Development of a fast method for quantitative measurement of hyperpolarized 129Xe dynamics in mouse brain

Preclinical MRI Using Hyperpolarized 129Xe

Although critical for development of novel therapies, understanding altered lung function in disease models is challenging because the transport and diffusion of gases over short distances, on which proper function relies, is not readily visualized. In this review we summarize progress introducing hyperpolarized ¹²⁹Xe imaging as a method to follow these processes in vivo. The work is organized in sections highlighting methods to observe the gas replacement effects of breathing (Gas Dynamics during the Breathing Cycle) and gas diffusion throughout the parenchymal airspaces (3). We then describe the spectral signatures indicative of gas dissolution and uptake (4), and how these features can be used to follow the gas as it enters the tissue and capillary bed, is taken up by hemoglobin in the red blood cells (5), re-enters the gas phase prior to exhalation (6), or is carried via the vasculature to other organs and body structures (7). We conclude with a discussion of practical imaging and spectroscopy techniques that deliver quantifiable metrics despite the small size, rapid motion and decay of signal and coherence characteristic of the magnetically inhomogeneous lung in preclinical models (8).

Hyperpolarized 129 Xe imaging of the brain: Achievements and future challenges

Hyperpolarized (HP) xenon-129 (¹²⁹ Xe) brain MRI is a promising imaging modality currently under extensive development. HP ¹²⁹ Xe is nontoxic, capable of dissolving in pulmonary blood, and is extremely sensitive to the local environment. After dissolution in the pulmonary blood, HP ¹²⁹ Xe travels with the blood flow to the brain and can be used for functional imaging such as perfusion imaging, hemodynamic response detection, and blood-brain barrier permeability assessment. HP ¹²⁹ Xe MRI imaging of the brain has been performed in animals, healthy human subjects, and in patients with Alzheimer's disease and stroke. In this review, the overall progress in the field of HP ¹²⁹ Xe brain imaging is discussed, along with various imaging approaches and pulse sequences used to optimize HP ¹²⁹ Xe brain MRI. In addition, current challenges and limitations of HP ¹²⁹ Xe brain imaging are discussed, as well as possible methods for their mitigation. Finally, potential pathways for further development are also discussed. HP ¹²⁹ Xe MRI of the brain has the potential to become a valuable novel perfusion imaging technique and has the potential to be used in the clinical setting in the future.

Gut Barrier Dysfunction and Type 2 Immunity: Implications for Compulsive Behavior

To date, much of the focus of gut-brain axis research has been on gut microbiota regulation of anxiety and stress-related behaviors. Much less attention has been directed to potential connections between gut microbiota and compulsive behavior. Here, we discuss a potential link between gut barrier dysfunction and compulsive behavior that is mediated through "type 2" rather than "type 1" inflammation. We examine connections between compulsive behavior and type 2 inflammation in Tourette syndrome, obsessive-compulsive disorder, autism, addiction, and post-traumatic stress disorder. Next, we discuss potential connections between gut barrier dysfunction, type 2 inflammation, and compulsive behavior. We posit a potential mechanism whereby gut barrier dysfunction-associated type 2 inflammation may drive compulsive behavior through histamine regulation of dopamine neurotransmission. Finally, we discuss the possibility of exploiting the greater accessibility of the gut relative to the brain in identifying targets to treat compulsive behavior disorders.

Hyperpolarized 129 Xe MRI of the rat brain with chemical shift saturation recovery and spiral-IDEAL readout

PURPOSE

To demonstrate the feasibility of ¹²⁹ Xe chemical shift saturation recovery (CSSR) combined with spiral-IDEAL imaging for simultaneous measurement of the time-course of red blood cell (RBC) and brain tissue signals in the rat brain.

METHODS

Images of both the RBC and brain tissue ¹²⁹ Xe signals from the brains of five rats were obtained using interleaved spiral-IDEAL imaging following chemical shift saturation pulses applied at multiple CSSR delay times, τ. A linear fit of the signals to τ was used to calculate the slope of the signal for both RBC and brain tissue compartments on a voxel-by-voxel basis. Gas transfer was evaluated by measuring the ratio of the whole brain tissue-to-RBC signal intensities as a function of τ. To investigate the relationship between the CSSR images and gas transfer in the brain, the experiments were repeated during hypercapnic ventilation.

RESULTS

Hypercapnia, affected the ratio of the tissue-to-RBC signal intensity (p = 0.026), consistent with an increase in gas transfer.

CONCLUSION

CSSR with spiral-IDEAL imaging is feasible for acquisition of ¹²⁹ Xe RBC and brain tissue time-course images in the rat brain. Differences in the time-course of the signal intensity ratios are consistent with gas transfer changes expected under hypercapnic conditions.

Nanoparticle-Based Contrast Agents for 129Xe HyperCEST NMR and MRI Applications

Spin hyperpolarization techniques have enabled important advancements in preclinical and clinical MRI applications to overcome the intrinsic low sensitivity of nuclear magnetic resonance. Functionalized xenon biosensors represent one of these approaches. They combine two amplification strategies, namely, spin exchange optical pumping (SEOP) and chemical exchange saturation transfer (CEST). The latter one requires host structures that reversibly bind the hyperpolarized noble gas. Different nanoparticle approaches have been implemented and have enabled molecular MRI with 129Xe at unprecedented sensitivity. This review gives an overview of the Xe biosensor concept, particularly how different nanoparticles address various critical aspects of gas binding and exchange, spectral dispersion for multiplexing, and targeted reporter delivery. As this concept is emerging into preclinical applications, comprehensive sensor design will be indispensable in translating the outstanding sensitivity potential into biomedical molecular imaging applications.

Magnetic resonance imaging with hyperpolarized agents: methods and applications

In the past decade, hyperpolarized (HP) contrast agents have been under active development for MRI applications to address the twin challenges of functional and quantitative imaging. Both HP helium (³He) and xenon (¹²⁹Xe) gases have reached the stage where they are under study in clinical research. HP ¹²⁹Xe, in particular, is poised for larger scale clinical research to investigate asthma, chronic obstructive pulmonary disease, and fibrotic lung diseases. With advances in polarizer technology and unique capabilities for imaging of ¹²⁹Xe gas exchange into lung tissue and blood, HP ¹²⁹Xe MRI is attracting new attention. In parallel, HP ¹³C and ¹⁵N MRI methods have steadily advanced in a wide range of pre-clinical research applications for imaging metabolism in various cancers and cardiac disease. The HP [1-¹³C] pyruvate MRI technique, in particular, has undergone phase I trials in prostate cancer and is poised for investigational new drug trials at multiple institutions in cancer and cardiac applications. This review treats the methodology behind both HP gases and HP ¹³C and ¹⁵N liquid state agents. Gas and liquid phase HP agents share similar technologies for achieving non-equilibrium polarization outside the field of the MRI scanner, strategies for image data acquisition, and translational challenges in moving from pre-clinical to clinical research. To cover the wide array of methods and applications, this review is organized by numerical section into (1) a brief introduction, (2) the physical and biological properties of the most common polarized agents with a brief summary of applications and methods of polarization, (3) methods for image acquisition and reconstruction specific to improving data acquisition efficiency for HP MRI, (4) the main physical properties that enable unique measures of physiology or metabolic pathways, followed by a more detailed review of the literature describing the use of HP agents to study: (5) metabolic pathways in cancer and cardiac disease and (6) lung function in both pre-clinical and clinical research studies, concluding with (7) some future directions and challenges, and (8) an overall summary.

Hyperpolarized (129) Xe MRI using isobutene as a new quenching gas

The use of a quenching gas, isobutene, with a low vapor pressure was investigated to enhance the utility of hyperpolarized (129) Xe (HP Xe) MRI. Xenon mixed with isobutene was hyperpolarized using a home-built apparatus for continuously producing HP Xe. The isobutene was then readily liquefied and separated almost totally by continuous condensation at about 173 K, because the vapor pressure of isobutene (0.247 kPa) is much lower than that of Xe (157 kPa). Finally, the neat Xe gas was continuously delivered to mice by spontaneous inhalation. The HP Xe MRI was enhanced twofold in polarization level and threefold in signal intensity when isobutene was adopted as the quenching gas instead of N2 . The usefulness of the HP Xe MRI was verified by application to pulmonary functional imaging of spontaneously breathing mice, where the parameters of fractional ventilation (ra ) and gas exchange (fD ) were evaluated, aiming at future extension to preclinical studies. This is the first application of isobutene as a quenching gas for HP Xe MRI.

Oxygen-dependent hyperpolarized (129) Xe brain MR

Hyperpolarized (HP) (129) Xe MR offers unique advantages for brain functional imaging (fMRI) because of its extremely high sensitivity to different chemical environments and the total absence of background noise in biological tissues. However, its advancement and applications are currently plagued by issues of signal strength. Generally, xenon atoms found in the brain after inhalation are transferred from the lung via the bloodstream. The longitudinal relaxation time (T1 ) of HP (129) Xe is inversely proportional to the pulmonary oxygen concentration in the lung because oxygen molecules are paramagnetic. However, the T1 of (129) Xe is proportional to the pulmonary oxygen concentration in the blood, because the higher pulmonary oxygen concentration will result in a higher concentration of diamagnetic oxyhemoglobin. Accordingly, there should be an optimal pulmonary oxygen concentration for a given quantity of HP (129) Xe in the brain. In this study, the relationship between pulmonary oxygen concentration and HP (129) Xe signal in the brain was analyzed using a theoretical model and measured through in vivo experiments. The results from the theoretical model and experiments in rats are found to be in good agreement with each other. The optimal pulmonary oxygen concentration predicted by the theoretical model was 21%, and the in vivo experiments confirmed the presence of such an optimal ratio by reporting measurements between 25% and 35%. These findings are helpful for improving the (129) Xe signal in the brain and make the most of the limited spin polarization available for brain experiments. Copyright © 2016 John Wiley & Sons, Ltd.

Microglia trigger astrocyte-mediated neuroprotection via purinergic gliotransmission

Microglia are highly sensitive to even small changes in the brain environment, such as invasion of non-hazardous toxicants or the presymptomatic state of diseases. However, the physiological or pathophysiological consequences of their responses remain unknown. Here, we report that cultured microglia sense low concentrations of the neurotoxicant methylmercury (MeHg^low) and provide neuroprotection against MeHg, for which astrocytes are also required. When exposed to MeHg^low, microglia exocytosed ATP via p38 MAPK- and vesicular nucleotide transporter (VNUT)-dependent mechanisms. Astrocytes responded to the microglia-derived ATP via P2Y₁ receptors and released interleukin-6 (IL-6), thereby protecting neurons against MeHg^low. These neuroprotective actions were also observed in organotypic hippocampal slices from wild-type mice, but not in slices prepared from VNUT knockout or P2Y₁ receptor knockout mice. These findings suggest that microglia sense and respond to even non-hazardous toxicants such as MeHg^low and change their phenotype into a neuroprotective one, for which astrocytic support is required.