Biological magnetic resonance imaging using laser-polarized 129Xe

Efficient resolution of racemic crown-shaped cyclotriveratrylene derivatives and isolation and characterization of the intermediate saddle isomer

The preparative resolution of a trifunctionalized C₃-symmetrical chiral cyclotriveratrylene derivative was achieved via high-performance liquid chromatography (HPLC) on a chiral stationary phase. This approach is a promising alternative to the previously reported resolution through formation of diastereomeric esters because it involves fewer synthetic steps and is less prone to thermal (re)racemization. During these studies an intermediate saddle conformer could also be isolated and characterized by ¹H and ¹³C NMR spectroscopy. The HPLC separation method was further developed in order to allow investigations on the racemization behavior of the cyclotriveratrylene derivative.

MRI and CT lung biomarkers: Towards an in vivo understanding of lung biomechanics

BACKGROUND

The biomechanical properties of the lung are necessarily dependent on its structure and function, both of which are complex and change over time and space. This makes in vivo evaluation of lung biomechanics and a deep understanding of lung biomarkers, very challenging. In patients and animal models of lung disease, in vivo evaluations of lung structure and function are typically made at the mouth and include spirometry, multiple-breath gas washout tests and the forced oscillation technique. These techniques, and the biomarkers they provide, incorporate the properties of the whole organ system including the parenchyma, large and small airways, mouth, diaphragm and intercostal muscles. Unfortunately, these well-established measurements mask regional differences, limiting their ability to probe the lung's gross and micro-biomechanical properties which vary widely throughout the organ and its subcompartments. Pulmonary imaging has the advantage in providing regional, non-invasive measurements of healthy and diseased lung, in vivo. Here we summarize well-established and emerging lung imaging tools and biomarkers and how they may be used to generate lung biomechanical measurements.

METHODS

We review well-established and emerging lung anatomical, microstructural and functional imaging biomarkers generated using synchrotron x-ray tomographic-microscopy (SRXTM), micro-x-ray computed-tomography (micro-CT), clinical CT as well as magnetic resonance imaging (MRI).

FINDINGS

Pulmonary imaging provides measurements of lung structure, function and biomechanics with high spatial and temporal resolution. Imaging biomarkers that reflect the biomechanical properties of the lung are now being validated to provide a deeper understanding of the lung that cannot be achieved using measurements made at the mouth.

Relaxation Dynamics of Nuclear Long-Lived Spin States in Propane and Propane-d6 Hyperpolarized by Parahydrogen

We report a systematic study of relaxation dynamics of hyperpolarized (HP) propane and HP propane-d6 prepared by heterogeneous pairwise parahydrogen addition to propylene and propylene-d6 respectively. Long-lived spin states (LLS) created for these molecules at the low magnetic field of 0.0475 T were employed for this study. The parahydrogen-induced overpopulation of a HP propane LLS decays exponentially with time constant (TLLS) approximately 3-fold greater than the corresponding T1 values. Both TLLS and T1 increase linearly with propane pressure in the range from 1 atm (the most biomedically relevant conditions for pulmonary MRI) to 5 atm. The TLLS value of HP propane gas at 1 atm is ~3 s. Deuteration of the substrate (propylene-d6) yields hyperpolarized propane-d6 gas with TLLS values approximately 20% shorter than those of hyperpolarized fully protonated propane gas, indicating that deuteration does not benefit the lifetime of the LLS HP state. The use of pH2 or Xe/N2 buffering gas during heterogeneous hydrogenation reaction (leading to production of 100% HP propane (no buffering gas) versus 43% HP propane gas (with 57% buffering gas) composition mixtures) results in (i) no significant changes in T1, (ii) decrease of TLLS values (by 35±7% and 8±7% respectively); and (iii) an increase of the polarization levels of HP propane gas with a propane concentration decrease (by 1.6±0.1-fold and 1.4±0.1-fold respectively despite the decrease in TLLS, which leads to disproportionately greater polarization losses during HP gas transport). Moreover, we demonstrate the feasibility of HP propane cryo-collection (which can be potentially useful for preparing larger amounts of concentrated HP propane, when buffering gas is employed), and TLLS of liquefied HP propane reaches 14.7 seconds, which is greater than the TLLS value of HP propane gas at any pressure studied. Finally, we have explored the utility of using a partial Spin-Lock Induced Crossing (SLIC) radio frequency (RF) pulse sequence for converting the overpopulated LLS into observable 1H nuclear magnetization at low magnetic field. We find that (i) the bulk of the overpopulated LLS is retained even when the optimal or near-optimal values of SLIC pulse duration are employed, and (ii) the overpopulated LLS of propane is also relatively immune to strong RF pulses-thereby, indicating that LLS is highly suitable as a spin-polarization reservoir in the context of NMR/MRI detection applications. The presented findings may be useful for improving the levels of polarization of HP propane produced by HET-PHIP via the use of an inert buffer gas; increasing the lifetime of the HP state during preparation and storage; and developing efficient approaches for ultrafast MR imaging of HP propane in the context of biomedical applications of HP propane gas, including its potential use as an inhalable contrast agent.

Single breath-hold measurement of pulmonary gas exchange and diffusion in humans with hyperpolarized 129 Xe MR

Pulmonary diseases usually result in changes of the blood-gas exchange function in the early stages. Gas exchange across the respiratory membrane and gas diffusion in the alveoli can be quantified using hyperpolarized ¹²⁹ Xe MR via chemical shift saturation recovery (CSSR) and diffusion-weighted imaging (DWI), respectively. Generally, CSSR and DWI data have been collected in separate breaths in humans. Unfortunately, the lung inflation level cannot be the exactly same in different breaths, which causes fluctuations in blood-gas exchange and pulmonary microstructure. Here we combine CSSR and DWI obtained with compressed sensing, to evaluate the gas diffusion and exchange function within a single breath-hold in humans. A new parameter, namely the perfusion factor of the respiratory membrane (SVR_d/g ), is proposed to evaluate the gas exchange function. Hyperpolarized ¹²⁹ Xe MR data are compared with pulmonary function tests and computed tomography examinations in healthy young, age-matched control, and chronic obstructive pulmonary disease human cohorts. SVR_d/g decreases as the ventilation impairment and emphysema index increase. Our results indicate that the proposed method has the potential to detect the extent of lung parenchyma destruction caused by age and pulmonary diseases, and it would be useful in the early diagnosis of pulmonary diseases in clinical practice.

Highly and Adaptively Undersampling Pattern for Pulmonary Hyperpolarized 129Xe Dynamic MRI

Hyperpolarized (HP) gas (e.g., ³He or ¹²⁹Xe) dynamic MRI could visualize the lung ventilation process, which provides characteristics regarding lung physiology and pathophysiology. Compressed sensing (CS) is generally used to increase the temporal resolution of such dynamic MRI. Nevertheless, the acceleration factor of CS is constant, which results in difficulties in precisely observing and/or measuring dynamic ventilation process due to bifurcating network structure of the lung. Here, an adaptive strategy is proposed to highly undersample pulmonary HP dynamic k-space data, according to the characteristics of both lung structure and gas motion. After that, a valid reconstruction algorithm is developed to reconstruct dynamic MR images, considering the low-rank, global sparsity, gas-inflow effects, and joint sparsity. Both the simulation and the in vivo results verify that the proposed approach outperforms the state-of-the-art methods both in qualitative and quantitative comparisons. In particular, the proposed method acquires 33 frames within 6.67 s (more than double the temporal resolution of the recently proposed strategy), and achieves high-image quality [the improvements are 29.63%, 3.19%, 2.08%, and 13.03% regarding the mean absolute error (MAE), structural similarity index (SSIM), quality index based on local variance (QILV), and contrast-to-noise ratio (CNR) comparisons]. This provides accurate structural and functional information for early detection of obstructive lung diseases.

Patch-based lung ventilation estimation using multi-layer supervoxels

Patch-based approaches have received substantial attention over the recent years in medical imaging. One of their potential applications may be to provide more anatomically consistent ventilation maps estimated on dynamic lung CT. An assessment of regional lung function may act as a guide for radiotherapy, ensuring a more accurate treatment plan. This in turn, could spare well-functioning parts of the lungs. We present a novel method for lung ventilation estimation from dynamic lung CT imaging, combining a supervoxel-based image representation with deformations estimated during deformable image registration, performed between peak breathing phases. For this we propose a method that tracks changes of the intensity of previously extracted supervoxels. For the evaluation of the method we calculate correlation of the estimated ventilation maps with static ventilation images acquired from hyperpolarized Xenon129 MRI. We also investigate the influence of different image registration methods used to estimate deformations between the peak breathing phases in the dynamic CT imaging. We show that our method performs favorably to other ventilation estimation methods commonly used in the field, independently of the image registration method applied to dynamic CT. Due to the patch-based approach of our method, it may be physiologically more consistent with lung anatomy than previous methods relying on voxel-wise relationships. In our method the ventilation is estimated for supervoxels, which tend to group spatially close voxels with similar intensity values. The proposed method was evaluated on a dataset consisting of three lung cancer patients undergoing radiotherapy treatment, and this resulted in a correlation of 0.485 with XeMRI ventilation images, compared with 0.393 for the intensity-based approach, 0.231 for the Jacobian-based method and 0.386 for the Hounsfield units averaging method, on average. Within the limitation of the small number of cases analyzed, results suggest that the presented technique may be advantageous for CT-based ventilation estimation. The results showing higher values of correlation of the proposed method demonstrate the potential of our method to more accurately mimic the lung physiology.

Clinical-Scale Batch-Mode Production of Hyperpolarized Propane Gas for MRI

NMR spectroscopy and imaging (MRI) are two of the most important methods to study structure, function, and dynamics from atom to organism scale. NMR approaches often suffer from an insufficient sensitivity, which, however, can be transiently boosted using hyperpolarization techniques. One of these techniques is parahydrogen-induced polarization, which has been used to produce catalyst-free hyperpolarized propane gas with proton polarization that is 3 orders of magnitude greater than equilibrium thermal polarization at a 1.5 T field of a clinical MRI scanner. Here we show that more than 0.3 L of hyperpolarized propane gas can be produced in 2 s. This production rate is more than an order of magnitude greater than that demonstrated previously, and the reported production rate is comparable to that employed for in-human MRI using HP noble gas (e.g., ¹²⁹Xe) produced via a spin exchange optical pumping (SEOP) hyperpolarization technique. We show that high polarization values can be retained despite the significant increase in the production rate of hyperpolarized propane. The enhanced signals of produced hyperpolarized propane gas were revealed by stopped-flow MRI visualization at 4.7 T. Achieving this high production rate enables the future use of this compound (already approved for unlimited use in foods by the corresponding regulating agencies, e.g., FDA in the USA, and more broadly as an E944 food additive) as a new inhalable contrast agent for diagnostic detection via MRI.

Advanced pulmonary MRI to quantify alveolar and acinar duct abnormalities: Current status and future clinical applications

There are serious clinical gaps in our understanding of chronic lung disease that require novel, sensitive, and noninvasive in vivo measurements of the lung parenchyma to measure disease pathogenesis and progressive changes over time as well as response to treatment. Until recently, our knowledge and appreciation of the tissue changes that accompany lung disease has depended on ex vivo biopsy and concomitant histological and stereological measurements. These measurements have revealed the underlying pathologies that drive lung disease and have provided important observations about airway occlusion, obliteration of the terminal bronchioles and airspace enlargement, or fibrosis and their roles in disease initiation and progression. ex vivo tissue stereology and histology are the established gold standards and, more recently, micro-computed tomography (CT) measurements of ex vivo tissue samples has also been employed to reveal new mechanistic findings about the progression of obstructive lung disease in patients. While these approaches have provided important understandings using ex vivo analysis of excised samples, recently developed hyperpolarized noble gas MRI methods provide an opportunity to noninvasively measure acinar duct and terminal airway dimensions and geometry in vivo, and, without radiation burden. Therefore, in this review we summarize emerging pulmonary MRI morphometry methods that provide noninvasive in vivo measurements of the lung in patients with bronchopulmonary dysplasia and chronic obstructive pulmonary disease, among others. We discuss new findings, future research directions, as well as clinical opportunities to address current gaps in patient care and for testing of new therapies. Level of Evidence: 5 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2019;50:28-40.

An initial investigation of hyperpolarized gas tagging magnetic resonance imaging in evaluating deformable image registration-based lung ventilation

BACKGROUND

Deformable image registration (DIR)-based lung ventilation mapping is attractive due to its simplicity, and also challenging due to its susceptibility to errors and uncertainties. In this study, we explored the use of 3D Hyperpolarized (HP) gas tagging MRI to evaluate DIR-based lung ventilation.

METHOD AND MATERIAL

Three healthy volunteers included in this study underwent both 3D HP gas tagging MRI (t-MRI) and 3D proton MRI (p-MRI) using balanced steady-state free precession pulse sequence at end of inhalation and end of exhalation. We first obtained the reference displacement vector fields (DVFs) from the t-MRIs by tracking the motion of each tagging grid between the exhalation and the inhalation phases. Then, we determined DIR-based DVFs from the p-MRIs by registering the images at the two phases with two commercial DIR algorithms. Lung ventilations were calculated from both the reference DVFs and the DIR-based DVFs using the Jacobian method and then compared using cross correlation and mutual information.

RESULTS

The DIR-based lung ventilations calculated using p-MRI varied considerably from the reference lung ventilations based on t-MRI among all three subjects. The lung ventilations generated using Velocity AI were preferable for the better spatial homogeneity and accuracy compared to the ones using MIM, with higher average cross correlation (0.328 vs 0.262) and larger average mutual information (0.528 vs 0.323).

CONCLUSION

We demonstrated that different DIR algorithms resulted in different lung ventilation maps due to underlining differences in the DVFs. HP gas tagging MRI provides a unique platform for evaluating DIR-based lung ventilation.

COPD biomarkers and phenotypes: opportunities for better outcomes with precision imaging

A number of chronic diseases have benefited from both imaging and personalised medicine, but unfortunately, for patients with chronic obstructive pulmonary disease (COPD), there has been little clinical uptake or recognition of the key advances in thoracic imaging that might help detect disease early, or, perhaps more importantly, might help develop and phenotype patients for novel or personalised therapies that may halt disease progression. We outline our vision for how computed tomography and magnetic resonance imaging may be used to better inform COPD patient care, and, perhaps more importantly, how these may be used to help develop new therapies directed at early disease. We think that imaging and precision medicine should be considered and used together as "precision imaging" at specific stages of COPD when the major pathologies may be more responsive to therapy. While "precision medicine" is the tailoring of medical treatment to individual patients, we define "precision imaging" as the tailoring of specific therapies and interventions to individual patients with a detailed quantitative understanding of their specific imaging phenotypes and measurements. Finally, we stress the importance of "seeing" the pathology, because without this understanding, you can neither treat nor cure patients with COPD.

A framework for Fourier-decomposition free-breathing pulmonary 1 H MRI ventilation measurements

PURPOSE

To develop a rapid Fourier decomposition (FD) free-breathing pulmonary ¹ H MRI (FDMRI) image processing and biomarker pipeline for research use.

METHODS

We acquired MRI in 20 asthmatic subjects using a balanced steady-state free precession (bSSFP) sequence optimized for ventilation imaging. 2D ¹ H MRI series were segmented by enforcing the spatial similarity between adjacent images and the right-to-left lung volume-ratio. The segmented lung series were co-registered using a coarse-to-fine deformable registration framework that used dual optimization techniques. All pairwise registrations were implemented in parallel and FD was performed to generate 2D ventilation-weighted maps and ventilation-defect-percent (VDP). Lung segmentation and registration accuracy were evaluated by comparing algorithm and manual lung-masks, deformed manual lung-masks, and fiducials in the moving and fixed images using Dice-similarity-coefficient (DSC), mean-absolute-distance (MAD), and target-registration-error (TRE). The relationship of FD-VDP and ³ He-VDP was evaluated using the Pearson-correlation-coefficient (r) and Bland Altman analysis. Algorithm reproducibility was evaluated using the coefficient-of-variation (CoV) and intra-class-correlation-coefficient (ICC) for segmentation, registration, and FD-VDP components.

RESULTS

For lung segmentation, there was a DSC of 95 ± 1.5% and MAD of 2.3 ± 0.5 mm, and for registration there was a DSC of 97 ± 0.8%, MAD of 1.6 ± 0.4 mm and TRE of 3.6 ± 1.2 mm. Reproducibility for segmentation DSC (CoV/ICC = 0.5%/0.92), registration TRE (CoV/ICC = 0.4%/0.98), and FD-VDP (Cov/ICC = 3.9%/0.97) was high. The pipeline required 10 min/subject. FD-VDP was correlated with ³ He-VDP (r = 0.69, P < 0.001) although there was a bias toward lower FD-VDP (bias = -4.9%).

CONCLUSIONS

We developed and evaluated a pipeline that provides a rapid and precise method for FDMRI ventilation maps.

Heterogeneous Parahydrogen Pairwise Addition to Cyclopropane

Hyperpolarized gases revolutionize functional pulmonary imaging. Hyperpolarized propane is a promising emerging contrast agent for pulmonary MRI. Unlike hyperpolarized noble gases, proton-hyperpolarized propane gas can be imaged using conventional MRI scanners with proton imaging capability. Moreover, it is non-toxic odorless anesthetic. Furthermore, propane hyperpolarization can be accomplished by pairwise addition of parahydrogen to propylene. Here, we demonstrate the feasibility of propane hyperpolarization via hydrogenation of cyclopropane with parahydrogen. 1 H propane polarization up to 2.4 % is demonstrated here using 82 % parahydrogen enrichment and heterogeneous Rh/TiO2 hydrogenation catalyst. This level of polarization is several times greater than that obtained with propylene as a precursor under the same conditions despite the fact that direct pairwise addition of parahydrogen to cyclopropane may also lead to formation of propane with NMR-invisible hyperpolarization due to magnetic equivalence of nascent parahydrogen protons in two CH3 groups. NMR-visible hyperpolarized propane demonstrated here can be formed only via a reaction pathway involving cleavage of at least one C-H bond in the reactant molecule. The resulting NMR signal enhancement of hyperpolarized propane was sufficient for 2D gradient echo MRI of ∼5.5 mL phantom with 1×1 mm2 spatial resolution and 64×64 imaging matrix despite relatively low chemical conversion of cyclopropane substrate.

19 F MRI of the Lungs Using Inert Fluorinated Gases: Challenges and New Developments

Fluorine-19 (¹⁹ F) MRI using inhaled inert fluorinated gases is an emerging technique that can provide functional images of the lungs. Inert fluorinated gases are nontoxic, abundant, relatively inexpensive, and the technique can be performed on any MRI scanner with broadband multinuclear imaging capabilities. Pulmonary ¹⁹ F MRI has been performed in animals, healthy human volunteers, and in patients with lung disease. In this review, the technical requirements of ¹⁹ F MRI are discussed, along with various imaging approaches used to optimize the image quality. Lung imaging is typically performed in humans using a gas mixture containing 79% perfluoropropane (PFP) or sulphur hexafluoride (SF₆ ) and 21% oxygen. In lung diseases, such as asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF), ventilation defects are apparent in regions that the inhaled gas cannot access. ¹⁹ F lung images are typically acquired in a single breath-hold, or in a time-resolved, multiple breath fashion. The former provides measurements of the ventilation defect percent (VDP), while the latter provides measurements of gas replacement (ie, fractional ventilation). Finally, preliminary comparisons with other functional lung imaging techniques are discussed, such as Fourier decomposition MRI and hyperpolarized gas MRI. Overall, functional ¹⁹ F lung MRI is expected to complement existing proton-based structural imaging techniques, and the combination of structural and functional lung MRI will provide useful outcome measures in the future management of pulmonary diseases in the clinic. Level of Evidence: 3 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:343-354.

Quantitative evaluation of pulmonary gas-exchange function using hyperpolarized 129 Xe CEST MRS and MRI

Hyperpolarized ¹²⁹ Xe gas MR has been a powerful tool for evaluating pulmonary structure and function due to the extremely high enhancement in spin polarization, the good solubility in the pulmonary parenchyma, and the excellent chemical sensitivity to its surrounding environment. Generally, the quantitative structural and functional information of the lung are evaluated using hyperpolarized ¹²⁹ Xe by employing the techniques of chemical shift saturation recovery (CSSR) and xenon polarization transfer contrast (XTC). Hyperpolarized ¹²⁹ Xe chemical exchange saturation transfer (Hyper-CEST) is another method for quantifying the exchange information of hyperpolarized ¹²⁹ Xe by using the exchange of xenon signals according to its different chemical shifts, and it has been widely used in biosensor studies in vitro. However, the feasibility of using hyperpolarized ¹²⁹ Xe CEST to quantify the pulmonary gas exchange function in vivo is still unclear. In this study, the technique of CEST was used to quantitatively evaluate the gas exchange in the lung globally and regionally via hyperpolarized ¹²⁹ Xe MRS and MRI, respectively. A new parameter, the pulmonary apparent gas exchange time constant (T_app ), was defined, and it increased from 0.63 s to 0.95 s in chronic obstructive pulmonary disease (COPD) rats (induced by cigarette smoke and lipopolysaccharide exposure) versus the controls with a significant difference (P = 0.001). Additionally, the spatial distribution maps of T_app in COPD rats' pulmonary parenchyma showed a regionally obvious increase compared with healthy rats. These results indicated that hyperpolarized ¹²⁹ Xe CEST MR was an effective method for globally and regionally quantifying the pulmonary gas exchange function, which would be helpful in diagnosing lung diseases that are related to gas exchange, such as COPD.

Development of a pulmonary imaging biomarker pipeline for phenotyping of chronic lung disease

We designed and generated pulmonary imaging biomarker pipelines to facilitate high-throughput research and point-of-care use in patients with chronic lung disease. Image processing modules and algorithm pipelines were embedded within a graphical user interface (based on the .NET framework) for pulmonary magnetic resonance imaging (MRI) and x-ray computed-tomography (CT) datasets. The software pipelines were generated using C++ and included: (1) inhaled He3/Xe129 MRI ventilation and apparent diffusion coefficients, (2) CT-MRI coregistration for lobar and segmental ventilation and perfusion measurements, (3) ultrashort echo-time H1 MRI proton density measurements, (4) free-breathing Fourier-decomposition H1 MRI ventilation/perfusion and free-breathing H1 MRI specific ventilation, (5) multivolume CT and MRI parametric response maps, and (6) MRI and CT texture analysis and radiomics. The image analysis framework was implemented on a desktop workstation/tablet to generate biomarkers of regional lung structure and function related to ventilation, perfusion, lung tissue texture, and integrity as well as multiparametric measures of gas trapping and airspace enlargement. All biomarkers were generated within 10 min with measurement reproducibility consistent with clinical and research requirements. The resultant pulmonary imaging biomarker pipeline provides real-time and automated lung imaging measurements for point-of-care and high-throughput research.

Inhaled Xenon Washout as a Biomarker of Alzheimer's Disease

Biomarkers have the potential to aid in the study of Alzheimer’s disease (AD); unfortunately, AD biomarker values often have a high degree of overlap between healthy and AD individuals. This study investigates the potential utility of a series of novel AD biomarkers, the sixty second 129Xe retention time, and the xenon washout parameter, based on the washout of hyperpolarized 129Xe from the brain of AD participants following inhalation. The xenon washout parameter is influenced by cerebral perfusion, T1 relaxation of xenon, and the xenon partition coefficient, all factors influenced by AD. Participants with AD (n = 4) and healthy volunteers (n = 4) were imaged using hyperpolarized 129Xe magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) to determine the amount of retained xenon in the brain. At 60 s after the breath hold, AD patients retained significantly higher amounts of 129Xe compared to healthy controls. Data was fit to a pharmacokinetic model and the xenon washout parameter was extracted. Xenon washout in white and grey matter occurs at a slower rate in Alzheimer’s participants (129Xe half-life time of 42 s and 43 s, respectively) relative to controls (20 s and 16 s, respectively). Following larger scale clinical trials for validation, the xenon washout parameter has the potential to become a useful biomarker for the support of AD diagnosis.

Effects of superparamagnetic iron oxide nanoparticles on the longitudinal and transverse relaxation of hyperpolarized xenon gas

SuperParamagnetic Iron Oxide Nanoparticles (SPIONs) are often used in magnetic resonance imaging experiments to enhance Magnetic Resonance (MR) sensitivity and specificity. While the effect of SPIONs on the longitudinal and transverse relaxation time of ¹H spins has been well characterized, their effect on highly diffusive spins, like those of hyperpolarized gases, has not. For spins diffusing in linear magnetic field gradients, the behavior of the magnetization is characterized by the relative size of three length scales: the diffusion length, the structural length, and the dephasing length. However, for spins diffusing in non-linear gradients, such as those generated by iron oxide nanoparticles, that is no longer the case, particularly if the diffusing spins experience the non-linearity of the gradient. To this end, 3D Monte Carlo simulations are used to simulate the signal decay and the resulting image contrast of hyperpolarized xenon gas near SPIONs. These simulations reveal that signal loss near SPIONs is dominated by transverse relaxation, with little contribution from T₁ relaxation, while simulated image contrast and experiments show that diffusion provides no appreciable sensitivity enhancement to SPIONs.

Considering low-rank, sparse and gas-inflow effects constraints for accelerated pulmonary dynamic hyperpolarized 129Xe MRI

Dynamic hyperpolarized (HP) ¹²⁹Xe MRI is able to visualize the process of lung ventilation, which potentially provides unique information about lung physiology and pathophysiology. However, the longitudinal magnetization of HP ¹²⁹Xe is nonrenewable, making it difficult to achieve high image quality while maintaining high temporal-spatial resolution in the pulmonary dynamic MRI. In this paper, we propose a new accelerated dynamic HP ¹²⁹Xe MRI scheme incorporating the low-rank, sparse and gas-inflow effects (L + S + G) constraints. According to the gas-inflow effects of HP gas during the lung inspiratory process, a variable-flip-angle (VFA) strategy is designed to compensate for the rapid attenuation of the magnetization. After undersampling k-space data, an effective reconstruction algorithm considering the low-rank, sparse and gas-inflow effects constraints is developed to reconstruct dynamic MR images. In this way, the temporal and spatial resolution of dynamic MR images is improved and the artifacts are lessened. Simulation and in vivo experiments implemented on the phantom and healthy volunteers demonstrate that the proposed method is not only feasible and effective to compensate for the decay of the magnetization, but also has a significant improvement compared with the conventional reconstruction algorithms (P-values are less than 0.05). This confirms the superior performance of the proposed designs and their ability to maintain high quality and temporal-spatial resolution.

Hyperpolarized gas MRI in pulmonology

Lung diseases have a high prevalence amongst the world population and their early diagnosis has been pointed out to be key for successful treatment. However, there is still a lack of non-invasive examination methods with sensitivity to early, local deterioration of lung function. Proton-based lung MRI is particularly challenging due to short T2* times and low proton density within the lung tissue. Hyperpolarized gas MRI is aan emerging technology providing a richness of methodologies which overcome the aforementioned problems. Unlike proton-based MRI, lung MRI of hyperpolarized gases may rely on imaging of spins in the lung's gas spaces or inside the lung tissue and thereby add substantial value and diagnostic potential to lung MRI. This review article gives an introduction to the MR physics of hyperpolarized media and presents the current state of hyperpolarized gas MRI of 3Headvasd and 129Xe in pulmonology. Key applications, ranging from static and dynamic ventilation imaging as well as oxygen-pressure mapping to 129Xe dissolved-phase imaging and spectroscopy are presented. Hyperpolarized gas MRI is compared to alternative examination methods based on MRI and future directions of hyperpolarized gas MRI are discussed.

Brain Imaging Using Hyperpolarized 129Xe Magnetic Resonance Imaging

Hyperpolarized (HP) ¹²⁹Xe magnetic resonance imaging (MRI) is a novel iteration of traditional MRI that relies on detecting the spins of ¹H. Since ¹²⁹Xe is a gaseous signal source, it can be used for lung imaging. Additionally, ¹²⁹Xe dissolves in the blood stream and can therefore be detectable in the brain parenchyma and vasculature. In this work, we provide detailed information on the protocols that we have developed to image ¹²⁹Xe within the brains of both rodents and human subjects.

Spin-Lattice Relaxation of Hyperpolarized Metronidazole in Signal Amplification by Reversible Exchange in Micro-Tesla Fields

Simultaneous reversible chemical exchange of parahydrogen and to-be-hyperpolarized substrate on metal centers enables spontaneous transfer of spin order from parahydrogen singlet to nuclear spins of the substrate. When performed at sub-micro-Tesla magnetic field, this technique of NMR Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH). SABRE-SHEATH has been shown to hyperpolarize nitrogen-15 sites of a wide range of biologically interesting molecules to a high polarization level (P > 20%) in one minute. Here, we report on a systematic study of 1H, 13C and 15N spin-lattice relaxation (T1) of metronidazole-13C2-15N2 in SABRE-SHEATH hyperpolarization process. In micro-Tesla range, we find that all 1H, 13C and 15N spins studied share approximately the same T1 values (ca. 4 s at the conditions studied) due to mixing of their Zeeman levels, which is consistent with the model of relayed SABRE-SHEATH effect. These T1 values are significantly lower than those at higher magnetic (i.e. the Earth's magnetic field and above), which exceed 3 minutes in some cases. Moreover, these relatively short T1 values observed below 1 micro-Tesla limit the polarization build-up process of SABRE-SHEATH- thereby, limiting maximum attainable 15N polarization. The relatively short nature of T1 values observed below 1 micro-Tesla is primarily caused by intermolecular interactions with quadrupolar iridium centers or dihydride protons of the employed polarization transfer catalyst, whereas intramolecular spin-spin interactions with 14N quadrupolar centers have significantly smaller contribution. The presented experimental results and their analysis will be beneficial for more rational design of SABRE-SHEATH (i) polarization transfer catalyst, and (ii) hyperpolarized molecular probes in the context of biomedical imaging and other applications.

Cyclodextrin-Based Pseudorotaxanes: Easily Conjugatable Scaffolds for Synthesizing Hyperpolarized Xenon-129 Magnetic Resonance Imaging Agents

Hyperpolarized (HP) xenon-129 (Xe) magnetic resonance (MR) imaging has the potential to detect biological analytes with high sensitivity and high resolution when coupled with xenon-encapsulating molecular probes. Despite the development of numerous HP Xe probes, one of the challenges that has hampered the translation of these agents from in vitro demonstration to in vivo testing is the difficulty in synthesizing the Xe-encapsulating cage molecule. In this study, we demonstrate that a pseudorotaxane, based on a γ-cyclodextrin macrocycle, is easily synthesized in one step and is detectable using HyperCEST-enhanced ¹²⁹Xe MR spectroscopy.

Plant metabolism as studied by NMR spectroscopy

The study of plant metabolism impacts a broad range of domains such as plant cultural practices, plant breeding, human or animal nutrition, phytochemistry and green biotechnologies. Plant metabolites are extremely diverse in terms of structure or compound families as well as concentrations. This review attempts to illustrate how NMR spectroscopy, with its broad variety of experimental approaches, has contributed widely to the study of plant primary or specialized metabolism in very diverse ways. The review presents recent developments of one-dimensional and multi-dimensional NMR methods to study various aspects of plant metabolism. Through recent examples, it highlights how NMR has proved to be an invaluable tool for the global characterization of sample composition within metabolomic studies, and shows some examples of use for targeted phytochemistry, with a special focus on compound identification and quantitation. In such cases, NMR approaches are often used to provide snapshots of the plant sample composition. The review also covers dynamic aspects of metabolism, with a description of NMR techniques to measure metabolic fluxes - in most cases after stable isotope labelling. It is mainly intended for NMR specialists who would be interested to learn more about the potential of their favourite technique in plant sciences and about specific details of NMR approaches in this field. Therefore, as a practical guide, a paragraph on the specific precautions that should be taken for sample preparation is also included. In addition, since the quality of NMR metabolic studies is highly dependent on approaches to data processing and data sharing, a specific part is dedicated to these aspects. The review concludes with perspectives on the emerging methods that could change significantly the role of NMR in the field of plant metabolism by boosting its sensitivity. The review is illustrated throughout with examples of studies selected to represent diverse applications of liquid-state or HR-MAS NMR.

Optically polarized 3He

This article reviews the physics and technology of producing large quantities of highly spin-polarized 3He nuclei using spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping. Both technical developments and deeper understanding of the physical processes involved have led to substantial improvements in the capabilities of both methods. For SEOP, the use of spectrally narrowed lasers and K-Rb mixtures has substantially increased the achievable polarization and polarizing rate. For MEOP nearly lossless compression allows for rapid production of polarized 3He and operation in high magnetic fields has likewise significantly increased the pressure at which this method can be performed, and revealed new phenomena. Both methods have benefitted from development of storage methods that allow for spin-relaxation times of hundreds of hours, and specialized precision methods for polarimetry. SEOP and MEOP are now widely applied for spin-polarized targets, neutron spin filters, magnetic resonance imaging, and precision measurements.

Nanomolar small-molecule detection using a genetically encoded 129Xe NMR contrast agent

Genetically encoded magnetic resonance imaging (MRI) contrast agents enable non-invasive detection of specific biomarkers in vivo.

Genetically encoded magnetic resonance imaging (MRI) contrast agents enable non-invasive detection of specific biomarkers in vivo. Here, we employed the hyper-CEST ¹²⁹Xe NMR technique to quantify maltose (32 nM to 1 mM) through its modulation of conformational change and xenon exchange in maltose binding protein (MBP). Remarkably, no hyper-CEST signal was observed for MBP in the absence of maltose, making MBP an ultrasensitive “smart” contrast agent. The resonance frequency of ¹²⁹Xe bound to MBP was greatly downfield-shifted (Δδ = 95 ppm) from the ¹²⁹Xe_(aq) peak, which facilitated detection in E. coli as well as multiplexing with TEM-1 β-lactamase. Finally, a Val to Ala mutation at the MBP–Xe binding site yielded 34% more contrast than WT, with ¹²⁹Xe resonance frequency shifted 59 ppm upfield from WT. We conclude that engineered MBPs constitute a new class of genetically encoded, analyte-sensitive molecular imaging agents detectable by ¹²⁹Xe NMR/MRI.

Assessment of pulmonary microstructural changes by hyperpolarized 129Xe diffusion-weighted imaging in an elastase-instilled rat model of emphysema

BACKGROUND

The purpose of this study was to explore the feasibility of hyperpolarized ¹²⁹Xe diffusion-weighted imaging (DWI) for the evaluation of pulmonary microstructural changes in the presence of pancreatic porcine elastase (PPE)-induced pulmonary emphysema rat model.

METHODS

Sixteen male Sprague-Dawley (SD) rats were randomly divided into two groups, the emphysema model group and control group. Experimental emphysematous models were made by instilling elastase into rat lungs of model group, the control group were instilled with isodose saline. Hyperpolarized ¹²⁹Xe magnetic resonance imaging (MRI) and histology were performed in all 16 rats after 30 days. DWIs were performed on a Bruker 7.0 T micro MRI, and the apparent diffusion coefficients (ADCs) were measured in all rats. Mean linear intercepts (MLIs) of pulmonary alveoli were measured on histology. The statistical analyses were performed about the correlation between the mean ADC of hyperpolarized ¹²⁹Xe in the whole lung and MLI of pulmonary histology metric.

RESULTS

The pulmonary emphysematous model was successfully confirmed by the histology and all scans were also successful. The ADC value of ¹²⁹Xe in the model group (0.0313±0.0005 cm²/s) was significantly increased compared with that of the control group (0.0288±0.0007 cm²/s, P<0.0001). Morphological differences such as MLI of pulmonary alveoli were observed between the two groups, the MLI of pulmonary alveoli in model group significantly increased (91±5 µm) than that of control group (50±3 µm, P<0.0001). Furthermore, the ADCs was moderately correlated with MLIs (r=0.724, P<0.01).

CONCLUSIONS

These results indicate that ¹²⁹Xe ADC value can quantitatively reflect the alveolar space enlargement and it is a promising biomarker for the detection of pulmonary emphysema.

MRI of chemical reactions and processes

As magnetic resonance imaging (MRI) can spatially resolve a wealth of molecular information available from nuclear magnetic resonance (NMR), it is able to non-invasively visualise the composition, properties and reactions of a broad range of spatially-heterogeneous molecular systems. Hence, MRI is increasingly finding applications in the study of chemical reactions and processes in a diverse range of environments and technologies. This article will explain the basic principles of MRI and how it can be used to visualise chemical composition and molecular properties, providing an overview of the variety of information available. Examples are drawn from the disciplines of chemistry, chemical engineering, environmental science, physics, electrochemistry and materials science. The review introduces a range of techniques used to produce image contrast, along with the chemical and molecular insight accessible through them. Methods for mapping the distribution of chemical species, using chemical shift imaging or spatially-resolved spectroscopy, are reviewed, as well as methods for visualising physical state, temperature, current density, flow velocities and molecular diffusion. Strategies for imaging materials with low signal intensity, such as those containing gases or low sensitivity nuclei, using compressed sensing, para-hydrogen or polarisation transfer, are discussed. Systems are presented which encapsulate the diversity of chemical and physical parameters observable by MRI, including one- and two-phase flow in porous media, chemical pattern formation, phase transformations and hydrodynamic (fingering) instabilities. Lastly, the emerging area of electrochemical MRI is discussed, with studies presented on the visualisation of electrochemical deposition and dissolution processes during corrosion and the operation of batteries, supercapacitors and fuel cells.

Inside information on xenon adsorption in porous organic cages by NMR

A solid porous molecular crystal formed from an organic cage, CC3, has unprecedented performance for the separation of rare gases. Here, xenon was used as an internal reporter providing extraordinarily versatile information about the gas adsorption phenomena in the cage and window cavities of the material. 129Xe NMR measurements combined with state-of-the-art quantum chemical calculations allowed the determination of the occupancies of the cavities, binding constants, thermodynamic parameters as well as the exchange rates of Xe between the cavities. Chemical exchange saturation transfer (CEST) experiments revealed a minor window cavity site with a significantly lower exchange rate than other sites. Diffusion measurements showed significantly reduced mobility of xenon with loading. 129Xe spectra also revealed that the cage cavity sites are preferred at lower loading levels, due to more favourable binding, whereas window sites come to dominate closer to saturation because of their greater prevalence.

Interrogating Metabolism in Brain Cancer

This article reviews existing and emerging techniques of interrogating metabolism in brain cancer from well-established proton magnetic resonance spectroscopy to the promising hyperpolarized metabolic imaging and chemical exchange saturation transfer and emerging techniques of imaging inflammation. Some of these techniques are at an early stage of development and clinical trials are in progress in patients to establish the clinical efficacy. It is likely that in vivo metabolomics and metabolic imaging is the next frontier in brain cancer diagnosis and assessing therapeutic efficacy; with the combined knowledge of genomics and proteomics a complete understanding of tumorigenesis in brain might be achieved.

Toward Hyperpolarized 19 F Molecular Imaging via Reversible Exchange with Parahydrogen

Fluorine-19 has high NMR detection sensitivity-similar to that of protons-owing to its large gyromagnetic ratio and high natural abundance (100 %). Unlike protons, however, fluorine-19 (¹⁹ F) has a negligible occurrence in biological objects, as well as a more sensitive chemical shift. As a result, in vivo ¹⁹ F NMR spectroscopy and MR imaging offer advantages of negligible background signal and sensitive reporting of the local molecular environment. Here we report on NMR hyperpolarization of ¹⁹ F nuclei using reversible exchange reactions with parahydrogen gas as the source of nuclear spin order. NMR signals of 3-fluoropyridine were enhanced by ≈100 fold, corresponding to 0.3 % ¹⁹ F nuclear spin polarization (at 9.4 T), using about 50 % parahydrogen. While future optimization efforts will likely significantly increase the hyperpolarization levels, we already demonstrate the utility of ¹⁹ F hyperpolarization for high-resolution hyperpolarized ¹⁹ F imaging and hyperpolarized ¹⁹ F pH sensing.

Diffusion lung imaging with hyperpolarized gas MRI

Lung imaging using conventional 1 H MRI presents great challenges because of the low density of lung tissue, lung motion and very fast lung tissue transverse relaxation (typical T2 * is about 1-2 ms). MRI with hyperpolarized gases (3 He and 129 Xe) provides a valuable alternative because of the very strong signal originating from inhaled gas residing in the lung airspaces and relatively slow gas T2 * relaxation (typical T2 * is about 20-30 ms). However, in vivo human experiments should be performed very rapidly - usually during a single breath-hold. In this review, we describe the recent developments in diffusion lung MRI with hyperpolarized gases. We show that a combination of the results of modeling of gas diffusion in lung airspaces and diffusion measurements with variable diffusion-sensitizing gradients allows the extraction of quantitative information on the lung microstructure at the alveolar level. From an MRI scan of less than 15 s, this approach, called in vivo lung morphometry, allows the provision of quantitative values and spatial distributions of the same physiological parameters as measured by means of 'standard' invasive stereology (mean linear intercept, surface-to-volume ratio, density of alveoli, etc.). In addition, the approach makes it possible to evaluate some advanced Weibel parameters characterizing lung microstructure: average radii of alveolar sacs and ducts, as well as the depth of their alveolar sleeves. Such measurements, providing in vivo information on the integrity of pulmonary acinar airways and their changes in different diseases, are of great importance and interest to a broad range of physiologists and clinicians. We also discuss a new type of experiment based on the in vivo lung morphometry technique combined with quantitative computed tomography measurements, as well as with gradient echo MRI measurements of hyperpolarized gas transverse relaxation in the lung airspaces. Such experiments provide additional information on the blood vessel volume fraction, specific gas volume and length of the acinar airways, and allow the evaluation of lung parenchymal and non-parenchymal tissue. Copyright © 2015 John Wiley & Sons, Ltd.

In vivo detection of cucurbit[6]uril, a hyperpolarized xenon contrast agent for a xenon magnetic resonance imaging biosensor

The Hyperpolarized gas Chemical Exchange Saturation Transfer (HyperCEST) Magnetic Resonance (MR) technique has the potential to increase the sensitivity of a hyperpolarized xenon-129 MRI contrast agent. Signal enhancement is accomplished by selectively depolarizing the xenon within a cage molecule which, upon exchange, reduces the signal in the dissolved phase pool. Herein we demonstrate the in vivo detection of the cucurbit[6]uril (CB6) contrast agent within the vasculature of a living rat. Our work may be used as a stepping stone towards using the HyperCEST technique as a molecular imaging modality.

NMR Hyperpolarization Techniques of Gases

Nuclear spin polarization can be significantly increased through the process of hyperpolarization, leading to an increase in the sensitivity of nuclear magnetic resonance (NMR) experiments by 4-8 orders of magnitude. Hyperpolarized gases, unlike liquids and solids, can often be readily separated and purified from the compounds used to mediate the hyperpolarization processes. These pure hyperpolarized gases enabled many novel MRI applications including the visualization of void spaces, imaging of lung function, and remote detection. Additionally, hyperpolarized gases can be dissolved in liquids and can be used as sensitive molecular probes and reporters. This Minireview covers the fundamentals of the preparation of hyperpolarized gases and focuses on selected applications of interest to biomedicine and materials science.

Challenges and perspectives in quantitative NMR

This perspective article summarizes, from the author's point of view at the beginning of 2016, the major challenges and perspectives in the field of quantitative NMR. The key concepts in quantitative NMR are first summarized; then, the most recent evolutions in terms of resolution and sensitivity are discussed, as well as some potential future research directions in this field. A particular focus is made on methodologies capable of boosting the resolution and sensitivity of quantitative NMR, which could open application perspectives in fields where the sample complexity and the analyte concentrations are particularly challenging. These include multi-dimensional quantitative NMR and hyperpolarization techniques such as para-hydrogen-induced polarization or dynamic nuclear polarization. Because quantitative NMR cannot be dissociated from the key concepts of analytical chemistry, i.e. trueness and precision, the methodological developments are systematically described together with their level of analytical performance. Copyright © 2016 John Wiley & Sons, Ltd.

A method for imaging and spectroscopy using γ-rays and magnetic resonance

Magnetic resonance imaging (MRI) provides fine spatial resolution, spectral sensitivity and a rich variety of contrast mechanisms for diagnostic medical applications. Nuclear imaging using γ-ray cameras offers the benefits of using small quantities of radioactive tracers that seek specific targets of interest within the body. Here we describe an imaging and spectroscopic modality that combines favourable aspects of both approaches. Spatial information is encoded into the spin orientations of tiny amounts of a polarized radioactive tracer using pulses of both radio-frequency electromagnetic radiation and magnetic-field gradients, as in MRI. However, rather than detecting weak radio-frequency signals, imaging information is obtained through the detection of γ-rays. A single γ-ray detector can be used to acquire an image; no γ-ray camera is needed. We demonstrate the feasibility of our technique by producing images and spectra from a glass cell containing only about 4 × 10(13) atoms (about 1 millicurie) of the metastable isomer (131m)Xe that were polarized using the laser technique of spin-exchange optical pumping. If the cell had instead been filled with water and imaged using conventional MRI, then it would have contained more than 10(24) water molecules. The high sensitivity of our modality expands the breadth of applications of magnetic resonance, and could lead to a new class of radioactive tracers.

SABRE hyperpolarisation of vitamin B3 as a function of pH

NMR sensitivity enhanced through SABRE hyperpolarisation and pH manipulation enables the use of vitamin B3 as a pH probe.

In this work we describe how the signal enhancements obtained through the SABRE process in methanol-d₄ solution are significantly affected by pH. Nicotinic acid (vitamin B3, NA) is used as the agent, and changing pH is shown to modify the level of polarisation transfer by over an order of magnitude, with significant improvements being seen in terms of the signal amplitude and relaxation rate at high pH values. These observations reveal that manipulating pH to improve SABRE enhancements levels may improve the potential of this method to quantify low concentrations of analytes in mixtures. ¹H NMR spectroscopy results link this change to the form of the SABRE catalyst, which changes with pH, resulting in dramatic changes in the magnitude of the ligand exchange rates. The presented data also uses the fact that the chemical shifts of the nicotinic acids NMR resonances are affected by pH to establish that hyperpolarised ¹H-based pH mapping with SABRE is possible. Moreover, the strong polarisation transfer field dependence shown in the amplitudes of the associated higher order longitudinal terms offers significant opportunities for the rapid detection of hyperpolarised NA in H₂O itself without solvent suppression. ¹H and ¹³C MRI images of hyperpolarised vitamin B3 in a series of test phantoms are presented that show pH dependent intensity and contrast. This study therefore establishes that when the pH sensitivity of NA is combined with the increase in signal gain provided for by SABRE hyperpolarisation, a versatile pH probe results.

Iridium Cyclooctene Complex That Forms a Hyperpolarization Transfer Catalyst before Converting to a Binuclear C-H Bond Activation Product Responsible for Hydrogen Isotope Exchange

[IrCl(COE)2]2 (1) reacts with pyridine (py) and H2 to form crystallographically characterized IrCl(H)2(COE)(py)2 (2). 2 undergoes py loss to form 16-electron IrCl(H)2(COE)(py) (3), with equivalent hydride ligands. When this reaction is studied with parahydrogen, 1 efficiently achieves hyperpolarization of free py (and nicotinamide, nicotine, 5-aminopyrimidine, and 3,5-lutudine) via signal amplification by reversible exchange (SABRE) and hence reflects a simple and readily available precatayst for this process. 2 reacts further over 48 h at 298 K to form crystallographically characterized (Cl)(H)(py)(μ-Cl)(μ-H)(κ-μ-NC5H4)Ir(H)(py)2 (4). This dimer is active in the hydrogen isotope exchange process that is used in radiopharmaceutical preparations. Furthermore, while [Ir(H)2(COE)(py)3]PF6 (6) forms upon the addition of AgPF6 to 2, its stability precludes its efficient involvement in SABRE.

Imaging of Flow through the Nasal Cavity and Paranasal Sinuses: Preliminary Work with Hyperpolarized Xenon

Experience has been gained at several imaging sites with the use of hyperpolarized gases to image the lungs and the proximal airways. The theoretical advantage to using a polarized noble gas is the dramatically higher polarization values when compared with the small fraction of the hydrogen spin polarization currently utilized in magnetic resonance imaging. Using polarized noble gases, the potential increased signal enhancement is on the order of 100 fold. Hyperpolarized gases provide the ability to image air flow and air-containing structures. Utilization of specially tuned coils on standard clinical imaging systems has enabled imaging of the lungs and airways, using the hyperpolarized gas as a positive contrast agent. We have begun work on imaging the real-time air flow through the nasal cavity and paranasal sinuses, about which little is currently known. Rapid acquisition techniques allow us to perform dynamic imaging of the pattern of airflow through the nasal cavity, and gain information regarding the patterns of air exchange within the paranasal sinuses. This technique may lead to understanding of air flow in normal and disease states.

An Expanded Palette of Xenon-129 NMR Biosensors

Molecular imaging holds considerable promise for elucidating biological processes in normal physiology as well as disease states, by determining the location and relative concentration of specific molecules of interest. Proton-based magnetic resonance imaging (¹H MRI) is nonionizing and provides good spatial resolution for clinical imaging but lacks sensitivity for imaging low-abundance (i.e., submicromolar) molecular markers of disease or environments with low proton densities. To address these limitations, hyperpolarized (hp) ¹²⁹Xe NMR spectroscopy and MRI have emerged as attractive complementary methodologies. Hyperpolarized xenon is nontoxic and can be readily delivered to patients via inhalation or injection, and improved xenon hyperpolarization technology makes it feasible to image the lungs and brain for clinical applications.

In order to target hp ¹²⁹Xe to biomolecular targets of interest, the concept of “xenon biosensing” was first proposed by a Berkeley team in 2001. The development of xenon biosensors has since focused on modifying organic host molecules (e.g., cryptophanes) via diverse conjugation chemistries and has brought about numerous sensing applications including the detection of peptides, proteins, oligonucleotides, metal ions, chemical modifications, and enzyme activity. Moreover, the large (∼300 ppm) chemical shift window for hp ¹²⁹Xe bound to host molecules in water makes possible the simultaneous identification of multiple species in solution, that is, multiplexing. Beyond hyperpolarization, a 10⁶-fold signal enhancement can be achieved through a technique known as hyperpolarized ¹²⁹Xe chemical exchange saturation transfer (hyper-CEST), which shows great potential to meet the sensitivity requirement in many applications.

This Account highlights an expanded palette of hyper-CEST biosensors, which now includes cryptophane and cucurbit[6]uril (CB[6]) small-molecule hosts, as well as genetically encoded gas vesicles and single proteins. In 2015, we reported picomolar detection of commercially available CB[6] via hyper-CEST. Inspired by the versatile host–guest chemistry of CB[6], our lab and others developed “turn-on” strategies for CB[6]-hyper-CEST biosensing, demonstrating detection of protein analytes in complex media and specific chemical events. CB[6] is starting to be employed for in vivo imaging applications. We also recently determined that TEM-1 β-lactamase can function as a single-protein reporter for hyper-CEST and observed useful saturation contrast for β-lactamase expressed in bacterial and mammalian cells. These newly developed small-molecule and genetically encoded xenon biosensors offer significant potential to extend the scope of hp ¹²⁹Xe toward molecular MRI.

Hyperpolarized MRS: New tool to study real-time brain function and metabolism

The advent of dissolution dynamic nuclear polarization (DNP) led to the emergence of a new kind of magnetic resonance (MR) measurements providing the opportunity to probe metabolism in vivo in real time. It has been shown that, following the injection of hyperpolarized substrates prepared using dissolution DNP, specific metabolic bioprobes that can be used to differentiate between healthy and pathological tissue in preclinical and clinical studies can be readily detected by MR thanks to the tremendous signal enhancement. The present article aims at reviewing the studies of cerebral function and metabolism based on the use of hyperpolarized MR. The constraints and future opportunities that this technology could offer are discussed.

The role of hyperpolarized 129xenon in MR imaging of pulmonary function

In the last two decades, functional imaging of the lungs using hyperpolarized noble gases has entered the clinical stage. Both helium (³He) and xenon (¹²⁹Xe) gas have been thoroughly investigated for their ability to assess both the global and regional patterns of lung ventilation. With advances in polarizer technology and the current transition towards the widely available ¹²⁹Xe gas, this method is ready for translation to the clinic. Currently, hyperpolarized (HP) noble gas lung MRI is limited to selected academic institutions; yet, the promising results from initial clinical trials have drawn the attention of the pulmonary medicine community. HP ¹²⁹Xe MRI provides not only 3-dimensional ventilation imaging, but also unique capabilities for probing regional lung physiology. In this review article, we aim to (1) provide a brief overview of current ventilation MR imaging techniques, (2) emphasize the role of HP ¹²⁹Xe MRI within the array of different imaging strategies, (3) discuss the unique imaging possibilities with HP ¹²⁹Xe MRI, and (4) propose clinical applications.

Management of COPD: Is there a role for quantitative imaging?

While the recent development of quantitative imaging methods have led to their increased use in the diagnosis and management of many chronic diseases, medical imaging still plays a limited role in the management of chronic obstructive pulmonary disease (COPD). In this review we highlight three pulmonary imaging modalities: computed tomography (CT), magnetic resonance imaging (MRI) and optical coherence tomography (OCT) imaging and the COPD biomarkers that may be helpful for managing COPD patients. We discussed the current role imaging plays in COPD management as well as the potential role quantitative imaging will play by identifying imaging phenotypes to enable more effective COPD management and improved outcomes.

Do twisted laser beams evoke nuclear hyperpolarization?

The hyperpolarization of nuclear spins promises great advances in chemical analysis and medical diagnosis by substantially increasing the sensitivity of nuclear magnetic resonance (NMR). Current methods to produce a hyperpolarized sample, however, are arduous, time-consuming or costly and require elaborate equipment. Recently, a much simpler approach was introduced that holds the potential, if harnessed appropriately, to revolutionize the production of hyperpolarized spins. It was reported that high levels of hyperpolarization in nuclear spins can be created by irradiation with a laser beam carrying orbital angular momentum (twisted light). Aside from these initial reports however, no further experimental verification has been presented. In addition, this effect has so far evaded a critical theoretical examination. In this contribution, we present the first independent attempt to reproduce the effect. We exposed a sample of immersion oil or a fluorocarbon liquid that was placed within a low-field NMR spectrometer to Laguerre-Gaussian and Bessel laser beams at a wavelength of 514.5nm and various topological charges. We acquired (1)H and (19)F NMR free induction decay data, either during or alternating with the irradiation that was parallel to B0. We observed an irregular increase in NMR signal in experiments where the sample was exposed to beams with higher values of the topological charge. However, at no time did the effect reach statistical significance of 95%. Given the measured sensitivity of our setup, we estimate that a possible effect did not exceed a hyperpolarization (at 5mT) of 0.14-6%, depending on the assumed hyperpolarized volume. It should be noted though, that there were some differences between our setup and the previous implementation of the experiment, which may have inhibited the full incidence of this effect. To approach a theoretical description of this effect, we considered the interaction of an electron with a plane wave, which is known to be able to induce electronic (e.g. in rubidium) and subsequent nuclear hyperpolarization. Compared to the plane wave, the additional transitions caused by a twisted wave are of the order of 10(-3) less. This suggests that the twist of the laser is unlikely to be responsible for the hyperpolarization of nuclear spins, unless a new mechanism of momentum transfer is identified.