Effects of isoflurane anesthesia on hyperpolarized (13)C metabolic measurements in rat brain

Effect of reducing isoflurane level on glucosamine uptake in the mouse brain during magnetic resonance imaging studies

Anesthesia is often required during magnetic resonance imaging (MRI) examinations in animal studies. Anesthetic drugs differ in their capacity to interfere with homeostatic mechanisms responsible for glucose metabolism in the brain, which may create a constraint in the study design. Recent studies suggest that the chemical exchange saturation transfer (CEST) MRI scanning technique can detect localized metabolic changes in rodent brains induced by the uptake of glucose or its analogs; however, most of these studies do not account for the impact of anesthesia type on the brain metabolism. Herein, we aimed to evaluate the effect of reduced isoflurane levels on the preclinical imaging of glucosamine (GlcN) uptake in healthy mouse brains to establish optimal conditions for future brain imaging studies using the CEST MRI technique. The commonly used anesthesia protocol for longitudinal MRI examinations using 1.5% isoflurane level was compared to that using a mixture of low isoflurane (0.8%) level combined with midazolam (2 mg/kg, SC). Magnetization transfer ratio asymmetry (MTRasym) and area under the curve (AUC) analyses were used to characterize GlcN signals in the brain. The results indicated that mice injected with GlcN and anesthetized with 1.5% isoflurane exhibited low and insignificant changes in the MTRasym and AUC signals in the frontal cortex, whereas mice administered with 0.8% isoflurane combined with midazolam demonstrated a significant increase in these signals in the frontal cortex. This study highlights the diverse GlcN metabolic changes observed in mouse brains under variable levels of isoflurane anesthesia using the CEST MRI method. The results suggest that it is feasible to maintain anesthesia with low-dose isoflurane by integrating midazolam, which may enable the investigation of GlcN uptake in the brain. Thus, reducing isoflurane levels may support studies into mouse brain metabolism using the CEST MRI method and should be considered in future studies.

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

PURPOSE

Pyruvate, produced from either glucose, glycogen, or lactate, is the dominant precursor of cerebral oxidative metabolism. Pyruvate dehydrogenase (PDH) flux is a direct measure of cerebral mitochondrial function and metabolism. Detection of [13 C]bicarbonate in the brain from hyperpolarized [1-13 C]pyruvate using carbon-13 (13 C) MRI provides a unique opportunity for assessing PDH flux in vivo. This study is to assess changes in cerebral PDH flux in response to visual stimuli using in vivo 13 C MRS with hyperpolarized [1-13 C]pyruvate.

METHODS

From seven sedentary adults in good general health, time-resolved [13 C]bicarbonate production was measured in the brain using 90° flip angles with minimal perturbation of its precursors, [1-13 C]pyruvate and [1-13 C]lactate, to test the hypothesis that the appearance of [13 C]bicarbonate signals in the brain reflects the metabolic changes associated with neuronal activation. With a separate group of healthy participants (n = 3), the likelihood of the bolus-injected [1-13 C]pyruvate being converted to [1-13 C]lactate prior to decarboxylation was investigated by measuring [13 C]bicarbonate production with and without [1-13 C]lactate saturation.

RESULTS

In the course of visual stimulation, the measured [13 C]bicarbonate signal normalized to the total 13 C signal in the visual cortex increased by 17.1% ± 15.9% (p = 0.017), whereas no significant change was detected in [1-13 C]lactate. Proton BOLD fMRI confirmed the regional activation in the visual cortex with the stimuli. Lactate saturation decreased bicarbonate-to-pyruvate ratio by 44.4% ± 9.3% (p < 0.01).

CONCLUSION

We demonstrated the utility of 13 C MRS with hyperpolarized [1-13 C]pyruvate for assessing the activation of cerebral PDH flux via the detection of [13 C]bicarbonate production.

Investigating the origin of the 13 C lactate signal in the anesthetized healthy rat brain in vivo after hyperpolarized [1-13 C]pyruvate injection

The goal of this study was to investigate the origin of brain lactate (Lac) signal in the healthy anesthetized rat after injection of hyperpolarized (HP) [1-13 C]pyruvate (Pyr). Dynamic two-dimensional spiral chemical shift imaging with flow-sensitizing gradients revealed reduction in both vascular and brain Pyr, while no significant dependence on the level of flow suppression was detected for Lac. These results support the hypothesis that the HP metabolites predominantly reside in different compartments in the brain (i.e., Pyr in the blood and Lac in the parenchyma). Data from high-resolution metabolic imaging of [1-13 C]Pyr further demonstrated that Lac detected in the brain was not from contributions of vascular signal attributable to partial volume effects. Additionally, metabolite distributions and kinetics measured with dynamic imaging after injection of HP [1-13 C]Lac were similar to Pyr data when Pyr was used as the substrate. These data do not support the hypothesis that Lac observed in the brain after Pyr injection was generated in other organs and then transported across the blood-brain barrier (BBB). Together, the presented results provide further evidence that even in healthy anesthetized rats, the transport of HP Pyr across the BBB is sufficiently fast to permit detection of its metabolic conversion to Lac within the brain.

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.

Metabolism of hyperpolarised [1-13 C]pyruvate in awake and anaesthetised rat brains

The use of hyperpolarised ¹³ C pyruvate for nononcological neurological applications has not been widespread so far, possibly due to delivery issues limiting the visibility of metabolites. First proof-of-concept results have indicated that metabolism can be detected in human brain, and this may supersede the results obtained in preclinical settings. One major difference between the experimental setups is that preclinical MRI/MRS routinely uses anaesthesia, which alters both haemodynamics and metabolism. Here, we used hyperpolarised [1-¹³ C]pyruvate to compare brain metabolism in awake rats and under isoflurane, urethane or medetomidine anaesthesia. Spectroscopic [1-¹³ C]pyruvate time courses measured sequentially showed that pyruvate-to-bicarbonate and pyruvate-to-lactate labelling rates were lower in isoflurane animals than awake animals. An increased bicarbonate-to-lactate ratio was observed in the medetomidine group compared with other groups. The study shows that hyperpolarised [1-¹³ C]pyruvate experiments can be performed in awake rats, thus avoiding anaesthesia-related issues. The results suggest that haemodynamics probably dominate the observed pyruvate-to-metabolite labelling rates and area-under-time course ratios of referenced to pyruvate. On the other hand, the results obtained with medetomidine suggest that the ratios are also modulated by the underlying cerebral metabolism. However, the ratios between intracellular metabolites were unchanged in awake compared with isoflurane-anaesthetised rats.

In Vivo 13C Magnetic Resonance Spectroscopy for Assessing Brain Biochemistry in Health and Disease

Magnetic resonance spectroscopy (MRS) is a non-invasive technique that contributes to the elucidation of brain biochemistry. ¹³C MRS enables the detection of specific neurochemicals and their neuroenergetic correlation with neuronal function. The synergistic outcome of ¹³C MRS and the infusion of ¹³C-labeled substrates provide an understanding of neurometabolism and the role of glutamate/gamma-aminobutyric acid (GABA) neurotransmission in diseases, such as Alzheimer's disease, schizophrenia, and bipolar disorder. Moreover, ¹³C MRS provides a window into the altered flux rate of different pathways, including the tricarboxylic acid cycle (TCA) and the glutamate/glutamine/GABA cycle, in health and in various diseases. Notably, the metabolic flux rate of the TCA cycle often decreases in neurodegenerative diseases. Additionally, ¹³C MRS can be used to investigate several psychiatric and neurological disorders as it directly reflects the real-time production and alterations of key brain metabolites. This review aims to highlight the chronology, the technological advancements, and the applications of ¹³C MRS in various brain diseases.

Probing Cerebral Metabolism with Hyperpolarized 13C Imaging after Opening the Blood-Brain Barrier with Focused Ultrasound

Transient disruption of the blood-brain barrier (BBB) with focused ultrasound (FUS) is an emerging clinical method to facilitate targeted drug delivery to the brain. The focal noninvasive disruption of the BBB can be applied to promote the local delivery of hyperpolarized substrates. In this study, we investigated the effects of FUS on imaging brain metabolism using two hyperpolarized ¹³C-labeled substrates in rodents: [1-¹³C]pyruvate and [1-¹³C]glycerate. The BBB is a rate-limiting factor for pyruvate delivery to the brain, and glycerate minimally passes through the BBB. First, cerebral imaging with hyperpolarized [1-¹³C]pyruvate resulted in an increase in total ¹³C signals (p = 0.05) after disrupting the BBB with FUS. Significantly higher levels of both [1-¹³C]lactate (lactate/total ¹³C signals, p = 0.01) and [¹³C]bicarbonate (p = 0.008) were detected in the FUS-applied brain region as compared to the contralateral FUS-unaffected normal-appearing brain region. The application of FUS without opening the BBB in a separate group of rodents resulted in comparable lactate and bicarbonate productions between the FUS-applied and the contralateral brain regions. Second, ¹³C imaging with hyperpolarized [1-¹³C]glycerate after opening the BBB showed increased [1-¹³C]glycerate delivery to the FUS-applied region (p = 0.04) relative to the contralateral side, and [1-¹³C]lactate production was consistently detected from the FUS-applied region. Our findings suggest that FUS accelerates the delivery of hyperpolarized molecules across the BBB and provides enhanced sensitivity to detect metabolic products in the brain; therefore, hyperpolarized ¹³C imaging with FUS may provide new opportunities to study cerebral metabolic pathways as well as various neurological pathologies.

The metabolic representation of ischemia in rat brain slices: A hyperpolarized 13 C magnetic resonance study

The ischemic penumbra in stroke is not clearly defined by today's available imaging tools. This study aimed to develop a model system and noninvasive biomarkers of ischemic brain tissue for an examination that might potentially be performed in humans, very quickly, in the course of stroke triage. Perfused rat brain slices were used as a model system and 31 P spectroscopy verified that the slices were able to recover from an ischemic insult of about 3.5 min of perfusion arrest. This was indicated as a return to physiological pH and adenosine triphosphate levels. Instantaneous changes in lactate dehydrogenase (LDH) and pyruvate dehydrogenase (PDH) activities were monitored and quantified by the metabolic conversions of hyperpolarized [1-13 C]pyruvate to [1-13 C]lactate and [13 C]bicarbonate, respectively, using 13 C spectroscopy. In a control group (n = 8), hyperpolarized [1-13 C]pyruvate was administered during continuous perfusion of the slices. In the ischemia group (n = 5), the perfusion was arrested 30 s prior to administration of hyperpolarized [1-13 C]pyruvate and perfusion was not resumed throughout the measurement time (approximately 3.5 min). Following about 110 s of the ischemic insult, LDH activity increased by 80.4 ± 13.5% and PDH activity decreased by 47.8 ± 25.3%. In the control group, the mean LDH/PDH ratio was 16.6 ± 3.3, and in the ischemia group, the LDH/PDH ratio reached an average value of 38.7 ± 16.9. The results suggest that monitoring the activity of LDH and PDH, and their relative activities, using hyperpolarized [1-13 C]pyruvate, could serve as an imaging biomarker to characterize the changes in the ischemic penumbra.

Schulte R, Laustsen C. Comprehensive Literature Review of Hyperpolarized Carbon-13 MRI: The Road to Clinical Application

This review provides a comprehensive assessment of the development of hyperpolarized (HP) carbon-13 metabolic MRI from the early days to the present with a focus on clinical applications. The status and upcoming challenges of translating HP carbon-13 into clinical application are reviewed, along with the complexity, technical advancements, and future directions. The road to clinical application is discussed regarding clinical needs and technological advancements, highlighting the most recent successes of metabolic imaging with hyperpolarized carbon-13 MRI. Given the current state of hyperpolarized carbon-13 MRI, the conclusion of this review is that the workflow for hyperpolarized carbon-13 MRI is the limiting factor.

LDH and PDH Activities in the Ischemic Brain and the Effect of Reperfusion-An Ex Vivo MR Study in Rat Brain Slices Using Hyperpolarized [1-13C]Pyruvate

Ischemic stroke is a leading cause for neurologic disability worldwide, for which reperfusion is the only available treatment. Neuroimaging in stroke guides treatment, and therefore determines the clinical outcome. However, there are currently no imaging biomarkers for the status of the ischemic brain tissue. Such biomarkers could potentially be useful for guiding treatment in patients presenting with ischemic stroke. Hyperpolarized 13C MR of [1-13C]pyruvate is a clinically translatable method used to characterize tissue metabolism non-invasively in a relevant timescale. The aim of this study was to utilize hyperpolarized [1-13C]pyruvate to investigate the metabolic consequences of an ischemic insult immediately during reperfusion and upon recovery of the brain tissue. The rates of lactate dehydrogenase (LDH) and pyruvate dehydrogenase (PDH) were quantified by monitoring the rates of [1-13C]lactate and [13C]bicarbonate production from hyperpolarized [1-13C]pyruvate. 31P NMR of the perfused brain slices showed that this system is suitable for studying ischemia and recovery following reperfusion. This was indicated by the levels of the high-energy phosphates (tissue viability) and the chemical shift of the inorganic phosphate signal (tissue pH). Acidification, which was observed during the ischemic insult, has returned to baseline level following reperfusion. The LDH/PDH activity ratio increased following ischemia, from 47.0 ± 12.7 in the control group (n = 6) to 217.4 ± 121.3 in the ischemia-reperfusion group (n = 6). Following the recovery period (ca. 1.5 h), this value had returned to its pre-ischemia (baseline) level, suggesting the LDH/PDH enzyme activity ratio may be used as a potential indicator for the status of the ischemic and recovering brain.

[68Ga]Ga-NODAGA-E[(cRGDyK)]2 PET and hyperpolarized [1-13C] pyruvate MRSI (hyperPET) in canine cancer patients: simultaneous imaging of angiogenesis and the Warburg effect

PURPOSE

Cancer has a multitude of phenotypic expressions and identifying these are important for correct diagnosis and treatment selection. Clinical molecular imaging such as positron emission tomography can access several of these hallmarks of cancer non-invasively. Recently, hyperpolarized magnetic resonance spectroscopy with [1-13C] pyruvate has shown great potential to probe metabolic pathways. Here, we investigate simultaneous dual modality clinical molecular imaging of angiogenesis and deregulated energy metabolism in canine cancer patients.

METHODS

Canine cancer patients (n = 11) underwent simultaneous [68Ga]Ga-NODAGA-E[(cRGDyK)]2 (RGD) PET and hyperpolarized [1-13C]pyruvate-MRSI (hyperPET). Standardized uptake values and [1-13C]lactate to total 13C ratio were quantified and compared generally and voxel-wise.

RESULTS

Ten out of 11 patients showed clear tumor uptake of [68Ga]Ga-NODAGA-RGD at both 20 and 60 min after injection, with an average SUVmean of 1.36 ± 0.23 g/mL and 1.13 ± 0.21 g/mL, respectively. A similar pattern was seen for SUVmax values, which were 2.74 ± 0.41 g/mL and 2.37 ± 0.45 g/mL. The [1-13C]lactate generation followed patterns previously reported. We found no obvious pattern or consistent correlation between the two modalities. Voxel-wise tumor values of RGD uptake and lactate generation analysis revealed a tendency for each canine cancer patient to cluster in separated groups.

CONCLUSION

We demonstrated combined imaging of [68Ga]Ga-NODAGA-RGD-PET for angiogenesis and hyperpolarized [1-13C]pyruvate-MRSI for probing energy metabolism. The results suggest that [68Ga]Ga-NODAGA-RGD-PET and [1-13C]pyruvate-MRSI may provide complementary information, indicating that hyperPET imaging of angiogenesis and energy metabolism is able to aid in cancer phenotyping, leading to improved therapy planning.

Pilot Study of Hyperpolarized 13C Metabolic Imaging in Pediatric Patients with Diffuse Intrinsic Pontine Glioma and Other CNS Cancers

BACKGROUND AND PURPOSE

Pediatric CNS tumors commonly present challenges for radiographic interpretation on conventional MR imaging. This study sought to investigate the safety and tolerability of hyperpolarized carbon-13 (HP-¹³C) metabolic imaging in pediatric patients with brain tumors.

MATERIALS AND METHODS

Pediatric patients 3 to 18 years of age who were previously diagnosed with a brain tumor and could undergo MR imaging without sedation were eligible to enroll in this safety study of HP [1-¹³C]pyruvate. Participants received a one-time injection of HP [1-¹³C]pyruvate and were imaged using dynamic HP-¹³C MR imaging. We assessed 2 dose levels: 0.34 mL/kg and the highest tolerated adult dose of 0.43 mL/kg. Participants were monitored throughout imaging and for 60 minutes postinjection, including pre- and postinjection electrocardiograms and vital sign measurements.

RESULTS

Between February 2017 and July 2019, ten participants (9 males; median age, 14 years; range, 10-17 years) were enrolled, of whom 6 completed injection of HP [1-¹³C]pyruvate and dynamic HP-¹³C MR imaging. Four participants failed to undergo HP-¹³C MR imaging due to technical failures related to generating HP [1-¹³C]pyruvate or MR imaging operability. HP [1-¹³C]pyruvate was well-tolerated in all participants who completed the study, with no dose-limiting toxicities or adverse events observed at either 0.34 (n = 3) or 0.43 (n = 3) mL/kg. HP [1-¹³C]pyruvate demonstrated characteristic conversion to [1-¹³C]lactate and [¹³C]bicarbonate in the brain. Due to poor accrual, the study was closed after only 3 participants were enrolled at the highest dose level.

CONCLUSIONS

Dynamic HP-¹³C MR imaging was safely performed in 6 pediatric patients with CNS tumors and demonstrated HP [1-¹³C]pyruvate brain metabolism.

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.

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

PURPOSE

The purpose of this study was to directly compare two isotopic metabolic imaging approaches, hyperpolarized (HP) ¹³ C MRI and deuterium metabolic imaging (DMI), for imaging specific closely related segments of cerebral glucose metabolism at 4.7 T.

METHODS

Comparative HP-¹³ C and DMI neuroimaging experiments were conducted consecutively in normal rats during the same scanning session. Localized conversions of [1-¹³ C]pyruvate and [6,6-² H₂ ]glucose to their respective downstream metabolic products were measured by spectroscopic imaging, using an identical 2D-CSI sequence with parameters optimized for the respective experiments. To facilitate direct comparison, a pair of substantially equivalent 2.5-cm double-tuned X/¹ H RF surface coils was developed. For improved results, multidimensional low-rank reconstruction was applied to denoise the raw DMI data.

RESULTS

Localized conversion of HP [1-¹³ C]pyruvate to [1-¹³ C]lactate, and [6,6-² H₂ ]glucose to [3,3-² H₂ ]lactate and Glx-d (glutamate and glutamine), was detected in rat brain by spectroscopic imaging at 4.7 T. The SNR and spatial resolution of HP-¹³ C MRI was superior to DMI but limited to a short time window, whereas the lengthy DMI acquisition yielded maps of not only lactate, but also Glx production, albeit with relatively poor spectral discrimination between metabolites at this field strength. Across the individual rats, there was an apparent inverse correlation between cerebral production of HP [1-¹³ C]lactate and Glx-d, along with a trend toward increased [3,3-² H₂ ]lactate.

CONCLUSION

The HP-¹³ C MRI and DMI methods are both feasible at 4.7 T and have significant potential for metabolic imaging of specific segments of glucose metabolism.

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.

Imaging Brain Metabolism Using Hyperpolarized 13C Magnetic Resonance Spectroscopy

Aberrant metabolism is a key factor in many neurological disorders. The ability to measure such metabolic impairment could lead to improved detection of disease progression, and development and monitoring of new therapeutic approaches. Hyperpolarized ¹³C magnetic resonance spectroscopy (MRS) is a developing imaging technique that enables non-invasive measurement of enzymatic activity in real time in living organisms. Primarily applied in the fields of cancer and cardiac disease so far, this metabolic imaging method has recently been used to investigate neurological disorders. In this review, we summarize the preclinical research developments in this emerging field, and discuss future prospects for this exciting technology, which has the potential to change the clinical paradigm for patients with neurological disorders.

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.

Autonomous cryogenic RF receive coil for 13 C imaging of rodents at 3 T

PURPOSE

To develop an autonomous, in-bore, MR-compatible cryostat cooled with liquid nitrogen that provides full-day operation, and to demonstrate that the theoretical signal-to-noise benefit can be achieved for ¹³ C imaging at 3 T (32.13 MHz).

METHODS

The cryogenic setup uses a vacuum-insulated fiberglass cryostat, which indirectly cools a cold finger where the RF coil is attached. The cryostat was evacuated before use and had a reservoir of liquid nitrogen for full-day operation. A 30 × 40 mm² copper coil was mounted inside the cryostat with a 3-mm distance to the sample. Two examples of in vivo experiments of rat brain metabolism after a hyperpolarized [1-¹³ C]pyruvate injection are reported.

RESULTS

A coil Q-factor ratio of Q_88K /Q_290K = 550/280 was obtained, and the theoretical SNR enhancement was verified with MR measurements. We achieved a coil temperature of 88 K and a preamplifier temperature of 77 K. A 2-fold overall SNR enhancement was achieved, compared with the best case at room temperature. The thermal performance of the coil was adequate for in vivo experiments, with an autonomy of 5 hours consuming 6 L of LN₂ , extendable to over 12 hours by LN₂ refilling.

CONCLUSION

Cryogenic surface coils can be highly beneficial for ¹³ C imaging, provided that the coil-to-sample distance remains short. An autonomous, in-bore cryostat was developed that achieved the theoretical improvement in SNR.

Imaging Hyperpolarized Pyruvate and Lactate after Blood-Brain Barrier Disruption with Focused Ultrasound

Imaging of hyperpolarized ¹³C-labeled substrates has emerged as an important magnetic resonance (MR) technique to study metabolic pathways in real time in vivo. Even though this technique has found its way to clinical trials, in vivo dynamic nuclear polarization is still mostly applied in preclinical models. Its tremendous increase in signal-to-noise ratio (SNR) overcomes the intrinsically low MR sensitivity of the ¹³C nucleus and allows real-time metabolic imaging in small structures like the mouse brain. However, applications in brain research are limited as delivery of hyperpolarized compounds is restrained by the blood–brain barrier (BBB). A local noninvasive disruption of the BBB could facilitate delivery of hyperpolarized substrates and create opportunities to study metabolic pathways in the brain that are generally not within reach. In this work, we designed a setup to apply BBB disruption in the mouse brain by MR-guided focused ultrasound (FUS) prior to MR imaging of ¹³C-enriched hyperpolarized [1-¹³C]-pyruvate and its conversion to [1-¹³C]-lactate. To overcome partial volume issues, we optimized a fast multigradient-echo imaging method (temporal resolution of 2.4 s) with an in-plane spatial resolution of 1.6 × 1.6 mm², without the need of processing large amounts of spectroscopic data. We demonstrated the feasibility to apply ¹³C imaging in less than 1 h after FUS treatment and showed a locally disrupted BBB during the time window of the whole experiment. From detected hyperpolarized pyruvate and lactate signals in both FUS-treated and untreated mice, we conclude that even at high spatial resolution, signals from the blood compartment dominate in the ¹³C images, leaving the interpretation of hyperpolarized signals in the mouse brain challenging.

Quantifying normal human brain metabolism using hyperpolarized [1-13C]pyruvate and magnetic resonance imaging

Hyperpolarized 13C Magnetic Resonance Imaging (13C-MRI) provides a highly sensitive tool to probe tissue metabolism in vivo and has recently been translated into clinical studies. We report the cerebral metabolism of intravenously injected hyperpolarized [1-13C]pyruvate in the brain of healthy human volunteers for the first time. Dynamic acquisition of 13C images demonstrated 13C-labeling of both lactate and bicarbonate, catalyzed by cytosolic lactate dehydrogenase and mitochondrial pyruvate dehydrogenase respectively. This demonstrates that both enzymes can be probed in vivo in the presence of an intact blood-brain barrier: the measured apparent exchange rate constant (kPL) for exchange of the hyperpolarized 13C label between [1-13C]pyruvate and the endogenous lactate pool was 0.012 ± 0.006 s-1 and the apparent rate constant (kPB) for the irreversible flux of [1-13C]pyruvate to [13C]bicarbonate was 0.002 ± 0.002 s-1. Imaging also revealed that [1-13C]pyruvate, [1-13C]lactate and [13C]bicarbonate were significantly higher in gray matter compared to white matter. Imaging normal brain metabolism with hyperpolarized [1-13C]pyruvate and subsequent quantification, have important implications for interpreting pathological cerebral metabolism in future studies.

Brain metabolism under different anesthetic conditions using hyperpolarized [1-13 C]pyruvate and [2-13 C]pyruvate

Carbon-13 NMR spectroscopy (13 C MRS) offers the unique capability to measure brain metabolic rates in vivo. Hyperpolarized 13 C reduces the time required to assess brain metabolism from hours to minutes when compared with conventional 13 C MRS. This study investigates metabolism of hyperpolarized [1-13 C]pyruvate and [2-13 C]pyruvate in the rat brain in vivo under various anesthetics: pentobarbital, isoflurane, α-chloralose, and morphine. The apparent metabolic rate from pyruvate to lactate modeled using time courses obtained after injection of hyperpolarized [1-13 C]pyruvate was significantly greater for isoflurane than for all other anesthetic conditions, and significantly greater for morphine than for α-chloralose. The apparent metabolic rate from pyruvate to bicarbonate was significantly greater for morphine than for all other anesthetic conditions, and significantly lower for pentobarbital than for α-chloralose. Results show that relative TCA cycle rates determined from hyperpolarized 13 C data are consistent with rates previously measured using conventional 13 C MRS under similar anesthetic conditions, and that using morphine for sedation greatly improves detection of downstream metabolic products compared with other anesthetics.

Brain metabolism under different anesthetic conditions using hyperpolarized [1-13 C]pyruvate and [2-13 C]pyruvate

Carbon-13 NMR spectroscopy (¹³ C MRS) offers the unique capability to measure brain metabolic rates in vivo. Hyperpolarized ¹³ C reduces the time required to assess brain metabolism from hours to minutes when compared with conventional ¹³ C MRS. This study investigates metabolism of hyperpolarized [1-¹³ C]pyruvate and [2-¹³ C]pyruvate in the rat brain in vivo under various anesthetics: pentobarbital, isoflurane, α-chloralose, and morphine. The apparent metabolic rate from pyruvate to lactate modeled using time courses obtained after injection of hyperpolarized [1-¹³ C]pyruvate was significantly greater for isoflurane than for all other anesthetic conditions, and significantly greater for morphine than for α-chloralose. The apparent metabolic rate from pyruvate to bicarbonate was significantly greater for morphine than for all other anesthetic conditions, and significantly lower for pentobarbital than for α-chloralose. Results show that relative TCA cycle rates determined from hyperpolarized ¹³ C data are consistent with rates previously measured using conventional ¹³ C MRS under similar anesthetic conditions, and that using morphine for sedation greatly improves detection of downstream metabolic products compared with other anesthetics.

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.

Development of methods and feasibility of using hyperpolarized carbon-13 imaging data for evaluating brain metabolism in patient studies

Purpose

Hyperpolarized 13C metabolic imaging is a non-invasive imaging modality for evaluating real-time metabolism. The purpose of this study was to develop and implement experimental strategies for using [1-13C]pyruvate to probe in vivo metabolism for patients with brain tumors and other neurological diseases.

Methods

The 13C RF coils and pulse sequences were tested in a phantom and were performed using a 3T whole body scanner. Samples of [1-13C]pyruvate were polarized using a SPINlab system. Dynamic 13C data were acquired from eight patients previously diagnosed with brain tumors, who had received treatment and were being followed with serial MR scans.

Results

The phantom studies produced good quality spectra with a reduction in signal intensity in the center due to the reception profiles of the 13C receive coils. Dynamic data obtained from a 3 cm slice through a patient’s brain following injection with [1-13C]pyruvate showed the anticipated arrival of the agent, its conversion to lactate and bicarbonate, and subsequent reduction in signal intensity. A similar temporal pattern was observed in 2D dynamic patient studies, with signals corresponding to pyruvate, lactate and bicarbonate being in normal appearing brain but only pyruvate and lactate being detected in regions corresponding to the anatomic lesion. Physiological monitoring and follow-up confirmed that there were no adverse events associated with the injection.

Conclusions

This study has presented the first application of hyperpolarized 13C metabolic imaging in patients with brain tumor and demonstrated the safety and feasibility of using hyperpolarized [1-13C]pyruvate to evaluate in vivo brain metabolism.

Development of methods and feasibility of using hyperpolarized carbon-13 imaging data for evaluating brain metabolism in patient studies

PURPOSE

Hyperpolarized ¹³C metabolic imaging is a non-invasive imaging modality for evaluating real-time metabolism. The purpose of this study was to develop and implement experimental strategies for using [1-¹³C]pyruvate to probe in vivo metabolism for patients with brain tumors and other neurological diseases.

METHODS

The ¹³C RF coils and pulse sequences were tested in a phantom and were performed using a 3T whole body scanner. Samples of [1-¹³C]pyruvate were polarized using a SPINlab system. Dynamic ¹³C data were acquired from eight patients previously diagnosed with brain tumors, who had received treatment and were being followed with serial MR scans.

RESULTS

The phantom studies produced good quality spectra with a reduction in signal intensity in the center due to the reception profiles of the ¹³C receive coils. Dynamic data obtained from a 3 cm slice through a patient’s brain following injection with [1-¹³C]pyruvate showed the anticipated arrival of the agent, its conversion to lactate and bicarbonate, and subsequent reduction in signal intensity. A similar temporal pattern was observed in 2D dynamic patient studies, with signals corresponding to pyruvate, lactate and bicarbonate being in normal appearing brain but only pyruvate and lactate being detected in regions corresponding to the anatomic lesion. Physiological monitoring and follow-up confirmed that there were no adverse events associated with the injection.

CONCLUSIONS

This study has presented the first application of hyperpolarized ¹³C metabolic imaging in patients with brain tumor and demonstrated the safety and feasibility of using hyperpolarized [1-¹³C]pyruvate to evaluate in vivo brain metabolism.

Real-time ex-vivo measurement of brain metabolism using hyperpolarized [1-13C]pyruvate

The ability to directly monitor in vivo brain metabolism in real time in a matter of seconds using the dissolution dynamic nuclear polarization technology holds promise to aid the understanding of brain physiology in health and disease. However, translating the hyperpolarized signal observed in the brain to cerebral metabolic rates is not straightforward, as the observed in vivo signals reflect also the influx of metabolites produced in the body, the cerebral blood volume, and the rate of transport across the blood brain barrier. We introduce a method to study rapid metabolism of hyperpolarized substrates in the viable rat brain slices preparation, an established ex vivo model of the brain. By retrospective evaluation of tissue motion and settling from analysis of the signal of the hyperpolarized [1-13C]pyruvate precursor, the T1s of the metabolites and their rates of production can be determined. The enzymatic rates determined here are in the range of those determined previously with classical biochemical assays and are in agreement with hyperpolarized metabolite relative signal intensities observed in the rodent brain in vivo.

Assessing the influence of isoflurane anesthesia on cardiac metabolism using hyperpolarized [1-13 C]pyruvate

Isoflurane is a frequently used anesthetic in small-animal dissolution dynamic nuclear polarization-magnetic resonance imaging (DNP-MRI) studies. Although the literature suggests interactions with mitochondrial metabolism, the influence of the compound on cardiac metabolism has not been assessed systematically to date. In the present study, the impact of low versus high isoflurane concentration was examined in a crossover experiment in healthy rats. The results revealed that cardiac metabolism is modulated by isoflurane concentration, showing increased [1-¹³ C]lactate and reduced [¹³ C]bicarbonate production during high isoflurane relative to low isoflurane dose [average differences: +16% [1-¹³ C]lactate/total myocardial carbon, -22% [¹³ C]bicarbonate/total myocardial carbon; +51% [1-¹³ C]lactate/[¹³ C]bicarbonate]. These findings emphasize that reproducible anesthesia is important when studying cardiac metabolism. As the depth of anesthesia is difficult to control in an experimental animal setting, careful study design is required to exclude confounding factors.

Dynamic nuclear polarisation: The future of imaging in oncology?

As clinical oncology evolves with new treatment options becoming available, there is an increasing demand on anatomic imaging for the assessment of patients at different stages. Imaging with hyperpolarized 13C-labelled cell substrates has the potential to become a powerful tool in many steps of clinical evaluation, offering a new metabolic metric and therefore a more personalised approach to treatment response. This articles explores the metabolic basis and potential for translation of hyperpolarised pyruvate as a dynamic nuclear polarisation probe in clinical oncology.

In vivo assessment of intracellular redox state in rat liver using hyperpolarized [1-13 C]Alanine

PURPOSE

The intracellular lactate to pyruvate concentration ratio is a commonly used tissue assay biomarker of redox, being proportional to free cytosolic [NADH]/[NAD⁺ ]. In this study, we assessed the use of hyperpolarized [1-¹³ C]alanine and the subsequent detection of the intracellular products of [1-¹³ C]pyruvate and [1-¹³ C]lactate as a useful substrate for assessing redox levels in the liver in vivo.

METHODS

Animal experiments were conducted to measure in vivo metabolism at baseline and after ethanol infusion. A solution of 80-mM hyperpolarized [1-¹³ C]alanine was injected intravenously at baseline (n = 8) and 45 min after ethanol infusion (n = 4), immediately followed by the dynamic acquisition of ¹³ C MRS spectra.

RESULTS

In vivo rat liver spectra showed peaks from [1-¹³ C] alanine and the products of [1-¹³ C]lactate, [1-¹³ C]pyruvate, and ¹³ C-bicarbonate. A significantly increased ¹³ C-lactate/¹³ C-pyruvate ratio was observed after ethanol infusion (8.46 ± 0.58 at baseline versus 13.58 ± 0.69 after ethanol infusion; P < 0.001) consistent with the increased NADH produced by liver metabolism of ethanol to acetaldehyde and then acetate. A decrease in ¹³ C-bicarbonate production was also noted, potentially reflecting ethanol-induced mitochondrial redox changes.

CONCLUSION

A method to measure in vivo tissue redox using hyperpolarized [1-¹³ C]alanine is presented, with the validity of the proposed ¹³ C-pyruvate/¹³ C-lactate metric tested using an ethanol challenge to alter liver redox state. Magn Reson Med 77:1741-1748, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Simultaneous in vivo assessment of cardiac and hepatic metabolism in the diabetic rat using hyperpolarized MRS

Understanding and assessing diabetic metabolism is vital for monitoring disease progression and improving treatment of patients. In vivo assessments, using MRI and MRS, provide non-invasive and accurate measurements, and the development of hyperpolarized ¹³ C spectroscopy in particular has been demonstrated to provide valuable metabolic data in real time. Until now, studies have focussed on individual organs. However, diabetes is a systemic disease affecting multiple tissues in the body. Therefore, we have developed a technique to simultaneously measure metabolism in both the heart and liver during a single acquisition. A hyperpolarized ¹³ C MRS protocol was developed to allow acquisition of metabolic data from the heart and liver during a single scan. This protocol was subsequently used to assess metabolism in the heart and liver of seven control male Wistar rats and seven diabetic rats (diabetes was induced by three weeks of high-fat feeding and a 30 mg/kg injection of streptozotocin). Using our new acquisition, we observed decreased cardiac and hepatic pyruvate dehydrogenase flux in our diabetic rat model. These diabetic rats also had increased blood glucose levels, decreased insulin, and increased hepatic triglycerides. Decreased production of hepatic [1-¹³ C]alanine was observed in the diabetic group, but this change was not present in the hearts of the same diabetic animals. We have demonstrated the ability to measure cardiac and hepatic metabolism simultaneously, with sufficient sensitivity to detect metabolic alterations in both organs. Further, we have non-invasively observed the different reactions of the heart and liver to the metabolic challenge of diabetes.

Hyperpolarized MRS: New tool to study real-time brain function and metabolism

The advent of dissolution dynamic nuclear polarization (DNP) led to the emergence of a new kind of magnetic resonance (MR) measurements providing the opportunity to probe metabolism in vivo in real time. It has been shown that, following the injection of hyperpolarized substrates prepared using dissolution DNP, specific metabolic bioprobes that can be used to differentiate between healthy and pathological tissue in preclinical and clinical studies can be readily detected by MR thanks to the tremendous signal enhancement. The present article aims at reviewing the studies of cerebral function and metabolism based on the use of hyperpolarized MR. The constraints and future opportunities that this technology could offer are discussed.

Hyperpolarized (13)C MR imaging detects no lactate production in mutant IDH1 gliomas: Implications for diagnosis and response monitoring

Metabolic imaging of brain tumors using (13)C Magnetic Resonance Spectroscopy (MRS) of hyperpolarized [1-(13)C] pyruvate is a promising neuroimaging strategy which, after a decade of preclinical success in glioblastoma (GBM) models, is now entering clinical trials in multiple centers. Typically, the presence of GBM has been associated with elevated hyperpolarized [1-(13)C] lactate produced from [1-(13)C] pyruvate, and response to therapy has been associated with a drop in hyperpolarized [1-(13)C] lactate. However, to date, lower grade gliomas had not been investigated using this approach. The most prevalent mutation in lower grade gliomas is the isocitrate dehydrogenase 1 (IDH1) mutation, which, in addition to initiating tumor development, also induces metabolic reprogramming. In particular, mutant IDH1 gliomas are associated with low levels of lactate dehydrogenase A (LDHA) and monocarboxylate transporters 1 and 4 (MCT1, MCT4), three proteins involved in pyruvate metabolism to lactate. We therefore investigated the potential of (13)C MRS of hyperpolarized [1-(13)C] pyruvate for detection of mutant IDH1 gliomas and for monitoring of their therapeutic response. We studied patient-derived mutant IDH1 glioma cells that underexpress LDHA, MCT1 and MCT4, and wild-type IDH1 GBM cells that express high levels of these proteins. Mutant IDH1 cells and tumors produced significantly less hyperpolarized [1-(13)C] lactate compared to GBM, consistent with their metabolic reprogramming. Furthermore, hyperpolarized [1-(13)C] lactate production was not affected by chemotherapeutic treatment with temozolomide (TMZ) in mutant IDH1 tumors, in contrast to previous reports in GBM. Our results demonstrate the unusual metabolic imaging profile of mutant IDH1 gliomas, which, when combined with other clinically available imaging methods, could be used to detect the presence of the IDH1 mutation in vivo.

Hyperpolarized (13)C-lactate to (13)C-bicarbonate ratio as a biomarker for monitoring the acute response of anti-vascular endothelial growth factor (anti-VEGF) treatment

Hyperpolarized [1-(13)C]pyruvate MRS provides a unique imaging opportunity to study the reaction kinetics and enzyme activities of in vivo metabolism because of its favorable imaging characteristics and critical position in the cellular metabolic pathway, where it can either be reduced to lactate (reflecting glycolysis) or converted to acetyl-coenzyme A and bicarbonate (reflecting oxidative phosphorylation). Cancer tissue metabolism is altered in such a way as to result in a relative preponderance of glycolysis relative to oxidative phosphorylation (i.e. Warburg effect). Although there is a strong theoretical basis for presuming that readjustment of the metabolic balance towards normal could alter tumor growth, a robust noninvasive in vivo tool with which to measure the balance between these two metabolic processes has yet to be developed. Until recently, hyperpolarized (13)C-pyruvate imaging studies had focused solely on [1-(13)C]lactate production because of its strong signal. However, without a concomitant measure of pyruvate entry into the mitochondria, the lactate signal provides no information on the balance between the glycolytic and oxidative metabolic pathways. Consistent measurement of (13)C-bicarbonate in cancer tissue, which does provide such information, has proven difficult, however. In this study, we report the reliable measurement of (13)C-bicarbonate production in both the healthy brain and a highly glycolytic experimental glioblastoma model using an optimized (13)C MRS imaging protocol. With the capacity to obtain signal in all tumors, we also confirm for the first time that the ratio of (13)C-lactate to (13)C-bicarbonate provides a more robust metric relative to (13)C-lactate for the assessment of the metabolic effects of anti-angiogenic therapy. Our data suggest a potential application of this ratio as an early biomarker to assess therapeutic effectiveness. Furthermore, although further study is needed, the results suggest that anti-angiogenic treatment results in a rapid normalization in the relative tissue utilization of glycolytic and oxidative phosphorylation by tumor tissue.

Studies of Metabolism Using (13)C MRS of Hyperpolarized Probes

First described in 2003, the dissolution dynamic nuclear polarization (DNP) technique, combined with (13)C magnetic resonance spectroscopy (MRS), has since been used in numerous metabolic studies and has become a valuable metabolic imaging method. DNP dramatically increases the level of polarization of (13)C-labeled compounds resulting in an increase in the signal-to-noise ratio (SNR) of over 50,000 fold for the MRS spectrum of hyperpolarized compounds. The high SNR enables rapid real-time detection of metabolism in cells, tissues, and in vivo. This chapter will present a comprehensive review of the DNP approaches that have been used to monitor metabolism in living systems. First, the list of (13)C DNP probes developed to date will be presented, with a particular focus on the most commonly used probe, namely [1-(13)C] pyruvate. In the next four sections, we will then describe the different factors that need to be considered when designing (13)C DNP probes for metabolic studies, conducting in vitro or in vivo hyperpolarized experiments, as well as acquiring, analyzing, and modeling hyperpolarized (13)C data.

Dynamic hyperpolarized carbon-13 MR metabolic imaging of nonhuman primate brain

PURPOSE

To investigate hyperpolarized (13) C metabolic imaging methods in the primate brain that can be translated into future clinical trials for patients with brain cancer.

METHODS

(13) C coils and pulse sequences designed for use in humans were tested in phantoms. Dynamic (13) C data were obtained from a healthy cynomolgus monkey brain using the optimized (13) C coils and pulse sequences. The metabolite kinetics were estimated from two-dimensional localized (13) C dynamic imaging data from the nonhuman primate brain.

RESULTS

Pyruvate and lactate signal were observed in both the brain and the surrounding tissues with the maximum signal-to-noise ratio of 218 and 29 for pyruvate and lactate, respectively. Apparent rate constants for the conversion of pyruvate to lactate and the ratio of lactate to pyruvate showed a difference between brain and surrounding tissues.

CONCLUSION

The feasibility of using hyperpolarized [1-(13) C]-pyruvate for assessing in vivo metabolism in a healthy nonhuman primate brain was demonstrated using a hyperpolarized (13) C imaging experimental setup designed for studying patients with brain tumors. The kinetics of the metabolite conversion suggests that this approach may be useful in future studies of human neuropathology.