the obese rats showed significantly increased ratios of [1-(13)C] lactate/[1-(13)C] pyruvate and [1-(13)C] alanine/[1-(13)C] pyruvate (p=0.05). The (13)C spectral outcomes are positively correlated with the enzyme levels of ALT and LDH in the HFD-induced obesity. The [1-(13)C] lactate and [1-(13)C] alanine are potentially considered as noninvasive biomarkers for the HFD-induced obesity.