Aaron M. Coffey

Near-unity nuclear polarization with an open-source 129Xe hyperpolarizer for NMR and MRI

Significance Lung diseases comprise the third leading cause of death in the United States and could benefit from new imaging modalities. “Hyperpolarized” xenon-129 can overcome the ordinarily weak MRI signals from low-density species in lung space or dissolved in tissue; however, clinical progress has been slowed by the difficulty in preparing large amounts of hyperpolarized xenon with high magnetization, as well as the cost and limited availability of xenon hyperpolarization devices. We describe a unique low-cost “open-source” xenon “hyperpolarizer,” characterize its ability to produce xenon-129 with high magnetization, and demonstrate its utility for human lung imaging. The exquisite NMR spectral sensitivity and negligible reactivity of hyperpolarized xenon-129 (HP129Xe) make it attractive for a number of magnetic resonance applications; moreover, HP129Xe embodies an alternative to rare and nonrenewable 3He. However, the ability to reliably and inexpensively produce large quantities of HP129Xe with sufficiently high 129Xe nuclear spin polarization (PXe) remains a significant challenge—particularly at high Xe densities. We present results from our “open-source” large-scale (∼1 L/h) 129Xe polarizer for clinical, preclinical, and materials NMR and MRI research. Automated and composed mostly of off-the-shelf components, this “hyperpolarizer” is designed to be readily implementable in other laboratories. The device runs with high resonant photon flux (up to 200 W at the Rb D1 line) in the xenon-rich regime (up to 1,800 torr Xe in 500 cc) in either single-batch or stopped-flow mode, negating in part the usual requirement of Xe cryocollection. Excellent agreement is observed among four independent methods used to measure spin polarization. In-cell PXe values of ∼90%, ∼57%, ∼50%, and ∼30% have been measured for Xe loadings of ∼300, ∼500, ∼760, and ∼1,570 torr, respectively. PXe values of ∼41% and ∼28% (with ∼760 and ∼1,545 torr Xe loadings) have been measured after transfer to Tedlar bags and transport to a clinical 3 T scanner for MR imaging, including demonstration of lung MRI with a healthy human subject. Long “in-bag” 129Xe polarization decay times have been measured (T1 ∼38 min and ∼5.9 h at ∼1.5 mT and 3 T, respectively)—more than sufficient for a variety of applications.

Microtesla SABRE Enables 10% Nitrogen-15 Nuclear Spin Polarization

Parahydrogen is demonstrated to efficiently transfer its nuclear spin hyperpolarization to nitrogen-15 in pyridine and nicotinamide (vitamin B3 amide) by conducting “signal amplification by reversible exchange” (SABRE) at microtesla fields within a magnetic shield. Following transfer of the sample from the magnetic shield chamber to a conventional NMR spectrometer, the 15N NMR signals for these molecules are enhanced by ∼30,000- and ∼20,000-fold at 9.4 T, corresponding to ∼10% and ∼7% nuclear spin polarization, respectively. This method, dubbed “SABRE in shield enables alignment transfer to heteronuclei” or “SABRE-SHEATH”, promises to be a simple, cost-effective way to hyperpolarize heteronuclei. It may be particularly useful for in vivo applications because of longer hyperpolarization lifetimes, lack of background signal, and facile chemical-shift discrimination of different species.

Irreversible Catalyst Activation Enables Hyperpolarization and Water Solubility for NMR Signal Amplification by Reversible Exchange

Activation of a catalyst [IrCl(COD)(IMes)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; COD = cyclooctadiene)] for signal amplification by reversible exchange (SABRE) was monitored by in situ hyperpolarized proton NMR at 9.4 T. During the catalyst-activation process, the COD moiety undergoes hydrogenation that leads to its complete removal from the Ir complex. A transient hydride intermediate of the catalyst is observed via its hyperpolarized signatures, which could not be detected using conventional nonhyperpolarized solution NMR. SABRE enhancement of the pyridine substrate can be fully rendered only after removal of the COD moiety; failure to properly activate the catalyst in the presence of sufficient substrate can lead to irreversible deactivation consistent with oligomerization of the catalyst molecules. Following catalyst activation, results from selective RF-saturation studies support the hypothesis that substrate polarization at high field arises from nuclear cross-relaxation with hyperpolarized 1H spins of the hydride/orthohydrogen spin bath. Importantly, the chemical changes that accompanied the catalyst’s full activation were also found to endow the catalyst with water solubility, here used to demonstrate SABRE hyperpolarization of nicotinamide in water without the need for any organic cosolvent—paving the way to various biomedical applications of SABRE hyperpolarization methods.

The Feasibility of Formation and Kinetics of NMR Signal Amplification by Reversible Exchange (SABRE) at High Magnetic Field (9.4 T)

1H NMR signal amplification by reversible exchange (SABRE) was observed for pyridine and pyridine-d5 at 9.4 T, a field that is orders of magnitude higher than what is typically utilized to achieve the conventional low-field SABRE effect. In addition to emissive peaks for the hydrogen spins at the ortho positions of the pyridine substrate (both free and bound to the metal center), absorptive signals are observed from hyperpolarized orthohydrogen and Ir-complex dihydride. Real-time kinetics studies show that the polarization build-up rates for these three species are in close agreement with their respective 1H T1 relaxation rates at 9.4 T. The results suggest that the mechanism of the substrate polarization involves cross-relaxation with hyperpolarized species in a manner similar to the spin-polarization induced nuclear Overhauser effect. Experiments utilizing pyridine-d5 as the substrate exhibited larger enhancements as well as partial H/D exchange for the hydrogen atom in the ortho position of pyridine and concomitant formation of HD molecules. While the mechanism of polarization enhancement does not explicitly require chemical exchange of hydrogen atoms of parahydrogen and the substrate, the partial chemical modification of the substrate via hydrogen exchange means that SABRE under these conditions cannot rigorously be referred to as a non-hydrogenative parahydrogen induced polarization process.

In Situ Detection of PHIP at 48 mT: Demonstration Using a Centrally Controlled Polarizer

Presented here is a centrally controlled, automated parahydrogen-based polarizer with in situ detection capability. A 20% polarization, corresponding to a 5,000,000-fold signal enhancement at 48 mT, is demonstrated on 2-hydroxyethyl-1-(13)C-propionate-d(2,3,3) using a double-tuned antenna and pulsed polarization transfer. In situ detection is a refinement of first-generation devices enabling fast calibration of rf pulses and B(0), quality assurance of hyperpolarized contrast agents, and stand-alone operation without the necessity of high-field MR spectrometers. These features are essential for biomedical applications of parahydrogen-based hyperpolarization and for clinical translation. We demonstrate the flexibility of the device by recording (13)C signal decay due to longitudinal relaxation of a hyperpolarized contrast agent at 48 mT corresponding to 2 MHz proton frequency. This appears to be the longest recorded T(1) (101 ± 7 s) for a (13)C hyperpolarized contrast agent in water.

Heterogeneous Solution NMR Signal Amplification by Reversible Exchange

A novel variant of an iridium-based organometallic catalyst was synthesized and used to enhance the NMR signals of pyridine in a heterogeneous phase by immobilization on polymer microbead solid supports. Upon administration of parahydrogen (pH2) gas to a methanol mixture containing the HET-SABRE catalyst particles and the pyridine, up to fivefold enhancements were observed in the (1)H NMR spectra after sample transfer to high field (9.4 T). Importantly, enhancements were not due to any residual catalyst molecules in solution, thus supporting the true heterogeneity of the SABRE process. Further significant improvements may be expected by systematic optimization of experimental parameters. Moreover, the heterogeneous catalyst is easy to separate and recycle, thus opening a door to future potential applications varying from spectroscopic studies of catalysis, to imaging metabolites in the body without concern of contamination from expensive and potentially toxic metal catalysts or accompanying organic molecules.

A pulsed injection parahydrogen generator and techniques for quantifying enrichment.

A device is presented for efficiently enriching parahydrogen by pulsed injection of ambient hydrogen gas. Hydrogen input to the generator is pulsed at high pressure to a catalyst chamber making thermal contact with the cold head of a closed-cycle cryocooler maintained between 15 and 20K. The system enables fast production (0.9 standard liters per minute) and allows for a wide range of production targets. Production rates can be systematically adjusted by varying the actuation sequence of high-pressure solenoid valves, which are controlled via an open source microcontroller to sample all combinations between fast and thorough enrichment by varying duration of hydrogen contact in the catalyst chamber. The entire enrichment cycle from optimization to quantification and storage kinetics are also described. Conversion of the para spin-isomer to orthohydrogen in borosilicate tubes was measured at 8 min intervals over a period of 64 h with a 12 T NMR spectrometer. These relaxation curves were then used to extract initial enrichment by exploiting the known equilibrium (relaxed) distribution of spin isomers with linear least squares fitting to a single exponential decay curve with an estimated error less than or equal to 1%. This procedure is time-consuming, but requires only one sample pressurized to atmosphere. Given that tedious matching to external references are unnecessary with this procedure, we find it to be useful for periodic inspection of generator performance. The equipment and procedures offer a variation in generator design that eliminate the need to meter flow while enabling access to increased rates of production. These tools for enriching and quantifying parahydrogen have been in steady use for 3 years and should be helpful as a template or as reference material for building and operating a parahydrogen production facility.

Parahydrogen Induced Polarization of 1-13C-Phospholactate-d2 for Biomedical Imaging with >30,000,000-fold NMR Signal Enhancement in Water

The synthetic protocol for preparation of 1-13C-phosphoenolpyruvate-d2, precursor for parahydrogen-induced polarization (PHIP) of 1-13C-phospholactate-d2, is reported. 13C nuclear spin polarization of 1-13C-phospholactate-d2 was increased by >30,000,000-fold (5.75 mT) in water. The reported 13C polarization level approaching unity (>15.6%), long lifetime of 13C hyperpolarized 1-13C-phospholactate-d2 (58 ± 4 s versus 36 ± 2 s for nondeuterated form at 47.5 mT), and large production quantities (52 μmoles in 3 mL) in aqueous medium make this compound useful as a potential contrast agent for the molecular imaging of metabolism and other applications.

PASADENA hyperpolarized 13C phospholactate.

We demonstrate that potassium 1-(13)C-phosphoenolpyruvate becomes hyperpolarized potassium 1-(13)C-phospholactate with (13)C T(1) = 36 s after molecular hydrogenation by PASADENA (Parahydrogen and Synthesis Allows Dramatically Enhanced Nuclear Alignment). This proof-of-principle study was conducted with a fully protonated molecular precursor. (13)C was polarized to a level of 1%, corresponding to nearly 4000-fold sensitivity enhancement at 3 T. The relevant homo- and heteronuclear spin-spin couplings are reported.

15N Hyperpolarization of Imidazole-15N2 for Magnetic Resonance pH Sensing via SABRE-SHEATH

15N nuclear spins of imidazole-15N2 were hyperpolarized using NMR signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH). A 15N NMR signal enhancement of ∼2000-fold at 9.4 T is reported using parahydrogen gas (∼50% para-) and ∼0.1 M imidazole-15N2 in methanol:aqueous buffer (∼1:1). Proton binding to a 15N site of imidazole occurs at physiological pH (pKa ∼ 7.0), and the binding event changes the 15N isotropic chemical shift by ∼30 ppm. These properties are ideal for in vivo pH sensing. Additionally, imidazoles have low toxicity and are readily incorporated into a wide range of biomolecules. 15N-Imidazole SABRE-SHEATH hyperpolarization potentially enables pH sensing on scales ranging from peptide and protein molecules to living organisms.