Parahydrogen-Induced Carbon-13 Radiofrequency Amplification by Stimulated Emission of Radiation

Rapid in situ carbon-13 hyperpolarization and imaging of acetate and pyruvate esters without external polarizer

Hyperpolarized 13C MRI visualizes real-time metabolic processes in vivo. In this study, we achieved high 13C polarization in situ in the bore of an MRI system for precursor molecules of most widely employed hyperpolarized agents: [1-13C]acetate and [1-13C]pyruvate ethyl esters in their perdeuterated forms, enhancing hyperpolarization lifetimes, hyperpolarized to P13C ≈ 28% at 80 mM concentration and P13C ≈ 19% at 10 mM concentration, respectively. Using vinyl esters as unsaturated Parahydrogen-Induced Polarization via Side-Arm Hydrogenation (PHIP-SAH) precursors and our novel polarization setup, we achieved these hyperpolarization levels by fast side-arm hydrogenation in acetone-d6 at elevated temperatures (up to 90°C) and hydrogenation pressures (up to 32 bar). We optimized the hyperpolarization process, reducing it to under 10 s, and employed advanced pulse sequences to enhance the polarization transfer efficiency. The hyperpolarization system has a small footprint, allowing it to be positioned in the same magnet, where 13C MRI is performed. We exemplified the utility of the design with sub-second in situ 13C MRI of ethyl [1-13C]pyruvate-d6. However, challenges remain in side-arm cleavage and purification in the MRI system to extract highly polarized aqueous agent solutions. Our results showcase efficient and rapid 13C hyperpolarization of these metabolite precursors in an MRI system with minimal additional hardware, promising to enhance future throughput and access to hyperpolarized 13C MRI.

Through-bond and through-space radiofrequency amplification by stimulated emission of radiation

Radio Amplification by Stimulated Emission of Radiation (RASER) is a phenomenon observed during nuclear magnetic resonance (NMR) experiments with strongly negatively polarized systems. This phenomenon may be utilized for the production of very narrow NMR lines, background-free NMR spectroscopy, and excitation-free sensing of chemical transformations. Recently, novel methods of producing RASER by ParaHydrogen-Induced Polarization (PHIP) were introduced. Here, we show that pairwise addition of parahydrogen to various propargylic compounds induces RASER activity of other protons beyond those chemically introduced in the reaction. In high-field PHIP, negative polarization initiating RASER is transferred via intramolecular cross-relaxation. When parahydrogen is added in Earth's field followed by adiabatic transfer to a high field, RASER activity of other protons is induced via both J-couplings and cross-relaxation. This through-bond and through-space induction of RASER holds potential for the ongoing development and expansion of RASER applications and can potentially enhance spectral resolution in two-dimensional NMR spectroscopy techniques.

Toward Ultra-High-Quality-Factor Wireless Masing Magnetic Resonance Sensing

It has recently been shown that a bolus of hyperpolarized nuclear spins can yield stimulated emission signals similar in nature to maser signals, potentially enabling new ways of sensing hyperpolarized contrast media, including most notably [1-13C]pyruvate that is under evaluation in over 50 clinical trials for metabolic imaging of cancer. The stimulated NMR signal emissions lasting for minutes do not require radio-frequency excitation, offering unprecedented advantages compared to conventional MR sensing. However, creating nuclear spin maser emission is challenging in practice due to stringent fundamental requirements, making practical in vivo applications hardly possible using conventional passive MR detectors. Here, we demonstrate the utility of a wireless NMR maser detector, the quality factor of which was enhanced 22-fold (to 1,670) via parametric pumping. This active-feedback technique breaks the intrinsic fundamental limit of NMR detector circuit quality factor. We show the use of parametric pumping to reduce the threshold requirement for inducing nuclear spin masing at 300 MHz resonance frequency in a preclinical MRI scanner. Indeed, stimulated emission from hyperpolarized protons was obtained under highly unfavorable conditions of low magnetic field homogeneity (T2* of 3 ms). Greater gains of the quality factor of the MR detector (up to 1 million) were also demonstrated.

Development of a fully automated workstation for conducting routine SABRE hyperpolarization

SABRE is emerging as a fast, simple and low-cost hyperpolarization method because of its ability to regenerate enhanced NMR signals. Generally, SABRE hyperpolarization has been performed predominantly manually, leading to variations in reproducibility and efficiency. Recent advances in SABRE include the development of automated shuttling systems to address previous inconsistencies. However, the operational complexity of such systems and the challenges of integration with existing workflows hinder their widespread adoption. This work presents a fully automated lab workstation based on a benchtop NMR spectrometer, specifically designed to facilitate SABRE of different nuclei across different polarization fields. We demonstrated the capability of this system through a series of routine SABRE experimental protocols, including consecutive SABRE hyperpolarization with high reproducibility (average standard deviation of 1.03%), optimization polarization of 13C nuclei respect to the polarization transfer field, and measurement of polarization buildup rate or decay time across a wide range of magnetic fields. Furthermore, we have iteratively optimized the durations for pulsed SABRE-SHEATH 13C pyruvate. The constructed SABRE workstation offers full automation, high reproducibility, and functional diversification, making it a practical tool for conducting routine SABRE hyperpolarization experiments. It provides a robust platform for high-throughput and reliable SABRE and X-SABRE hyperpolarization studies.

MATRESHCA: Microtesla Apparatus for Transfer of Resonance Enhancement of Spin Hyperpolarization via Chemical Exchange and Addition

We present an integrated, open-source device for parahydrogen-based hyperpolarization processes in the microtesla field regime with a cost of components of less than $7000. The device is designed to produce a batch of 13C and 15N hyperpolarized (HP) compounds via hydrogenative or non-hydrogenative parahydrogen-induced polarization methods that employ microtesla magnetic fields for efficient polarization transfer of parahydrogen-derived spin order to X-nuclei (e.g., 13C and 15N). The apparatus employs a layered structure (reminiscent of a Russian doll "Matryoshka") that includes a nonmagnetic variable-temperature sample chamber, a microtesla magnetic field coil (operating in the range of 0.02-75 microtesla), a three-layered mu-metal shield (to attenuate the ambient magnetic field), and a magnetic shield degaussing coil placed in the overall device enclosure. The gas-handling manifold allows for parahydrogen-gas flow and pressure control (up to 9.2 bar of total parahydrogen pressure). The sample temperature can be varied either using a water bath or a PID-controlled heat exchanger in the range from -12 to 80 °C. This benchtop device measures 62 cm (length) × 47 cm (width) × 47 cm (height), weighs 30 kg, and requires only connections to a high-pressure parahydrogen gas supply and a single 110/220 VAC power source. The utility of the device has been demonstrated using an example of parahydrogen pairwise addition to form HP ethyl [1-13C]acetate (P13C = 7%, [c] = 1 M). Moreover, the Signal Amplification By Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) technique was employed to demonstrate efficient hyperpolarization of 13C and 15N spins in a wide range of biologically relevant molecules, including [1-13C]pyruvate (P13C = 14%, [c] = 27 mM), [1-13C]-α-ketoglutarate (P13C = 17%), [1-13C]ketoisocaproate (P13C = 18%), [15N3]metronidazole (P15N = 13%, [c] = 20 mM), and others. While the vast majority of the utility studies have been performed in standard 5 mm NMR tubes, the sample chamber of the device can accommodate a wide range of sample container sizes and geometries of up to 1 L sample volume. The device establishes an integrated, simple, inexpensive, and versatile equipment gateway needed to facilitate parahydrogen-based hyperpolarization experiments ranging from basic science to preclinical applications; indeed, detailed technical drawings and a bill of materials are provided to support the ready translation of this design to other laboratories.

Carbon-13 Radiofrequency Amplification by Stimulated Emission of Radiation of the Hyperpolarized Ketone and Hemiketal Forms of Allyl [1-13C]Pyruvate

13C hyperpolarized pyruvate is an emerging MRI contrast agent for sensing molecular events in cancer and other diseases with aberrant metabolic pathways. This metabolic contrast agent can be produced via several hyperpolarization techniques. Despite remarkable success in research settings, widespread clinical adoption faces substantial roadblocks because the current sensing technology utilized to sense this contrast agent requires the excitation of 13C nuclear spins that also need to be synchronized with MRI field gradient pulses. Here, we demonstrate sensing of hyperpolarized allyl [1-13C]pyruvate via the stimulated emission of radiation that mitigates the requirements currently blocking broader adoption. Specifically, 13C Radiofrequency Amplification by Stimulated Emission of Radiation (13C RASER) was obtained after pairwise addition of parahydrogen to a pyruvate precursor, detected in a commercial inductive detector with a quality factor (Q) of 32 for sample concentrations as low as 0.125 M with 13C polarization of 4%. Moreover, parahydrogen-induced polarization allowed for the preparation of a mixture of ketone and hemiketal forms of hyperpolarized allyl [1-13C]pyruvate, which are separated by 10 ppm in 13C NMR spectra. This is a good model system to study the simultaneous 13C RASER signals of multiple 13C species. This system models the metabolic production of hyperpolarized [1-13C]lactate from hyperpolarized [1-13C]pyruvate, which has a similar chemical shift difference. Our results show that 13C RASER signals can be obtained from both species simultaneously when the emission threshold is exceeded for both species. On the other hand, when the emission threshold is exceeded only for one of the hyperpolarized species, 13C stimulated emission is confined to this species only, therefore enabling the background-free detection of individual hyperpolarized 13C signals. The reported results pave the way to novel sensing approaches of 13C hyperpolarized pyruvate, potentially unlocking hyperpolarized 13C MRI on virtually any MRI system─an attractive vision for the future molecular imaging and diagnostics.

13C Radiofrequency Amplification by Stimulated Emission of Radiation Threshold Sensing of Chemical Reactions

Conventional nuclear magnetic resonance (NMR) enables detection of chemicals and their transformations by exciting nuclear spin ensembles with a radio-frequency pulse followed by detection of the precessing spins at their characteristic frequencies. The detected frequencies report on chemical reactions in real time and the signal amplitudes scale with concentrations of products and reactants. Here, we employ Radiofrequency Amplification by Stimulated Emission of Radiation (RASER), a quantum phenomenon producing coherent emission of 13C signals, to detect chemical transformations. The 13C signals are emitted by the negatively hyperpolarized biomolecules without external radio frequency pulses and without any background signal from other, nonhyperpolarized spins in the ensemble. Here, we studied the hydrolysis of hyperpolarized ethyl-[1-13C]acetate to hyperpolarized [1-13C]acetate, which was analyzed as a model system by conventional NMR and 13C RASER. The chemical transformation of 13C RASER-active species leads to complete and abrupt disappearance of reactant signals and delayed, abrupt reappearance of a frequency-shifted RASER signal without destroying 13C polarization. The experimentally observed "quantum" RASER threshold is supported by simulations.