C NMR is a powerful tool for monitoring metabolic fluxes in vivo.The recent availability of automated dynamic nuclear polarization equipment for hyperpolarizing 13 C nuclei now offers the potential to measure metabolic fluxes through select enzyme-catalyzed steps with substantially improved sensitivity. Here, we investigated the metabolism of hyperpolarized [1-13 C1]pyruvate in a widely used model for physiology and pharmacology, the perfused rat heart. Dissolved 13 CO2, the immediate product of the first step of the reaction catalyzed by pyruvate dehydrogenase, was observed with a temporal resolution of Ϸ1 s along with H 13 CO 3 ؊ , the carbon dioxide ͉ heart ͉ hyperpolarization ͉ NMR spectroscopy ͉ pyruvate N oninvasive measures of flux through specific enzymecatalyzed reactions remain an important goal in physiology and clinical medicine. Standard radionuclide imaging methods do not provide information about individual reactions because the measured signal represents the weighted sum of the tracer plus the biochemical products produced by tissue. 13 C NMR spectroscopy is much more powerful in this regard because it can easily differentiate between specific 13 C-labeled products of biochemical reactions (1). The sensitivity enhancement gained by hyperpolarization of 13 C nuclei (2) offers the possibility of using noninvasive 13 C NMR spectroscopy and imaging to measure f luxes through individual enzyme-catalyzed reactions.[1-13 C 1 ]Pyruvate, for example, is a substrate that is avidly metabolized in most tissues. The individual metabolic products of this tracer, [1-13 C 1 ]lactate, [1-13 C 1 ]alanine, and H 13 CO 3 Ϫ , can be separately detected in vivo because of the inherent chemical shift dispersion of 13 C NMR (3). Because decarboxylation of [1-13 C 1 ]pyruvate via pyruvate dehydrogenase (PDH) must produce 13 CO 2 , the appearance of H 13 CO 3 Ϫ potentially directly reflects flux through PDH. A substantially reduced H 13 CO 3 Ϫ signal in hearts after coronary occlusion and reflow was previously attributed to an effect of transient ischemia on the tricarboxylic acid (TCA) cycle (3). However, it is known that the heart switches rapidly among a wide variety of substrates to supply acetyl-CoA (4, 5), and because fats and ketones are not metabolized via PDH the rate of production of H 13 CO 3 Ϫ from [1-13 C 1 ]pyruvate in vivo should be sensitive to the availability of other substrates.Noninvasive detection of flux through PDH would unquestionably be of value in understanding potentially high-impact therapies for heart disease. Pharmacological and metabolic interventions that would be expected to increase flux through PDH have been examined since the 1960s with the goal of protecting ischemic myocardium (6-9) or improving function in the failing heart (10, 11). However, because of the complex interconnections between fatty acid and carbohydrate oxidation, it has been difficult to separate effects of metabolism through a specific reaction, PDH, from effects on oxygen consumption, myocardial efficiency, and overa...

In the heart, detection of hyperpolarized [ 13 H epatic metabolism is comprised of a network of exergonic reactions that facilitate energy capture, such as fatty acid oxidation, carbohydrate oxidation and flux through the tricarboxylic acid (TCA) cycle, and endergonic reactions required for gluconeogenesis, lipogenesis, and amino acid biosynthesis. The dysregulation of these pathways underlie the pathology of a variety of diseases, including diabetes, cancer, and inborn errors of metabolism, making their diagnostic evaluation critically important. In particular, the hepatic TCA cycle is an important diagnostic target because its intermediates integrate lipid, carbohydrate, and amino acid metabolism during normal physiology. As examples, oxaloacetate is the common intermediate by which all noncarbohydrate sources enter gluconeogenesis; citrate is required to shuttle acetyl-CoA from the mitochondria for cytosolic lipogenesis; and the ketoacids of the TCA cycle serve as intermediates of amino acid synthesis and catabolism. The interconnectivity of these hepatic pathways makes their examination difficult but worthwhile because technologies that detect these fluxes provide important advances for basic and clinical investigation of mechanisms of disease.Dynamic nuclear polarization (DNP) of 13 C is a powerful molecular imaging technology that uses principles of magnetic resonance (MR) (1). Prepolarization of 13 C-labeled tracers enhances the sensitivity of MR by 10,000-fold or more and enables studies of cancer (2), hepatic metabolism (3, 4), and cardiac metabolism (5-7). In the heart, metabolism of hyperpolarized [1-13 C]pyruvate to 13 CO 2 and [ 13 C]bicarbonate is caused exclusively by flux through the pyruvate dehydrogenase complex (PDH). Consequently, 13 CO 2 detection is not necessarily indicative of flux in the TCA cycle, because non-PDH sources of acetyl-CoA (e.g., fat oxidation) dominate the metabolism of certain tissues, like the liver. Recently, DNP was used to observe altered cytosolic redox state in liver following ethanol administration (3), and hyperpolarized bicarbonate was observed in vivo after administration of [1-13 C]lactate, but analysis of oxidative versus biosynthetic metabolism in the liver has not been reported. In contrast to the heart, hepatic anaplerosis is four-to sixfold higher than acetyl-CoA oxidation (8)(9)(10) C]pyruvate, followed by rapid but incomplete "backwards" equilibration with the symmetric intermediate, fumarate. The polarization profiles of these hepatic metabolites changed as anticipated during feeding and fasting. Signal from hyperpolarized [ 13 C]bicarbonate was also observed. A 13 C isotopomer analysis of glutamate isolated from separate livers demonstrated that flux through PEPCK is ∼sevenfold higher than flux through PDH, indicating that flux through PEPCK may be a significant source of the The authors declare no conflict of interest.

This study aimed to investigate the role of regional f 0 inhomogeneity in spiral hyperpolarized 13 C image quality and to develop measures to alleviate these effects. Methods: Field map correction of hyperpolarized 13 C cardiac imaging using spiral readouts was evaluated in healthy subjects. Spiral readouts with differing duration (26 and 45 ms) but similar resolution were compared with respect to off-resonance performance and image quality. An f 0 map-based image correction based on the multifrequency interpolation (MFI) method was implemented and compared to correction using a global frequency shift alone. Estimation of an unknown frequency shift was performed by maximizing a sharpness objective based on the Sobel variance. The apparent full width half at maximum (FWHM) of the myocardial wall on [ 13 C]bicarbonate was used to estimate blur. Results: Mean myocardial wall FWHM measurements were unchanged with the short readout pre-correction (14.1 ± 2.9 mm) and post-MFI correction (14.1 ± 3.4 mm), but significantly decreased in the long waveform (20.6 ± 6.6 mm uncorrected, 17.7 ± 7.0 corrected, P = .007). Bicarbonate signal-to-noise ratio (SNR) of the images acquired with the long waveform were increased by 1.4 ± 0.3 compared 158 | REED Et al. How to cite this article: Reed GD, Ma J, Park JM, et al. Characterization and compensation of f 0 inhomogeneity artifact in spiral hyperpolarized 13 C imaging of the human heart.

The activity of specific enzyme-catalyzed reactions may be detected in vivo by 13C NMR of hyperpolarized (HP) substrates. The signal from HP substrates and products, acquired over time, have been fit to a number of different mathematical models to determine fluxes, but these models have not been critically compared. In this study, two-pool and three-pool first-order models were constructed to measure flux through lactate dehydrogenase in isolated glioblastoma cells by NMR detection of lactate and pyruvate following addition of hyperpolarized [1-13C]pyruvate. Mass spectrometry (MS) was used to independently monitor 13C enrichment in intra- and extracellular lactate. Six models were evaluated using time dependent pyruvate C2 and lactate C1 HP NMR data acquired by use of selective excitation pulses plus 13C enrichment data from intracellular and extracellular lactate measured by MS. A three-pool bi-directional model provided the most accurate description of pyruvate metabolism in these cells. With computed values for the T1 of pyruvate and lactate as well as the effect of pulsing, the initial flux through lactate dehydrogenase (LDH) was well-determined by both the two-pool bidirectional and unidirectional models when only HP data was available. The three-pool model was necessary to fit the combined data from both MS and HP, but the simpler two-pool exchange model was sufficient to determine the 13C lactate concentration when the lactate appearance was measured only by HP.