Background: Non-invasive tumor characterization and monitoring are among the key goals of medical imaging. Using hyperpolarized 13C-labelled metabolic probes fast metabolic pathways can be probed in real-time, providing new opportunities for tumor characterization. In this in vitro study, we investigated whether measurement of apparent diffusion coefficient (ADC) measurements and magnetic resonance spectroscopy (MRS) of co-polarized 13C-labeled pyruvic acid and fumaric acid can non-invasively detect both necrosis and changes in lactate export, which are parameters indicative of tumor aggressiveness. Methods:13C-labeled pyruvic acid and fumaric acid were co-polarized in a preclinical hyperpolarizer and the dissolved compounds were added to prepared samples of 8932 pancreatic cancer and MCF-7 breast carcinoma cells. Extracellular lactate concentrations and cell viability were measured in separate assays. Results: The mean ratios of the ADC values of lactate and pyruvate (ADClac/ADCpyr) between MCF-7 (0.533 ± 0.015, n = 3) and 8932 pancreatic cancer cells (0.744 ± 0.064, n = 3) showed a statistically significant difference (p = 0.048). 8932 cells had higher extracellular lactate concentrations in the extracellular medium (22.97 ± 2.53 ng/µl) compared with MCF-7 cells (7.52 ± 0.59 ng/µl; p < 0.001). Fumarate-to-malate conversion was only detectable in necrotic cells, thereby allowing clear differentiation between necrotic and viable cells. Conclusion: We provide evidence that MRS of hyperpolarized 13C-labelled pyruvic acid and fumaric acid, with their respective conversions to lactate and malate, are useful for characterization of necrosis and lactate efflux in tumor cells.

The goal of the research described in this thesis was to improve current imaging methods for nuclear magnetic resonance acquisitions with hyperpolarised biomarkers, and to apply these improved techniques to preclinical in vivo studies in order to validate their performance. Beginning with advanced pulse sequence design and reconstruction methods, a broad spectrum of biomedical applications is discussed and new methods to increase the diagnostic value of hyperpolarised examinations are presented.

Individual tumor characterization and treatment response monitoring based on current medical imaging methods remain challenging. This work investigates hyperpolarized (13) C compounds in an orthotopic rat hepatocellular carcinoma (HCC) model system before and after transcatheter arterial embolization (TAE). HCC ranks amongst the top six most common cancer types in humans and accounts for one-third of cancer-related deaths worldwide. Early therapy response monitoring could aid in the development of personalized therapy approaches and novel therapeutic concepts. Measurements with selectively (13) C-labeled and hyperpolarized urea, pyruvate and fumarate were performed in tumor-bearing rats before and after TAE. Two-dimensional, slice-selective MRSI was used to obtain spatially resolved maps of tumor perfusion, cell energy metabolic conversion rates and necrosis, which were additionally correlated with immunohistochemistry. All three injected compounds, taken together with their respective metabolites, exhibited similar signal distributions. TAE induced a decrease in blood flow into the tumor and thus a decrease in tumor to muscle and tumor to liver ratios of urea, pyruvate and its metabolites, alanine and lactate, whereas conversion rates remained stable or increased on TAE in tumor, muscle and liver tissue. Conversion from fumarate to malate successfully indicated individual levels of necrosis, and global malate signals after TAE suggested the washout of fumarase or malate itself on necrosis. This study presents a combination of three (13) C compounds as novel candidate biomarkers for a comprehensive characterization of genetically and molecularly diverse HCC using hyperpolarized MRSI, enabling the simultaneous detection of differences in tumor perfusion, metabolism and necrosis. If, as in this study, bolus dynamics are not required and qualitative perfusion information is sufficient, the desired information could be extracted from hyperpolarized fumarate and pyruvate alone, acquired at higher fields with better spectral separation. Copyright © 2016 John Wiley & Sons, Ltd.

Magnetic resonance spectroscopy (MRS) of hyperpolarized 13C pyruvate and its metabolites in large animal models is a powerful tool for assessing cardiac metabolism in patho-physiological conditions. In 13C studies, a high signal-to-noise ratio (SNR) is crucial to overcome the intrinsic data quality limitation due to the low molar concentration of certain metabolites as well as the low flux of conversion. Since 13C-MRS is essentially a semi-quantitative technique, the SNR of the spectra acquired in different myocardial segments should be homogeneous. MRS coil design plays an important role in achieving both targets. In this study, a 16-channel receive surface coil was designed for 13C hyperpolarized studies of the pig heart with a clinical 3-T scanner. The coil performance was characterized by phantom experiments and compared with that of a birdcage coil used in transmit/receive mode. Segmental signal distribution in the left ventricle (LV) was assessed by experiments on six healthy mini pigs. The proposed coil showed a significant increase in SNR for the LV wall close to the coil surface with respect to that for the birdcage but also significant segmental inhomogeneity. Hence, the use of the 16-channel coil is recommended for studies of septal and anterior LV walls.

Most tumours exhibit a high rate of glycolysis and predominantly produce energy by lactic acid fermentation. To maintain energy production and prevent toxicity, the lactate generated needs to be rapidly transported out of the cell. This is achieved by monocarboxylate transporters (MCTs), which therefore play an essential role in cancer metabolism and development. In vivo experiments were performed on eight male Fisher F344 rats bearing a subcutaneous mammary carcinoma after injection of hyperpolarised [1-(13) C]pyruvate. A Gd(III)DO3A complex that binds to pyruvate and its metabolites was used to efficiently destroy the extracellular magnetisation after hyperpolarised lactate had been formed. Moreover, a pulse sequence including a frequency-selective saturation pulse was designed so that the pyruvate magnetisation could be destroyed to exclude effects arising from further conversion. Given this preparation, metabolite transport out of the cell manifested as additional decay and apparent cell membrane transporter rates could thus be obtained using a reference measurement without a relaxation agent. In addition to slice-selective spectra, spatially resolved maps of apparent membrane transporter activity were acquired using a single-shot spiral gradient readout. A considerable increase in decay rate was detected for lactate, indicating rapid transport out of the cell. The alanine signal was unaltered, which corresponds to a slower efflux rate. This technique could allow for better understanding of tumour metabolism and progression, and enable treatment response measurements for MCT-targeted cancer therapies. Moreover, it provides vital insights into the signal kinetics of hyperpolarised [1-(13) C]pyruvate examinations. Copyright © 2016 John Wiley & Sons, Ltd.

PurposeWe characterized the performance of a novel hyperpolarized perfusion marker, α‐trideuteromethyl[15N]glutamine, for direct comparison with a 13C‐based hyperpolarized perfusion marker, [13C, 15N2]urea.MethodsA hardware platform and pulse sequence for in vivo 15N experiments were established. Hyperpolarized solutions of α‐trideuteromethyl[15N]glutamine and [13C, 15N2]urea were injected into healthy male Lewis rats. Kidney slice images were acquired using a single‐shot spiral readout. Both compounds were compared to determine in vivo signal lifetime and tracer distribution. Mass spectrometry was performed to evaluate excretion of the compound.ResultsCompared with 13C‐labeled urea, a significantly increased signal lifetime was observed. While the urea signal was gone after 90 s, decay of the glutamine compound was sufficiently slow to obtain a quantifiable signal, even after 5 min. The glutamine derivative showed strong localization in the kidneys with little background signal. Effective T1 of α‐trideuteromethyl[15N]glutamine was approximately eight‐fold higher than that of urea. Mass spectrometry results confirmed rapid excretion within the time scale of the measurement.ConclusionHyperpolarized α‐trideuteromethyl[15N]glutamine is a highly promising candidate for renal studies because of its long signal lifetime, strong localization and rapid excretion. Magn Reson Med 76:1900–1904, 2016. © 2016 International Society for Magnetic Resonance in Medicine

The aim of this study was to characterise and compare widely used acquisition strategies for hyperpolarised (13)C imaging. Free induction decay chemical shift imaging (FIDCSI), echo-planar spectroscopic imaging (EPSI), IDEAL spiral chemical shift imaging (ISPCSI) and spiral chemical shift imaging (SPCSI) sequences were designed for two different regimes of spatial resolution. Their characteristics were studied in simulations and in tumour-bearing rats after injection of hyperpolarised [1-(13)C]pyruvate on a clinical 3-T scanner. Two or three different sequences were used on the same rat in random order for direct comparison. The experimentally obtained lactate signal-to-noise ratio (SNR) in the tumour matched the simulations. Differences between the sequences were mainly found in the encoding efficiency, gradient demand and artefact behaviour. Although ISPCSI and SPCSI offer high encoding efficiencies, these non-Cartesian trajectories are more prone than EPSI and FIDCSI to artefacts from various sources. If the encoding efficiency is sufficient for the desired application, EPSI has been proven to be a robust choice. Otherwise, faster spiral acquisition schemes are recommended. The conclusions found in this work can be applied directly to clinical applications.

Dynamic nuclear polarisation has enabled real-time metabolic imaging of pyruvate and its metabolites. Conventional imaging sequences rely on predefined settings and do not account for intersubject variations in biological parameters such as perfusion. We present a fully automatic real-time bolus tracking sequence for hyperpolarised substrates which starts the imaging acquisition at a defined point on the bolus curve. This reduces artefacts due to signal change and allows for a more efficient use of hyperpolarised magnetisation. For single time point imaging methods, bolus tracking enables a more reliable and consistent quantification of metabolic activity. An RF excitation with a small flip angle is used to obtain slice-selective pyruvate tracking information in rats. Moreover, in combination with a copolarised urea and pyruvate injection, spectrally selective tracking on urea allows obtaining localised bolus tracking information without depleting the pyruvate signal. Particularly with regard to clinical application, the bolus tracking technique could provide an important step towards a routine assessment protocol which removes operator dependencies and ensures comparable results.

Hyperpolarized C-acetate for the detection of metabolic response of the heart to a stress protocol Ulrich Koellisch, Concetta V. Gringeri, Giaime Rancan, Markus Durst, Markus Schwaiger, Marion I. Menzel, Axel Haase, and Rolf F. Schulte IMETUM, Technical University München, Munich, Germany, GE Global Research, Munich, Germany, Nuclear Medicine, Technical University München, Munich, Germany, Technical University München, Munich, Germany Introduction: Acetate metabolism plays an important role particularly in myocardial cells [1]. The carboxylate molecule is taken up into the cytosol and gets converted to Acetyl-Carnitine (ALCAR) via Acetyl Coenzyme A (Fig. 1). Hyperpolarized MRS using [1-13C]acetate gives the ability to differentiate between normal and pathological metabolic rates. Specifically the conversion of [1-13C]acetate to [1-13C]ALCAR can be a negative marker for viability of myocardial cells and disorders like cardiomyopathy as it occurs in ischemia [1,5] or as a positive marker for changes of fatty acid metabolism in diabetes mellitus. The bottleneck of the proposed method on a clinical MRscanner is the low SNR of the ALCAR-signal. To adress this problem a stress protocol, previously developed for 11C-acetate-PET [6] was applied in order to enhance the myocardial acetate consumption. Furthermore a SNR-optimized spectro-spatial pulse sequence was designed. The aim of this study was to investigate the increase of ALCAR-production under stress in the short time scale of a hyperpolarized MRS experiment in respect to the ALCAR-toAcetate ratios in the rat heart. Methods: A spectro-spatial rf-pulse was used to exploit the magnetization efficiently, acquiring the ALCAR signal with a higher flip-angle (20°) than acetate (5°) (Fig. 2). The pulse was designed to be short (8 sub-lobes, duration 15,5ms, isodelay 7ms) [4], in order to minimize the T2*decay during the pulse, therefore allowing a residual excitation of acetate during the ALCAR acquisition (Fig. 3). A slice of 12mm, containing the whole heart was excited. The measurement started with the injection of acetate, the alternating excitations on the two frequencies had a repetition time of 5s for each. The flip angle corrected signal intensities of each species for 60s after injection were summed up after Rice correction of each spectrum, finally the total signal-ratio was calculated. For the preparation of the substrate, [1-13C]acetate sodium salt (4.5 M) was dissolved in glycerol doped with 30mM OXO63 radical and 1.6mM of Dotarem®. This was polarized in a 3.35T Hypersense DNP polarizer for 75 min. A 120mM acetate solution was injected into the rat tail vein inside the MR-scanner (dose 5ml/kg). All in vivo studies were performed on healthy Sprague-Dawley male rats (n = 6) on a 3T GE HDx system equipped with a dual-tuned 1H-13C-volume coil. Each animal underwent two MRI sessions: One prior to Dobutamine treatment (control) and one after infusion of Dobutamine (5μg/min/kg for 2min; 10μg/min/kg for 2 min; 20μg/min/kg for 9 min) [6].

Thermal T1 measurements for frequently used C hyperpolarization agents at clinically available field strengths Stephan Düwel, Patrick Christ, Ulrich Köllisch, Markus Durst, Concetta V Gringeri, Franz Schilling, Marion I Menzel, Rolf F Schulte, Steffen Glaser, Markus Schwaiger, and Axel Haase Institute of Medical Engineering, Technische Universität München, Garching, Germany, Department of Chemistry, Technische Universität München, Garching, Germany, GE Global Research, Garching, Germany, Institute of Nuclear Medicine, Klinikum Rechts der Isar, Munich, Germany

Compensating for Metabolite Dynamics in 13C Chemical Shift Separation Elena Nasonova, Markus Durst, Concetta Gringeri, Eliane V. Farrell, Michael Friebe, Axel Haase, Markus Schwaiger, and Rolf F. Schulte Chair of Computer Aided Medical Procedures, TU München, Munich, Germany, Zentralinstitut für Medizintechnik (IMETUM), Munich, Germany, GE Global Research, Munich, Germany, Department for Nuclear Medicine, TU München, Munich, Germany

Titus Lanz, Markus Durst, Matthias Müller, Francesca Frijia, Giulio Govanetti, Luca Menichetti, Massimo Lombardi, Jan-Henrik Ardenkjaer-Larsen, and Rolf F Schulte Rapid Biomedical, Rimpar, Germany, GE Global Research, Munich, Germany, Technical University Munich, Munich, Germany, Fondazione G. Monasterio CNRRegione Toscana, Pisa, Italy, Institute of Clinical Physiology of CNR, Pisa, Italy, GE Healthcare, Copenhagen, Denmark, Danish Technical University, Copenhagen, Denmark

Hyperpolarized [2-C]-D-fructose Uptake and Metabolism in Brain Tissue Kamil Lorenc, Eugen Kubala, Concetta V. Gringeri, Markus Durst, Ulrich Koellisch, Annette Frank, Markus Schwaiger, Steffen J. Glaser, Rolf F. Schulte, and Marion I. Menzel Nalecz Institute of Biocybernetics and Biomedical Engineering, PAS, Warsaw, Poland, Klinikum rechts der Isar, TU München, Munich, Germany, Department of Chemistry, TU München, Munich, Germany, GE Global Research, Munich, Germany, Institute of Medical Engineering, TU München, Munich, Germany

Investigation of Cell Membrane Transport and Compartmentalisation of Hyperpolarised Metabolites using a GdDO3A Relaxation Agent Markus Durst, Ulrich Koellisch, Francesca Reineri, Valeria Daniele, Concetta Gringeri, Annette Frank, Marion I. Menzel, Axel Haase, Rolf F. Schulte, and Silvio Aime IMETUM, Technische Universität München, Garching, Germany, GE Global Research, Garching, Germany, Department Chemistry I.F.M. and Molecular Imaging Center, University of Torino, Torino, Italy, Klinikum rechts der Isar, Technische Universität München, München, Germany

Introduction Hyperpolarized C metabolic imaging benefits from the enormous increase of signal, however this polarization must be used efficiently due to the irreversible T1-decay and the loss of magnetization at every excitation. For this purpose spiral trajectories with long readout times are favorable. Compared to shorter trajectories they are more sensitive to off-resonance effects, which lead to blurring artifacts. In this work, a signal model is presented which allows searching for the reconstruction frequencies of each metabolite without the need of a FID. Furthermore, the model was extended with a B0-map described by a linear combination of polynomial basis-functions. The parameters of the field-map are calculated simultaneously with the metabolite images using an iterative joint-estimation approach based on data consistency and a least squares minimization.

Scalar coupling relaxation, which is usually only associated with closely resonant nuclei (e.g., 79Br–13C), can be a very effective relaxation mechanism. While working on hyperpolarized [5-13C]glutamine, fast liquid-state polarization decay during transfer to the MRI scanner was observed. This behavior could hypothetically be explained by substantial T1 shortening due to a scalar coupling contribution (type II) to the relaxation caused by the fast-relaxing quadrupolar 14N adjacent to the 13C nucleus in the amide group. This contribution is only effective in low magnetic fields (i.e., less than 800 μT) and prevents the use of molecules bearing the 13C-amide group as hyperpolarized MRS/MRI probes. In the present work, this hypothesis is explored both theoretically and experimentally. The results show that high hyperpolarization levels can be retained using either a 15N-labeled amide or by applying a magnetic field during transfer of the sample from the polarizer to the MRI scanner.

Ulrich Köllisch, Rolf F Schulte, Markus Durst, Jan Henrik Ardenkjaer-Larsen, Francesca Frijia, Luca Menichetti, Massimo Lombardi, Axel Haase, and Florian Wiesinger Institute of Medical Engineering, Technische Universität München, Garching, Germany, GE Global Research, Garching, Germany, GE Healthcare, Brøndby, Denmark, Fondazione CNR-Regione Toscana G.Monasterio, Pisa, Italy, Institute of Clinical Physiology of CNR, Pisa, Italy

Within the last decade hyperpolarized [1‐ 13 C] pyruvate chemical‐shift imaging has demonstrated impressive potential for metabolic MR imaging for a wide range of applications in oncology, cardiology, and neurology. In this work, a highly efficient pulse sequence is described for time‐resolved, multislice chemical shift imaging of the injected substrate and obtained downstream metabolites. Using spectral‐spatial excitation in combination with single‐shot spiral data acquisition, the overall encoding is evenly distributed between excitation and signal reception, allowing the encoding of one full two‐dimensional metabolite image per excitation. The signal‐to‐noise ratio can be flexibly adjusted and optimized using lower flip angles for the pyruvate substrate and larger ones for the downstream metabolites. Selectively adjusting the excitation of the down‐stream metabolites to 90° leads to a so‐called “saturation‐recovery” scheme with the detected signal content being determined by forward conversion of the available pyruvate. In case of repetitive excitations, the polarization is preserved using smaller flip angles for pyruvate. Metabolic exchange rates are determined spatially resolved from the metabolite images using a simplified two‐site exchange model. This novel contrast is an important step toward more quantitative metabolic imaging. Goal of this work was to derive, analyze, and implement this “saturation‐recovery metabolic exchange rate imaging” and demonstrate its capabilities in four rats bearing subcutaneous tumors. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.

Introduction: C-labeled fumarate is a promising candidate for hyperpolarised in-vivo applications because it could allow the early detection of necrosis [1]. When the cell membrane integrity breaks down due to necrosis, the fumarase enzyme can be accessed by the injected fumarate and is rapidly converted to malate which therefore can be used to quantify the amount of tissue damage. This could for example be used to assess treatment response for various types of cancer. The goal of this work was to design and implement an efficient sequence for the in-vivo quantification of fumarate and its metabolite. The proposed sequence combines a multiband excitation pulse specifically tailored to the spectrum of fumarate and malate with IDEAL spiral imaging. Saturation recovery spectra are acquired for malate to analyse the metabolic kinetics. First measurements in an orthotopic rat hepatocellular carcinoma (HCC) model were successfully performed before and after transcatheter arterial embolisation (TAE) to validate the effectiveness of the method.