Magnetic resonance (MR) is one of the most versatile and useful physical effects used for human imaging, chemical analysis, and the elucidation of molecular structures. However, its full potential is rarely used, because only a small fraction of the nuclear spin ensemble is polarized, that is, aligned with the applied static magnetic field. Hyperpolarization methods seek other means to increase the polarization and thus the MR signal. A unique source of pure spin order is the entangled singlet spin state of dihydrogen, parahydrogen (pH2 ), which is inherently stable and long-lived. When brought into contact with another molecule, this "spin order on demand" allows the MR signal to be enhanced by several orders of magnitude. Considerable progress has been made in the past decade in the area of pH2 -based hyperpolarization techniques for biomedical applications. It is the goal of this Review to provide a selective overview of these developments, covering the areas of spin physics, catalysis, instrumentation, preparation of the contrast agents, and applications.

Parahydrogen-Based Hyperpolarization for Biomedicine

Studies to enhance the hyperpolarization level in PHIP-SAH-produced C13-pyruvate.

Hyperpolarisation of nuclear spins is important in overcoming sensitivity and resolution limitations of magnetic resonance imaging and nuclear magnetic resonance spectroscopy. Current hyperpolarisation techniques require high magnetic fields, low temperatures, or catalysts. Alternatively, the emergence of room temperature spin qubits has opened new pathways to achieve direct nuclear spin hyperpolarisation. Employing a microwave-free cross-relaxation induced polarisation protocol applied to a nitrogen vacancy qubit, we demonstrate quantum probe hyperpolarisation of external molecular nuclear spins to ~50% under ambient conditions, showing a single qubit increasing the polarisation of ~106 nuclear spins by six orders of magnitude over the thermal background. Results are verified against a detailed theoretical treatment, which also describes how the system can be scaled up to a universal quantum hyperpolarisation platform for macroscopic samples. Our results demonstrate the prospects for this approach to nuclear spin hyperpolarisation for molecular imaging and spectroscopy and its potential to extend beyond into other scientific areas.

/pdf/quantum-probe-hyperpolarisation-of-molecular-nuclear-spins-2rmpngtcg2.pdf

Quantum probe hyperpolarisation of molecular nuclear spins.

■ CONTENTS Analysis of pH2 Polarizers B Parahydrogen Addition B Spin-Order Transfer C Purification and Quality Assurance C Components and Capabilities of a Polarizer C Review of Published Instruments D Parahydrogen Generators D Spin Order Transfer (SOT) E SOT Conditions E SOT at Millitesla Fields G SOT Conditions G Published Setups G Challenges H SOT at Ultralow Fields (ULF) I SOT Conditions I Published Setups I Challenges I Magnetic Field Cycling (MFC) I SOT Conditions I Published Setups J Challenges L PHIP of Gases L Published Setups L Challenges M Purification M Catalyst Scavenging M Liquid−Liquid Phase Separation O Heterogeneous Catalysts O Precipitation P Other Promising Developments P PHIP-on-a-Chip P Radio Amplification by Stimulated Emission of Radiation (RASER) P PHIP-X P Toward Clinical Application P Conclusions Q Author Information Q Corresponding Authors Q Authors Q Author Contributions R Notes R Biographies R Acknowledgments S References T

Instrumentation for Hydrogenative Parahydrogen-Based Hyperpolarization Techniques.

Nitroxides occupy a privileged position among plausible metal-free magnetic resonance imaging (MRI) contrast agents (CAs) due to their inherently low-toxicity profiles; nevertheless, their translational development has been hindered by a lack of appropriate contrast sensitivity. Nanostructured materials with high nitroxide densities, where each individual nitroxide within a macromolecular construct contributes to the image contrast, could address this limitation, but the synthesis of such materials remains challenging. Here, we report a modular and scalable synthetic approach to nitroxide-based brush-arm star polymer (BASP) organic radical CAs (ORCAs) with high nitroxide loadings. The optimized ∼30 nm diameter “BASP-ORCA3” displays outstanding T2 sensitivity with a very high molecular transverse relaxivity (r2 > 1000 mM–1 s–1). BASP-ORCA3 further exhibits excellent stability in vivo, no acute toxicity, and highly desirable pharmacokinetic and biodistribution profiles for longitudinal detection of tumors b...

/pdf/triply-loaded-nitroxide-brush-arm-star-polymers-enable-metal-90y7fd9n65.pdf

Triply Loaded Nitroxide Brush-Arm Star Polymers Enable Metal-Free Millimetric Tumor Detection by Magnetic Resonance Imaging.

Hyperpolarized (HP) tracers dramatically increase the sensitivity of magnetic resonance imaging (MRI) to monitor metabolism non-invasively and in vivo. Their production, however, requires an extra polarizing device (polarizer) whose complexity, operation and cost can exceed that of an MRI system itself. Furthermore, the lifetime of HP tracers is short and some of the enhancement is lost during transfer to the application site. Here, we present the production of HP tracers in water without an external polarizer: by Synthesis Amid the Magnet Bore, A Dramatically Enhanced Nuclear Alignment (SAMBADENA) is achieved within seconds, corresponding to a hyperpolarization of ∼20%. As transfer of the tracer is no longer required, SAMBADENA may permit a higher polarization at the time of detection at a fraction of the cost and complexity of external polarizers. This development is particularly promising in light of the recently extended portfolio of biomedically relevant para-hydrogen-tracers and may lead to new diagnostic applications. Hyperpolarized MRI uses molecules with a nuclear spin polarization beyond the thermodynamic equilibrium to enhance imaging contrast. Here, Schmidtet al. enable a single MRI system to both generate a hyperpolarized tracer and perform imaging, eliminating the need for an external polarizer.

/pdf/liquid-state-carbon-13-hyperpolarization-generated-in-an-mri-4bjqmd907e.pdf

Liquid-state carbon-13 hyperpolarization generated in an MRI system for fast imaging.

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Liquid-state carbon-13 hyperpolarization generated in an MRI system for fast imaging.