Comprehensive Literature Review of Hyperpolarized Carbon-13 MRI: The Road to Clinical Application

Commentary to "Task activation results in regional 13C-lactate signal increase in the human brain"

Metabolism is fundamental to functional brain imaging. While functional MRI (fMRI) has greatly benefited neuroscience, 13C-MRS measures coupling between neuroenergetics and neurotransmission. However, a hyperpolarized 13C-MRI study in human brain shows increased 13C-lactate (i.e., cytosolic aerobic glycolysis) with no 13C-bicarbonate change (i.e., mitochondrial oxidation) within fMRI-defined activated areas. We discuss (dis)advantages of hyperpolarized vs. non-hyperpolarized 13C experiments and metabolic implications regarding the lactate increase: Is lactate a fuel for oligodendrocytes, astrocytes, or neurons? Is lactate a neuromodulator or a vasomodulator? Is lactate a byproduct of astrocytic glycogenolysis? Caveats aside, there is great enthusiasm for hyperpolarized 13C-fMRI.

Evaluation of an integrated variable flip angle protocol to estimate coil B1 for hyperpolarized MRI

PURPOSE

The purpose of this work is to validate a simple and versatile integrated variable flip angle (VFA) method for mapping B1 in hyperpolarized MRI, which can be used to correct signal variations due to coil inhomogeneity.

THEORY AND METHODS

Simulations were run to assess performance of the VFA B1 mapping method compared to the currently used constant flip angle (CFA) approach. Simulation results were used to inform the design of VFA sequences, validated in four volunteers for hyperpolarized xenon-129 imaging of the lungs and another four volunteers for hyperpolarized carbon-13 imaging of the human brain. B1 maps obtained were used to correct transmit and receive inhomogeneity in the images.

RESULTS

Simulations showed improved performance of the VFA approach over the CFA approach with reduced sensitivity to T1. For xenon-129, the B1 maps accurately reflected the variation of signal depolarization, but in some cases could not be used to correct for coil receive inhomogeneity due to a lack of transmit-receive reciprocity resulting from suboptimal coil positioning. For carbon-13, the B1 maps showed good agreement with a separately acquired B1 map of a phantom and were effectively used to correct coil-induced signal inhomogeneity.

CONCLUSION

A simple, versatile, and effective VFA B1 mapping method was implemented and evaluated. Inclusion of the B1 mapping method in hyperpolarized imaging studies can enable more robust signal quantification.

Quantum life science: biological nano quantum sensors, quantum technology-based hyperpolarized MRI/NMR, quantum biology, and quantum biotechnology

The emerging field of quantum life science combines principles from quantum physics and biology to study fundamental life processes at the molecular level. Quantum mechanics, which describes the properties of small particles, can help explain how quantum phenomena such as tunnelling, superposition, and entanglement may play a role in biological systems. However, capturing these effects in living systems is a formidable challenge, as it involves dealing with dissipation and decoherence caused by the surrounding environment. We overview the current status of the quantum life sciences from technologies and topics in quantum biology. Technologies such as biological nano quantum sensors, quantum technology-based hyperpolarized MRI/NMR, high-speed 2D electronic spectrometers, and computer simulations are being developed to address these challenges. These interdisciplinary fields have the potential to revolutionize our understanding of living organisms and lead to advancements in genetics, molecular biology, medicine, and bioengineering.

K-Means Clustering of Hyperpolarised 13C-MRI Identifies Intratumoral Perfusion/Metabolism Mismatch in Renal Cell Carcinoma as the Best Predictor of the Highest Grade

Background: Early and accurate grading of renal cell carcinoma (RCC) improves patient risk stratification and has implications for clinical management and mortality. However, current diagnostic approaches using imaging and renal mass biopsy have limited specificity and may lead to undergrading. Methods: This study explored the use of hyperpolarised [1-13C]pyruvate MRI (HP 13C-MRI) to identify the most aggressive areas within the tumour of patients with clear cell renal cell carcinoma (ccRCC) as a method to guide biopsy targeting and to reduce undergrading. Six patients with ccRCC underwent presurgical HP 13C-MRI and conventional contrast-enhanced MRI. From the imaging data, three k-means clusters were computed by combining the kPL as a marker of metabolic activity, and the 13C-pyruvate signal-to-noise ratio (SNRPyr) as a perfusion surrogate. The combined clusters were compared to those derived from individual parameters and to those derived from the percentage of enhancement on the nephrographic phase (%NG). The diagnostic performance of each cluster was assessed based on its ability to predict the highest histological tumour grade in postsurgical tissue samples. The postsurgical tissue samples underwent immunohistochemical staining for the pyruvate transporter (monocarboxylate transporter 1, MCT1), as well as RNA and whole-exome sequencing. Results: The clustering approach combining SNRPyr and kPL demonstrated the best performance for predicting the highest tumour grade: specificity 85%; sensitivity 64%; positive predictive value 82%; and negative predictive value 68%. Epithelial MCT1 was identified as the major determinant of the HP 13C-MRI signal. The perfusion/metabolism mismatch cluster showed an increased expression of metabolic genes and markers of aggressiveness. Conclusions: This study demonstrates the potential of using HP 13C-MRI-derived metabolic clusters to identify intratumoral variations in tumour grade with high specificity. This work supports the use of metabolic imaging to guide biopsies to the most aggressive tumour regions and could potentially reduce sampling error.

Quo Vadis Hyperpolarized 13C MRI?

Over the last two decades, hyperpolarized 13C MRI has gained significance in both preclinical and clinical studies, hereby relying on technologies like PHIP-SAH (ParaHydrogen-Induced Polarization-Side Arm Hydrogenation), SABRE (Signal Amplification by Reversible Exchange), and dDNP (dissolution Dynamic Nuclear Polarization), with dDNP being applied in humans. A clinical dDNP polarizer has enabled studies across 24 sites, despite challenges like high cost and slow polarization. Parahydrogen-based techniques like SABRE and PHIP offer faster, more cost-efficient alternatives but require molecule-specific optimization. The focus has been on imaging metabolism of hyperpolarized probes, which requires long T1, high polarization and rapid contrast generation. Efforts to establish novel probes, improve acquisition techniques and enhance data analysis methods including artificial intelligence are ongoing. Potential clinical value of hyperpolarized 13C MRI was demonstrated primarily for treatment response assessment in oncology, but also in cardiology, nephrology, hepatology and CNS characterization. In this review on biomedical hyperpolarized 13C MRI, we summarize important and recent advances in polarization techniques, probe development, acquisition and analysis methods as well as clinical trials. Starting from those we try to sketch a trajectory where the field of biomedical hyperpolarized 13C MRI might go.

The Role of Proton Magnetic Resonance Spectroscopy in Neonatal and Fetal Brain Research

The biochemical composition and structure of the brain are in a rapid change during the exuberant stage of fetal and neonatal development. 1H-MRS is a noninvasive tool that can evaluate brain metabolites in healthy fetuses and infants as well as those with neurological diseases. This review aims to provide readers with an understanding of 1) the basic principles and technical considerations relevant to 1H-MRS in the fetal-neonatal brain and 2) the role of 1H-MRS in early fetal-neonatal development brain research. We performed a PubMed search to identify original studies using 1H-MRS in neonates and fetuses to establish the clinical applications of 1H-MRS. The eligible studies for this review included original research with 1H-MRS applications to the fetal-neonatal brain in healthy and high-risk conditions. We ran our search between 2000 and 2023, then added in several high-impact landmark publications from the 1990s. A total of 366 results appeared. After, we excluded original studies that did not include fetuses or neonates, non-proton MRS and non-neurological studies. Eventually, 110 studies were included in this literature review. Overall, the function of 1H-MRS in healthy fetal-neonatal brain studies focuses on measuring the change of metabolite concentrations during neurodevelopment and the physical properties of the metabolites such as T1/T2 relaxation times. For high-risk neonates, studies in very low birth weight preterm infants and full-term neonates with hypoxic-ischemic encephalopathy, along with examining the associations between brain biochemistry and cognitive neurodevelopment are most common. Additional high-risk conditions included infants with congenital heart disease or metabolic diseases, as well as fetuses of pregnant women with hypertensive disorders were of specific interest to researchers using 1H-MRS. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 2.

Dual inhibition of oxidative phosphorylation and glycolysis exerts a synergistic antitumor effect on colorectal and gastric cancer by creating energy depletion and preventing metabolic switch

Pyruvate is situated at the intersection of oxidative phosphorylation (OXPHOS) and glycolysis, which are the primary energy-producing pathways in cells. Cancer therapies targeting these pathways have been previously documented, indicating that inhibiting one pathway may lead to functional compensation by the other, resulting in an insufficient antitumor effect. Thus, effective cancer treatment necessitates concurrent and comprehensive suppression of both. However, whether a metabolic switch between the metabolic pathways occurs in colorectal and gastric cancer cells and whether blocking it by inhibiting both pathways has an antitumor effect remain to be determined. In the present study, we used two small molecules, namely OXPHOS and glycolysis inhibitors, to target pyruvate metabolic pathways as a cancer treatment in these cancer cells. OXPHOS and glycolysis inhibition each augmented the other metabolic pathway in vitro and in vivo. OXPHOS inhibition alone enhanced glycolysis and showed antitumor effects on colorectal and gastric cancer cells in vitro and in vivo. Moreover, glycolysis inhibition in addition to OXPHOS inhibition blocked the metabolic switch from OXPHOS to glycolysis, causing an energy depletion and deterioration of the tumor microenvironment that synergistically enhanced the antitumor effect of OXPHOS inhibitors. In addition, using hyperpolarized 13C-magnetic resonance spectroscopic imaging (HP-MRSI), which enables real-time and in vivo monitoring of molecules containing 13C, we visualized how the inhibitors shifted the flux of pyruvate and how this dual inhibition in colorectal and gastric cancer mouse models altered the two pathways. Integrating dual inhibition of OXPHOS and glycolysis with HP-MRSI, this therapeutic model shows promise as a future "cancer theranostics" treatment option.

PyAMARES, an Open-Source Python Library for Fitting Magnetic Resonance Spectroscopy Data

Background/Objectives: Magnetic resonance spectroscopy (MRS) is a valuable tool for studying metabolic processes in vivo. While numerous quantification methods exist, the advanced method for accurate, robust, and efficient spectral fitting (AMARES) is among the most used. This study introduces pyAMARES, an open-source Python implementation of AMARES, addressing the need for a flexible, user-friendly, and versatile MRS quantification tool within the Python ecosystem. Methods: PyAMARES was developed as a Python library, implementing the AMARES algorithm with additional features such as multiprocessing capabilities and customizable objective functions. The software was validated against established AMARES implementations (OXSA and jMRUI) using both simulated and in vivo MRS data. Monte Carlo simulations were conducted to assess robustness and accuracy across various signal-to-noise ratios and parameter perturbations. Results: PyAMARES utilizes spreadsheet-based prior knowledge and fitting parameter settings, enhancing flexibility and ease of use. It demonstrated comparable performance to existing software in terms of accuracy, precision, and computational efficiency. In addition to conventional AMARES fitting, pyAMARES supports fitting without prior knowledge, frequency-selective AMARES, and metabolite residual removal from mobile macromolecule (MM) spectra. Utilizing multiple CPU cores significantly enhances the performance of pyAMARES. Conclusions: PyAMARES offers a robust, flexible, and user-friendly solution for MRS quantification within the Python ecosystem. Its open-source nature, comprehensive documentation, and integration with popular data science tools enhance reproducibility and collaboration in MRS research. PyAMARES bridges the gap between traditional MRS fitting methods and modern machine learning frameworks, potentially accelerating advancements in metabolic studies and clinical applications.

Design, Synthesis, and Assessment of Tricarboxylic Acid Cycle Probes

Hyperpolarized 13C magnetic resonance spectroscopy can provide unique insights into metabolic activity in vivo. Despite the advantages of this technology, certain metabolic pathways such as the tricarboxylic acid (TCA) cycle are more challenging to examine due to the limitations associated with currently available hyperpolarized 13C probes. In this report, we systematically employ computational analyses, synthetic techniques, and in vitro studies to facilitate the design of new chemical probes for the TCA cycle. This platform allows for the rapid identification of probe scaffolds that are amenable to hyperpolarized 13C experimentation. Using these results, we have developed two 13C-labeled chemical probes, [1,4-13C2]-dipropyl succinate and [1,4-13C2]-diallyl succinate, which are employed in hyperpolarized 13C metabolic studies.

Long-lived enhanced magnetization-A practical metabolic MRI contrast material

Noninvasive tracking of biochemical processes in the body is paramount in diagnostic medicine. Among the leading techniques is spectroscopic magnetic resonance imaging (MRI), which tracks metabolites with an amplified (hyperpolarized) magnetization signal injected into the subject just before scanning. Traditionally, the brief enhanced magnetization period of these agents limited clinical imaging. We propose a solution based on amalgamating two materials-one having diagnostic-metabolic activity and the other characterized by robust magnetization retention. This combination slows the magnetization decay in the diagnostic metabolic probe, which receives continuously replenished magnetization from the companion material. Thus, it extends the magnetization lifetime in some of our measurements to beyond 4 min, with net magnetization enhanced by more than four orders of magnitude. This could allow the metabolic probes to remain magnetized from injection until they reach the targeted organ, improving tissue signatures in clinical imaging. Upon validation, this metabolic MRI technique promises wide-ranging clinical applications, including diagnostic imaging, therapeutic monitoring, and posttreatment surveillance.

Efficient Parahydrogen-Induced 13C Hyperpolarization on a Microfluidic Device

We show the direct production and detection of 13C-hyperpolarized fumarate by parahydrogen-induced polarization (PHIP) in a microfluidic lab-on-a-chip (LoC) device and achieve 8.5% 13C polarization. This is the first demonstration of 13C-hyperpolarization of a metabolite by PHIP in a microfluidic device. LoC technology allows the culture of mammalian cells in a highly controlled environment, providing an important tool for the life sciences. In-situ preparation of hyperpolarized metabolites greatly enhances the ability to quantify metabolic processes in such systems by microfluidic NMR. PHIP of 1H nuclei has been successfully implemented in microfluidic systems, with mass sensitivities in the range of pmol/s. However, metabolic NMR requires high-yield production of hyperpolarized metabolites with longer spin life times than is possible with 1H. This can be achieved by transfer of the polarization onto 13C nuclei, which exhibit much longer T1 relaxation times. We report an improved microfluidic PHIP device, optimized using a finite element model, that enables the direct and efficient production of 13C-hyperpolarized fumarate.

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.

NMRduino: A modular, open-source, low-field magnetic resonance platform

The NMRduino is a compact, cost-effective, sub-MHz NMR spectrometer that utilizes readily available open-source hardware and software components. One of its aims is to simplify the processes of instrument setup and data acquisition control to make experimental NMR spectroscopy accessible to a broader audience. In this introductory paper, the key features and potential applications of NMRduino are described to highlight its versatility both for research and education.

Mice and minipigs with compromised expression of the Alzheimer's disease gene SORL1 show cerebral metabolic disturbances on hyperpolarized [1-13C]pyruvate and sodium MRI

The sortilin-related receptor 1 (SORL1) gene, encoding the cellular endosomal sorting-related receptor with A-type repeats (SORLA), is now established as a causal gene for Alzheimer's disease. As the latest addition to the list of causal genes, the pathophysiological effects and biomarker potential of SORL1 variants remain relatively undiscovered. Metabolic dysfunction is, however, well described in patients with Alzheimer's disease and is used as an imaging biomarker in clinical diagnosis settings. To understand the metabolic consequences of loss-of-function SORL1 mutations, we applied two metabolic MRI technologies, sodium (23Na) MRI and MRI with hyperpolarized [1-13C]pyruvate, in minipigs and mice with compromised expression of SORL1. At the age analysed here, both animal models display no conventional imaging evidence of neurodegeneration but show biochemical signs of elevated amyloid production, thus representing the early preclinical disease. With hyperpolarized MRI, the exchange from [1-13C]pyruvate to [1-13C]lactate and 13C-bicarbonate was decreased by 32 and 23%, respectively, in the cerebrum of SORL1-haploinsufficient minipigs. A robust 11% decrease in the sodium content was observed with 23Na-MRI in the same minipigs. Comparably, the brain sodium concentration gradually decreased from control to SORL1 haploinsufficient (-11%) to SORL1 knockout mice (-23%), suggesting a gene dose dependence in the metabolic dysfunction. The present study highlights that metabolic MRI technologies are sensitive to the functional, metabolic consequences of Alzheimer's disease and Alzheimer's disease-linked genotypes. Further, the study suggests a potential avenue of research into the mechanisms of metabolic alterations by SORL1 mutations and their potential role in neurodegeneration.

Molecular imaging of tumor metabolism: Insight from pyruvate- and glucose-based deuterium MRI studies

Cancer diagnosis by metabolic MRI proposes to follow the fate of glycolytic precursors such as pyruvate or glucose, and their in vivo conversion into lactate. This study compares the 2H MRI outlooks afforded by these metabolites when targeting a pancreatic cancer model. Exogenously injected [3,3',3″-2H3]-pyruvate was visible only briefly; it generated a deuterated lactate signal throughout the body that faded after ~5 min, showing a minor concentration bias at the rims of the tumors. [6,6'-2H2]-glucose by contrast originated a lactate signal that localized clearly within the tumors, persisting for over an hour. Investigations alternating deuterated and nondeuterated glucose injections revealed correlations between the lactate generation and the glucose available at the tumor, evidencing a continuous and avid glucose consumption generating well-localized lactate signatures as driven by the Warburg effect. This is by contrast to the transient and more promiscuous pyruvate-to-lactate transformation, which seemed subject to transporter and kinetics effects. The consequences of these observations within metabolic MRI are briefly discussed.

Parallel detection of chemical reactions in a microfluidic platform using hyperpolarized nuclear magnetic resonance

The sensitivity of NMR may be enhanced by more than four orders of magnitude via dissolution dynamic nuclear polarization (dDNP), potentially allowing real-time, in situ analysis of chemical reactions. However, there has been no widespread use of the technique for this application and the major limitation has been the low experimental throughput caused by the time-consuming polarization build-up process at cryogenic temperatures and fast decay of the hyper-intense signal post dissolution. To overcome this limitation, we have developed a microfluidic device compatible with dDNP-MR spectroscopic imaging methods for detection of reactants and products in chemical reactions in which up to 8 reactions can be measured simultaneously using a single dDNP sample. Multiple MR spectroscopic data sets can be generated under the same exact conditions of hyperpolarized solute polarization, concentration, pH, and temperature. A proof-of-concept for the technology is demonstrated by identifying the reactants in the decarboxylation of pyruvate via hydrogen peroxide (e.g. 2-hydroperoxy-2-hydroxypropanoate, peroxymonocarbonate and CO2). dDNP-MR allows tracing of fast chemical reactions that would be barely detectable at thermal equilibrium by MR. We envisage that dDNP-MR spectroscopic imaging combined with microfluidics will provide a new high-throughput method for dDNP enhanced MR analysis of multiple components in chemical reactions and for non-destructive in situ metabolic analysis of hyperpolarized substrates in biological samples for laboratory and preclinical research.

High-sensitivity deuterium metabolic MRI differentiates acute pancreatitis from pancreatic cancers in murine models

Deuterium metabolic imaging (DMI) is a promising tool for investigating a tumor's biology, and eventually contribute in cancer diagnosis and prognosis. In DMI, [6,6'-2H2]-glucose is taken up and metabolized by different tissues, resulting in the formation of HDO but also in an enhanced formation of [3,3'-2H2]-lactate at the tumor site as a result of the Warburg effect. Recent studies have shown DMI's suitability to highlight pancreatic cancer in murine models by [3,3'-2H2]-lactate formation; an important question is whether DMI can also differentiate between these tumors and pancreatitis. This differentiation is critical, as these two diseases are hard to distinguish today radiologically, but have very different prognoses requiring distinctive treatments. Recent studies have shown the limitations that hyperpolarized MRI faces when trying to distinguish these pancreatic diseases by monitoring the [1-13C1]-pyruvate→[1-13C1]-lactate conversion. In this work, we explore DMI's capability to achieve such differentiation. Initial tests used a multi-echo (ME) SSFP sequence, to identify any metabolic differences between tumor and acute pancreatitis models that had been previously elicited very similar [1-13C1]-pyruvate→[1-13C1]-lactate conversion rates. Although ME-SSFP provides approximately 5 times greater signal-to-noise ratio (SNR) than the standard chemical shift imaging (CSI) experiment used in DMI, no lactate signal was observed in the pancreatitis model. To enhance lactate sensitivity further, we developed a new, weighted-average, CSI-SSFP approach for DMI. Weighted-average CSI-SSFP improved DMI's SNR by another factor of 4 over ME-SSFP-a sensitivity enhancement that sufficed to evidence natural abundance 2H fat in abdominal images, something that had escaped the previous approaches even at ultrahigh (15.2 T) MRI fields. Despite these efforts to enhance DMI's sensitivity, no lactate signal could be detected in acute pancreatitis models (n = 10; [3,3'-2H2]-lactate limit of detection < 100 µM; 15.2 T). This leads to the conclusion that pancreatic tumors and acute pancreatitis may be clearly distinguished by DMI, based on their different abilities to generate deuterated lactate.

Lactate and Lactylation: Clinical Applications of Routine Carbon Source and Novel Modification in Human Diseases

Cell metabolism generates numerous intermediate metabolites that could serve as feedback and feed-forward regulation substances for posttranslational modification. Lactate, a metabolic product of glycolysis, has recently been conceptualized to play a pleiotropic role in shaping cell identities through metabolic rewiring and epigenetic modifications. Lactate-derived carbons, sourced from glucose, mediate the crosstalk among glycolysis, lactate, and lactylation. Furthermore, the multiple metabolic fates of lactate make it an ideal substrate for metabolic imaging in clinical application. Several studies have identified the crucial role of protein lactylation in human diseases associated with cell fate determination, embryonic development, inflammation, neoplasm, and neuropsychiatric disorders. Herein, this review will focus on the metabolic fate of lactate-derived carbon to provide useful information for further research and therapeutic approaches in human diseases. We comprehensively discuss its role in reprogramming and modification during the regulation of glycolysis, the clinical translation prospects of the hyperpolarized lactate signal, lactyl modification in human diseases, and its application with other techniques and omics.

Functional lung imaging using novel and emerging MRI techniques

Respiratory diseases are leading causes of death and disability in the world. While early diagnosis is key, this has proven difficult due to the lack of sensitive and non-invasive tools. Computed tomography is regarded as the gold standard for structural lung imaging but lacks functional information and involves significant radiation exposure. Lung magnetic resonance imaging (MRI) has historically been challenging due to its short T2 and low proton density. Hyperpolarised gas MRI is an emerging technique that is able to overcome these difficulties, permitting the functional and microstructural evaluation of the lung. Other novel imaging techniques such as fluorinated gas MRI, oxygen-enhanced MRI, Fourier decomposition MRI and phase-resolved functional lung imaging can also be used to interrogate lung function though they are currently at varying stages of development. This article provides a clinically focused review of these contrast and non-contrast MR imaging techniques and their current applications in lung disease.

Deuterium metabolic imaging and hyperpolarized 13C-MRI of the normal human brain at clinical field strength reveals differential cerebral metabolism

Deuterium metabolic imaging (DMI) and hyperpolarized 13C-pyruvate MRI (13C-HPMRI) are two emerging methods for non-invasive and non-ionizing imaging of tissue metabolism. Imaging cerebral metabolism has potential applications in cancer, neurodegeneration, multiple sclerosis, traumatic brain injury, stroke, and inborn errors of metabolism. Here we directly compare these two non-invasive methods at 3 T for the first time in humans and show how they simultaneously probe both oxidative and non-oxidative metabolism. DMI was undertaken 1-2 h after oral administration of [6,6'-2H2]glucose, and 13C-MRI was performed immediately following intravenous injection of hyperpolarized [1-13C]pyruvate in ten and nine normal volunteers within each arm respectively. DMI was used to generate maps of deuterium-labelled water, glucose, lactate, and glutamate/glutamine (Glx) and the spectral separation demonstrated that DMI is feasible at 3 T. 13C-HPMRI generated maps of hyperpolarized carbon-13 labelled pyruvate, lactate, and bicarbonate. The ratio of 13C-lactate/13C-bicarbonate (mean 3.7 ± 1.2) acquired with 13C-HPMRI was higher than the equivalent 2H-lactate/2H-Glx ratio (mean 0.18 ± 0.09) acquired using DMI. These differences can be explained by the route of administering each probe, the timing of imaging after ingestion or injection, as well as the biological differences in cerebral uptake and cellular physiology between the two molecules. The results demonstrate these two metabolic imaging methods provide different yet complementary readouts of oxidative and reductive metabolism within a clinically feasible timescale. Furthermore, as DMI was undertaken at a clinical field strength within a ten-minute scan time, it demonstrates its potential as a routine clinical tool in the future.

Imaging Neurodegenerative Metabolism in Amyotrophic Lateral Sclerosis with Hyperpolarized [1-13C]pyruvate MRI

The cause of amyotrophic lateral sclerosis (ALS) is still unknown, and consequently, early diagnosis of the disease can be difficult and effective treatment is lacking. The pathology of ALS seems to involve specific disturbances in carbohydrate metabolism, which may be diagnostic and therapeutic targets. Magnetic resonance imaging (MRI) with hyperpolarized [1-¹³C]pyruvate is emerging as a technology for the evaluation of pathway-specific changes in the brain’s metabolism. By imaging pyruvate and the lactate and bicarbonate it is metabolized into, the technology is sensitive to the metabolic changes of inflammation and mitochondrial dysfunction. In this study, we performed hyperpolarized MRI of a patient with newly diagnosed ALS. We found a lateralized difference in [1-¹³C]pyruvate-to-[1-¹³C]lactate exchange with no changes in exchange from [1-¹³C]pyruvate to ¹³C-bicarbonate. The 40% increase in [1-¹³C]pyruvate-to-[1-¹³C]lactate exchange corresponded with the patient’s symptoms and presentation with upper-motor neuron affection and cortical hyperexcitability. The data presented here demonstrate the feasibility of performing hyperpolarized MRI in ALS. They indicate potential in pathway-specific imaging of dysfunctional carbohydrate metabolism in ALS, an enigmatic neurodegenerative disease.

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.

Hyperpolarized 13 C MRI Reveals Large Changes in Pyruvate Metabolism During Digestion in Snakes

Purpose

Methods

Results

Conclusion

Temperature Cycling Enables Efficient 13C SABRE-SHEATH Hyperpolarization and Imaging of [1-13C]-Pyruvate

Molecular metabolic imaging in humans is dominated by positron emission tomography (PET). An emerging nonionizing alternative is hyperpolarized MRI of 13C-pyruvate, which is innocuous and has a central role in metabolism. However, similar to PET, hyperpolarized MRI with dissolution dynamic nuclear polarization (d-DNP) is complex costly, and requires significant infrastructure. In contrast, Signal Amplification By Reversible Exchange (SABRE) is a fast, cheap, and scalable hyperpolarization technique. SABRE in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) can transfer polarization from parahydrogen to 13C in pyruvate; however, polarization levels remained low relative to d-DNP (1.7% with SABRE-SHEATH versus ≈60% with DNP). Here we introduce a temperature cycling method for SABRE-SHEATH that enables >10% polarization on [1-13C]-pyruvate, sufficient for successful in vivo experiments. First, at lower temperatures, ≈20% polarization is accumulated on SABRE catalyst-bound pyruvate, which is released into free pyruvate at elevated temperatures. A kinetic model of differential equations is developed that explains this effect and characterizes critical relaxation and buildup parameters. With the large polarization, we demonstrate the first 13C pyruvate images with a cryogen-free MRI system operated at 1.5 T, illustrating that inexpensive hyperpolarization methods can be combined with low-cost MRI systems to obtain a broadly available, yet highly sensitive metabolic imaging platform.

Hyperpolarized MRI - An update and future perspectives

In recent years, hyperpolarized ¹³C magnetic resonance spectroscopic (MRS) imaging has emerged as a complementary metabolic imaging approach. Hyperpolarization via dissolution dynamic nuclear polarization is a technique that enhances the MR signal of ¹³C-enriched molecules by a factor of > 10⁴, enabling detection downstream metabolites in a variety of intracellular metabolic pathways. The aim of the present review is to provide the reader with an update on hyperpolarized ¹³C MRS imaging and to assess the future clinical potential of the technology. Several carbon-based probes have been used in hyperpolarized studies. However, the first and most widely used ¹³C-probe in clinical studies is [1-¹³C]pyruvate. In this probe, the enrichment of ¹³C is performed at the first carbon position as the only modification. Hyperpolarized [1-¹³C]pyruvate MRS imaging can detect intracellular production of [1-¹³C]lactate and ¹³C-bicarbonate non-invasively and in real time without the use of ionizing radiation. Thus, by probing the balance between oxidative and glycolytic metabolism, hyperpolarized [1-¹³C]pyruvate MRS imaging can image the Warburg effect in malignant tumors and detect the hallmarks of ischemia or viability in the myocardium. An increasing number of clinical studies have demonstrated that clinical hyperpolarized ¹³C MRS imaging is not only possible, but also it provides metabolic information that was previously inaccessible by non-invasive techniques. Although the technology is still in its infancy and several technical improvements are warranted, it is of paramount importance that nuclear medicine physicians gain knowledge of the possibilities and pitfalls of the technique. Hyperpolarized ¹³C MRS imaging may become an integrated feature in combined metabolic imaging of the future.