Hyperpolarized 15N caffeine, a potential probe of liver function and perfusion.

PURPOSE: To demonstrate hyperpolarization of 15N-caffeine and report exploratory findings as a potential probe of liver function and perfusion. METHODS: An amorphous formulation of [1,3-15N2]caffeine was developed for hyperpolarization via dissolution dynamic nuclear polarization. Polarizer hardware was augmented to support monitoring of solid-state 15N MR signals during the buildup of hyperpolarization. Liquid state hyperpolarized 15N MR signals were obtained in a preclinical 3T magnet by interfacing an external spectrometer console with home-built RF surface coils. 15N signal decay constants were estimated in H2O and in vivo in liver and brain regions of rats at 3 T. Decays were also measured at 9.4 T to assess the effect of B0, and in the presence of albumin to assess the impact of protein binding. RESULTS: Polarization levels of 3.5% and aqueous T1 relaxation times of nearly 200 s were attained for both N1 and N3 positions at 3 T. Shorter apparent decay constants were observed in vivo, ranging from 25 s to 43 s, with modest extensions possible by exploiting competitive binding of iophenoxate with plasma albumin. Downstream products of caffeine could not be detected on in vivo 15N-MR spectra of the liver region, even with metabolic stimulation by ß $$ \beta $$ -naphthoflavone treatment. Considering the high perfusion rate of brain, persistence of caffeine signal in this region is consistent with potential value as a perfusion imaging agent. CONCLUSION: These results establish the feasibility of hyperpolarization of hyperpolarized 15N-caffeine, but further work is necessary to establish the role of this new agent to probe liver metabolism and perfusion.

A vendor-neutral EPI sequence for hyperpolarized 13C MRI.

PURPOSE: To develop a flexible, vendor-neutral EPI sequence for hyperpolarized 13C metabolic imaging. METHODS: An open-source EPI sequence consisting of a metabolite-specific spectral-spatial RF excitation pulse and a customizable EPI readout was created using the Pulseq framework. To explore the flexibility of our sequence, we tested several versions of the sequence including a symmetric 3D readout with different spatial resolutions for each metabolite (1.0 cm3 and 1.5 cm3). A multichamber phantom constructed with a Shepp-Logan geometry, containing two chambers filled with either natural abundance 13C compounds or hyperpolarized (HP) [1-13C]pyruvate, was used to test each sequence. For experiments involving HP [1-13C]pyruvate, a single chamber was prefilled with nicotinamide adenine dinucleotide hydride and lactate dehydrogenase to facilitate the conversion of [1-13C]pyruvate to [1-13C]lactate. All experiments were performed on a Siemens Prisma 3T scanner. RESULTS: All the sequence variations localized natural-abundance 13C ethylene glycol and methanol to the appropriate compartment of the multichamber phantom. [1-13C]pyruvate was detectable in both chambers following the injection of HP [1-13C]pyruvate, whereas [1-13C]lactate was only found in the chamber containing nicotinamide adenine dinucleotide hydride and lactate dehydrogenase. The conversion rate from [1-13C]pyruvate to [1-13C]lactate (kPL) was 0.01 s-1 (95% confidence interval [0.00, 0.02]). CONCLUSION: We have developed and tested a vendor-neutral EPI sequence for imaging HP 13C agents. We have made all of our sequence creation and image reconstruction code freely available online for other investigators to use.

Distinguishing metabolic signals of liver tumors from surrounding liver cells using hyperpolarized 13 C MRI and gadoxetate.

PURPOSE: To use the hepatocyte-specific gadolinium-based contrast agent gadoxetate combined with hyperpolarized (HP) [1-13 C]pyruvate MRI to selectively suppress metabolic signals from normal hepatocytes while preserving the signals arising from tumors. METHODS: Simulations were performed to determine the expected changes in HP 13 C MR signal in liver and tumor under the influence of gadoxetate. CC531 colon cancer cells were implanted into the livers of five Wag/Rij rats. Liver and tumor metabolism were imaged at 3 T using HP [1-13 C] pyruvate chemical shift imaging before and 15 min after injection of gadoxetate. Area under the curve for pyruvate and lactate were measured from voxels containing at least 75% of normal-appearing liver or tumor. RESULTS: Numerical simulations predicted a 36% decrease in lactate-to-pyruvate (L/P) ratio in liver and 16% decrease in tumor. In vivo, baseline L/P ratio was 0.44 ± 0.25 in tumors versus 0.21 ± 0.08 in liver (p = 0.09). Following administration of gadoxetate, mean L/P ratio decreased by an average of 0.11 ± 0.06 (p < 0.01) in normal-appearing liver. In tumors, mean L/P ratio post-gadoxetate did not show a statistically significant change from baseline. Compared to baseline levels, the relative decrease in L/P ratio was significantly greater in liver than in tumors (-0.52 ± 0.16 vs. -0.19 ± 0.25, p < 0.05). CONCLUSIONS: The intracellular hepatobiliary contrast agent showed a greater effect suppressing HP 13 C MRI metabolic signals (through T1 shortening) in normal-appearing liver when compared to tumors. The combined use of HP MRI with selective gadolinium contrast agents may allow more selective imaging in HP 13 C MRI.

Hyperpolarized [1-13 C] pyruvate MRSI to detect metabolic changes in liver in a methionine and choline-deficient diet rat model of fatty liver disease.

PURPOSE: Nonalcoholic fatty liver disease is an important cause of chronic liver disease. There are limited methods for monitoring metabolic changes during progression to steatohepatitis. Hyperpolarized 13 C MRSI (HP 13 C MRSI) was used to measure metabolic changes in a rodent model of fatty liver disease. METHODS: Fifteen Wistar rats were placed on a methionine- and choline-deficient (MCD) diet for 1-18 weeks. HP 13 C MRSI, T2 -weighted imaging, and fat-fraction measurements were obtained at 3 T. Serum aspartate aminotransaminase, alanine aminotransaminase, and triglycerides were measured. Animals were sacrificed for histology and measurement of tissue lactate dehydrogenase (LDH) activity. RESULTS: Animals lost significant weight (13.6% ± 2.34%), an expected characteristic of the MCD diet. Steatosis, inflammation, and mild fibrosis were observed. Liver fat fraction was 31.7% ± 4.5% after 4 weeks and 22.2% ± 4.3% after 9 weeks. Lactate-to-pyruvate and alanine-to-pyruvate ratios decreased significantly over the study course; were negatively correlated with aspartate aminotransaminase and alanine aminotransaminase (r = -[0.39-0.61]); and were positively correlated with triglycerides (r = 0.59-0.60). Despite observed decreases in hyperpolarized lactate signal, LDH activity increased by a factor of 3 in MCD diet-fed animals. Observed decreases in lactate and alanine hyperpolarized signals on the MCD diet stand in contrast to other studies of liver injury, where lactate and alanine increased. Observed hyperpolarized metabolite changes were not explained by alterations in LDH activity, suggesting that changes may reflect co-factor depletion known to occur as a result of oxidative stress in the MCD diet. CONCLUSION: HP 13 C MRSI can noninvasively measure metabolic changes in the MCD model of chronic liver disease.

Current methods for hyperpolarized [1-13C]pyruvate MRI human studies.

MRI with hyperpolarized (HP) 13C agents, also known as HP 13C MRI, can measure processes such as localized metabolism that is altered in numerous cancers, liver, heart, kidney diseases, and more. It has been translated into human studies during the past 10 years, with recent rapid growth in studies largely based on increasing availability of HP agent preparation methods suitable for use in humans. This paper aims to capture the current successful practices for HP MRI human studies with [1-13C]pyruvate-by far the most commonly used agent, which sits at a key metabolic junction in glycolysis. The paper is divided into four major topic areas: (1) HP 13C-pyruvate preparation; (2) MRI system setup and calibrations; (3) data acquisition and image reconstruction; and (4) data analysis and quantification. In each area, we identified the key components for a successful study, summarized both published studies and current practices, and discuss evidence gaps, strengths, and limitations. This paper is the output of the "HP 13C MRI Consensus Group" as well as the ISMRM Hyperpolarized Media MR and Hyperpolarized Methods and Equipment study groups. It further aims to provide a comprehensive reference for future consensus, building as the field continues to advance human studies with this metabolic imaging modality.

Multi-band echo-planar spectroscopic imaging of hyperpolarized 13 C probes in a compact preclinical PET/MR scanner.

PURPOSE: Hyperpolarized (HP) 13 C MRI has enabled real-time imaging of specific enzyme-catalyzed metabolic reactions, but advanced pulse sequences are necessary to capture the dynamic, localized metabolic information. Herein we describe the design, implementation, and testing of a rapid and efficient HP 13 C pulse sequence strategy on a cryogen-free simultaneous positron emission tomography/MR molecular imaging platform with compact footprint. METHODS: We developed an echo planar spectroscopic imaging pulse sequence incorporating multi-band spectral-spatial radiofrequency (SSRF) pulses for spatially coregistered excitation of 13 C metabolites with differential individual flip angles. Excitation profiles were measured in phantoms, and the SSRF-echo planar spectroscopic imaging sequence was tested in rats in vivo and compared to conventional echo planar spectroscopic imaging. The new sequence was applied for 2D dynamic metabolic imaging of HP [1-13 C]pyruvate and its molecular analog [1-13 C] α -ketobutyrate at a spatial resolution of 5 mm × 5 mm × 20 mm and temporal resolution of 4 s. We also obtained simultaneous 18 F-fluorodeoxyglucose positron emission tomography data for comparison with HP [1-13 C]pyruvate data acquired during the same scan session. RESULTS: Measured SSRF excitation profiles corresponded well to Bloch simulations. Multi-band SSRF excitation facilitated efficient sampling of the multi-spectral kinetics of [1-13 C]pyruvate and [1-13 C] α - ketobutyrate . Whereas high pyruvate to lactate conversion was observed in liver, corresponding reduction of α -ketobutyrate to [1-13 C] α -hydroxybutyrate ( α HB) was largely restricted to the kidneys and heart, consistent with the known expression pattern of lactate dehydrogenase B. CONCLUSION: Advanced 13 C SSRF imaging approaches are feasible on our compact positron emission tomography/MR platform, maximizing the potential of HP 13 C technology and facilitating direct comparison with positron emission tomography.

Whole-Abdomen Metabolic Imaging of Healthy Volunteers Using Hyperpolarized [1-13 C]pyruvate MRI.

BACKGROUND: Hyperpolarized 13 C MRI quantitatively measures enzyme-catalyzed metabolism in cancer and metabolic diseases. Whole-abdomen imaging will permit dynamic metabolic imaging of several abdominal organs simultaneously in healthy and diseased subjects. PURPOSE: Image hyperpolarized [1-13 C]pyruvate and products in the abdomens of healthy volunteers, overcoming challenges of motion, magnetic field variations, and spatial coverage. Compare hyperpolarized [1-13 C]pyruvate metabolism across abdominal organs of healthy volunteers. STUDY TYPE: Prospective technical development. SUBJECTS: A total of 13 healthy volunteers (8 male), 21-64 years (median 36). FIELD STRENGTH/SEQUENCE: A 3 T. Proton: T1 -weighted spoiled gradient echo, T2 -weighted single-shot fast spin echo, multiecho fat/water imaging. Carbon-13: echo-planar spectroscopic imaging, metabolite-specific echo-planar imaging. ASSESSMENT: Transmit magnetic field was measured. Variations in main magnetic field (ΔB0 ) determined using multiecho proton acquisitions were compared to carbon-13 acquisitions. Changes in ΔB0 were measured after localized shimming. Improvements in metabolite signal-to-noise ratio were calculated. Whole-organ regions of interests were drawn over the liver, spleen, pancreas, and kidneys by a single investigator. Metabolite signals, time-to-peak, decay times, and mean first-order rate constants for pyruvate-to-lactate (kPL ) and alanine (kPA ) conversion were measured in each organ. STATISTICAL TESTS: Linear regression, one-sample Kolmogorov-Smirnov tests, paired t-tests, one-way ANOVA, Tukey's multiple comparisons tests. P ≤ 0.05 considered statistically significant. RESULTS: Proton ΔB0 maps correlated with carbon-13 ΔB0 maps (slope = 0.93, y-intercept = -2.88, R2  = 0.73). Localized shimming resulted in mean frequency offset within ±25 Hz for all organs. Metabolite SNR significantly increased after denoising. Mean kPL and kPA were highest in liver, followed by pancreas, spleen, and kidneys (all comparisons with liver were significant). DATA CONCLUSION: Whole-abdomen coverage with hyperpolarized carbon-13 MRI was feasible despite technical challenges. Multiecho gradient echo 1 H acquisitions accurately predicted chemical shifts observed using carbon-13 spectroscopy. Carbon-13 acquisitions benefited from local shimming. Metabolite energetics in the abdomen compiled for healthy volunteers can be used to design larger clinical trials in patients with metabolic diseases. EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 1.

Comparison of hyperpolarized 13 C and non-hyperpolarized deuterium MRI approaches for imaging cerebral glucose metabolism at 4.7 T.

PURPOSE: The purpose of this study was to directly compare two isotopic metabolic imaging approaches, hyperpolarized (HP) 13 C MRI and deuterium metabolic imaging (DMI), for imaging specific closely related segments of cerebral glucose metabolism at 4.7 T. METHODS: Comparative HP-13 C and DMI neuroimaging experiments were conducted consecutively in normal rats during the same scanning session. Localized conversions of [1-13 C]pyruvate and [6,6-2 H2 ]glucose to their respective downstream metabolic products were measured by spectroscopic imaging, using an identical 2D-CSI sequence with parameters optimized for the respective experiments. To facilitate direct comparison, a pair of substantially equivalent 2.5-cm double-tuned X/1 H RF surface coils was developed. For improved results, multidimensional low-rank reconstruction was applied to denoise the raw DMI data. RESULTS: Localized conversion of HP [1-13 C]pyruvate to [1-13 C]lactate, and [6,6-2 H2 ]glucose to [3,3-2 H2 ]lactate and Glx-d (glutamate and glutamine), was detected in rat brain by spectroscopic imaging at 4.7 T. The SNR and spatial resolution of HP-13 C MRI was superior to DMI but limited to a short time window, whereas the lengthy DMI acquisition yielded maps of not only lactate, but also Glx production, albeit with relatively poor spectral discrimination between metabolites at this field strength. Across the individual rats, there was an apparent inverse correlation between cerebral production of HP [1-13 C]lactate and Glx-d, along with a trend toward increased [3,3-2 H2 ]lactate. CONCLUSION: The HP-13 C MRI and DMI methods are both feasible at 4.7 T and have significant potential for metabolic imaging of specific segments of glucose metabolism.

55 Mn-based fiducial markers for rapid and automated RF coil localization for hyperpolarized 13 C MRI.

PURPOSE: To use fiducial markers containing manganese 55 to rapidly localize carbon 13 (13 C) RF coils for correcting images for B1 variation. METHODS: Hollow high-density polyethylene spheres were filled with 3M sodium permanganate and affixed to a rectangular 13 C-tuned RF coil. The relative positions of the markers and coil conductors were mapped using CT. Marker positions were measured by MRI using a series of 1D projections and automated peak detection. Once the coil location was determined, coil sensitivity was estimated using a quasi-static calculation. Simulations were performed to determine the minimum number of projections required for robust localization. Phantom experiments were used to confirm the accuracy of marker localization as well as the calculated coil sensitivity. Finally, in vivo validation was performed using hyperpolarized 13 C pyruvate in a rat model. RESULTS: In simulations, our algorithm was accurate in determining marker positions when at least 6 projections were used (RMSE 1.4 ± 0.9 mm). These estimates were verified in phantom experiments, where markers locations were determined with an RMS accuracy of 1.3 mm. A minimum SNR of 4 was required for automated detection to perform accurately. Computed coil sensitivity had a median error of 17% when taken over the entire measured area and 5.7% over a central region. In a rat, correction for nonuniform reception and flip angle was able to normalize the signals arising from asymmetrically positioned kidneys. CONCLUSION: Manganese 55 fiducial markers are an inexpensive and reliable method for rapidly localizing 13 C RF coils and correcting 13 C images for B1 variation without user intervention.

15 N-carnitine, a novel endogenous hyperpolarized MRI probe with long signal lifetime.

PURPOSE: The purpose of this study was to investigate hyperpolarization and in vivo imaging of [15 N]carnitine, a novel endogenous MRI probe with long signal lifetime. METHODS: L-[15 N]carnitine-d9 was hyperpolarized by the method of dynamic nuclear polarization followed by rapid dissolution. The T1 signal lifetimes were estimated in aqueous solution and in vivo following intravenous injection in rats, using a custom-built dual-tuned 15 N/1 H RF coil at 4.7 T. 15 N chemical shift imaging and 15 N fast spin-echo images of rat abdomen were acquired 3 minutes after [15 N]carnitine injection. RESULTS: Estimated T1 times of [15 N]carnitine at 4.7 T were 210 seconds (in H2 O) and 160 seconds (in vivo), with an estimated polarization level of 10%. Remarkably, the [15 N]carnitine coherence was detectable in rat abdomen for 5 minutes after injection for the nonlocalized acquisition. No downstream metabolites were detected on localized or nonlocalized 15 N spectra. Diffuse liver enhancement was detected on 15 N fast spin-echo imaging 3 minutes after injection, with mean hepatic SNR of 18 ± 5 at a spatial resolution of 4 × 4 mm. CONCLUSION: This study showed the feasibility of hyperpolarizing and imaging the biodistribution of HP [15 N]carnitine.

Cancer in the crosshairs: targeting cancer metabolism with hyperpolarized carbon-13 MRI technology.

Magnetic resonance (MR)-based hyperpolarized (HP) 13 C metabolic imaging is under active pursuit as a new clinical diagnostic method for cancer detection, grading, and monitoring of therapeutic response. Following the tremendous success of metabolic imaging by positron emission tomography, which already plays major roles in clinical oncology, the added value of HP 13 C MRI is emerging. Aberrant glycolysis and central carbon metabolism is a hallmark of many forms of cancer. The chemical transformations associated with these pathways produce metabolites ranging in general from three to six carbons, and are dependent on the redox state and energy charge of the tissue. The significant changes in chemistry associated with flux through these pathways imply that HP imaging can take advantage of the underlying chemical shift information encoded into an MR experiment to produce images of the injected substrate as well as its metabolites. However, imaging of HP metabolites poses unique constraints on pulse sequence design related to detection of X-nuclei, decay of the HP magnetization due to T1 , and the consumption of HP signal by the inspection pulses. Advancements in the field continue to depend critically on customization of MRI systems and pulse sequences for optimized detection of HP 13 C signals, focused largely on extracting the maximum amount of information during the short lifetime of the HP magnetization. From a clinical perspective, the success of HP 13 C MRI of cancer will largely depend upon the utility of HP pyruvate for the detection of lactate pools associated with the Warburg effect, though several other agents are also under investigation, with novel agents continually being formulated. In this review, the salient aspects of HP 13 C imaging will be highlighted, with an emphasis on both technological challenges and the biochemical aspects of HP experimental design.

Pulse sequence considerations for quantification of pyruvate-to-lactate conversion kPL in hyperpolarized 13 C imaging.

Hyperpolarized 13 C MRI takes advantage of the unprecedented 50 000-fold signal-to-noise ratio enhancement to interrogate cancer metabolism in patients and animals. It can measure the pyruvate-to-lactate conversion rate, kPL , a metabolic biomarker of cancer aggressiveness and progression. Therefore, it is crucial to evaluate kPL reliably. In this study, three sequence components and parameters that modulate kPL estimation were identified and investigated in model simulations and through in vivo animal studies using several specifically designed pulse sequences. These factors included a magnetization spoiling effect due to RF pulses, a crusher gradient-induced flow suppression, and intrinsic image weightings due to relaxation. Simulation showed that the RF-induced magnetization spoiling can be substantially improved using an inputless kPL fitting. In vivo studies found a significantly higher apparent kPL with an additional gradient that leads to flow suppression (kPL,FID-Delay,Crush /kPL,FID-Delay  = 1.37 ± 0.33, P < 0.01, N = 6), which agrees with simulation outcomes (12.5% kPL error with Δv = 40 cm/s), indicating that the gradients predominantly suppressed flowing pyruvate spins. Significantly lower kPL was found using a delayed free induction decay (FID) acquisition versus a minimum-TE version (kPL,FID-Delay /kPL,FID  = 0.67 ± 0.09, P < 0.01, N = 5), and the lactate peak had broader linewidth than pyruvate (Δωlactate /Δωpyruvate  = 1.32 ± 0.07, P < 0.000 01, N = 13). This illustrated that lactate's T2 *, shorter than that of pyruvate, can affect calculated kPL values. We also found that an FID sequence yielded significantly lower kPL versus a double spin-echo sequence that includes spin-echo spoiling, flow suppression from crusher gradients, and more T2 weighting (kPL,DSE /kPL,FID  = 2.40 ± 0.98, P < 0.0001, N = 7). In summary, the pulse sequence, as well as its interaction with pharmacokinetics and the tissue microenvironment, can impact and be optimized for the measurement of kPL . The data acquisition and analysis pipelines can work synergistically to provide more robust and reproducible kPL measures for future preclinical and clinical studies.

Direct assessment of renal mitochondrial redox state using hyperpolarized 13 C-acetoacetate.

PURPOSE: The purpose of this study was to investigate the hyperpolarized ketone body 13 C-acetoacetate (AcAc) and its conversion to 13 C-ß-hydroxybutyrate (ßOHB) in vivo, catalyzed by ß-hydroxybutyrate dehydrogenase (BDH), as a novel direct marker of mitochondrial redox state. METHODS: [1,3-13 C2 ]AcAc was synthesized by hydrolysis of the ethyl ester, and hyperpolarized via dissolution DNP. Cold storage under basic conditions resulted in sufficient chemical stability for use in hyperpolarized (HP) MRI studies. Polarizations and relaxation times of HP [1,3-13 C2 ]AcAc were measured in a clinical 3T MRI scanner, and 8 rats were scanned by dynamic HP 13 C MR spectroscopy of a slab through the kidneys. Four rats were scanned after acute treatment with high dose metformin (125 mg/kg, intravenous), which is known to modulate mitochondrial redox via inhibition of mitochondrial complex I. An additional metformin-treated rat was scanned by abdominal 2D CSI (8 mm × 8 mm). RESULTS: Polarizations of 7 ± 1% and 7 ± 3%, and T1 relaxation times of 58 ± 5 s and 52 ± 3 s, were attained at the C1 and C3 positions, respectively. Rapid conversion of HP AcAc to ßOHB was detected in rat kidney in vivo, via the C1 label. The product HP ßOHB was resolved from closely resonating acetate. Conversion to ßOHB was also detected via 2D CSI, in both kidney as well as liver regions. Metformin treatment resulted in a significant increase (40%, P = 0.01) of conversion of HP AcAc to ßOHB. CONCLUSION: Rapid conversion of HP AcAc to ßOHB was observed in rat kidney in vivo and is a promising new non-invasive marker of mitochondrial redox state. Magn Reson Med 79:1862-1869, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

In vivo hyperpolarization transfer in a clinical MRI scanner.

PURPOSE: The purpose of this study was to investigate the feasibility of in vivo 13 C->1 H hyperpolarization transfer, which has significant potential advantages for detecting the distribution and metabolism of hyperpolarized 13 C probes in a clinical MRI scanner. METHODS: A standalone pulsed 13 C RF transmit channel was developed for operation in conjunction with the standard 1 H channel of a clinical 3T MRI scanner. Pulse sequences for 13 C power calibration and polarization transfer were programmed on the external hardware and integrated with a customized water-suppressed 1 H MRS acquisition running in parallel on the scanner. The newly developed RF system was tested in both phantom and in vivo polarization transfer experiments in 1 JCH -coupled systems: phantom experiments in thermally polarized and hyperpolarized [2-13 C]glycerol, and 1 H detection of [2-13 C]lactate generated from hyperpolarized [2-13 C]pyruvate in rat liver in vivo. RESULTS: Operation of the custom pulsed 13 C RF channel resulted in effective 13 C->1 H hyperpolarization transfer, as confirmed by the characteristic antiphase appearance of 1 H-detected, 1 JCH -coupled doublets. In conjunction with a pulse sequence providing 190-fold water suppression in vivo, 1 H detection of hyperpolarized [2-13 C]lactate generated in vivo was achieved in a rat liver slice. CONCLUSION: The results show clear feasibility for effective 13 C->1 H hyperpolarization transfer in a clinical MRI scanner with customized heteronuclear RF system.

Sensitivity enhancement for detection of hyperpolarized 13 C MRI probes with 1 H spin coupling introduced by enzymatic transformation in vivo.

PURPOSE: Although 1 H spin coupling is generally avoided in probes for hyperpolarized (HP) 13 C MRI, enzymatic transformations of biological interest can introduce large 13 C-1 H couplings in vivo. The purpose of this study was to develop and investigate the application of 1 H decoupling for enhancing the sensitivity for detection of affected HP 13 C metabolic products. METHODS: A standalone 1 H decoupler system and custom concentric 13 C/1 H paddle coil setup were integrated with a clinical 3T MRI scanner for in vivo 13 C MR studies using HP [2-13 C]dihydroxyacetone, a novel sensor of hepatic energy status. Major 13 C-1 H coupling JCH = ∼150 Hz) is introduced after adenosine triphosphate-dependent enzymatic transformation of HP [2-13 C]dihydroxyacetone to [2-13 C]glycerol-3-phosphate in vivo. Application of WALTZ-16 1 H decoupling for elimination of large 13 C-1 H couplings was first tested in thermally polarized glycerol phantoms and then for in vivo HP MR studies in three rats, scanned both with and without decoupling. RESULTS: As configured, 1 H-decoupled 13 C MR of thermally polarized glycerol and the HP metabolic product [2-13 C]glycerol-3-phosphate was achieved at forward power of approximately 15 W. High-quality 3-s dynamic in vivo HP 13 C MR scans were acquired with decoupling duty cycle of 5%. Application of 1 H decoupling resulted in sensitivity enhancement of 1.7-fold for detection of metabolic conversion of [2-13 C]dihydroxyacetone to HP [2-13 C]glycerol-3-phosphate in vivo. CONCLUSIONS: Application of 1 H decoupling provides significant sensitivity enhancement for detection of HP 13 C metabolic products with large 1 H spin couplings, and is therefore expected to be useful for preclinical and potentially clinical HP 13 C MR studies. Magn Reson Med 80:36-41, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Investigating tumor perfusion by hyperpolarized 13 C MRI with comparison to conventional gadolinium contrast-enhanced MRI and pathology in orthotopic human GBM xenografts.

PURPOSE: Dissolution dynamic nuclear polarization (DNP) enables the acquisition of 13 C magnetic resonance data with a high sensitivity. Recently, metabolically inactive hyperpolarized 13 C-labeled compounds have shown to be potentially useful for perfusion imaging. The purpose of this study was to validate hyperpolarized perfusion imaging methods by comparing with conventional gadolinium (Gd)-based perfusion MRI techniques and pathology. METHODS: Dynamic 13 C data using metabolically inactive hyperpolarized bis-1,1-(hydroxymethyl)-[1-13 C]cyclopropane-d8 (HMCP) were obtained from an orthotopic human glioblastoma (GBM) model for the characterization of tumor perfusion and compared with standard Gd-based dynamic susceptibility contrast (DSC) MRI data and immunohistochemical analysis from resected brains. RESULTS: Distinct HMCP perfusion characteristics were observed within the GBM tumors compared with contralateral normal brain tissue. The perfusion parameters obtained from the hyperpolarized HMCP data in tumor were strongly correlated with normalized peak height measured from the DSC images. The results from immunohistochemical analysis supported these findings by showing a high level of vascular staining for tumor that exhibited high levels of hyperpolarized HMCP signal. CONCLUSION: The results from this study have demonstrated that hyperpolarized HMCP data can be used as an indicator of tumor perfusion in an orthotopic xenograft model for GBM. Magn Reson Med 77:841-847, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Combining hyperpolarized 13 C MRI with a liver-specific gadolinium contrast agent for selective assessment of hepatocyte metabolism.

PURPOSE: Hyperpolarized 13 C MRI is a powerful tool for studying metabolism, but can lack tissue specificity. Gadoxetate is a gadolinium-based MRI contrast agent that is selectively taken into hepatocytes. The goal of this project was to investigate whether gadoxetate can be used to selectively suppress the hyperpolarized signal arising from hepatocytes, which could in future studies be applied to generate specificity for signal from abnormal cell types. METHODS: Baseline gadoxetate uptake kinetics were measured using T1 -weighted contrast enhanced imaging. Relaxivity of gadoxetate was measured for [1-13 C]pyruvate, [1-13 C]lactate, and [1-13 C]alanine. Four healthy rats were imaged with hyperpolarized [1-13 C]pyruvate using a three-dimensional (3D) MRSI sequence prior to and 15 min following administration of gadoxetate. The lactate:pyruvate ratio and alanine:pyruvate ratios were measured in liver and kidney. RESULTS: Overall, the hyperpolarized signal decreased approximately 60% as a result of pre-injection of gadoxetate. In liver, the lactate:pyruvate and alanine:pyruvate ratios decreased 42% and 78%, respectively (P < 0.05) following gadoxetate administration. In kidneys, these ratios did not change significantly. Relaxivity of gadoxetate for [1-13 C]alanine was 12.6 times higher than relaxivity of gadoxetate for [1-13 C]pyruvate, explaining the greater selective relaxation effect on alanine. CONCLUSIONS: The liver-specific gadolinium contrast-agent gadoxetate can selectively suppress normal hepatocyte contributions to hyperpolarized 13 C MRI signals. Magn Reson Med 77:2356-2363, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Multiband spectral-spatial RF excitation for hyperpolarized [2-13 C]dihydroxyacetone 13 C-MR metabolism studies.

PURPOSE: To develop a specialized multislice, single-acquisition approach to detect the metabolites of hyperpolarized (HP) [2-13 C]dihydroxyacetone (DHAc) to probe gluconeogenesis in vivo, which have a broad 144 ppm spectral range (∼4.6 kHz at 3T). A novel multiband radio-frequency (RF) excitation pulse was designed for independent flip angle control over five to six spectral-spatial (SPSP) excitation bands, each corrected for chemical shift misregistration effects. METHODS: Specialized multiband SPSP RF pulses were designed, tested, and applied to investigate HP [2-13 C]DHAc metabolism in kidney and liver of fasted rats with dynamic 13 C-MR spectroscopy and an optimal flip angle scheme. For comparison, experiments were also performed with narrow-band slice-selective RF pulses and a sequential change of the frequency offset to cover the five frequency bands of interest. RESULTS: The SPSP pulses provided a controllable spectral profile free of baseline distortion with improved signal to noise of the metabolite peaks, allowing for quantification of the metabolic products. We observed organ-specific differences in DHAc metabolism. There was two to five times more [2-13 C]phosphoenolpyruvate and about 19 times more [2-13 C]glycerol 3-phosphate in the liver than in the kidney. CONCLUSION: A multiband SPSP RF pulse covering a spectral range over 144 ppm enabled in vivo characterization of HP [2-13 C]DHAc metabolism in rat liver and kidney. Magn Reson Med 77:1419-1428, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Monitoring acute metabolic changes in the liver and kidneys induced by fructose and glucose using hyperpolarized [2-13 C]dihydroxyacetone.

PURPOSE: To investigate acute changes in glucose metabolism in liver and kidneys in vivo after a bolus injection of either fructose or glucose, using hyperpolarized [2-13 C]dihydroxyacetone. METHODS: Spatially registered, dynamic, multislice MR spectroscopy was acquired for the metabolic products of [2-13 C]dihydroxyacetone in liver and kidneys. Metabolism was probed in 13 fasted rats at three time points: 0, 70, and 140 min. At 60 min, rats were injected intravenously with fructose (n = 5) or glucose (n = 4) at 0.8 g/kg to initiate acute response. Controls (n = 4) did not receive a carbohydrate challenge. RESULTS: Ten minutes after fructose infusion, levels of [2-13 C]phosphoenolpyruvate and [2-13 C]glycerol-3-phosphate halved in liver: 51% (P = 0.0010) and 47% (P = 0.0001) of baseline, respectively. Seventy minutes later, levels returned to baseline. The glucose challenge did not alter the signals significantly, nor did repeated administration of the dihydroxyacetone imaging bolus. In kidneys, no statistically significant changes were detected after sugar infusion other than a 20% increase of the glycerol-3-phosphate signal between 10 and 80 min after fructose injection (P = 0.0028). CONCLUSION: Hyperpolarized [2-13 C]dihydroxyacetone detects a real-time, transient metabolic response of the liver to an acute fructose challenge. Observed effects possibly include ATP depletion and changes in the unlabeled pool sizes of glycolytic intermediates. Magn Reson Med 77:65-73, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Spectrally selective three-dimensional dynamic balanced steady-state free precession for hyperpolarized C-13 metabolic imaging with spectrally selective radiofrequency pulses.

PURPOSE: Balanced steady-state free precession (bSSFP) sequences can provide superior signal-to-noise ratio efficiency for hyperpolarized (HP) carbon-13 (13 C) magnetic resonance imaging by efficiently utilizing the nonrecoverable magnetization, but managing their spectral response is challenging in the context of metabolic imaging. A new spectrally selective bSSFP sequence was developed for fast imaging of multiple HP 13 C metabolites with high spatiotemporal resolution. THEORY AND METHODS: This novel approach for bSSFP spectral selectivity incorporates optimized short-duration spectrally selective radiofrequency pulses within a bSSFP pulse train and a carefully chosen repetition time to avoid banding artifacts. RESULTS: The sequence enabled subsecond 3D dynamic spectrally selective imaging of 13 C metabolites of copolarized [1-13 C]pyruvate and [13 C]urea at 2-mm isotropic resolution, with excellent spectral selectivity (∼100:1). The sequence was successfully tested in phantom studies and in vivo studies with normal mice. CONCLUSION: This sequence is expected to benefit applications requiring dynamic volumetric imaging of metabolically active 13 C compounds at high spatiotemporal resolution, including preclinical studies at high field and, potentially, clinical studies. Magn Reson Med 78:963-975, 2017. © 2016 International Society for Magnetic Resonance in Medicine.