Hyperpolarized [2-13C]pyruvate MR molecular imaging with whole brain coverage

Simultaneous noninvasive quantification of redox and downstream glycolytic fluxes reveals compartmentalized brain metabolism

Brain metabolism across anatomic regions and cellular compartments plays an integral role in many aspects of neuronal function. Changes in key metabolic pathway fluxes, including oxidative and reductive energy metabolism, have been implicated in a wide range of brain diseases. Given the complex nature of the brain and the need for understanding compartmentalized metabolism noninvasively in vivo, new tools are required. Herein, using hyperpolarized (HP) magnetic resonance imaging coupled with in vivo isotope tracing, we develop a platform to simultaneously probe redox and energy metabolism in the murine brain. By combining HP dehydroascorbate and pyruvate, we are able to visualize increased lactate production in the white matter and increased redox capacity in the deep gray matter. Leveraging positional labeling, we show differences in compartmentalized tricarboxylic acid cycle entry versus downstream flux to glutamate. These findings lay the foundation for clinical translation of the proposed approach to probe brain metabolism.

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.

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.

Advanced Hyperpolarized 13C Metabolic Imaging Protocol for Patients with Gliomas: A Comprehensive Multimodal MRI Approach

This study aimed to implement a multimodal 1H/HP-13C imaging protocol to augment the serial monitoring of patients with glioma, while simultaneously pursuing methods for improving the robustness of HP-13C metabolic data. A total of 100 1H/HP [1-13C]-pyruvate MR examinations (104 HP-13C datasets) were acquired from 42 patients according to the comprehensive multimodal glioma imaging protocol. Serial data coverage, accuracy of frequency reference, and acquisition delay were evaluated using a mixed-effects model to account for multiple exams per patient. Serial atlas-based HP-13C MRI demonstrated consistency in volumetric coverage measured by inter-exam dice coefficients (0.977 ± 0.008, mean ± SD; four patients/11 exams). The atlas-derived prescription provided significantly improved data quality compared to manually prescribed acquisitions (n = 26/78; p = 0.04). The water-based method for referencing [1-13C]-pyruvate center frequency significantly reduced off-resonance excitation relative to the coil-embedded [13C]-urea phantom (4.1 ± 3.7 Hz vs. 9.9 ± 10.7 Hz; p = 0.0007). Significantly improved capture of tracer inflow was achieved with the 2-s versus 5-s HP-13C MRI acquisition delay (p = 0.007). This study demonstrated the implementation of a comprehensive multimodal 1H/HP-13C MR protocol emphasizing the monitoring of steady-state/dynamic metabolism in patients with glioma.

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.