Jimin Ren

MRI detection of glycogen in vivo by using chemical exchange saturation transfer imaging (glycoCEST).

Detection of glycogen in vivo would have utility in the study of normal physiology and many disorders. Presently, the only magnetic resonance (MR) method available to study glycogen metabolism in vivo is 13C MR spectroscopy, but this technology is not routinely available on standard clinical scanners. Here, we show that glycogen can be detected indirectly through the water signal by using selective radio frequency (RF) saturation of the hydroxyl protons in the 0.5- to 1.5-ppm frequency range downfield from water. The resulting saturated spins are rapidly transferred to water protons via chemical exchange, leading to partial saturation of the water signal, a process now known as chemical exchange saturation transfer. This effect is demonstrated in glycogen phantoms at magnetic field strengths of 4.7 and 9.4 T, showing improved detection at higher field in adherence with MR exchange theory. Difference images obtained during RF irradiation at 1.0 ppm upfield and downfield of the water signal showed that glycogen metabolism could be followed in isolated, perfused mouse livers at 4.7 T before and after administration of glucagon. Glycogen breakdown was confirmed by measuring effluent glucose and, in separate experiments, by 13C NMR spectroscopy. This approach opens the way to image the distribution of tissue glycogen in vivo and to monitor its metabolism rapidly and noninvasively with MRI.

Composition of adipose tissue and marrow fat in humans by 1H NMR at 7 Tesla

Proton NMR spectroscopy at 7 Tesla (7T) was evaluated as a new method to quantify human fat composition noninvasively. In validation experiments, the composition of a known mixture of triolein, tristearin, and trilinolein agreed well with measurements by 1H NMR spectroscopy. Triglycerides in calf subcutaneous tissue and tibial bone marrow were examined in 20 healthy subjects by 1H spectroscopy. Ten well-resolved proton resonances from triglycerides were detected using stimulated echo acquisition mode sequence and small voxel (∼0.1 ml), and T1 and T2 were measured. Triglyceride composition was not different between calf subcutaneous adipose tissue and tibial marrow for a given subject, and its variation among subjects, as a result of diet and genetic differences, fell in a narrow range. After correction for differential relaxation effects, the marrow fat composition was 29.1 ± 3.5% saturated, 46.4 ± 4.8% monounsaturated, and 24.5 ± 3.1% diunsaturated, compared with adipose fat composition, 27.1 ± 4.2% saturated, 49.6 ± 5.7% monounsaturated, and 23.4 ± 3.9% diunsaturated. Proton spectroscopy at 7T offers a simple, fast, noninvasive, and painless method for obtaining detailed information about lipid composition in humans, and the sensitivity and resolution of the method may facilitate longitudinal monitoring of changes in lipid composition in response to diet, exercise, and disease.

Molecular Mechanisms of Hepatic Steatosis and Insulin Resistance in the AGPAT2-Deficient Mouse Model of Congenital Generalized Lipodystrophy

Mutations in 1-acylglycerol-3-phosphate-O-acyltransferase 2 (AGPAT2) cause congenital generalized lipodystrophy. To understand the molecular mechanisms underlying the metabolic complications associated with AGPAT2 deficiency, Agpat2 null mice were generated. Agpat2(-/-) mice develop severe lipodystrophy affecting both white and brown adipose tissue, extreme insulin resistance, diabetes, and hepatic steatosis. The expression of lipogenic genes and rates of de novo fatty acid biosynthesis were increased approximately 4-fold in Agpat2(-/-) mouse livers. The mRNA and protein levels of monoacylglycerol acyltransferase isoform 1 were markedly increased in the livers of Agpat2(-/-) mice, suggesting that the alternative monoacylglycerol pathway for triglyceride biosynthesis is activated in the absence of AGPAT2. Feeding a fat-free diet reduced liver triglycerides by approximately 50% in Agpat2(-/-) mice. These observations suggest that both dietary fat and hepatic triglyceride biosynthesis via a monoacylglycerol pathway may contribute to hepatic steatosis in Agpat2(-/-) mice.

A Concentration-Independent Method to Measure Exchange Rates in PARACEST Agents

The efficiency of chemical exchange dependent saturation transfer (CEST) agents is largely determined by their water or proton exchange kinetics, yet methods to measure such exchange rates are variable and many are not applicable to in vivo measurements. In this work, the water exchange kinetics of two prototype paramagnetic agents (PARACEST) are compared by using data from classic NMR line‐width measurements, by fitting CEST spectra to the Bloch equations modified for chemical exchange, and by a method where CEST intensity is measured as a function of applied amplitude of radiofrequency field. A relationship is derived that provides the water exchange rate from the X‐intercept of a plot of steady‐state CEST intensity divided by reduction in signal caused by CEST irradiation versus 1/ω12, referred to here as an omega plot. Furthermore, it is shown that this relationship is independent of agent concentration. Exchange rates derived from omega plots using either high‐resolution CEST NMR data or CEST data obtained by imaging agree favorably with exchange rates measured by the more commonly used Bloch fitting and line‐width methods. Thus, this new method potentially allows access to a direct measure of exchange rates in vivo, where the agent concentration is typically unknown. Magn Reson Med 63:625–632, 2010.

Imaging the tissue distribution of glucose in livers using a PARACEST sensor

Noninvasive imaging of glucose in tissues could provide important insights about glucose gradients in tissue, the origins of gluconeogenesis, or perhaps differences in tissue glucose utilization in vivo. Direct spectral detection of glucose in vivo by 1H NMR is complicated by interfering signals from other metabolites and the much larger water signal. One potential way to overcome these problems is to use an exogenous glucose sensor that reports glucose concentrations indirectly through the water signal by chemical exchange saturation transfer (CEST). Such a method is demonstrated here in mouse liver perfused with a Eu3+‐based glucose sensor containing two phenylboronate moieties as the recognition site. Activation of the sensor by applying a frequency‐selective presaturation pulse at 42 ppm resulted in a 17% decrease in water signal in livers perfused with 10 mM sensor and 10 mM glucose compared with livers with the same amount of sensor but without glucose. It was shown that livers perfused with 5 mM sensor but no glucose can detect glucose exported from hepatocytes after hormonal stimulation of glycogenolysis. CEST images of livers perfused in the magnet responded to changes in glucose concentrations demonstrating that the method has potential for imaging the tissue distribution of glucose in vivo. Magn Reson Med 60:1047–1055, 2008.

1H MRS of intramyocellular lipids in soleus muscle at 7 T: Spectral simplification by using long echo times without water suppression

The popular short echo time 1H MR spectroscopy acquisition method for detection of intramyocellular lipids suffers from spectral overlap due to the large, broad, and asymmetric extramyocellular lipid signals, the time‐consuming practice of selecting “lean” voxels for spectroscopy, and the overlap of the extramyocellular lipid signal with the creatine methyl 1H signal at ∼3 parts per million (ppm), commonly used as an internal standard. Using an alternative acquisition strategy, spectra with well‐resolved intramyocellular lipids resonances were acquired from large volumes (10 to 15 mL) of human soleus muscle in less than 5 min by single‐voxel 7‐T 1H MR spectroscopy, using an echo time of 280 ms. From the high‐resolution spectra, an average intramyocellular lipid concentration of 7.7 ± 3.5 mmol/kg muscle was found for 25 healthy subjects (male/female 17/8; age 29.4 ± 6.6 years). Since water suppression was not required, the 1H signals from unsaturated intracellular triglycerides at about 5.3 ppm were easily detected, which, in combination with the well‐determined (CH2)n/CH3 intensity ratio at long echo time, enabled assessment of the composition of triglycerides in the intramyocellular lipids compartment. Long‐echo single‐voxel spectroscopy at 7 T offers rapid and convenient acquisition of high‐resolution spectra from human soleus muscle. Magn Reson Med, 2010.

In vivo determination of human breast fat composition by 1H magnetic resonance spectroscopy at 7 T

The role of diet and fat consumption in the pathogenesis of breast cancer is an important subject. We report a method for noninvasive determination of lipid composition in human breast by proton magnetic resonance spectroscopy (MRS) at 7 T. Two respiratory‐triggered TE‐averaged stimulated echo acquisition mode (STEAM) acquisitions were performed on the adipose tissue of 10 healthy volunteers where the second acquisition had all gradients inverted. This acquisition protocol allows the suppression of modulation sidebands that complicate spectral analysis at the short TEavg = 24.5 ms. The entire acquisition takes ∼10 min. Ten lipid peaks were typically resolved. T1 and T2 were also measured and used to correct the peak intensities. The calculated average lipid composition for saturated was 28.7 ± 8.4%, monounsaturated, 48.5 ± 7.9%, and polyunsaturated, 22.7 ± 3.1%, in close agreement with reported values from subcutaneous adipose measurements. Intrasubject variability was 2.0, 1.6, and 3.6% for the saturated, monounsaturated, and polyunsaturated fractions, respectively. In conclusion, we have shown that a chemical analysis of lipids in breast tissue can be determined quite simply, quickly, and noninvasively by proton MRS at 7 T. Magn Reson Med, 2011.

31P‐MRS of healthy human brain: ATP synthesis, metabolite concentrations, pH, and T1 relaxation times

The conventional method for measuring brain ATP synthesis is 31P saturation transfer (ST), a technique typically dependent on prolonged pre‐saturation with γ‐ATP. In this study, ATP synthesis rate in resting human brain is evaluated using EBIT (exchange kinetics by band inversion transfer), a technique based on slow recovery of γ‐ATP magnetization in the absence of B1 field following co‐inversion of PCr and ATP resonances with a short adiabatic pulse. The unidirectional rate constant for the Pi → γ‐ATP reaction is 0.21 ± 0.04 s−1 and the ATP synthesis rate is 9.9 ± 2.1 mmol min−1 kg−1 in human brain (n = 12 subjects), consistent with the results by ST. Therefore, EBIT could be a useful alternative to ST in studying brain energy metabolism in normal physiology and under pathological conditions. In addition to ATP synthesis, all detectable 31P signals are analyzed to determine the brain concentration of phosphorus metabolites, including UDPG at around 10 ppm, a previously reported resonance in liver tissues and now confirmed in human brain. Inversion recovery measurements indicate that UDPG, like its diphosphate analogue NAD, has apparent T1 shorter than that of monophosphates (Pi, PMEs, and PDEs) but longer than that of triphosphate ATP, highlighting the significance of the 31P–31P dipolar mechanism in T1 relaxation of polyphosphates. Another interesting finding is the observation of approximately 40% shorter T1 for intracellular Pi relative to extracellular Pi, attributed to the modulation by the intracellular phosphoryl exchange reaction Pi ↔ γ‐ATP. The sufficiently separated intra‐ and extracellular Pi signals also permit the distinction of pH between intra‐ and extracellular environments (pH 7.0 versus pH 7.4). In summary, quantitative 31P MRS in combination with ATP synthesis, pH, and T1 relaxation measurements may offer a promising tool to detect biochemical alterations at early stages of brain dysfunctions and diseases. Copyright

Dynamic Monitoring of Carnitine and Acetylcarnitine in the Trimethylamine Signal after Exercise in Human Skeletal Muscle by 7T 1H MRS

A trimethylamine (TMA) moiety is present in carnitine and acetylcarnitine, and both molecules play critical roles in muscle metabolism. At 7 T, the chemical shift dispersion was sufficient to routinely resolve the TMA signals from carnitine at 3.20 and from acetylcarnitine at 3.17 ppm in the 1H‐MRS (Magnetic Resonance Spectroscopy) of human soleus muscle with a temporal resolution of about 2 min. In healthy, sedentary adults, the concentration of acetylcarnitine increased nearly 10‐fold, to 4.1 ± 1.0 mmol/kg, in soleus muscle after 5 min of calf‐raise exercise and recovered to a baseline concentration of 0.5 ± 0.3 mmol/kg. While the half‐time for decay of acetylcarnitine was the same whether measured from the TMA signal (18.8 ± 5.6 min) or measured from the methyl signal (19.4 ± 6.1 min), the detection of acetylcarnitine by its TMA signal in soleus has the advantage of higher sensitivity and without overlapping from lipid signals. Although the activity of carnitine acetyltransferase is sufficient to allow equilibrium between carnitine and coenzyme‐A pools, the exchange in TMA signal between carnitine and acetylcarnitine is slow in soleus following exercise on 7T 1H‐NMR time scale. The TMA signal provides a simple and direct measure of the relative amounts of carnitine and acetylcarnitine. Magn Reson Med, 2013.

Orientation of lipid strands in the extracellular compartment of muscle: Effect on quantitation of intramyocellular lipids

Single‐voxel 1H NMR spectra from gastrocnemius and soleus muscle were acquired in healthy volunteers at 7T with the objective of measuring the concentration of intramyocellular lipid [IMCL] (note: throughout this article, square brackets indicate concentration). However, significant asymmetry in the resonance assigned to the methylene protons (‐CH2‐)n in extramyocellular lipids (EMCL) interfered with fitting the spectra. Since muscle fibers in these tissues are generally not parallel to B0, the influence of variable orientation in strands of extracellular fat was examined using a mathematical model. Modest variation in orientation produced asymmetric lineshapes that were qualitatively similar to typical observations at 7T. Analysis of simulated spectra by fitting with a Voigt function overestimated [IMCL]/[EMCL] except when EMCL fibers were nearly parallel to B0. Estimates of [IMCL]/[EMCL] were improved by including variations in fiber orientation in the lineshape analysis (fiber orientation modeling, or FOM). Calculated [IMCL] using FOM, 4.8 ± 2.2 mmol/kg wet weight, was lower compared to most previous reports in soleus. Magn Reson Med 61:16–21, 2009.