Functional activation of pyruvate dehydrogenase in human brain using hyperpolarized [1-13 C]pyruvate.

PURPOSE: Pyruvate, produced from either glucose, glycogen, or lactate, is the dominant precursor of cerebral oxidative metabolism. Pyruvate dehydrogenase (PDH) flux is a direct measure of cerebral mitochondrial function and metabolism. Detection of [13 C]bicarbonate in the brain from hyperpolarized [1-13 C]pyruvate using carbon-13 (13 C) MRI provides a unique opportunity for assessing PDH flux in vivo. This study is to assess changes in cerebral PDH flux in response to visual stimuli using in vivo 13 C MRS with hyperpolarized [1-13 C]pyruvate. METHODS: From seven sedentary adults in good general health, time-resolved [13 C]bicarbonate production was measured in the brain using 90° flip angles with minimal perturbation of its precursors, [1-13 C]pyruvate and [1-13 C]lactate, to test the hypothesis that the appearance of [13 C]bicarbonate signals in the brain reflects the metabolic changes associated with neuronal activation. With a separate group of healthy participants (n = 3), the likelihood of the bolus-injected [1-13 C]pyruvate being converted to [1-13 C]lactate prior to decarboxylation was investigated by measuring [13 C]bicarbonate production with and without [1-13 C]lactate saturation. RESULTS: In the course of visual stimulation, the measured [13 C]bicarbonate signal normalized to the total 13 C signal in the visual cortex increased by 17.1% ± 15.9% (p = 0.017), whereas no significant change was detected in [1-13 C]lactate. Proton BOLD fMRI confirmed the regional activation in the visual cortex with the stimuli. Lactate saturation decreased bicarbonate-to-pyruvate ratio by 44.4% ± 9.3% (p < 0.01). CONCLUSION: We demonstrated the utility of 13 C MRS with hyperpolarized [1-13 C]pyruvate for assessing the activation of cerebral PDH flux via the detection of [13 C]bicarbonate production.

Dual-phase imaging of cardiac metabolism using hyperpolarized pyruvate.

PURPOSE: Previous cardiac imaging studies using hyperpolarized (HP) [1-13 C]pyruvate were acquired at end-diastole (ED). Little is known about the interaction between cardiac cycle and metabolite content in the myocardium. In this study, we compared images of HP pyruvate and products at end-systole (ES) and ED. METHODS: A dual-phase 13 C MRI sequence was implemented to acquire two sequential HP images within a single cardiac cycle at ES and ED during successive R-R intervals in an interleaved manner. Each healthy volunteer (N = 3) received two injections of HP [1-13 C]pyruvate for the dual-phase imaging on the short-axis and the vertical long-axis planes. Spatial distribution of HP 13 C metabolites at each cardiac phase was correlated to multiphase 1 H MRI to confirm the mechanical changes. Ratios of myocardial HP metabolites were compared between ES and ED. Segmental analysis was performed on the midcavity short-axis plane. RESULTS: In addition to mechanical changes, metabolic profiles of the heart detected by HP [1-13 C]pyruvate differed between ES and ED. The myocardial signal of [13 C]bicarbonate relative to [1-13 C]lactate was significantly smaller at ED than the ratio at ES (p < .05), particularly in mid-anterior and mid-inferoseptal segments. The distinct metabolic profiles in the myocardium likely reflect the technical aspects of the imaging approach such as the coronary flow in addition to the cyclical changes in metabolism. CONCLUSION: The study demonstrates that metabolic profiles of the heart, measured by HP [1-13 C]pyruvate, are affected by the cardiac cycle in which that the data are acquired.

Dynamic 13 C MR spectroscopy as an alternative to imaging for assessing cerebral metabolism using hyperpolarized pyruvate in humans.

PURPOSE: This study is to investigate time-resolved 13 C MR spectroscopy (MRS) as an alternative to imaging for assessing pyruvate metabolism using hyperpolarized (HP) [1-13 C]pyruvate in the human brain. METHODS: Time-resolved 13 C spectra were acquired from four axial brain slices of healthy human participants (n = 4) after a bolus injection of HP [1-13 C]pyruvate. 13 C MRS with low flip-angle excitations and a multichannel 13 C/1 H dual-frequency radiofrequency (RF) coil were exploited for reliable and unperturbed assessment of HP pyruvate metabolism. Slice-wise areas under the curve (AUCs) of 13 C-metabolites were measured and kinetic analysis was performed to estimate the production rates of lactate and HCO3- . Linear regression analysis between brain volumes and HP signals was performed. Region-focused pyruvate metabolism was estimated using coil-wise 13 C reconstruction. Reproducibility of HP pyruvate exams was presented by performing two consecutive injections with a 45-minutes interval. RESULTS: [1-13 C]Lactate relative to the total 13 C signal (tC) was 0.21-0.24 in all slices. [13 C] HCO3- /tC was 0.065-0.091. Apparent conversion rate constants from pyruvate to lactate and HCO3- were calculated as 0.014-0.018 s-1 and 0.0043-0.0056 s-1 , respectively. Pyruvate/tC and lactate/tC were in moderate linear relationships with fractional gray matter volume within each slice. White matter presented poor linear regression fit with HP signals, and moderate correlations of the fractional cerebrospinal fluid volume with pyruvate/tC and lactate/tC were measured. Measured HP signals were comparable between two consecutive exams with HP [1-13 C]pyruvate. CONCLUSIONS: Dynamic MRS in combination with multichannel RF coils is an affordable and reliable alternative to imaging methods in investigating cerebral metabolism using HP [1-13 C]pyruvate.

Preoperative imaging of glioblastoma patients using hyperpolarized 13C pyruvate: Potential role in clinical decision making.

BACKGROUND: Glioblastoma remains incurable despite treatment with surgery, radiation therapy, and cytotoxic chemotherapy, prompting the search for a metabolic pathway unique to glioblastoma cells.13C MR spectroscopic imaging with hyperpolarized pyruvate can demonstrate alterations in pyruvate metabolism in these tumors. METHODS: Three patients with diagnostic MRI suggestive of a glioblastoma were scanned at 3 T 1-2 days prior to tumor resection using a 13C/1H dual-frequency RF coil and a 13C/1H-integrated MR protocol, which consists of a series of 1H MR sequences (T2 FLAIR, arterial spin labeling and contrast-enhanced [CE] T1) and 13C spectroscopic imaging with hyperpolarized [1-13C]pyruvate. Dynamic spiral chemical shift imaging was used for 13C data acquisition. Surgical navigation was used to correlate the locations of tissue samples submitted for histology with the changes seen on the diagnostic MR scans and the 13C spectroscopic images. RESULTS: Each tumor was histologically confirmed to be a WHO grade IV glioblastoma with isocitrate dehydrogenase wild type. Total hyperpolarized 13C signals detected near the tumor mass reflected altered tissue perfusion near the tumor. For each tumor, a hyperintense [1-13C]lactate signal was detected both within CE and T2-FLAIR regions on the 1H diagnostic images (P = .008). [13C]bicarbonate signal was maintained or decreased in the lesion but the observation was not significant (P = .3). CONCLUSIONS: Prior to surgical resection, 13C MR spectroscopic imaging with hyperpolarized pyruvate reveals increased lactate production in regions of histologically confirmed glioblastoma.

Hyperpolarized 13C MR Spectroscopy Depicts in Vivo Effect of Exercise on Pyruvate Metabolism in Human Skeletal Muscle.

Background Pyruvate dehydrogenase (PDH) and lactate dehydrogenase are essential for adenosine triphosphate production in skeletal muscle. At the onset of exercise, oxidation of glucose and glycogen is quickly enabled by dephosphorylation of PDH. However, direct measurement of PDH flux in exercising human muscle is daunting, and the net effect of covalent modification and other control mechanisms on PDH flux has not been assessed. Purpose To demonstrate the feasibility of assessing PDH activation and changes in pyruvate metabolism in human skeletal muscle after the onset of exercise using carbon 13 (13C) MRI with hyperpolarized (HP) [1-13C]-pyruvate. Materials and Methods For this prospective study, sedentary adults in good general health (mean age, 42 years ± 18 [standard deviation]; six men) were recruited from August 2019 to September 2020. Subgroups of the participants were injected with HP [1-13C]-pyruvate at resting, during plantar flexion exercise, or 5 minutes after exercise during recovery. In parallel, hydrogen 1 arterial spin labeling MRI was performed to estimate muscle tissue perfusion. An unpaired t test was used for comparing 13C data among the states. Results At rest, HP [1-13C]-lactate and [1-13C]-alanine were detected in calf muscle, but [13C]-bicarbonate was negligible. During moderate flexion-extension exercise, total HP 13C signals (tC) increased 2.8-fold because of increased muscle perfusion (P = .005), and HP [1-13C]-lactate-to-tC ratio increased 1.7-fold (P = .04). HP [13C]-bicarbonate-to-tC ratio increased 8.4-fold (P = .002) and returned to the resting level 5 minutes after exercise, whereas the lactate-to-tC ratio continued to increase to 2.3-fold as compared with resting (P = .008). Conclusion Lactate and bicarbonate production from hyperpolarized (HP) [1-carbon 13 {13C}]-pyruvate in skeletal muscle rapidly reflected the onset and the termination of exercise. These results demonstrate the feasibility of imaging skeletal muscle metabolism using HP [1-13C]-pyruvate MRI and the sensitivity of in vivo pyruvate metabolism to exercise states. © RSNA, 2021 Online supplemental material is available for this article.

Cardiac T2∗ measurement of hyperpolarized 13 C metabolites using metabolite-selective multi-echo spiral imaging.

PURPOSE: Noninvasive imaging with hyperpolarized (HP) pyruvate can capture in vivo cardiac metabolism. For proper quantification of the metabolites and optimization of imaging parameters, understanding MR characteristics such as T2∗ s of the HP signals is critical. This study is to measure in vivo cardiac T2∗ s of HP [1-13 C]pyruvate and the products in rodents and humans. METHODS: A dynamic 13 C multi-echo spiral imaging sequence that acquires [13 C]bicarbonate, [1-13 C]lactate, and [1-13 C]pyruvate images in an interleaved manner was implemented for a clinical 3 Tesla system. T2∗ of each metabolite was calculated from the multi-echo images by fitting the signal decay of each region of interest mono-exponentially. The performance of measuring T2∗ using the sequence was first validated using a 13 C phantom and then with rodents following a bolus injection of HP [1-13 C]pyruvate. In humans, T2∗ of each metabolite was calculated for left ventricle, right ventricle, and myocardium. RESULTS: Cardiac T2∗ s of HP [1-13 C]pyruvate, [1-13 C]lactate, and [13 C]bicarbonate in rodents were measured as 24.9 ± 5.0, 16.4 ± 4.7, and 16.9 ± 3.4 ms, respectively. In humans, T2∗ of [1-13 C]pyruvate was 108.7 ± 22.6 ms in left ventricle and 129.4 ± 8.9 ms in right ventricle. T2∗ of [1-13 C]lactate was 40.9 ± 8.3, 44.2 ± 5.5, and 43.7 ± 9.0 ms in left ventricle, right ventricle, and myocardium, respectively. T2∗ of [13 C]bicarbonate in myocardium was 64.4 ± 2.5 ms. The measurements were reproducible and consistent over time after the pyruvate injection. CONCLUSION: The proposed metabolite-selective multi-echo spiral imaging sequence reliably measures in vivo cardiac T2∗ s of HP [1-13 C]pyruvate and products.

Characterization and compensation of f0 inhomogeneity artifact in spiral hyperpolarized 13 C imaging of the human heart.

PURPOSE: This study aimed to investigate the role of regional f0 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 f0 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 to those acquired with the short waveform (predicted 1.32). Improvement of image quality was observed for all metabolites with f0 correction. CONCLUSIONS: f0 -map correction reduced blur and recovered signal from dropouts, particularly along the posterior myocardial wall. The low image SNR of [13 C]bicarbonate can be compensated with longer duration readouts but at the expense of increased f0 artifacts, which can be partially corrected for with the proposed methods.

Imaging Acute Metabolic Changes in Patients with Mild Traumatic Brain Injury Using Hyperpolarized [1-13C]Pyruvate.

Traumatic brain injury (TBI) involves complex secondary injury processes following the primary injury. The secondary injury is often associated with rapid metabolic shifts and impaired brain function immediately after the initial tissue damage. Magnetic resonance spectroscopic imaging (MRSI) coupled with hyperpolarization of 13C-labeled substrates provides a unique opportunity to map the metabolic changes in the brain after traumatic injury in real-time without invasive procedures. In this report, we investigated two patients with acute mild TBI (Glasgow coma scale 15) but no anatomical brain injury or hemorrhage. Patients were imaged with hyperpolarized [1-13C]pyruvate MRSI 1 or 6 days after head trauma. Both patients showed significantly reduced bicarbonate (HCO3 -) production, and one showed hyperintense lactate production at the injured sites. This study reports the feasibility of imaging altered metabolism using hyperpolarized pyruvate in patients with TBI, demonstrating the translatability and sensitivity of the technology to cerebral metabolic changes after mild TBI.

Hyperpolarized δ-[1-13 C]gluconolactone as a probe of the pentose phosphate pathway.

The pentose phosphate pathway (PPP) is thought to be upregulated in trauma (to produce excess NADPH) and in cancer (to provide ribose for nucleotide biosynthesis), but simple methods for detecting changes in flux through this pathway are not available. MRI of hyperpolarized 13 C-enriched metabolites offers considerable potential as a rapid, non-invasive tool for detecting changes in metabolic fluxes. In this study, hyperpolarized δ-[1-13 C]gluconolactone was used as a probe to detect flux through the oxidative portion of the pentose phosphate pathway (PPPox ) in isolated perfused mouse livers. The appearance of hyperpolarized (HP) H13 CO3- within seconds after exposure of livers to HP-δ-[1-13 C]gluconolactone demonstrates that this probe rapidly enters hepatocytes, becomes phosphorylated, and enters the PPPox pathway to produce HP-H13 CO3- after three enzyme catalyzed steps (6P-gluconolactonase, 6-phosphogluconate dehydrogenase, and carbonic anhydrase). Livers perfused with octanoate as their sole energy source show no change in production of H13 CO3- after exposure to low levels of H2 O2 , while livers perfused with glucose and insulin showed a twofold increase in H13 CO3- after exposure to peroxide. This indicates that flux through the PPPox is stimulated by H2 O2 in glucose perfused livers but not in livers perfused with octanoate alone. Subsequent perfusion of livers with non-polarized [1,2-13 C]glucose followed by 1 H NMR analysis of lactate in the perfusate verified that flux through the PPPox is indeed low in healthy livers and modestly higher in peroxide damaged livers. We conclude that hyperpolarized δ-[1-13 C]gluconolactone has the potential to serve as a metabolic imaging probe of this important biological pathway.