Radical pairs may play a role in xenon-induced general anesthesia

Understanding the mechanisms underlying general anesthesia would be a key step towards understanding consciousness. The process of xenon-induced general anesthesia has been shown to involve electron transfer, and the potency of xenon as a general anesthetic exhibits isotopic dependence. We propose that these observations can be explained by a mechanism in which the xenon nuclear spin influences the recombination dynamics of a naturally occurring radical pair of electrons. We develop a simple model inspired by the body of work on the radical-pair mechanism in cryptochrome in the context of avian magnetoreception, and we show that our model can reproduce the observed isotopic dependence of the general anesthetic potency of xenon in mice. Our results are consistent with the idea that radical pairs of electrons with entangled spins could be important for consciousness.

Quantification of Ventilation and Gas Uptake in Free-Breathing Mice With Hyperpolarized 129Xe MRI

Hyperpolarized ¹²⁹Xe magnetic resonance imaging is a powerful modality capable of assessing lung structure and function. While it has shown promise as a clinical tool for the longitudinal assessment of lung function, its utility as an investigative tool for animal models of pulmonary diseases is limited by the necessity of invasive intubation and mechanical ventilation procedures. In this paper, we overcame this limitation by developing a gas delivery system and implementing a set of imaging schemes to acquire high-resolution gas- and dissolved-phase images in free-breathing mice. Gradient echo pulse sequences were used to acquire both high- and low-resolution gas-phase images, and regional fractional ventilation was quantified by comparing signal buildup among low-resolution gas-phase images acquired at two flip-angles. Dissolved-phase images were acquired using both ultra-short echo time and chemical shift imaging sequences with discrete sets of flip-angle/repetition time combinations to visualize gas uptake and distribution throughout the body. Spectral features distinct to various anatomical regions were identified in images acquired using the latter sequence and were used for the quantification of gas arrival times for respective compartments.

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.

Imaging Human Brain Perfusion with Inhaled Hyperpolarized 129Xe MR Imaging

Purpose To evaluate the feasibility of directly imaging perfusion of human brain tissue by using magnetic resonance (MR) imaging with inhaled hyperpolarized xenon 129 (129Xe). Materials and Methods In vivo imaging with 129Xe was performed in three healthy participants. The combination of a high-yield spin-exchange optical pumping 129Xe polarizer, custom-built radiofrequency coils, and an optimized gradient-echo MR imaging protocol was used to achieve signal sensitivity sufficient to directly image hyperpolarized 129Xe dissolved in the human brain. Conventional T1-weighted proton (hydrogen 1 [1H]) images and perfusion images by using arterial spin labeling were obtained for comparison. Results Images of 129Xe uptake were obtained with a signal-to-noise ratio of 31 ± 9 and demonstrated structural similarities to the gray matter distribution on conventional T1-weighted 1H images and to perfusion images from arterial spin labeling. Conclusion Hyperpolarized 129Xe MR imaging is an injection-free means of imaging the perfusion of cerebral tissue. The proposed method images the uptake of inhaled xenon gas to the extravascular brain tissue compartment across the intact blood-brain barrier. This level of sensitivity is not readily available with contemporary MR imaging methods. ©RSNA, 2017.

129 Xe chemical shift in human blood and pulmonary blood oxygenation measurement in humans using hyperpolarized 129 Xe NMR

PURPOSE

To evaluate the dependency of the 129 Xe-red blood cell (RBC) chemical shift on blood oxygenation, and to use this relation for noninvasive measurement of pulmonary blood oxygenation in vivo with hyperpolarized 129 Xe NMR.

METHODS

Hyperpolarized 129 Xe was equilibrated with blood samples of varying oxygenation in vitro, and NMR was performed at 1.5 T and 3 T. Dynamic in vivo NMR during breath hold apnea was performed at 3 T on two healthy volunteers following inhalation of hyperpolarized 129 Xe.

RESULTS

The 129 Xe chemical shift in RBCs was found to increase nonlinearly with blood oxygenation at 1.5 T and 3 T. During breath hold apnea, the 129 Xe chemical shift in RBCs exhibited a periodic time modulation and showed a net decrease in chemical shift of ∼1 ppm over a 35 s breath hold, corresponding to a decrease of 7-10 % in RBC oxygenation. The 129 Xe-RBC signal amplitude showed a modulation with the same frequency as the 129 Xe-RBC chemical shift.

CONCLUSION

The feasibility of using the 129 Xe-RBC chemical shift to measure pulmonary blood oxygenation in vivo has been demonstrated. Correlation between 129 Xe-RBC signal and 129 Xe-RBC chemical shift modulations in the lung warrants further investigation, with the aim to better quantify temporal blood oxygenation changes in the cardiopulmonary vascular circuit. Magn Reson Med 77:1399-1408, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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.

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.

Hyperpolarized 129Xe MRI of the human lung

By permitting direct visualization of the airspaces of the lung, magnetic resonance imaging (MRI) using hyperpolarized gases provides unique strategies for evaluating pulmonary structure and function. Although the vast majority of research in humans has been performed using hyperpolarized (3)He, recent contraction in the supply of (3)He and consequent increases in price have turned attention to the alternative agent, hyperpolarized (129) Xe. Compared to (3)He, (129)Xe yields reduced signal due to its smaller magnetic moment. Nonetheless, taking advantage of advances in gas-polarization technology, recent studies in humans using techniques for measuring ventilation, diffusion, and partial pressure of oxygen have demonstrated results for hyperpolarized (129)Xe comparable to those previously demonstrated using hyperpolarized (3)He. In addition, xenon has the advantage of readily dissolving in lung tissue and blood following inhalation, which makes hyperpolarized (129)Xe particularly attractive for exploring certain characteristics of lung function, such as gas exchange and uptake, which cannot be accessed using (3)He. Preliminary results from methods for imaging (129) Xe dissolved in the human lung suggest that these approaches will provide new opportunities for quantifying relationships among gas delivery, exchange, and transport, and thus show substantial potential to broaden our understanding of lung disease. Finally, recent changes in the commercial landscape of the hyperpolarized-gas field now make it possible for this innovative technology to move beyond the research laboratory.

Multiple-exchange-time xenon polarization transfer contrast (MXTC) MRI: initial results in animals and healthy volunteers

Hyperpolarized xenon-129 is a noninvasive contrast agent for lung MRI, which upon inhalation dissolves in parenchymal structures, thus mirroring the gas-exchange process for oxygen in the lung. Multiple-exchange-time xenon polarization transfer contrast (MXTC) MRI is an implementation of the XTC MRI technique in four dimensions (three spatial dimensions plus exchange time). The aim of this study was to evaluate the sensitivity of MXTC MRI for the detection of microstructural deformations of the healthy lung in response to gravity-induced tissue compression and the degree of lung inflation. MXTC MRI was performed in four rabbits and in three healthy human volunteers. Two lung function parameters, one related to tissue- to alveolar-volume ratio and the other to average septal-wall thickness, were determined regionally. A significant gradient in MXTC MRI parameters, consistent with gravity-induced lung tissue deformation in the supine imaging position, was found at low lung volumes. At high lung volumes, parameters were generally lower and the gradient in parameter values was less pronounced. Results show that MXTC MRI permits the quantification of subtle changes in healthy lung microstructure. Further, only structures participating in gas exchange are represented in MXTC MRI data, which potentially makes the technique especially sensitive to pathological changes in lung microstructure affecting gas exchange.

Diffusion-weighted hyperpolarized 129Xe MRI in healthy volunteers and subjects with chronic obstructive pulmonary disease

Given its greater availability and lower cost, (129) Xe apparent diffusion coefficient (ADC) MRI offers an alternative to (3) He ADC MRI. To demonstrate the feasibility of hyperpolarized (129) Xe ADC MRI, we present results from healthy volunteers (HV), chronic obstructive pulmonary disease (COPD) subjects, and age-matched healthy controls (AMC). The mean parenchymal ADC was 0.036 ± 0.003 cm(2) sec(-1) for HV, 0.043 ± 0.006 cm(2) sec(-1) for AMC, and 0.056 ± 0.008 cm(2) sec(-1) for COPD subjects with emphysema. In healthy individuals, but not the COPD group, ADC decreased significantly in the anterior-posterior direction by ∼ 22% (P = 0.006, AMC; 0.0059, HV), likely because of gravity-induced tissue compression. The COPD group exhibited a significantly larger superior-inferior ADC reduction (∼ 28%) than the healthy groups (∼ 24%) (P = 0.00018, HV; P = 3.45 × 10(-5) , AMC), consistent with smoking-related tissue destruction in the superior lung. Superior-inferior gradients in healthy subjects may result from regional differences in xenon concentration. ADC was significantly correlated with pulmonary function tests (forced expiratory volume in 1 sec, r = -0.77, P = 0.0002; forced expiratory volume in 1 sec/forced vital capacity, r = -0.77, P = 0.0002; diffusing capacity of carbon monoxide in the lung/alveolar volume (V(A) ), r = -0.77, P = 0.0002). In healthy groups, ADC increased with age by 0.0002 cm(2) sec(-1) year(-1) (r = 0.56, P = 0.02). This study shows that (129) Xe ADC MRI is clinically feasible, sufficiently sensitive to distinguish HV from subjects with emphysema, and detects age- and posture-dependent changes.

Pulmonary perfusion and xenon gas exchange in rats: MR imaging with intravenous injection of hyperpolarized 129Xe

PURPOSE

To develop and demonstrate a method for regional evaluation of pulmonary perfusion and gas exchange based on intravenous injection of hyperpolarized xenon 129 ((129)Xe) and subsequent magnetic resonance (MR) imaging of the gas-phase (129)Xe emerging in the alveolar airspaces.

MATERIALS AND METHODS

Five Fischer 344 rats that weighed 200-425 g were prepared for imaging according to an institutional animal care and use committee-approved protocol. Rats were ventilated, and a 3-F catheter was placed in the jugular (n = 1) or a 24-gauge catheter in the tail (n = 4) vein. Imaging and spectroscopy of gas-phase (129)Xe were performed after injecting 5 mL of half-normal saline saturated with (129)Xe hyperpolarized to 12%. Corresponding ventilation images were obtained during conventional inhalation delivery of hyperpolarized (129)Xe.

RESULTS

Injections of (129)Xe-saturated saline were well tolerated and produced a strong gas-phase (129)Xe signal in the airspaces that resulted from (129)Xe transport through the pulmonary circulation and diffusion across the blood-gas barrier. After a single injection, the emerging (129)Xe gas could be detected separately from (129)Xe remaining in the blood and was imaged with an in-plane resolution of 1 x 1 mm and a signal-to-noise ratio of 25. Images in one rat revealed a matched ventilation-perfusion deficit, while images in another rat showed that xenon gas exchange was temporarily impaired after saline overload, with recovery of function 1 hour later.

CONCLUSION

MR imaging of gas-phase (129)Xe emerging in the pulmonary airspaces after intravenous injection has the potential to become a sensitive and minimally invasive new tool for regional evaluation of pulmonary perfusion and gas exchange.

SUPPLEMENTAL MATERIAL

http://radiology.rsnajnls.org/cgi/content/full/2513081550/DC1.

Pulmonary perfusion and xenon gas exchange in rats: MR imaging with intravenous injection of hyperpolarized 129Xe

PURPOSE

To develop and demonstrate a method for regional evaluation of pulmonary perfusion and gas exchange based on intravenous injection of hyperpolarized xenon 129 ((129)Xe) and subsequent magnetic resonance (MR) imaging of the gas-phase (129)Xe emerging in the alveolar airspaces.

MATERIALS AND METHODS

Five Fischer 344 rats that weighed 200-425 g were prepared for imaging according to an institutional animal care and use committee-approved protocol. Rats were ventilated, and a 3-F catheter was placed in the jugular (n = 1) or a 24-gauge catheter in the tail (n = 4) vein. Imaging and spectroscopy of gas-phase (129)Xe were performed after injecting 5 mL of half-normal saline saturated with (129)Xe hyperpolarized to 12%. Corresponding ventilation images were obtained during conventional inhalation delivery of hyperpolarized (129)Xe.

RESULTS

Injections of (129)Xe-saturated saline were well tolerated and produced a strong gas-phase (129)Xe signal in the airspaces that resulted from (129)Xe transport through the pulmonary circulation and diffusion across the blood-gas barrier. After a single injection, the emerging (129)Xe gas could be detected separately from (129)Xe remaining in the blood and was imaged with an in-plane resolution of 1 x 1 mm and a signal-to-noise ratio of 25. Images in one rat revealed a matched ventilation-perfusion deficit, while images in another rat showed that xenon gas exchange was temporarily impaired after saline overload, with recovery of function 1 hour later.

CONCLUSION

MR imaging of gas-phase (129)Xe emerging in the pulmonary airspaces after intravenous injection has the potential to become a sensitive and minimally invasive new tool for regional evaluation of pulmonary perfusion and gas exchange.

SUPPLEMENTAL MATERIAL

http://radiology.rsnajnls.org/cgi/content/full/2513081550/DC1.

Hyperpolarized (129)Xe dynamic study in mouse lung under spontaneous respiration: application to murine tumor B16BL6 melanoma

This is a study on the analysis of hyperpolarized (HP) (129)Xe dynamics applied in the lung of a pathological model mouse under spontaneous respiration. A novel parameter k(1)k(2) - a product of the rate constant for Xe transfer from gas to dissolved phase (k(1)) and from dissolved to gas phase (k(2)) - was shown to be derived successfully from the analysis of the HP (129)Xe dynamic MR experiment in the mouse lung under spontaneous respiration with the aid of a selective pre-saturation technique. A comparative study using healthy mice and model mice induced with lung cancer (by injection of murine tumor B16BL6 melanoma) was performed and a significant difference was found in the k(1)k(2) values of the two groups, that is, 0.020+/-0.007s(-2) (n=4) for healthy mice and 0.032+/-0.04s(-2) (n=3) for lung cancer model mice (p=0.04). Thus, the parameter obtained by our proposed method is considered useful for detection of lung tumors.

Functional lung imaging using hyperpolarized gas MRI

The noninvasive assessment of lung function using imaging is increasingly of interest for the study of lung diseases, including chronic obstructive pulmonary disease (COPD) and asthma. Hyperpolarized gas MRI (HP MRI) has demonstrated the ability to detect changes in ventilation, perfusion, and lung microstructure that appear to be associated with both normal lung development and disease progression. The physical characteristics of HP gases and their application to MRI are presented with an emphasis on current applications. Clinical investigations using HP MRI to study asthma, COPD, cystic fibrosis, pediatric chronic lung disease, and lung transplant are reviewed. Recent advances in polarization, pulse sequence development for imaging with Xe-129, and prototype low magnetic field systems dedicated to lung imaging are highlighted as areas of future development for this rapidly evolving technology.

Confirming the existence of five peaks in129Xe rat head spectra

A series of experiments were performed to investigate why two peaks (D and E) of the five dissolved phase peaks in hyperpolarized (129)Xe rat head spectra appeared inconsistently in previous work. Specifically, spectra were acquired under conditions of various shim states, anaesthetics, and arterial ligation. The shimming experiments showed that slice-shimming can be used to improve resolution of the dissolved phase peaks, but even so, subtle changes in the shim state that may dramatically alter the shape of peak E remain poorly understood. Also, the inability to shim gas spaces and tissue simultaneously may explain why inconsistent chemical shift values have been reported in the literature. A possible solution for this problem is suggested. The results of pre- and postligation spectra from the same animal indicated that two peaks (A and E) originate from brain. Changing the anaesthetic was found to have no effect on the number of dissolved peaks in xenon spectra.

129Xe spectra from the heads of rats with and without ligation of the external carotid and pterygopalatine arteries

After rats inhaled hyperpolarized (129)Xe gas, in vivo spectra from their heads revealed a dominant peak around 195 ppm, another easily resolvable peak near 189 ppm, a broad peak around 210 ppm, and two minor peaks around 198 ppm and 192 ppm. However, the source of each peak remains controversial. To further study the origin of each peak, we compared spectra obtained from the heads of normal rats with spectra taken from the heads of rats that had undergone ligation of the external carotid (ECA) and pterygopalatine (PPA) arteries, the major feeding vessels of nonbrain tissue in the rat head. The amplitude of the peak at around 189 ppm was greatly reduced in the ECA/PPA-ligated rats, while the peak around 195 ppm persisted. We conclude that the signal that originates from the rat brain after inhalation of (129)Xe gas is overwhelmingly dominated by the single resonance at 195 ppm.

Development of a hyperpolarized 129Xe system on 3T for the rat lungs

MRI (magnetic resonance imaging) with 129Xe has gained much attention as a diagnostic methodology because of its affinity for lipids and possible polarization. The quantitative estimation of net detectability and stability of hyperpolarized 129Xe in the dissolved phase in vivo is valuable to the development of clinical applications. The goal of this study was to develop a stable hyperpolarized 129Xe experimental 3T system to statistically analyze the dissolved-phase 129Xe signal in the rat lungs. The polarization of 129Xe with buffer gases at the optical pumping cell was measured under adiabatic fast passage against the temperature of an oven and laser absorption at the cell. The gases were insufflated into the lungs of Sprague-Dawley rats (n = 15, 400-550 g) through an endotracheal tube under spontaneous respiration. Frequency-selective spectroscopy was performed for the gas phase and dissolved phase. We analyzed the 129Xe signal in the dissolved phase to measure the chemical shift, T2*, delay and its ratio in a rat lungs on 3T. The polarizer was able to produce polarized gas (1.1+/-0.47%, 120 cm3) hundreds of times with the laser absorption ratio (25%) kept constant at the cell. The optimal buffer gas ratio of 25-50% rendered the maximum signal in the dissolved phase. Two dominant peaks of 211.8+/-0.9 and 201.1+/-0.6 ppm were observed with a delay of 0.4+/-0.9 and 0.9+/-1.0 s from the gas phase spectra. The ratios of their average signal to that of the gas phase were 5.6+/-5.2% and 4.4+/-4.7%, respectively. The T2* of the air space in the lungs was 2.5+/-0.5 ms, which was 3.8 times shorter than that in a syringe. We developed a hyperpolarized 129Xe experimental system using a 3T MRI scanner that yields sufficient volume and polarization and quantitatively analyzed the dissolved-phase 129Xe signal in the rat lungs.

Improvement of T1 Determination of Hyperpolarized 129Xe in Mouse Brain under Controlled-Flow

The method of determining the longitudinal relaxation time of hyperpolarized 129Xe in the mouse brain has been established in vivo with the ventilation technique under controlled-flow conditions. The uptake and washout processes for nine mice were traced through observation of time-dependent changes in NMR (nuclear magnetic resonance) signal amplitudes and analyzed by means of a two-compartment model, thus providing the quantitative value of 14.1+/-1.6 s as the relaxation time.

Xenon-129 MR imaging and spectroscopy of rat brain using arterial delivery of hyperpolarized xenon in a lipid emulsion

Hyperpolarized (129)Xe dissolved in a lipid emulsion constitutes an NMR tracer that can be injected into the blood stream, enabling blood-flow measurement and perfusion imaging. A small volume (0.15 ml) of this tracer was injected in 1.5 s in rat carotid and (129)Xe MR spectra and images were acquired at 2.35 T to evaluate the potential of this approach for cerebral studies. Xenon spectra consistently showed two resonances, at 194.5 ppm and 199.0 ppm relative to the gas peak. The signal-to-noise ratio (SNR) obtained for the two peaks was sufficient (ranging from 12 to 90) to follow their time courses. 2D transverse-projection xenon images were obtained with an in-plane resolution of 900 microm per pixel (SNR range 8-15). Histological analysis revealed no brain damage except in two rats that had received three injections.

[Measurement of local cerebral blood flow by computerized tomography with inhalation of stable xenon and curve-fitting method]

Non-invasive methods are described for estimating local cerebral blood flows (LCBF) and local partition coefficients (L lambda) during inhalation of 30% stable xenon gas (Xe) in oxygen during CT scanning. After the denitrogenation with pure oxygen breathing, 30% Xe is inhaled for four minutes to minimize subanesthetic effects with a rubber face-mask and the delivery system of Xe. Local time-delta Hounsfield units curve during the Xe wash-in and wash-out phase is utilized in order to calculate L lambda and LCBF using a least squares curve fitting analysis. Calculated L lambda and LCBF with the new method manifested reasonable distribution between the grey and white matters, and reproducibility was excellent in our study. Several case studies of patients with cerebral infarction are presented to demonstrate the characterization of L lambda and LCBF patterns in various tissues and theoretical grounds underlying the new method of curve fitting analysis are discussed.