Rapid multislice imaging of hyperpolarized 13C pyruvate and bicarbonate in the heart

MRI Application and Challenges of Hyperpolarized Carbon-13 Pyruvate in Translational and Clinical Cardiovascular Studies: A Literature Review

Cardiovascular disease shows, or may even be caused by, changes in metabolism. Hyperpolarized magnetic resonance spectroscopy and imaging is a technique that could assess the role of different aspects of metabolism in heart disease, allowing real-time metabolic flux assessment in vivo. In this review, we introduce the main hyperpolarization techniques. Then, we summarize the use of dedicated radiofrequency 13C coils, and report a state of the art of 13C data acquisition. Finally, this review provides an overview of the pre-clinical and clinical studies on cardiac metabolism in the healthy and diseased heart. We furthermore show what advances have been made to translate this technique into the clinic in the near future and what technical challenges still remain, such as exploring other metabolic substrates.

Model-constrained reconstruction accelerated with Fourier-based undersampling for hyperpolarized [1-13 C] pyruvate imaging

PURPOSE

Model-constrained reconstruction with Fourier-based undersampling (MoReFUn) is introduced to accelerate the acquisition of dynamic MRI using hyperpolarized [1-13 C]-pyruvate.

METHODS

The MoReFUn method resolves spatial aliasing using constraints introduced by a pharmacokinetic model that describes the signal evolution of both pyruvate and lactate. Acceleration was evaluated on three single-channel data sets: a numerical digital phantom that is used to validate the accuracy of reconstruction and model parameter restoration under various SNR and undersampling ratios, prospectively and retrospectively sampled data of an in vitro dynamic multispectral phantom, and retrospectively undersampled imaging data from a prostate cancer patient to test the fidelity of reconstructed metabolite time series.

RESULTS

All three data sets showed successful reconstruction using MoReFUn. In simulation and retrospective phantom data, the restored time series of pyruvate and lactate maintained the image details, and the mean square residual error of the accelerated reconstruction increased only slightly (< 10%) at a reduction factor up to 8. In prostate data, the quantitative estimation of the conversion-rate constant of pyruvate to lactate was achieved with high accuracy of less than 10% error at a reduction factor of 2 compared with the conversion rate derived from unaccelerated data.

CONCLUSION

The MoReFUn technique can be used as an effective and reliable imaging acceleration method for metabolic imaging using hyperpolarized [1-13 C]-pyruvate.

Parahydrogen-induced polarization and spin order transfer in ethyl pyruvate at high magnetic fields

Nuclear magnetic resonance has experienced great advances in developing and translating hyperpolarization methods into procedures for fundamental and clinical studies. Here, we propose the use of a wide-bore NMR for large-scale (volume- and concentration-wise) production of hyperpolarized media using parahydrogen-induced polarization. We discuss the benefits of radio frequency-induced parahydrogen spin order transfer, we show that 100% polarization is theoretically expected for homogeneous B0 and B1 magnetic fields for a three-spin system. Moreover, we estimated that the efficiency of spin order transfer is not significantly reduced when the B1 inhomogeneity is below ± 5%; recommendations for the sample size and RF coils are also given. With the latest breakthrough in the high-yield synthesis of 1-13C-vinyl pyruvate and its deuterated isotopologues, the high-field PHIP-SAH will gain increased attention. Some remaining challenges will be addressed shortly.

Performance and reproducibility of 13C and 15N hyperpolarization using a cryogen-free DNP polarizer

The setup, operational procedures and performance of a cryogen-free device for producing hyperpolarized contrast agents using dissolution dynamic nuclear polarization (dDNP) in a preclinical imaging center is described. The polarization was optimized using the solid-state, DNP-enhanced NMR signal to calibrate the sample position, microwave and NMR frequency and power and flip angle. The polarization of a standard formulation to yield ~ 4 mL, 60 mM 1-13C-pyruvic acid in an aqueous solution was quantified in five experiments to P(13C) = (38 ± 6) % (19 ± 1) s after dissolution. The mono-exponential time constant of the build-up of the solid-state polarization was quantified to (1032 ± 22) s. We achieved a duty cycle of 1.5 h that includes sample loading, monitoring the polarization build-up, dissolution and preparation for the next run. After injection of the contrast agent in vivo, pyruvate, pyruvate hydrate, lactate, and alanine were observed, by measuring metabolite maps. Based on this work sequence, hyperpolarized 15N urea was obtained (P(15N) = (5.6 ± 0.8) % (30 ± 3) s after dissolution).

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.

Cardio-Oncology: Understanding the Intersections Between Cardiac Metabolism and Cancer Biology

An important priority in the cardiovascular care of oncology patients is to reduce morbidity and mortality, and improve the quality of life in cancer survivors through cross-disciplinary efforts. The rate of survival in cancer patients has improved dramatically over the past decades. Nonetheless, survivors may be more likely to die from cardiovascular disease in the long term, secondary, not only to the potential toxicity of cancer therapeutics, but also to the biology of cancer. In this context, efforts from basic and translational studies are crucial to understanding the molecular mechanisms causal to cardiovascular disease in cancer patients and survivors, and identifying new therapeutic targets that may prevent and treat both diseases. This review aims to highlight our current understanding of the metabolic interaction between cancer and the heart, including potential therapeutic targets. An overview of imaging techniques that can support both research studies and clinical management is also provided. Finally, this review highlights opportunities and challenges that are necessary to advance our understanding of metabolism in the context of cardio-oncology.

Cardiac metabolic imaging using hyperpolarized [1-13 C]lactate as a substrate

Hyperpolarized (HP) [1-¹³ C]lactate is an attractive alternative to [1-¹³ C]pyruvate as a substrate to investigate cardiac metabolism in vivo: it can be administered safely at a higher dose and can be polarized to a degree similar to pyruvate via dynamic nuclear polarization. While ¹³ C cardiac experiments using HP lactate have been performed in small animal models, they have not been demonstrated in large animal models or humans. Utilizing the same hardware and data acquisition methods as the first human HP ¹³ C cardiac study, ¹³ C metabolic images were acquired following injections of HP [1-¹³ C]lactate in porcine hearts. Data were also acquired using HP [1-¹³ C]pyruvate for comparison. The ¹³ C bicarbonate signal was localized to the myocardium and had a similar appearance with both substrates for all animals. No ¹³ C pyruvate signal was detected in the experiments following injection of HP ¹³ C lactate. The signal-to-noise ratio (SNR) of injected lactate was 88 ± 4% of the SNR of injected pyruvate, and the SNR of bicarbonate in the experiments using lactate as the substrate was 52 ± 19% of the SNR in the experiments using pyruvate as the substrate. The lower SNR was likely due to the shorter T₁ of [1-¹³ C]lactate as compared with [1-¹³ C]pyruvate and the additional enzyme-catalyzed metabolic conversion step before the ¹³ C nuclei from [1-¹³ C]lactate were detected as ¹³ C bicarbonate. While challenges remain, the potential of HP lactate as a substrate for clinical metabolic imaging of human heart has been demonstrated.

Hyperpolarized Metabolic MRI-Acquisition, Reconstruction, and Analysis Methods

Hyperpolarized metabolic MRI with 13C-labeled agents has emerged as a powerful technique for in vivo assessments of real-time metabolism that can be used across scales of cells, tissue slices, animal models, and human subjects. Hyperpolarized contrast agents have unique properties compared to conventional MRI scanning and MRI contrast agents that require specialized imaging methods. Hyperpolarized contrast agents have a limited amount of available signal, irreversible decay back to thermal equilibrium, bolus injection and perfusion kinetics, cellular uptake and metabolic conversion kinetics, and frequency shifts between metabolites. This article describes state-of-the-art methods for hyperpolarized metabolic MRI, summarizing data acquisition, reconstruction, and analysis methods in order to guide the design and execution of studies.

Accelerated MR spectroscopic imaging-a review of current and emerging techniques

Over more than 30 years in vivo MR spectroscopic imaging (MRSI) has undergone an enormous evolution from theoretical concepts in the early 1980s to the robust imaging technique that it is today. The development of both fast and efficient sampling and reconstruction techniques has played a fundamental role in this process. State-of-the-art MRSI has grown from a slow purely phase-encoded acquisition technique to a method that today combines the benefits of different acceleration techniques. These include shortening of repetition times, spatial-spectral encoding, undersampling of k-space and time domain, and use of spatial-spectral prior knowledge in the reconstruction. In this way in vivo MRSI has considerably advanced in terms of spatial coverage, spatial resolution, acquisition speed, artifact suppression, number of detectable metabolites and quantification precision. Acceleration not only has been the enabling factor in high-resolution whole-brain 1 H-MRSI, but today is also common in non-proton MRSI (31 P, 2 H and 13 C) and applied in many different organs. In this process, MRSI techniques had to constantly adapt, but have also benefitted from the significant increase of magnetic field strength boosting the signal-to-noise ratio along with high gradient fidelity and high-density receive arrays. In combination with recent trends in image reconstruction and much improved computation power, these advances led to a number of novel developments with respect to MRSI acceleration. Today MRSI allows for non-invasive and non-ionizing mapping of the spatial distribution of various metabolites' tissue concentrations in animals or humans, is applied for clinical diagnostics and has been established as an important tool for neuro-scientific and metabolism research. This review highlights the developments of the last five years and puts them into the context of earlier MRSI acceleration techniques. In addition to 1 H-MRSI it also includes other relevant nuclei and is not limited to certain body regions or specific applications.

Cardiac T 2 ∗ 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 T 2 ∗ s of the HP signals is critical. This study is to measure in vivo cardiac T 2 ∗ 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. T 2 ∗ 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 T 2 ∗ 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, T 2 ∗ of each metabolite was calculated for left ventricle, right ventricle, and myocardium.

RESULTS

Cardiac T 2 ∗ 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, T 2 ∗ of [1-¹³ C]pyruvate was 108.7 ± 22.6 ms in left ventricle and 129.4 ± 8.9 ms in right ventricle. T 2 ∗ 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. T 2 ∗ 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 T 2 ∗ s of HP [1-¹³ C]pyruvate and products.

pH Dependence of T2 for Hyperpolarizable 13C-Labelled Small Molecules Enables Spatially Resolved pH Measurement by Magnetic Resonance Imaging

Hyperpolarized ¹³C magnetic resonance imaging often uses spin-echo-based pulse sequences that are sensitive to the transverse relaxation time T₂. In this context, local T₂-changes might introduce a quantification bias to imaging biomarkers. Here, we investigated the pH dependence of the apparent transverse relaxation time constant (denoted here as T₂) of six ¹³C-labelled molecules. We obtained minimum and maximum T₂ values within pH 1–13 at 14.1 T: [1-¹³C]acetate (T_2,min = 2.1 s; T_2,max = 27.7 s), [1-¹³C]alanine (T_2,min = 0.6 s; T_2,max = 10.6 s), [1,4-¹³C₂]fumarate (T_2,min = 3.0 s; T_2,max = 18.9 s), [1-¹³C]lactate (T_2,min = 0.7 s; T_2,max = 12.6 s), [1-¹³C]pyruvate (T_2,min = 0.1 s; T_2,max = 18.7 s) and ¹³C-urea (T_2,min = 0.1 s; T_2,max = 0.1 s). At 7 T, T₂-variation in the physiological pH range (pH 6.8–7.8) was highest for [1-¹³C]pyruvate (ΔT₂ = 0.95 s/0.1pH) and [1-¹³C]acetate (ΔT₂ = 0.44 s/0.1pH). Concentration, salt concentration, and temperature alterations caused T₂ variations of up to 45.4% for [1-¹³C]acetate and 23.6% for [1-¹³C]pyruvate. For [1-¹³C]acetate, spatially resolved pH measurements using T₂-mapping were demonstrated with 1.6 pH units accuracy in vitro. A strong proton exchange-based pH dependence of T₂ suggests that pH alterations potentially influence signal strength for hyperpolarized ¹³C-acquisitions.

Porphyrins as Versatile, Aggregation-Tolerant, and Biocompatible Polarizing Agents for Triplet Dynamic Nuclear Polarization of Biomolecules

Triplet dynamic nuclear polarization (triplet-DNP) achieves nuclear spin polarization at moderate temperatures by using spin polarization of photoexcited triplet electrons. The applications of triplet-DNP for biomolecules have been hampered because acenes, the only polarizing agents used so far, tend to aggregate and lose their polarization in biomolecular matrices. Here, we report for the first time use of porphyrins as polarizing agents of triplet-DNP and propose a new concept of aggregation-tolerant polarizing agents. Sodium salts of tetrakis(4-carboxyphenyl)porphyrin (TCPPNa) can be dispersed in amorphous as well as crystalline biomolecular matrices, and importantly, it can generate polarized triplet electrons even in a slightly aggregated state. Triplet-DNP of crystalline erythritol containing slightly aggregated TCPPNa can achieve more than 120-fold signal enhancement. Because TCPPNa is also the first biocompatible triplet-DNP polarizing agent, this work provides a crucial step forward for the biological and medical applications of triplet-DNP.

Characterization and compensation of f 0 inhomogeneity artifact in spiral hyperpolarized 13 C imaging of the human heart

PURPOSE

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 to those acquired with the short waveform (predicted 1.32). Improvement of image quality was observed for all metabolites with f 0 correction.

CONCLUSIONS

f 0 -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 f 0 artifacts, which can be partially corrected for with the proposed methods.

Correlation between lactate dehydrogenase/pyruvate dehydrogenase activities ratio and tissue pH in the perfused mouse heart: A potential noninvasive indicator of cardiac pH provided by hyperpolarized magnetic resonance

Cardiovascular diseases account for more than 30% of all deaths worldwide and many could be ameliorated with early diagnosis. Current cardiac imaging modalities can assess blood flow, heart anatomy and mechanical function. However, for early diagnosis and improved treatment, further functional biomarkers are needed. One such functional biomarker could be the myocardium pH. Although tissue pH is already determinable via MR techniques, and has been since the early 1990s, it remains elusive to use practically. The objective of this study was to explore the possibility to evaluate cardiac pH noninvasively, using in-cell enzymatic rates of hyperpolarized [1-¹³ C]pyruvate metabolism (ie, moles of product produced per unit time) determined directly in real time using magnetic resonance spectroscopy in a perfused mouse heart model. As a gold standard for tissue pH we used ³¹ P spectroscopy and the chemical shift of the inorganic phosphate (Pi) signal. The nonhomogenous pH distribution of the perfused heart was analyzed using a multi-parametric analysis of this signal, thus taking into account the heterogeneous nature of this characteristic. As opposed to the signal ratio of hyperpolarized [¹³ C]bicarbonate to [¹³ CO₂ ], which has shown correlation to pH in other studies, we investigated here the ratio of two intracellular enzymatic rates: lactate dehydrogenase (LDH) and pyruvate dehydrogenase (PDH), by way of determining the production rates of [1-¹³ C]lactate and [¹³ C]bicarbonate, respectively. The enzyme activities determined here are intracellular, while the pH determined using the Pi signal may contain an extracellular component, which could not be ruled out. Nevertheless, we report a strong correlation between the tissue pH and the LDH/PDH activities ratio. This work may pave the way for using the LDH/PDH activities ratio as an indicator of cardiac intracellular pH in vivo, in an MRI examination.

A variable resolution approach for improved acquisition of hyperpolarized 13 C metabolic MRI

PURPOSE

To ameliorate tradeoffs between a fixed spatial resolution and signal-to-noise ratio (SNR) for hyperpolarized 13 C MRI.

METHODS

In MRI, SNR is proportional to voxel volume but retrospective downsampling or voxel averaging only improves SNR by the square root of voxel size. This can be exploited with a metabolite-selective imaging approach that independently encodes each compound, yielding high-resolution images for the injected substrate and coarser resolution images for downstream metabolites, while maintaining adequate SNR for each. To assess the efficacy of this approach, hyperpolarized [1-13 C]pyruvate data were acquired in healthy Sprague-Dawley rats (n = 4) and in two healthy human subjects.

RESULTS

Compared with a constant resolution acquisition, variable-resolution data sets showed improved detectability of metabolites in pre-clinical renal studies with a 3.5-fold, 8.7-fold, and 6.0-fold increase in SNR for lactate, alanine, and bicarbonate data, respectively. Variable-resolution data sets from healthy human subjects showed cardiac structure and neuro-vasculature in the higher resolution pyruvate images (6.0 × 6.0 mm2 for cardiac and 7.5 × 7.5 mm2 for brain) that would otherwise be missed due to partial-volume effects and illustrates the level of detail that can be achieved with hyperpolarized substrates in a clinical setting.

CONCLUSION

We developed a variable-resolution strategy for hyperpolarized 13 C MRI using metabolite-selective imaging and demonstrated that it mitigates tradeoffs between a fixed spatial resolution and SNR for hyperpolarized substrates, providing both high resolution pyruvate and coarse resolution metabolite data sets in a single exam. This technique shows promise to improve future studies by maximizing metabolite SNR while minimizing partial-volume effects from the injected substrate.

MRI of [2-13 C]Lactate without J-coupling artifacts

PURPOSE

Imaging of [2-¹³ C]lactate, a metabolic product of [2-¹³ C]pyruvate, is over considerable interest in hyperpolarized ¹³ C studies. However, artifact-free imaging of a J-coupled nuclear spin species can be challenging due to the peak-splitting induced by the spin-spin interactions. In this work, two new techniques resolving these J-modulated artifacts are presented.

THEORY AND METHODS

The Product Operator Formalism (POF) of density matrix theory is used to both numerically and analytically derive the coherences arising during radiofrequency excitation and readout of a J-coupled spin system. A combination of computer simulations and experiments with [2-¹³ C]lactate and ¹³ C-formate phantoms are then used to verify the performance of two imaging methods. In the first approach, a quadrature imaging technique is used to eliminate scalar coupling artifacts via the combination of in-phase and quadrature images acquired at echo times differing by 1/2J with an echoplanar readout. The second approach employs a highly narrowband RF excitation pulse to image a single peak from the J-coupled doublet.

RESULTS

Simulations using a numerical Shepp-Logan phantom, in vitro experiments using thermally polarized [2-¹³ C]lactate, thermally and hyperpolarized ¹³ C-formate phantoms, and in vivo imaging of [2-¹³ C]lactate produced in rat brain following injection of hyperpolarized [2-¹³ C]pyruvate show artifact-free images and demonstrate potential utility of these methods.

CONCLUSION

The quadrature imaging and the narrowband excitation techniques resolve the J-coupling induced ghosting and blurring artifacts present with conventional MRI of J-coupled signals such as [2-¹³ C]lactate.

A multi spin echo pulse sequence with optimized excitation pulses and a 3D cone readout for hyperpolarized 13 C imaging

PURPOSE

Imaging tumor metabolism in vivo using hyperpolarized [1-13 C]pyruvate is a promising technique for detecting disease, monitoring disease progression, and assessing treatment response. However, the transient nature of the hyperpolarization and its depletion following excitation limits the available time for imaging. We describe here a single-shot multi spin echo sequence, which improves on previously reported sequences, with a shorter readout time, isotropic point spread function (PSF), and better signal-to-noise ratio.

METHODS

The sequence uses numerically optimized spectrally selective excitation pulses set to the resonant frequencies of pyruvate and lactate and a hyperbolic secant adiabatic refocusing pulse, all applied in the absence of slice selection gradients. The excitation pulses were designed to be resistant to the effects of B0 and B1 field inhomogeneity. The gradient readout uses a 3D cone trajectory composed of 13 cones, all fully refocused and distributed among 7 spin echoes. The maximal gradient amplitude and slew rate were set to 4 G/cm and 20 G/cm/ms, respectively, to demonstrate the feasibility of clinical translation.

RESULTS

The pulse sequence gave an isotropic PSF of 2.8 mm. The excitation profiles of the optimized pulses closely matched simulations and a 46.10 ± 0.04% gain in image SNR was observed compared to a conventional Shinnar-Le Roux excitation pulse. The sequence was demonstrated with dynamic imaging of hyperpolarized [1-13 C]pyruvate and [1-13 C]lactate in vivo.

CONCLUSION

The pulse sequence was capable of dynamic imaging of hyperpolarized 13 C labeled metabolites in vivo with relatively high spatial and temporal resolution and immunity to system imperfections.

A 3D hybrid-shot spiral sequence for hyperpolarized 13 C imaging

Purpose

Hyperpolarized imaging experiments have conflicting requirements of high spatial, temporal, and spectral resolution. Spectral-spatial RF excitation has been shown to form an attractive magnetization-efficient method for hyperpolarized imaging, but the optimum readout strategy is not yet known.

Methods

In this work, we propose a novel 3D hybrid-shot spiral sequence which features two constant density regions that permit the retrospective reconstruction of either high spatial or high temporal resolution images post hoc, (adaptive spatiotemporal imaging) allowing greater flexibility in acquisition and reconstruction.

Results

We have implemented this sequence, both via simulation and on a preclinical scanner, to demonstrate its feasibility, in both a ¹H phantom and with hyperpolarized ¹³C pyruvate in vivo.

Conclusions

This sequence forms an attractive method for acquiring hyperpolarized imaging datasets, providing adaptive spatiotemporal imaging to ameliorate the conflict of spatial and temporal resolution, with significant potential for clinical translation.

Metabolic Imaging and Biological Assessment: Platforms to Evaluate Acute Lung Injury and Inflammation

Pulmonary inflammation is a hallmark of several pulmonary disorders including acute lung injury and acute respiratory distress syndrome. Moreover, it has been shown that patients with hyperinflammatory phenotype have a significantly higher mortality rate. Despite this, current therapeutic approaches focus on managing the injury rather than subsiding the inflammatory burden of the lung. This is because of the lack of appropriate non-invasive biomarkers that can be used clinically to assess pulmonary inflammation. In this review, we discuss two metabolic imaging tools that can be used to non-invasively assess lung inflammation. The first method, Positron Emission Tomography (PET), is widely used in clinical oncology and quantifies flux in metabolic pathways by measuring uptake of a radiolabeled molecule into the cells. The second method, hyperpolarized ¹³C MRI, is an emerging tool that interrogates the branching points of the metabolic pathways to quantify the fate of metabolites. We discuss the differences and similarities between these techniques and discuss their clinical applications.

Dynamic volumetric hyperpolarized 13 C imaging with multi-echo EPI

PURPOSE

To generate dynamic, volumetric maps of hyperpolarized [1-¹³ C]pyruvate and its metabolic products in vivo.

METHODS

Maps of chemical species were generated with iterative least squares (IDEAL) reconstruction from multiecho echo-planar imaging (EPI) of phantoms of thermally polarized ¹³ C-labeled chemicals and mice injected with hyperpolarized [1-¹³ C]pyruvate on a preclinical 3T scanner. The quality of the IDEAL decomposition of single-shot and multishot phantom images was evaluated using quantitative results from a simple pulse-and-acquire sequence as the gold standard. Time course and area-under-the-curve plots were created to analyze the distribution of metabolites in vivo.

RESULTS

Improved separation of chemical species by IDEAL, evaluated by the amount of residual signal measured for chemicals not present in the phantoms, was observed as the number of EPI shots was increased from one to four. Dynamic three-dimensional metabolite maps of [1-¹³ C]pyruvate,[1-¹³ C]pyruvatehydrate, [1-¹³ C]lactate, [1-¹³ C]bicarbonate, and [1-¹³ C]alanine generated by IDEAL from interleaved multishot multiecho EPI of live mice were used to construct time course and area-under-the-curve graphs for the heart, kidneys, and liver, which showed good agreement with previously published results.

CONCLUSIONS

IDEAL decomposition of multishot multiecho 13C EPI images is a simple, yet robust method for generating high-quality dynamic volumetric maps of hyperpolarized [1-¹³ C]pyruvate and its products in vivo and has potential applications for the assessment of multiorgan metabolic phenomena.

Hyperpolarized 13C MRI: A novel approach for probing cerebral metabolism in health and neurological disease

Cerebral metabolism is tightly regulated and fundamental for healthy neurological function. There is increasing evidence that alterations in this metabolism may be a precursor and early biomarker of later stage disease processes. Proton magnetic resonance spectroscopy (1H-MRS) is a powerful tool to non-invasively assess tissue metabolites and has many applications for studying the normal and diseased brain. However, the technique has limitations including low spatial and temporal resolution, difficulties in discriminating overlapping peaks, and challenges in assessing metabolic flux rather than steady-state concentrations. Hyperpolarized carbon-13 magnetic resonance imaging is an emerging clinical technique that may overcome some of these spatial and temporal limitations, providing novel insights into neurometabolism in both health and in pathological processes such as glioma, stroke and multiple sclerosis. This review will explore the growing body of pre-clinical data that demonstrates a potential role for the technique in assessing metabolism in the central nervous system. There are now a number of clinical studies being undertaken in this area and this review will present the emerging clinical data as well as the potential future applications of hyperpolarized 13C magnetic resonance imaging in the brain, in both clinical and pre-clinical studies.

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

Hyperpolarization is an emerging method in magnetic resonance imaging that allows nuclear spin polarization of gases or liquids to be temporarily enhanced by up to five or six orders of magnitude at clinically relevant field strengths and administered at high concentration to a subject at the time of measurement. This transient gain in signal has enabled the non-invasive detection and imaging of gas ventilation and diffusion in the lungs, perfusion in blood vessels and tissues, and metabolic conversion in cells, animals, and patients. The rapid development of this method is based on advances in polarizer technology, the availability of suitable probe isotopes and molecules, improved MRI hardware and pulse sequence development. Acquisition strategies for hyperpolarized nuclei are not yet standardized and are set up individually at most sites depending on the specific requirements of the probe, the object of interest, and the MRI hardware. This review provides a detailed introduction to spatially resolved detection of hyperpolarized nuclei and summarizes novel and previously established acquisition strategies for different key areas of application.

Optimisation of pyruvate hyperpolarisation using SABRE by tuning the active magnetisation transfer catalyst

Hyperpolarisation techniques such as signal amplification by reversible exchange (SABRE) can deliver NMR signals several orders of magnitude larger than those derived under Boltzmann conditions. SABRE is able to catalytically transfer latent magnetisation from para-hydrogen to a substrate in reversible exchange via temporary associations with an iridium complex. SABRE has recently been applied to the hyperpolarisation of pyruvate, a substrate often used in many in vivo MRI studies. In this work, we seek to optimise the pyruvate-13C2 signal gains delivered through SABRE by fine tuning the properties of the active polarisation transfer catalyst. We present a detailed study of the effects of varying the carbene and sulfoxide ligands on the formation and behaviour of the active [Ir(H)2(η2-pyruvate)(sulfoxide)(NHC)] catalyst to produce a rationale for achieving high pyruvate signal gains in a cheap and refreshable manner. This optimisation approach allows us to achieve signal enhancements of 2140 and 2125-fold for the 1-13C and 2-13C sites respectively of sodium pyruvate-1,2-[13C2].

Dynamic hyperpolarized 13 C MR spectroscopic imaging using SPICE in mouse kidney at 9.4 T

This study aims to investigate the feasibility of dynamic hyperpolarized ¹³ C MR spectroscopic imaging (MRSI) using the SPectroscopic Imaging by exploiting spatiospectral CorrElation (SPICE) technique and an estimation of the spatially resolved conversion constant rate (k_pl ). An acquisition scheme comprising a single training dataset and several imaging datasets was proposed considering hyperpolarized ¹³ C circumstances. The feasibility and advantage of the scheme were investigated in two parts: (a) consistency of spectral basis over time and (b) accuracy of the estimated k_pl . The simulations and in vivo experiments support accurate k_pl estimation with consistent spectral bases. The proposed method was implemented in an enzyme phantom and via in vivo experiments. In the enzyme phantom experiments, spatially resolved homogeneous k_pl maps were observed. In the in vivo experiments, normal diet (ND) mice and high-fat diet (HFD) mice had k_pl (s^-1 ) values of medullar (ND: 0.0119 ± 0.0022, HFD: 0.0195 ± 0.0005) and cortical (ND: 0.0148 ±0.0023, HFD: 0.0224 ±0.0054) regions which were higher than vascular (ND: 0.0087 ±0.0013, HFD: 0.0132 ±0.0050) regions. In particular, the k_pl value in the medullar region exhibited a significant difference between the two diet groups. In summary, the feasibility of using modified SPICE for dynamic hyperpolarized ¹³ C MRSI was demonstrated via simulations and in vivo experiments. The consistency of spectral bases over time and the accuracy of the estimated k_pl values validate the proposed acquisition scheme, which comprises only a single training dataset. The proposed method improved the spatial resolution of dynamic hyperpolarized ¹³ C MRSI, which could be used for k_pl estimation using high signal-to-noise ratio spectral bases.

Fast Imaging for Hyperpolarized MR Metabolic Imaging

MRI with hyperpolarized carbon-13 agents has created a new type of noninvasive, in vivo metabolic imaging that can be applied in cell, animal, and human studies. The use of ¹³ C-labeled agents, primarily [1-¹³ C]pyruvate, enables monitoring of key metabolic pathways with the ability to image substrate and products based on their chemical shift. Over 10 sites worldwide are now performing human studies with this new approach for studies of cancer, heart disease, liver disease, and kidney disease. Hyperpolarized metabolic imaging studies must be performed within several minutes following creation of the hyperpolarized agent due to irreversible decay of the net magnetization back to equilibrium, so fast imaging methods are critical. The imaging methods must include multiple metabolites, separated based on their chemical shift, which are also undergoing rapid metabolic conversion (via label exchange), further exacerbating the challenges of fast imaging. This review describes the state-of-the-art in fast imaging methods for hyperpolarized metabolic imaging. This includes the approach and tradeoffs between three major categories of fast imaging methods-fast spectroscopic imaging, model-based strategies, and metabolite specific imaging-as well additional options of parallel imaging, compressed sensing, tailored RF flip angles, refocused imaging methods, and calibration methods that can improve the scan coverage, speed, signal-to-noise ratio (SNR), resolution, and/or robustness of these studies. To date, these approaches have produced extremely promising initial human imaging results. Improvements to fast hyperpolarized metabolic imaging methods will provide better coverage, SNR, resolution, and reproducibility for future human imaging studies. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 1.

Kinetic Modeling of Hyperpolarized Carbon-13 Pyruvate Metabolism in the Human Brain

Kinetic modeling of the in vivo pyruvate-to-lactate conversion is crucial to investigating aberrant cancer metabolism that demonstrates Warburg effect modifications. Non-invasive detection of alterations to metabolic flux might offer prognostic value and improve the monitoring of response to treatment. In this clinical research project, hyperpolarized [1-¹³C] pyruvate was intravenously injected in a total of 10 brain tumor patients to measure its rate of conversion to lactate ( k_PL ) and bicarbonate ( k_PB ) via echo-planar imaging. Our aim was to investigate new methods to provide k_PL and k_PB maps with whole-brain coverage. The approach was data-driven and addressed two main issues: selecting the optimal model for fitting our data and determining an appropriate goodness-of-fit metric. The statistical analysis suggested that an input-less model had the best agreement with the data. It was also found that selecting voxels based on post-fitting error criteria provided improved precision and wider spatial coverage compared to using signal-to-noise cutoffs alone.

Assessing the effect of hypoxia on cardiac metabolism using hyperpolarized 13 C magnetic resonance spectroscopy

Hypoxia plays a role in many diseases and can have a wide range of effects on cardiac metabolism depending on the extent of the hypoxic insult. Noninvasive imaging methods could shed valuable light on the metabolic effects of hypoxia on the heart in vivo. Hyperpolarized carbon-13 magnetic resonance spectroscopy (HP 13 C MRS) in particular is an exciting technique for imaging metabolism that could provide such information. The aim of our work was, therefore, to establish whether hyperpolarized 13 C MRS can be used to assess the in vivo heart's metabolism of pyruvate in response to systemic acute and chronic hypoxic exposure. Groups of healthy male Wistar rats were exposed to either acute (30 minutes), 1 week or 3 weeks of hypoxia. In vivo MRS of hyperpolarized [1-13 C] pyruvate was carried out along with assessments of physiological parameters and ejection fraction. Hematocrit was elevated after 1 week and 3 weeks of hypoxia. 30 minutes of hypoxia resulted in a significant reduction in pyruvate dehydrogenase (PDH) flux, whereas 1 or 3 weeks of hypoxia resulted in a PDH flux that was not different to normoxic animals. Conversion of hyperpolarized [1-13 C] pyruvate into [1-13 C] lactate was elevated following acute hypoxia, suggestive of enhanced anaerobic glycolysis. Elevated HP pyruvate to lactate conversion was also seen at the one week timepoint, in concert with an increase in lactate dehydrogenase (LDH) expression. Following three weeks of hypoxic exposure, cardiac metabolism of pyruvate was comparable with that observed in normoxia. We have successfully visualized the effects of systemic hypoxia on cardiac metabolism of pyruvate using hyperpolarized 13 C MRS, with differences observed following 30 minutes and 1 week of hypoxia. This demonstrates the potential of in vivo hyperpolarized 13 C MRS data for assessing the cardiometabolic effects of hypoxia in disease.

Using coligands to gain mechanistic insight into iridium complexes hyperpolarized with para-hydrogen

We report the formation of a series of novel [Ir(H)2(IMes)(α-13C2-carboxyimine)L] complexes in which the identity of the coligand L is varied. When examined with para-hydrogen, complexes in which L is benzylamine or phenethylamine show significant 1H hydride and 13C2 imine enhancements and may exist in 13C2 singlet spin order. Isotopic labeling techniques are used to double 13C2 enhancements (up to 750-fold) and singlet state lifetimes (up to 20 seconds) compared to those previously reported. Exchange spectroscopy and Density Functional Theory are used to investigate the stability and mechanism of rapid hydrogen exchange in these complexes, a process driven by dissociative coligand loss to form a key five coordinate intermediate. When L is pyridine or imidazole, competitive binding to such intermediates leads to novel complexes whose formation, kinetics, behaviour, structure, and hyperpolarization is investigated. The ratio of the observed PHIP enhancements were found to be affected not only by the hydrogen exchange rates but the identity of the coligands. This ligand reactivity is accompanied by decoherence of any 13C2 singlet order which can be preserved by isotopic labeling. Addition of a thiol coligand proved to yield a thiol oxidative addition product which is characterized by NMR and MS techniques. Significant 870-fold 13C enhancements of pyridine can be achieved using the Signal Amplification By Reversible Exchange (SABRE) process when α-carboxyimines are used to block active coordination sites. [Ir(H)2(IMes)(α-13C2-carboxyimine)L] therefore acts as unique sensors whose 1H hydride chemical shifts and corresponding hyperpolarization levels are indicative of the identity of a coligand and its binding strength.

Hyperpolarized 13C MRI: State of the Art and Future Directions

Hyperpolarized (HP) carbon 13 (¹³C) MRI is an emerging molecular imaging method that allows rapid, noninvasive, and pathway-specific investigation of dynamic metabolic and physiologic processes that were previously inaccessible to imaging. This technique has enabled real-time in vivo investigations of metabolism that are central to a variety of diseases, including cancer, cardiovascular disease, and metabolic diseases of the liver and kidney. This review provides an overview of the methods of hyperpolarization and ¹³C probes investigated to date in preclinical models of disease. The article then discusses the progress that has been made in translating this technology for clinical investigation. In particular, the potential roles and emerging clinical applications of HP [1-¹³C]pyruvate MRI will be highlighted. The future directions to enable the adoption of this technology to advance the basic understanding of metabolism, to improve disease diagnosis, and to accelerate treatment assessment are also detailed.

Kinetic Analysis of Hepatic Metabolism Using Hyperpolarized Dihydroxyacetone

Hyperpolarized carbon-13 magnetic resonance (HP-MR) is a new metabolic imaging method the does not use ionizing radiation. Due to the inherent chemical specificity of MR, not only tracer uptake but also downstream metabolism of the agent is detected in a straightforward manner. HP [2-¹³C] dihydroxyacetone (DHA) is a promising new agent that directly interrogates hepatic glucose metabolism. DHA has three metabolic fates in the liver: glucose production, glycerol production and potential inclusion into triglycerides, and oxidation in the tricarboxylic acid cycle. Each pathway is regulated by flux through multiple enzymes. Using Duhamel's formula, the kinetics of DHA metabolism is modeled, resulting in estimates of specific reaction rate constants. The multiple enzymatic steps that control DHA metabolism make more simplified methods for extracting kinetic data less than satisfactory. The described modeling paradigm effectively identifies changes in metabolism between gluconeogenic and glycogenolytic models of hepatic function.

Hyperpolarized [1-13 C] pyruvate as a possible diagnostic tool in liver disease

Introduction of hyperpolarized magnetic resonance in preclinical studies and lately translation to patients provides new detailed in vivo information of metabolic flux in organs. Hyperpolarized magnetic resonance based on 13 C enriched pyruvate is performed without ionizing radiation and allows quantification of the pyruvate conversion products: alanine, lactate and bicarbonate in real time. Thus, this methodology has a promising potential for in vivo monitoring of energetic alterations in hepatic diseases. Using 13 C pyruvate, we investigated the metabolism in the porcine liver before and after intravenous injection of glucose. The overall mean lactate to pyruvate ratio increased significantly after the injection of glucose whereas the bicarbonate to pyruvate ratio was unaffected, representative of the levels of pyruvate entering the tricarboxylic acid cycle. Similarly, alanine to pyruvate ratio did not change. The increased lactate to pyruvate ratio over time showed an exponential correlation with insulin, glucagon and free fatty acids. Together, these data, obtained by hyperpolarized 13 C magnetic resonance spectroscopy and by blood sampling, indicate a hepatic metabolic shift in glucose utilization following a glucose challenge. Our findings demonstrate the capacity of hyperpolarized 13 C magnetic resonance spectroscopy for quantifying hepatic substrate metabolism in accordance with well-known physiological processes. When combined with concentration of blood insulin, glucagon and free fatty acids in the blood, the results indicate the potential of hyperpolarized magnetic resonance spectroscopy as a future clinical method for quantification of hepatic substrate metabolism.

Hyperpolarized [1-13 C]pyruvate MRI can image the metabolic shift in cardiac metabolism between the fasted and fed state in a porcine model

PURPOSE

Owing to its noninvasive nature, hyperpolarized MRI may improve delineation of myocardial metabolic derangement in heart disease. However, consistency may depend on the changeable nature of cardiac metabolism in relation to whole-body metabolic state. This study investigates the impact of feeding status on cardiac hyperpolarized MRI in a large animal model resembling human physiology.

METHODS

Thirteen 30-kg pigs were subjected to an overnight fast, and 5 pigs were fed a carbohydrate-rich meal on the morning of the experiments. Vital parameters and blood samples were registered. All pigs were then scanned by hyperpolarized [1-¹³ C]pyruvate cardiac MRI, and results were compared between the 2 groups and correlated with circulating substrates and hormones.

RESULTS

The fed group had higher blood glucose concentration and mean arterial pressure than the fasted group. Plasma concentrations of free fatty acids (FFAs) were decreased in the fed group, whereas plasma insulin concentrations were similar between groups. Hyperpolarized MRI showed that fed animals had increased lactate/pyruvate, alanine/pyruvate, and bicarbonate/pyruvate ratios. Metabolic ratios correlated negatively with FFA levels.

CONCLUSION

Hyperpolarized MR can identify the effects of different metabolic states on cardiac metabolism in a large animal model. Unlike previous rodent studies, all metabolic derivatives of pyruvate increased in the myocardium of fed pigs. Carbohydrate-rich feeding seems to be a feasible model for standardized, large animal hyperpolarized MRI studies of myocardial carbohydrate metabolism.

Translation of Carbon-13 EPI for hyperpolarized MR molecular imaging of prostate and brain cancer patients

PURPOSE

To develop and translate a metabolite-specific imaging sequence using a symmetric echo planar readout for clinical hyperpolarized (HP) Carbon-13 (¹³ C) applications.

METHODS

Initial data were acquired from patients with prostate cancer (N = 3) and high-grade brain tumors (N = 3) on a 3T scanner. Samples of [1-¹³ C]pyruvate were polarized for at least 2 h using a 5T SPINlab system operating at 0.8 K. Following injection of the HP substrate, pyruvate, lactate, and bicarbonate (for brain studies) were sequentially excited with a singleband spectral-spatial RF pulse and signal was rapidly encoded with a single-shot echo planar readout on a slice-by-slice basis. Data were acquired dynamically with a temporal resolution of 2 s for prostate studies and 3 s for brain studies.

RESULTS

High pyruvate signal was seen throughout the prostate and brain, with conversion to lactate being shown across studies, whereas bicarbonate production was also detected in the brain. No Nyquist ghost artifacts or obvious geometric distortion from the echo planar readout were observed. The average error in center frequency was 1.2 ± 17.0 and 4.5 ± 1.4 Hz for prostate and brain studies, respectively, below the threshold for spatial shift because of bulk off-resonance.

CONCLUSION

This study demonstrated the feasibility of symmetric EPI to acquire HP ¹³ C metabolite maps in a clinical setting. As an advance over prior single-slice dynamic or single time point volumetric spectroscopic imaging approaches, this metabolite-specific EPI acquisition provided robust whole-organ coverage for brain and prostate studies while retaining high SNR, spatial resolution, and dynamic temporal resolution.

13C Pyruvate Transport Across the Blood-Brain Barrier in Preclinical Hyperpolarised MRI

Hyperpolarised MRI with Dynamic Nuclear Polarisation overcomes the fundamental thermodynamic limitations of conventional magnetic resonance, and is translating to human studies with several early-phase clinical trials in progress including early reports that demonstrate the utility of the technique to observe lactate production in human brain cancer patients. Owing to the fundamental coupling of metabolism and tissue function, metabolic neuroimaging with hyperpolarised [1-13C]pyruvate has the potential to be revolutionary in numerous neurological disorders (e.g. brain tumour, ischemic stroke, and multiple sclerosis). Through the use of [1-13C]pyruvate and ethyl-[1-13C]pyruvate in naïve brain, a rodent model of metastasis to the brain, or porcine brain subjected to mannitol osmotic shock, we show that pyruvate transport across the blood-brain barrier of anaesthetised animals is rate-limiting. We show through use of a well-characterised rat model of brain metastasis that the appearance of hyperpolarized [1-13C]lactate production corresponds to the point of blood-brain barrier breakdown in the disease. With the more lipophilic ethyl-[1-13C]pyruvate, we observe pyruvate production endogenously throughout the entire brain and lactate production only in the region of disease. In the in vivo porcine brain we show that mannitol shock permeabilises the blood-brain barrier sufficiently for a dramatic 90-fold increase in pyruvate transport and conversion to lactate in the brain, which is otherwise not resolvable. This suggests that earlier reports of whole-brain metabolism in anaesthetised animals may be confounded by partial volume effects and not informative enough for translational studies. Issues relating to pyruvate transport and partial volume effects must therefore be considered in pre-clinical studies investigating neuro-metabolism in anaesthetised animals, and we additionally note that these same techniques may provide a distinct biomarker of blood-brain barrier permeability in future studies.

Simultaneous multislice acquisition without trajectory modification for hyperpolarized 13 C experiments

Purpose

To investigate the feasibility of performing large FOV hyperpolarized ¹³C metabolic imaging using simultaneous multislice excitation.

Methods

A spectral‐spatial multislice excitation pulse was constructed by cosine modulation and incorporated into a ¹³C spiral imaging sequence. Phantom and in vivo pig experiments were performed to test the feasibility of simultaneous multislice data acquisition and image reconstruction. In vivo cardiac‐gated images of hyperpolarized pyruvate, bicarbonate, and lactate were obtained at 1 × 1 × 1 cm³ resolution over a 48 × 48 × 24 cm³ FOV with 2‐fold acceleration in the slice direction. Sensitivity encoding was used for image reconstruction with both autocalibrated and numerically calculated coil sensitivities.

Results

Simultaneous multislice images obtained with 2‐fold acceleration were comparable to reference unaccelerated images. Retained SNR figures greater than 80% were achieved over the part of the image containing the heart.

Conclusion

This method is anticipated to enable large FOV imaging studies using hyperpolarized ¹³C substrates, with an aim toward whole‐body exams that have to date been out of reach.

Probing the cardiac malate-aspartate shuttle non-invasively using hyperpolarized [1,2-13 C2 ]pyruvate

Previous studies have demonstrated that using hyperpolarized [2-¹³ C]pyruvate as a contrast agent can reveal ¹³ C signals from metabolites associated with the tricarboxylic acid (TCA) cycle. However, the metabolites detectable from TCA cycle-mediated oxidation of [2-¹³ C]pyruvate are the result of several metabolic steps. In the instance of the [5-¹³ C]glutamate signal, the amplitude can be modulated by changes to the rates of pyruvate dehydrogenase (PDH) flux, TCA cycle flux and metabolite pool size. Also key is the malate-aspartate shuttle, which facilitates the transport of cytosolic reducing equivalents into the mitochondria for oxidation via the malate-α-ketoglutarate transporter, a process coupled to the exchange of cytosolic malate for mitochondrial α-ketoglutarate. In this study, we investigated the mechanism driving the observed changes to hyperpolarized [2-¹³ C]pyruvate metabolism. Using hyperpolarized [1,2-¹³ C]pyruvate with magnetic resonance spectroscopy (MRS) in the porcine heart with different workloads, it was possible to probe ¹³ C-glutamate labeling relative to rates of cytosolic metabolism, PDH flux and TCA cycle turnover in a single experiment non-invasively. Via the [1-¹³ C]pyruvate label, we observed more than a five-fold increase in the cytosolic conversion of pyruvate to [1-¹³ C]lactate and [1-¹³ C]alanine with higher workload. ¹³ C-Bicarbonate production by PDH was increased by a factor of 2.2. Cardiac cine imaging measured a two-fold increase in cardiac output, which is known to couple to TCA cycle turnover. Via the [2-¹³ C]pyruvate label, we observed that ¹³ C-acetylcarnitine production increased 2.5-fold in proportion to the ¹³ C-bicarbonate signal, whereas the ¹³ C-glutamate metabolic flux remained constant on adrenergic activation. Thus, the ¹³ C-glutamate signal relative to the amount of ¹³ C-labeled acetyl-coenzyme A (acetyl-CoA) entering the TCA cycle was decreased by 40%. The data strongly suggest that NADH (reduced form of nicotinamide adenine dinucleotide) shuttling from the cytosol to the mitochondria via the malate-aspartate shuttle is limited on adrenergic activation. Changes in [5-¹³ C]glutamate production from [2-¹³ C]pyruvate may play an important future role in non-invasive myocardial assessment in patients with cardiovascular diseases, but careful interpretation of the results is required.

Imaging Regional Metabolic Changes in the Ischemic Rat Heart In Vivo Using Hyperpolarized [1-13C]Pyruvate

We evaluated the use of hyperpolarized ¹³C magnetic resonance imaging (MRI) in an open-chest rat model of myocardial infarction to image regional changes in myocardial metabolism. In total, 10 rats were examined before and after 30 minutes of occlusion of the left anterior descending coronary artery using hyperpolarized [1-¹³C]pyruvate. Cardiac metabolic images of [1-¹³C]pyruvate and its metabolites [1-¹³C]lactate, [1-¹³C]alanine, and [¹³C]bicarbonate were obtained before and after ischemia. Significant reduction in the [1-¹³C]alanine and [1-¹³C]lactate signals were observed in the ischemic region post ischemia. The severity of the ischemic insult was verified by increased blood levels of troponin I and by using late contrast-enhanced MRI that showed enhanced signal in the ischemic region. This study shows that hyperpolarized MRI can be used to image regional metabolic changes in the in vivo rat heart in an open-chest model of ischemia reperfusion. Hyperpolarized MRI enables new possibilities for evaluating changes in cardiac metabolism noninvasively and in real time, which potentially could be used for research to evaluate new treatments and metabolic interventions for myocardial ischemia and to apply knowledge to future application of the technique in humans.

Susceptibility-induced distortion correction in hyperpolarized echo planar imaging

Purpose

Echo planar imaging is an attractive rapid imaging readout that can image hyperpolarized compounds in vivo. By alternating the sign of the phase encoding gradient waveform, spatial offsets arising from uncertain frequency shifts can be determined. We show here that blip‐reversed echo planar imaging can also be used to correct for susceptibility and B₀ inhomogeneity effects that would otherwise produce image‐domain distortion in the heart.

Methods

Previously acquired blip‐reversed cardiac 3D‐Spectral‐Spatial echo planar imaging volumetric timecourses of hyperpolarized [1‐¹³C]pyruvate were distortion corrected by a deformation field estimated by reconstructing signal‐to‐noise ratio (SNR)‐weighted progressively subsampled temporally summed images of each metabolite.

Results

Reconstructing blip‐reversed data as proposed produced volumetric timecourses that overlaid with proton reference images more consistently than without such corrections.

Conclusion

The method proposed may form an attractive method to correct for image‐domain distortions in hyperpolarized echo planar imaging experiments. Magn Reson Med 79:2135–2141, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Development of high resolution 3D hyperpolarized carbon-13 MR molecular imaging techniques

The goal of this project was to develop and apply techniques for T2 mapping and 3D high resolution (1.5mm isotropic; 0.003cm3) 13C imaging of hyperpolarized (HP) probes [1-13C]lactate, [1-13C]pyruvate, [2-13C]pyruvate, and [13C,15N2]urea in vivo. A specialized 2D bSSFP sequence was implemented on a clinical 3T scanner and used to obtain the first high resolution T2 maps of these different hyperpolarized compounds in both rats and tumor-bearing mice. These maps were first used to optimize timings for highest SNR for single time-point 3D bSSFP acquisitions with a 1.5mm isotropic spatial resolution of normal rats. This 3D acquisition approach was extended to serial dynamic imaging with 2-fold compressed sensing acceleration without changing spatial resolution. The T2 mapping experiments yielded measurements of T2 values of >1s for all compounds within rat kidneys/vasculature and TRAMP tumors, except for [2-13C]pyruvate which was ~730ms and ~320ms, respectively. The high resolution 3D imaging enabled visualization the biodistribution of [1-13C]lactate, [1-13C]pyruvate, and [2-13C]pyruvate within different kidney compartments as well as in the vasculature. While the mouse anatomy is smaller, the resolution was also sufficient to image the distribution of all compounds within kidney, vasculature, and tumor. The development of the specialized 3D sequence with compressed sensing provided improved structural and functional assessments at a high (0.003cm3) spatial and 2s temporal resolution in vivo utilizing HP 13C substrates by exploiting their long T2 values. This 1.5mm isotropic resolution is comparable to 1H imaging and application of this approach could be extended to future studies of uptake, metabolism, and perfusion in cancer and other disease models and may ultimately be of value for clinical imaging.

Magnetic resonance imaging with hyperpolarized agents: methods and applications

In the past decade, hyperpolarized (HP) contrast agents have been under active development for MRI applications to address the twin challenges of functional and quantitative imaging. Both HP helium (³He) and xenon (¹²⁹Xe) gases have reached the stage where they are under study in clinical research. HP ¹²⁹Xe, in particular, is poised for larger scale clinical research to investigate asthma, chronic obstructive pulmonary disease, and fibrotic lung diseases. With advances in polarizer technology and unique capabilities for imaging of ¹²⁹Xe gas exchange into lung tissue and blood, HP ¹²⁹Xe MRI is attracting new attention. In parallel, HP ¹³C and ¹⁵N MRI methods have steadily advanced in a wide range of pre-clinical research applications for imaging metabolism in various cancers and cardiac disease. The HP [1-¹³C] pyruvate MRI technique, in particular, has undergone phase I trials in prostate cancer and is poised for investigational new drug trials at multiple institutions in cancer and cardiac applications. This review treats the methodology behind both HP gases and HP ¹³C and ¹⁵N liquid state agents. Gas and liquid phase HP agents share similar technologies for achieving non-equilibrium polarization outside the field of the MRI scanner, strategies for image data acquisition, and translational challenges in moving from pre-clinical to clinical research. To cover the wide array of methods and applications, this review is organized by numerical section into (1) a brief introduction, (2) the physical and biological properties of the most common polarized agents with a brief summary of applications and methods of polarization, (3) methods for image acquisition and reconstruction specific to improving data acquisition efficiency for HP MRI, (4) the main physical properties that enable unique measures of physiology or metabolic pathways, followed by a more detailed review of the literature describing the use of HP agents to study: (5) metabolic pathways in cancer and cardiac disease and (6) lung function in both pre-clinical and clinical research studies, concluding with (7) some future directions and challenges, and (8) an overall summary.

The use of hyperpolarized carbon-13 magnetic resonance for molecular imaging

Until recently, molecular imaging using magnetic resonance (MR) has been limited by the modality's low sensitivity, especially with non-proton nuclei. The advent of hyperpolarized (HP) MR overcomes this limitation by substantially enhancing the signal of certain biologically important probes through a process known as external nuclear polarization, enabling real-time assessment of tissue function and metabolism. The metabolic information obtained by HP MR imaging holds significant promise in the clinic, where it could play a critical role in disease diagnosis and therapeutic monitoring. This review will provide a comprehensive overview of the developments made in the field of hyperpolarized MR, including advancements in polarization techniques and delivery, probe development, pulse sequence optimization, characterization of healthy and diseased tissues, and the steps made towards clinical translation.

Single shot three-dimensional pulse sequence for hyperpolarized 13 C MRI

PURPOSE

Metabolic imaging with hyperpolarized 13 C-labeled cell substrates is a promising technique for imaging tissue metabolism in vivo. However, the transient nature of the hyperpolarization, and its depletion following excitation, limits the imaging time and the number of excitation pulses that can be used. We describe here a single-shot three-dimensional (3D) imaging sequence and demonstrate its capability to generate 13 C MR images in tumor-bearing mice injected with hyperpolarized [1-13 C]pyruvate.

METHODS

The pulse sequence acquires a stack-of-spirals at two spin echoes after a single excitation pulse and encodes the kz-dimension in an interleaved manner to enhance robustness to B0 inhomogeneity. Spectral-spatial pulses are used to acquire dynamic 3D images from selected hyperpolarized 13 C-labeled metabolites.

RESULTS

A nominal spatial/temporal resolution of 1.25 × 1.25 × 2.5 mm3  × 2 s was achieved in tumor images of hyperpolarized [1-13 C]pyruvate and [1-13 C]lactate acquired in vivo. Higher resolution in the z-direction, with a different k-space trajectory, was demonstrated in measurements on a thermally polarized [1-13 C]lactate phantom.

CONCLUSION

The pulse sequence is capable of imaging hyperpolarized 13 C-labeled substrates at relatively high spatial and temporal resolutions and is robust to moderate system imperfections. Magn Reson Med 77:740-752, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Imaging oxygen metabolism with hyperpolarized magnetic resonance: a novel approach for the examination of cardiac and renal function

Every tissue in the body critically depends on meeting its energetic demands with sufficient oxygen supply. Oxygen supply/demand imbalances underlie the diseases that inflict the greatest socio-economic burden globally. The purpose of this review is to examine how hyperpolarized contrast media, used in combination with MR data acquisition methods, may advance our ability to assess oxygen metabolism non-invasively and thus improve management of clinical disease. We first introduce the concept of hyperpolarization and how hyperpolarized contrast media have been practically implemented to achieve translational and clinical research. We will then analyse how incorporating hyperpolarized contrast media could enable realization of unmet technical needs in clinical practice. We will focus on imaging cardiac and renal oxygen metabolism, as both organs have unique physiological demands to satisfy their requirements for tissue oxygenation, their dysfunction plays a fundamental role in society’s most prevalent diseases, and each organ presents unique imaging challenges. It is our aim that this review attracts a multi-disciplinary audience and sparks collaborations that utilize an exciting, emergent technology to advance our ability to treat patients adversely affected by an oxygen supply/demand mismatch.

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.

Hyperpolarized 13C Metabolic MRI of the Human Heart: Initial Experience

Rationale:

Altered cardiac energetics is known to play an important role in the progression toward heart failure. A noninvasive method for imaging metabolic markers that could be used in longitudinal studies would be useful for understanding therapeutic approaches that target metabolism.

Objective:

To demonstrate the first hyperpolarized ¹³C metabolic magnetic resonance imaging of the human heart.

Methods and Results:

Four healthy subjects underwent conventional proton cardiac magnetic resonance imaging followed by ¹³C imaging and spectroscopic acquisition immediately after intravenous administration of a 0.1 mmol/kg dose of hyperpolarized [1-¹³C]pyruvate. All subjects tolerated the procedure well with no adverse effects reported ≤1 month post procedure. The [1-¹³C]pyruvate signal appeared within the chambers but not within the muscle. Imaging of the downstream metabolites showed ¹³C-bicarbonate signal mainly confined to the left ventricular myocardium, whereas the [1-¹³C]lactate signal appeared both within the chambers and in the myocardium. The mean ¹³C image signal:noise ratio was 115 for [1-¹³C]pyruvate, 56 for ¹³C-bicarbonate, and 53 for [1-¹³C]lactate.

Conclusions:

These results represent the first ¹³C images of the human heart. The appearance of ¹³C-bicarbonate signal after administration of hyperpolarized [1-¹³C]pyruvate was readily detected in this healthy cohort (n=4). This shows that assessment of pyruvate metabolism in vivo in humans is feasible using current technology.

Clinical Trial Registration:

URL: https://www.clinicaltrials.gov. Unique identifier: NCT02648009.

Application of Good's buffers to pH imaging using hyperpolarized (13)C MRI

N-(2-Acetamido)-2-aminoethanesulfonic acid (ACES), one of Good's buffers, was applied to pH imaging using hyperpolarized (13)C magnetic resonance spectroscopy. Rapid NMR- and MRI-based pH measurements were obtained by exploiting the sensitive pH-dependence of its (13)C chemical shift within the physiologic range.

Imaging Renal Urea Handling in Rats at Millimeter Resolution using Hyperpolarized Magnetic Resonance Relaxometry

In vivo spin spin relaxation time (T2) heterogeneity of hyperpolarized [13C,15N2]urea in the rat kidney was investigated. Selective quenching of the vascular hyperpolarized 13C signal with a macromolecular relaxation agent revealed that a long-T2 component of the [13C,15N2]urea signal originated from the renal extravascular space, thus allowing the vascular and renal filtrate contrast agent pools of the [13C,15N2]urea to be distinguished via multi-exponential analysis. The T2 response to induced diuresis and antidiuresis was performed with two imaging agents: hyperpolarized [13C,15N2]urea and a control agent hyperpolarized bis-1,1-(hydroxymethyl)-1-13C-cyclopropane-2H8. Large T2 increases in the inner-medullar and papilla were observed with the former agent and not the latter during antidiuresis. Therefore, [13C,15N2]urea relaxometry is sensitive to two steps of the renal urea handling process: glomerular filtration and the inner-medullary urea transporter (UT)-A1 and UT-A3 mediated urea concentrating process. Simple motion correction and subspace denoising algorithms are presented to aid in the multi exponential data analysis. Furthermore, a T2-edited, ultra long echo time sequence was developed for sub-2 mm3 resolution 3D encoding of urea by exploiting relaxation differences in the vascular and filtrate pools.