Design of spectral-spatial outer volume suppression RF pulses for tissue specific metabolic characterization with hyperpolarized 13C pyruvate

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.

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.

Cardiac applications of hyperpolarised magnetic resonance

Cardiovascular disease is the leading cause of death world-wide. It is increasingly recognised that cardiac pathologies show, or may even be caused by, changes in metabolism, leading to impaired cardiac energetics. The heart turns over 15 times its own weight in ATP every day and thus relies heavily on the availability of substrates and on efficient oxidation to generate this ATP. A number of old and emerging drugs that target different aspects of metabolism are showing promising results with regard to improved cardiac outcomes in patients. A non-invasive imaging technique that could assess the role of different aspects of metabolism in heart disease, as well as measure changes in cardiac energetics due to treatment, would be valuable in the routine clinical care of cardiac patients. Hyperpolarised magnetic resonance spectroscopy and imaging have revolutionised metabolic imaging, allowing real-time metabolic flux assessment in vivo for the first time. In this review we summarise metabolism in the healthy and diseased heart, give an introduction to the hyperpolarisation technique, 'dynamic nuclear polarisation' (DNP), and review the preclinical studies that have thus far explored healthy cardiac metabolism and different models of human heart disease. We furthermore show what advances have been made to translate this technique into the clinic, what technical challenges still remain and what unmet clinical needs and unexplored metabolic substrates still need to be assessed by researchers in this exciting and fast-moving field.

High resolution hyperpolarized 13 C MRSI using SPICE at 9.4T

PURPOSE

To test the feasibility of using the SPICE (SPectroscopic Imaging by exploiting spatiospectral CorrElation) technique, which uses the partial separability of spectroscopic data, for high resolution hyperpolarized (HP) ¹³ C spectroscopic imaging.

METHODS

Numerical simulations were performed to investigate the impact of transient HP signals on SPICE reconstruction. Furthermore, spectroscopic imaging exams from SPICE and conventional EPSI (echo-planar spectroscopic imaging) were simulated for comparison. For in vivo experiments, HP ¹³ C SPICE was performed in a mouse kidney by means of the injection of HP [1-¹³ C] pyruvate at 9.4T.

RESULTS

The variation of lactate/pyruvate from the simulated SPICE was less than 4% under various factors that affect the transient HP signal, suggesting that the impact is negligible. We found that while HP ¹³ C EPSI was limited to the low signal-to-noise ratio (SNR) of lactate, these limitations were mitigated through HP ¹³ C SPICE, facilitating the improved SNR of lactate and the distinction of tissues. Acquisition of a high resolution HP ¹³ C spectroscopic image was possible for the in vivo experiments. With the fine structural information, the acquired image showed higher signal of pyruvate and lactate in the renal cortices than in the medullas, which is known to be attributed to higher activity of lactate dehydrogenase.

CONCLUSION

The feasibility of HP ¹³ C SPICE was investigated. Simulation studies were conducted and in vivo experiments were performed in the mouse kidney at 9.4T. Results confirmed that a high resolution HP ¹³ C spectroscopic image with adequate spectral resolution can be obtained. Magn Reson Med 80:703-710, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Hyperpolarized [1-13C]-acetate Renal Metabolic Clearance Rate Mapping

¹¹C-acetate is a positron emission tomography (PET) tracer of oxidative metabolism, whereas hyperpolarized ¹³C-acetate can be used in magnetic resonance imaging (MRI) for investigating specific metabolic processes. The aims of this study were to examine if the kinetic formalism of ¹¹C-acetate PET in the kidneys is comparable to that of ¹³C-acetate MRI, and to compare the dynamic metabolic information of hyperpolarized ¹³C-acetate MRI with that obtained with ¹¹C-acetate PET. Rats were examined with dynamic hyperpolarized ¹³C-acetate MRI or ¹¹C-acetate PET before and after intravenous injection of furosemide, a loop diuretic known to alter both the hemodynamics and oxygen consumption in the kidney. The metabolic clearance rates (MCR) were estimated and compared between the two modalities experimentally in vivo and in simulations. There was a clear dependency on the mean transit time and MCR for both ¹³C-acetate and ¹¹C-acetate following furosemide administration, while no dependencies on the apparent renal perfusion were observed. This study demonstrated that hyperpolarized ¹³C-acetate MRI is feasible for measurements of the intrarenal energetic demand via the MCR, and that the quantitative measures are correlated with those measured by ¹¹C-acetate PET, even though the temporal window is more than 30 times longer with ¹¹C-acetate.

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

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

Flow-suppressed hyperpolarized 13 C chemical shift imaging using velocity-optimized bipolar gradient in mouse liver tumors at 9.4 T

PURPOSE

To optimize and investigate the influence of bipolar gradients for flow suppression in metabolic quantification of hyperpolarized ¹³ C chemical shift imaging (CSI) of mouse liver at 9.4 T.

METHODS

The trade-off between the amount of flow suppression using bipolar gradients and T2* effect from static spins was simulated. A free induction decay CSI sequence with alternations between the flow-suppressed and non-flow-suppressed acquisitions for each repetition time was developed and was applied to liver tumor-bearing mice via injection of hyperpolarized [1-¹³ C] pyruvate.

RESULTS

The in vivo results from flow suppression using the velocity-optimized bipolar gradient were comparable with the simulation results. The vascular signal was adequately suppressed and signal loss in stationary tissue was minimized. Application of the velocity-optimized bipolar gradient to tumor-bearing mice showed reduction in the vessel-derived pyruvate signal contamination, and the average lactate/pyruvate ratio increased by 0.095 (P < 0.05) in the tumor region after flow suppression.

CONCLUSION

Optimization of the bipolar gradient is essential because of the short ¹³ C T2* and high signal in venous flow in the mouse liver. The proposed velocity-optimized bipolar gradient can suppress the vascular signal, minimizing T2*-related signal loss in stationary tissues at 9.4 T. Magn Reson Med 78:1674-1682, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Assessing the transport rate of hyperpolarized pyruvate and lactate from the intra- to the extracellular space

The use of [1-(13) C]pyruvate hyperpolarized by means of dynamic nuclear polarization provides a direct way to track the metabolic transformations of this metabolite in vivo and in cell cultures. The identification of the intra- and extracellular contributions to the (13) C NMR resonances is not straightforward. In order to obtain information about the rate of pyruvate and lactate transport through the cellular membrane, we set up a method that relies on the sudden 'quenching' of the extracellular metabolites' signal. The paramagnetic Gd-tetraazacyclododecane triacetic acid (Gd-DO3A) complex was used to dramatically decrease the longitudinal relaxation time constants of the (13) C-carboxylate resonances of both pyruvate and lactate. When Gd-DO3A was added to an MCF-7 cellular culture, which had previously received a dose of hyperpolarized [1-(13) C]pyruvate, the contributions of the extracellular pyruvate and lactate signals were deleted. From the analysis of the decay curves of the (13) C-carboxylate resonances of pyruvate and lactate it was possible to extract information about the exchange rate of the two metabolites across the cellular membrane. In particular, it was found that, in the reported experimental conditions, the lactate transport from the intra- to the extracellular space is not much lower than the rate of lactate formation. The method reported herein is non-destructive and it could be translated to in vivo studies. It opens a route for the use of hyperpolarized pyruvate to assess altered activity of carboxylate transporter proteins that may occur in pathological conditions. Copyright © 2016 John Wiley & Sons, Ltd.

Cardiac perfusion imaging using hyperpolarized (13)C urea using flow sensitizing gradients

Purpose

To demonstrate the feasibility of imaging the first passage of a bolus of hyperpolarized ¹³C urea through the rodent heart using flow‐sensitizing gradients to reduce signal from the blood pool.

Methods

A flow‐sensitizing bipolar gradient was optimized to reduce the bright signal within the cardiac chambers, enabling improved contrast of the agent within the tissue capillary bed. The gradient was incorporated into a dynamic golden angle spiral ¹³C imaging sequence. Healthy rats were scanned during rest (n = 3) and under adenosine stress‐induced hyperemia (n = 3).

Results

A two‐fold increase in myocardial perfusion relative to rest was detected during adenosine stress‐induced hyperemia, consistent with a myocardial perfusion reserve of two in rodents.

Conclusion

The new pulse sequence was used to obtain dynamic images of the first passage of hyperpolarized ¹³C urea in the rodent heart, without contamination from bright signal within the neighboring cardiac lumen. This probe of myocardial perfusion is expected to enable new hyperpolarized ¹³C studies in which the cardiac metabolism/perfusion mismatch can be identified. Magn Reson Med, 2015. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Magn Reson Med 75:1474–1483, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance.

Application of flow sensitive gradients for improved measures of metabolism using hyperpolarized (13) c MRI

PURPOSE

To develop the use of bipolar gradients to suppress partial-volume and flow-related artifacts from macrovascular, hyperpolarized spins.

THEORY AND METHODS

Digital simulations were performed over a range of spatial resolutions and gradient strengths to determine the optimal bipolar gradient strength and duration to suppress flowing spins while minimizing signal loss from static tissue. In vivo experiments were performed to determine the efficacy of this technique to suppress vascular signal in the study of hyperpolarized [1-(13)C]pyruvate renal metabolism.

RESULTS

Digital simulations showed that in the absence of bipolar gradients, partial-volume artifacts from the vasculature were still present, causing underestimation of the apparent reaction rate of pyruvate to lactate (kP). The addition of a bipolar gradient with b = 32 s/mm(2) sufficiently suppressed the vascular signal without a substantial decrease in signal from static tissue. In vivo results corroborate digital simulations, with similar peak lactate signal to noise ratio (SNR) but substantially different kP in the presence of bipolar gradients.

CONCLUSION

The proposed approach suppresses signal from flowing spins while minimizing signal loss from static tissue, removing contaminating signal from the vasculature and increasing kinetic modeling accuracy without substantially sacrificing SNR or temporal resolution.

Using [1-(13) C]lactic acid for hyperpolarized (13) C MR cardiac studies

PURPOSE

Hyperpolarized [1-(13) C]lactate in solution may be a clinically relevant and safe substrate for real time MR investigations of key metabolic pathways. The potential of using hyperpolarized [1-(13) C]lactate for magnetic resonance studies of cardiac metabolism in vivo was explored.

METHODS

Neat [1-(13) C]lactic acid was hyperpolarized using the dynamic nuclear polarization process. Cardiac MR spectroscopy experiments were performed in vivo using hyperpolarized [1-(13) C]lactate and [1-(13) C]pyruvate in solutions.

RESULTS

A high degree of polarization was achieved for [1-(13) C]lactate in solution (16.7%). (13) C-bicarbonate was observed in rat hearts in vivo after either hyperpolarized [1-(13) C]lactate or hyperpolarized [1-(13) C]pyruvate was infused, but lower (13) C-bicarbonate to substrate ratio was observed with hyperpolarized [1-(13) C]lactate infusions. The response of (13) C-bicarbonate signal as a function of hyperpolarized [1-(13) C]lactate doses was also investigated and a saturation of (13) C-bicarbonate signal was observed at the highest dose of [1-(13) C]lactate used (0.69 mmol/kg).

CONCLUSION

This study demonstrated that the use of neat [1-(13) C]lactic acid as the DNP sample is a potential alternative to [1-(13) C]pyruvic acid for cardiac hyperpolarized (13) C MR studies. Hyperpolarized [1-(13) C]lactate may enable noninvasive assessment of cardiac PDH flux in cardiac patients in the near future.

In vivo measurement of apparent diffusion coefficients of hyperpolarized ¹³C-labeled metabolites

The combination of hyperpolarized MRS with diffusion weighting (dw) allows for determination of the apparent diffusion coefficient (ADC), which is indicative of the intra- or extracellular localization of the metabolite. Here, a slice-selective pulsed-gradient spin echo sequence was implemented to acquire a series of dw spectra from rat muscle in vivo to determine the ADCs of multiple metabolites after a single injection of hyperpolarized [1- ¹³C]pyruvate. An optimal control optimized universal-rotation pulse was used for refocusing to minimize signal loss caused by B1 imperfections. Non-dw spectra were acquired interleaved with the dw spectra and these were used to correct for signal decay during the acquisition as a result of T1 decay, pulse imperfections, flow etc. The data showed that the ADC values for [1- ¹³C]lactate (0.4-0.7 µm² /ms) and [1- ¹³C]alanine (0.4-0.9 µm² /ms) were about a factor of two lower than the ADC of [1- ¹³C]pyruvate (1.1-1.5 µm²/ms). This indicates a more restricted diffusion space for the former two metabolites consistent with lactate and alanine being intracellular. The higher ADC for pyruvate (similar to the proton ADC) reflected that the injected substance was not confined inside the muscle cells but also present extracellular.

Perfusion and diffusion sensitive 13C stimulated-echo MRSI for metabolic imaging of cancer

Metabolic imaging with hyperpolarized [1-(13)C]-pyruvate can rapidly probe tissue metabolic profiles in vivo and has been shown to provide cancer imaging biomarkers for tumor detection, progression, and response to therapy. This technique uses a bolus injection followed by imaging within 1-2 minutes. The observed metabolites include vascular components and their generation is also influenced by cellular transport. These factors complicate image interpretation, especially since [1-(13)C]lactate, a metabolic product that is a biomarker of cancer, is also produced by red blood cells. It would be valuable to understand the distribution of metabolites between the vasculature, interstitial space, and intracellular compartments. The purpose of this study was to better understand this compartmentalization by using a perfusion and diffusion-sensitive stimulated-echo acquisition mode (STEAM) MRSI acquisition method tailored to hyperpolarized substrates. Our results in mouse models showed that among metabolites, the injected substrate (13)C-pyruvate had the largest vascular fraction overall while (13)C-alanine had the smallest vascular fraction. We observed a larger vascular fraction of pyruvate and lactate in the kidneys and liver when compared to back muscle and prostate tumor tissue. Our data suggests that (13)C-lactate in prostate tumor tissue voxels was the most abundant labeled metabolite intracellularly. This was shown in STEAM images that highlighted abnormal cancer cell metabolism and suppressed vascular (13)C metabolite signals.

Dynamic nuclear polarization and optimal control spatial-selective 13C MRI and MRS

Aimed at (13)C metabolic magnetic resonance imaging (MRI) and spectroscopy (MRS) applications, we demonstrate that dynamic nuclear polarization (DNP) may be combined with optimal control 2D spatial selection to simultaneously obtain high sensitivity and well-defined spatial restriction. This is achieved through the development of spatial-selective single-shot spiral-readout MRI and MRS experiments combined with dynamic nuclear polarization hyperpolarized [1-(13)C]pyruvate on a 4.7 T pre-clinical MR scanner. The method stands out from related techniques by facilitating anatomic shaped region-of-interest (ROI) single metabolite signals available for higher image resolution or single-peak spectra. The 2D spatial-selective rf pulses were designed using a novel Krotov-based optimal control approach capable of iteratively fast providing successful pulse sequences in the absence of qualified initial guesses. The technique may be important for early detection of abnormal metabolism, monitoring disease progression, and drug research.

Spin echo measurements of the extravasation and tumor cell uptake of hyperpolarized [1-(13) C]lactate and [1-(13) C]pyruvate

PURPOSE

To assess the blood-tissue distribution of hyperpolarized (13) C-labeled molecules in vivo.

METHODS

Spin-echo experiments with simultaneous acquisition of the free induction decay (FID) signal following the excitation pulse and the spin-echo signal, were used to monitor hyperpolarized [1-(13) C]lactate, [1-(13) C]pyruvate, and the perfusion marker, [(13) C]HP001, following their intravenous injection into tumor-bearing mice. Apparent T2 relaxation times and diffusion coefficients were also measured.

RESULTS

An increasing tumor echo/FID ratio was observed for all three molecules, which could be explained by their extravasation into the tumor interstitial space, where T2 relaxation times were longer and diffusion coefficients smaller. Inhibition of the monocarboxylate transporter, which decreased by 40% the label exchange between pyruvate and lactate, reduced the increase in the echo/FID ratio for pyruvate and lactate, but not for HP001, demonstrating that some of the increase in the echo/FID ratio was due to cell uptake of lactate and pyruvate. The different relaxation and diffusion behavior of the intravascular and extravascular signals affected measurements of the apparent label exchange rate constants.

CONCLUSION

Simultaneous collection of both FID and echo signals can provide information on cell uptake thus giving further insight into the kinetics of hyperpolarized (13) C label exchange. Care is needed when comparing exchange rate constants determined in spin-echo-based studies.

Spin tagging for hyperpolarized ¹³C metabolic studies

In studies utilizing pre-polarized (13)C substrates to investigate metabolic activities in vivo, the metabolite signals observed in a region or a voxel contains a mixture of intracellular and extracellular components. This extracellular component arriving via perfusion may confound the measurements of metabolic flux or exchange rates. But if spin tagging is performed on the magnetization of the substrate, it may be possible to measure the signals of the metabolic products in the intracellular space that were derived from the tagged substrate spins locally. In this study, a spin tagging pulse sequence designed for acquiring data from spatially tagged longitudinal magnetization in hyperpolarized (13)C metabolic studies was presented and tested. Using a spectral-spatial RF pulse during the tagging preparation enabled the observation of metabolite signals derived exclusively from the tagged substrate in vivo.

Application of double spin echo spiral chemical shift imaging to rapid metabolic mapping of hyperpolarized [1-¹³C]-pyruvate

Undersampled spiral CSI (spCSI) using a free induction decay (FID) acquisition allows sub-second metabolic imaging of hyperpolarized ¹³C. Phase correction of the FID acquisition can be difficult, especially with contributions from aliased out-of-phase peaks. This work extends the spCSI sequence by incorporating double spin echo radiofrequency (RF) pulses to eliminate the need for phase correction and obtain high quality spectra in magnitude mode. The sequence also provides an added benefit of attenuating signal from flowing spins, which can otherwise contaminate signal in the organ of interest. The refocusing pulses can potentially lead to a loss of hyperpolarized magnetization in dynamic imaging due to flow of spins through the fringe field of the RF coil, where the refocusing pulses fail to provide complete refocusing. Care must be taken for dynamic imaging to ensure that the spins remain within the B₁-homogeneous sensitive volume of the RF coil.

Detection of inflammatory arthritis by using hyperpolarized 13C-pyruvate with MR imaging and spectroscopy

PURPOSE

To examine the feasibility of using magnetic resonance (MR) spectroscopy with hyperpolarized carbon 13 ((13)C)-labeled pyruvate to detect inflammation.

MATERIALS AND METHODS

The animal care and use committee approved all work with animals. Arthritis was induced in the right hind paw of six rats; the left hind paw served as an internal control. The lactate dehydrogenase-catalyzed conversion of pyruvate to lactate was measured in inflamed and control paws by using (13)C MR spectroscopy. Clinical and histologic data were obtained to confirm the presence and severity of arthritis. Hyperpolarized (13)C-pyruvate was intravenously injected into the rats before simultaneous imaging of both paws with (13)C MR spectroscopy. The Wilcoxon signed rank test was used to test for differences in metabolites between the control and arthritic paws.

RESULTS

All animals showed findings of inflammation in the affected paws and no signs of arthritis in the control paws at both visible inspection (clinical index of 3 for arthritic paws and 0 for control paws) and histologic examination (histologic score of 3-5 for arthritic paws and 0 for control paws). Analysis of the spectroscopic profiles of (13)C-pyruvate and converted (13)C-lactate showed an increase in the amount of (13)C-lactate in inflamed paws (median lactate-to-pyruvate ratio, 0.50; mean lactate-to-pyruvate ratio ± standard deviation, 0.52 ± 0.16) versus control paws (median lactate-to-pyruvate ratio, 0.27; mean lactate-to-pyruvate ratio, 0.32 ± 0.11) (P < .03). The ratio of (13)C-lactate to total (13)C was also significantly increased in inflamed paws compared with control paws (P < .03).

CONCLUSION

These results suggest that alterations in the conversion of pyruvate to lactate as detected with (13)C-MR spectroscopy may be indicative of the presence of inflammatory arthritis.

Applications of hyperpolarized agents in solutions

This chapter provides an overview of pulse sequences adapted to hyperpolarized MR imaging. Applications of hyperpolarized agents in aqueous solution are reviewed. Vascular (e.g., angiography, perfusion, and catheter tracking) as well as metabolic (e.g., oncology, cardiology, neurology, and pH mapping) applications are covered. Due to the rapid development of new applications for hyperpolarized agents, a review format has been used for this chapter instead of a strict protocol/procedure structure.