Relaxation Behavior of Laser-Polarized Xe Gas: Size Dependency and Wall Effect of the T1 Relaxation Time in Glass and Gelatin Bulbs

Size dependency of the relaxation time T₁ was measured for laser-polarized ¹²⁹Xe gas encapsulated in different sized cavities made by glass bulbs or gelatin capsules. The use of laser-polarized gas enhances the sensitivity a great deal, making it possible to measure the longer ¹²⁹Xe relaxation time in quite a short time. The size dependency is analyzed on the basis of the kinetic theory of gases and a relationship is derived in which the relaxation rate is connected with the square inverse of the diameter of the cavity. Such an analysis provides a novel parameter which denotes the wall effect on the relaxation rate when a gas molecule collides with the surface once in a second. The relaxation time of ¹²⁹Xe gas is also dependent on the material which forms the cavity. This dependency is large and the relaxation study using polarized ¹²⁹Xe gas is expected to offer important information about the state of the matter of the cavity wall.     

129Xe and131Xe NMR of xenon on silica surfaces at 77 K

Little is known about¹²⁹Xe NMR spectral features and spin-lattice relaxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (≤77 K), where exchange with gas-phase atoms is not a significant complication. We report¹²⁹Xe NMR experiments at 9,4 T that provide such information for xenon adsorbed onto the hydroxylated surface of a number of microporous silica samples at 77 K. A convenient design for these cryogenic experiments is described. Dynamics of surface-adsorbed xenon atoms on the time scale of seconds can be observed by¹²⁹Xe NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the¹²⁹Xe NMR chemical shifts. The peak maxima occur in the region ca. 180–316 ppm, considerably downfield of¹²⁹Xe shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4±1 ppm). The¹²⁹Xe spin-lattice relaxation timesT ₁ range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely shortT ₁ values (35 ms) are discussed. Preliminary¹³¹Xe and¹H NMR results are presented, as well as a method for greatly increasing the sensitivity of¹²⁹Xe NMR detection at low temperatures by using closely-spaced trains of rf pulses.     

Relaxation behavior of laser-polarized (129)Xe gas: size dependency and wall effect of the T(1) relaxation time in glass and gelatin bulbs

Size dependency of the relaxation time T(1) was measured for laser-polarized (129)Xe gas encapsulated in different sized cavities made by glass bulbs or gelatin capsules. The use of laser-polarized gas enhances the sensitivity a great deal, making it possible to measure the longer (129)Xe relaxation time in quite a short time. The size dependency is analyzed on the basis of the kinetic theory of gases and a relationship is derived in which the relaxation rate is connected with the square inverse of the diameter of the cavity. Such an analysis provides a novel parameter which denotes the wall effect on the relaxation rate when a gas molecule collides with the surface once in a second. The relaxation time of (129)Xe gas is also dependent on the material which forms the cavity. This dependency is large and the relaxation study using polarized (129)Xe gas is expected to offer important information about the state of the matter of the cavity wall.     

Consequences of Xe–H Cross Relaxation in Aqueous Solutions

We have investigated the transfer of polarization from ¹²⁹Xe to solute protons in aqueous solutions to determine the feasibility of using hyperpolarized xenon to enhance ¹H sensitivity in aqueous systems at or near room temperatures. Several solutes, each of different molecular weight, were dissolved in deuterium oxide and although large xenon polarizations were created, no significant proton signal enhancement was detected in -tyrosine, α-cyclodextrin, β-cyclodextrin, apomyoglobin, or myoglobin. Solute-induced enhancement of the ¹²⁹Xe spin–lattice relaxation rate was observed and depended on the size and structure of the solute molecule. The significant increase of the apparent spin–lattice relaxation rate of the solution phase ¹²⁹Xe by α-cyclodextrin and apomyoglobin indicates efficient cross relaxation. The slow relaxation of xenon in β-cyclodextrin and -tyrosine indicates weak coupling and inefficient cross relaxation. Despite the apparent cross-relaxation effects, all attempts to detect the proton enhancement directly were unsuccessful. Spin–lattice relaxation rates were also measured for Boltzmann ¹²⁹Xe in myoglobin. The cross-relaxation rates were determined from changes in ¹²⁹Xe relaxation rates in the α-cyclodextrin and myoglobin solutions. These cross-relaxation rates were then used to model ¹H signal gains for a range of ¹²⁹Xe to ¹H spin population ratios. These models suggest that in spite of very large ¹²⁹Xe polarizations, the ¹H gains will be less than 10% and often substantially smaller. In particular, dramatic ¹H signal enhancements in lung tissue signals are unlikely.     

MEASUREMENTS OF RELAXATION OF SPIN POLARIZED 129Xe NUCLEI IN HIGH MAGNETIC FIELDS

We observed the NMR signal of low-pressure gas ¹²⁹Xe by laser enhanced method on an MSL-400 NMR spectrometer and measured nuclear spin relaxations of ¹²⁹Xe gas at various temperatures. The relaxation rate constant of ¹²⁹Xe-¹³³Cs spin exchange was obtained as (6.8±0.5)×10^-16cm^-3s^-1.     

Free precession and transverse relaxation of hyperpolarized <Superscript>129</Superscript>Xe gas detected by SQUIDs in ultra-low magnetic fields

We studied the free precession of the nuclear magnetization of hyperpolarized ¹²⁹Xe gas in external magnetic fields as low as B₀ = 4.5 nT, using SQUIDs as magnetic flux detectors. The transverse relaxation was mainly caused by the restricted diffusion of ¹²⁹Xe in the presence of ambient magnetic field gradients. Its pressure dependence was measured in the range from 30 mbar to 850 mbar and compared quantitatively to theory. Motional narrowing was observed at low pressure, yielding transverse relaxation times of up to 8000 s.     

Intermolecular Dipole–Dipole Relaxation of Xe Dissolved in Water

Intermolecular ¹²⁹Xe–¹H nuclear Overhauser effects and ¹²⁹Xe longitudinal relaxation time measurements were used to demonstrate that the dipole–dipole coupling is the dominant relaxation mechanism for ¹²⁹Xe in water, at room temperature. ¹²⁹Xe–¹H cross-relaxation rates were derived to be ς_XeH 3.2 ± 0.3 × 10⁻³ s⁻¹, independent of xenon pressure (in the range of 1–10 bar) and of the presence of oxygen. Corresponding xenon–proton internuclear distances were calculated to be 2.69 ± 0.12 Å. Using the magnitude of the dipole–dipole coupling and the spin density ratio between dissolved xenon and bulk water, it is estimated that ¹²⁹Xe–¹H spin polarization-induced nuclear Overhauser effects would yield little net proton signal enhancement in water.     

Experiment and dynamic simulations of radiation damping of laser-polarized liquid<Superscript>129</Superscript>Xe at low magnetic field in a flow system

Radiation damping is generally observed when a sample with high spin concentration and high gyromagnetic ratio is placed in a high magnetic field. However, we firstly observed liquid-state¹²⁹Xe radiation damping with laser-enhanced nuclear polarization at low magnetic field in a flow system in which the polarization enhancement factor for the liquid-state¹²⁹Xe was estimated to be 5000, and, furthermore, theoretically simulated the envelopes of the¹²⁹Xe free induction decay and spectral lineshape in the presence of both relaxation and radiation damping with different pulse flip angles and ratios ofT ₂ ^* /T _rd. The radiation damping time constantT _rd of 5 ms was derived on the basis of the simulations. The reasons of depolarization and the further possible improvements were also discussed.     

Relaxation of hyperpolarized 129Xe in a deflating polymer bag

In magnetic resonance imaging with hyperpolarized (HP) noble gases, data is often acquired during prolonged gas delivery from a storage reservoir. However, little is known about the extent to which relaxation within the reservoir will limit the useful acquisition time. For quantitative characterization, 129Xe relaxation was studied in a bag made of polyvinyl fluoride (Tedlar). Particular emphasis was on wall relaxation, as this mechanism is expected to dominate. The HP 129Xe magnetization dynamics in the deflating bag were accurately described by a model assuming dissolution of Xe in the polymer matrix and dipolar relaxation with neighboring nuclear spins. In particular, the wall relaxation rate changed linearly with the surface-to-volume ratio and exhibited a relaxivity of κ=0.392±0.008 cm/h, which is in reasonable agreement with κ=0.331±0.051 cm/h measured in a static Tedlar bag. Estimates for the bulk gas-phase 129Xe relaxation yielded T1bulk=2.55±0.22 h, which is dominated by intrinsic Xe-Xe relaxation, with small additional contributions from magnetic field inhomogeneities and oxygen-induced relaxation. Calculations based on these findings indicate that relaxation may limit HP 129Xe experiments when slow gas delivery rates are employed as, for example, in mouse imaging or vascular infusion experiments.     

NMR of Xe on CO/Ir(1 1 1) and on multilayer Xe/Ir(1 1 1)

Nuclear magnetic resonance (NMR) is performed on monolayer (ML) amounts of adsorbed ¹²⁹Xe on a single crystal substrate. The inherently low sensitivity of NMR is overcome by using highly nuclear spin polarized ¹²⁹Xe that has been produced by optical pumping. A polarization of 0.8 is regularly achieved which is 10⁵ times the thermal (Boltzmann) polarization. The experiments are performed with a constant flux of xenon atoms impinging on the surface, typically 4 ML/s. The chemical shift (σ) of ¹²⁹Xe is highly sensitive to the Xe local environment. We measured profoundly different shifts for the Xe bulk, for the surface of the Xe bulk, and for Xe on CO/Ir(1 1 1). The growth of the bulk is seen in a phase transition like change of σ as a function of temperature at constant Xe flux. At temperatures where no bulk forms at a flux of 4 ML/s, the xenon exchange rate was measured by a spin inversion/recovery method. The exchange time of Xe is found to be 0.24 s at 63.4 K and 64.4 K and somewhat longer at 61.2 K. An analysis is given involving the desorption out of the second layer and fast mixing of first and second layer atoms at these temperatures.     

NMR spin-lattice relaxation of the129Xe nucleus of xenon gas dissolved in various isotropic liquids

The spin-lattice relaxation time of the¹²⁹Xe nucleus of natural xenon gas dissolved in various isotropic liquids, acetonitrile, benzene, carbon tetrachloride and cyclohexane, was studied as a function of temperature at the magnetic fields of 9.4 and 4.7 T. The utilization of hydrogenated and deuterated benzene and cyclohexane reveals that the intermolecular¹²⁹Xe-¹H dipole-dipole interaction constitutes an important relaxation mechanism in hydrogenated solvents. According to this interpretation the interaction is rather strongly temperature-dependent, and increases with increasing temperature. An important observation of an experimental nature is also noted, namely convective flow present in non-spinning sample tubes at elevated temperatures disturbs inversion-recovery measurements and leads to erroneous and unreliable relaxation time values.     

Present status of the 129Xe comagnetometer development for neutron EDM measurement

A ¹²⁹Xe comagnetometer designed for the measurement of neutron electric dipole moment (nEDM) as precisely as 1 × 10^?27e cm is presented. Highly nuclear spin polarized ¹²⁹Xe are introduced into an EDM cell where the ¹²⁹Xe spin precession is detected by means of the two-photon transition. The geometric phase effect (GPE) which generates the false nEDM was quantitatively discussed and the systematic error of nEDM from the GPE was estimated considering the buffer-gas suppression due to Xe atomic collisions. Research and development are in progress to construct the ¹²⁹Xe comagnetometer with a field sensitivity of 0.3 fT. At present, about 70 % nuclear spin polarized ¹²⁹Xe atoms have been obtained in a spin exchange opitial pumping cell, that are in the process of being transferred into the EDM cell via a cold trap.     

Magnetic resonance imaging of dissolved hyperpolarized Xe using a membrane-based continuous flow system

A technique for continuous production of solutions containing hyperpolarized ¹²⁹Xe is explored for MRI applications. The method is based on hollow fiber membranes which inhibit the formation of foams and bubbles. A systematic analysis of various carrier agents for hyperpolarized ¹²⁹Xe has been carried out, which are applicable as contrast agents for in vivo MRI. The image quality of different hyperpolarized Xe solutions is compared and MRI results obtained in a clinical as well as in a nonclinical MRI setting are provided. Moreover, we demonstrate the application of ¹²⁹Xe contrast agents produced with our dissolution method for lung MRI by imaging hyperpolarized ¹²⁹Xe that has been both dissolved in and outgassed from a carrier liquid in a lung phantom, illustrating its potential for the measurement of lung perfusion and ventilation.     

Frequency-selective laser optical pumping and spin exchange of cesium with129Xe and131Xe in a high magnetic field

We report the experimental results of frequency-selective laser optical pumping and spin exchange of Cs with¹²⁹Xe and¹³¹Xe in a high magnetic field of 11.74 T. Our results show that hyperpolarized¹²⁹Xe and¹³¹Xe nuclear magnetic resonance (NMR) signals exhibit alternating phases when the laser frequency for pumping the cesium atoms is changed, which is explained on the basis of the high-field optical pumping of Cs. We obtain about 3% polarization of the¹²⁹Xe. The electron-spin polarization of the Cs atoms has been measured to be about 22% with a simple NMR method.     

XeNA: An automated ‘open-source’ Xe hyperpolarizer for clinical use

Here we provide a full report on the construction, components, and capabilities of our consortium’s “open-source” large-scale (~ 1 L/h) ¹²⁹Xe hyperpolarizer for clinical, pre-clinical, and materials NMR/MRI (Nikolaou et al., Proc. Natl. Acad. Sci. USA, 110, 14150 (2013)). The ‘hyperpolarizer’ is automated and built mostly of off-the-shelf components; moreover, it is designed to be cost-effective and installed in both research laboratories and clinical settings with materials costing less than $125,000. The device runs in the xenon-rich regime (up to 1800 Torr Xe in 0.5 L) in either stopped-flow or single-batch mode—making cryo-collection of the hyperpolarized gas unnecessary for many applications. In-cell ¹²⁹Xe nuclear spin polarization values of ~ 30%–90% have been measured for Xe loadings of ~ 300–1600 Torr. Typical ¹²⁹Xe polarization build-up and T₁ relaxation time constants were ~ 8.5 min and ~ 1.9 h respectively under our spin-exchange optical pumping conditions; such ratios, combined with near-unity Rb electron spin polarizations enabled by the high resonant laser power (up to ~ 200 W), permit such high P_Xe values to be achieved despite the high in-cell Xe densities. Importantly, most of the polarization is maintained during efficient HP gas transfer to other containers, and ultra-long ¹²⁹Xe relaxation times (up to nearly 6 h) were observed in Tedlar bags following transport to a clinical 3 T scanner for MR spectroscopy and imaging as a prelude to in vivo experiments. The device has received FDA IND approval for a clinical study of chronic obstructive pulmonary disease subjects. The primary focus of this paper is on the technical/engineering development of the polarizer, with the explicit goals of facilitating the adaptation of design features and operative modes into other laboratories, and of spurring the further advancement of HP-gas MR applications in biomedicine.     

NMR Study of Laser polarized ~(129) Xe in Low Pressure Natural Xenon Gas

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NMR Study of Laser-polarized 129Xe in Low Pressure Natural Xenon Gas

The NMR signal from the laser-polarized t29 Xe in low-pressure natural xenon gas has been observed with a Bruker WP-80SY NMR spectrometer. The laser-polarized 129 Xe was produced by the method of laser pumping and spin exchange in a magnetic field of 1.87 Tesla. It is obtained experimentally that the nuclear spin relaxation rate 1/T1 of laser-polarized 129Xe are (4.03±1.97)×10-3/see～(2.21±0.78)×10-3/see in the range of the 3.33×103 Pa～8.29×104 Pa Xe gas pressures, the apparent wall relaxation rate 1/Tw* =(1.98±0.18)×10-3/see, and the relaxation rate coefficient C of 133Cs-129Xe spin exchange is (2.81±0.74)×10-16 em3/sec.     

NMR chemical shifts of129Xe dissolved in various oxygen and nitrogen substituted straight chain aliphatic compounds

Xenon-129 NMR spectra have been measured for dilute solutions of¹²⁹Xe dissolved in a series of n-alcohols and primary n-alkyl amines, as well as ethylene glycol and water. The chemical shifts recorded for¹²⁹Xe in the alcohols and amines are found to be linearly related to solvent composition expressed in terms of the individual methyl and methylene groups as well as the nitrogen and oxygen bearing functional groups making up these compounds. This behavior is consistent with a simple model which considers the van der Waals contribution to the chemical shift of¹²⁹Xe to result from pair interactions between a dissolved Xe atom and the individual methyl, methylene, and functional groups from which the solvent molecules are derived. The¹²⁹Xe chemical shifts caused by the ?CH₂?, ?NH₂, and ?OH groups are found to be quite similar in magnitude, each being approximately twice that of a ?CH₃ group.     

Enhanced NMR signal from the laser-polarized129Xe produced by spin exchange at a high magnetic field

Nuclear-magnetic-resonance (NMR) measurement of laser-polarized gaseous¹²⁹Xe produced by spin-exchange optical pumping with a narrow-linewidth laser at a high magnetic field of 4.7 T is reported. The samples are contained in the glass tubes. The nuclear spin polarization of the laserpolarized¹²⁹Xe is 3.9%, and this corresponds to an enhancement of 9· 10³ compared to the equilibrium value at 311 K and at the same magnetic field. The laser-enhanced¹²⁹Xe NMR signals can be used in MR imaging.     

Isotope effect in nuclear capture of negative muons in xenon

The lifetime of negative muons in the ¹²⁹Xe 1_s state was measured. The muon capture rate in ¹²⁹Xe is compared with that in the ^{132, 136}Xe isotopes. The capture rate was found to depend on the mass number of the cited isotopes. The experimental results are compared with the results of calculations by the semiempirical Goulard-Primakoff formula.