Mapping transmit and receive B1 using variable flip angle acquisition on a person-by-person basis for hyperpolarized Carbon-13 and Xenon-129 MRI
Kylie YeungZack RavetzKher Lik NgGabriele AbueidW. HickesKenneth JacobMitchel DanisaM DurrantRebecca MillsAyaka ShinozakiJordan McGingAaron AxfordSarah BirkhoelzerRolf F. SchulteOliver J. RiderAnthony McIntyreEmily FraserDamian J. TylerFergus GleesonJames T. Grist
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Hyperpolarized MRI allows the imaging of processes such as gas exchange and metabolism. Calibration of transmit and receive (Tx/Rx) B1 is required to account for coil inhomogeneities in reconstruction. A variable flip angle (VFA) method, which is fast and readily implemented, allows for a simultaneous B1/T1 measurement. In this study, a mathematical model of VFA calibration was developed, tested, validated in a hyperpolarized Carbon-13 (13C) phantom and in vivo with hyperpolarized Xenon-129 imaging of human lungs.Keywords:
Flip angle
The performance for x-ray spectrometry of neon-xenon gas proportional scintillation counters using a CsI-coated microstrip plate in direct contact with the scintillation gas as a VUV photosensor is investigated for different neon-xenon mixtures. At best operation conditions, the detector gain can reach values about 50% higher than those achieved with pure xenon filling. The highest gains and the best energy resolutions are achieved for xenon contents around 40%. However, the achieved energy resolutions are similar to those achieved with pure xenon. As in pure xenon and argon-xenon mixture gas-fillings, the detector performance is limited by optical positive feedback resulting from additional scintillation produced in the electron avalanche processes around the MSP-anodes. Best energy resolutions are achieved for positive feedback gains in the range of 1.1 to 1.2. The performance achieved with neon-xenon mixtures is inferior to that achieved with argon-xenon mixtures.
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目的: MRIを用いて嚥下機能評価を行うには, 超高速のパルス印加により極短時間で撮像可能なシーケンスが必要であり, steady-state free precession sequences (SSFPシーケンス) が適した撮像方法であると考える, SSFPシーケンスではflip angleの条件により種々の嚥下機能に関連する組織のコントラストやノイズ発生様相が変化し, 嚥下の動的観察における画質に影響する.そこで本研究では, SSFPシーケンスを用いた超高速撮像において, flip angleの条件が, 静止状態における嚥下関連組織の信号強度および視覚評価に及ぼす影響を明らかにし, 嚥下機能評価に適用できる至適flip angleの検討を行った.方法: 顎口腔機能に異常を認めない有歯顎者7名 (平均年齢30.1歳) を被験者とし, 1.5Tesla超伝導MRI装置を用いてSSFPシーケンスにより撮像を行った.撮像条件は, 嚥下運動に対応できることが前提であるため, 繰り返し時間 (repetition time, TR) とエコー時間 (echo time, TE) をいずれも極めて短くした超高速撮像が不可欠であるが, 信号強度の測定精度を向上させる目的で, パルス印加を同条件にして頚部の静止状態を撮像した.本研究では, 撮像にあたり嚥下機能領域のコントラストの可変パラメータであるflip angleを10°から100°まで, 10°間隔の10条件と設定し, この各条件で得られた画像における嚥下関連組織 (軟口蓋, 舌筋, 喉頭蓋, 甲状軟骨, オトガイ舌骨筋, 下顎骨, 舌骨) の信号強度変化および視覚評価の検討を行った.結果: 嚥下関連組織の信号強度はflip angleにより有意に変化した.得られた画像においてコントラストを形成する各嚥下関連組織の信号強度の差が最も多く出現したのは, flip angle10°であり, 次いでflip angle20°, 30°の順に多く, flip angle 40°～100°ではほぼ同数で少なかった.嚥下関連組織の信号雑音比 (signa1-to-noise ratio, SNR) を総じて高い値に維持する条件は, flip angle20°, 30°, 40°であった.視覚評価ではflip angle30°および40°が嚥下関連組織の識別および, 画像の明瞭さの点で優れていた.結論: SSFPシーケンスによる超高速MRIを用いた嚥下関連組織の撮像条件は, 信号強度変化と視覚評価から検討した結果, flip angle 30°が至適撮像条件であり, 嚥下機能評価の動的観察における解剖学的形態描出の明瞭化においても有効であることが示唆された.
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This study investigates the significance of flip angle, an imaging parameter, in enhancing Magnetic Resonance image quality under various imaging conditions. It specifically explores the extent to which the Ernst angle, an optimal flip angle, optimizes image quality under different imaging parameters. The investigation begins with a theoretical derivation of the Ernst angle, assuming steady state imaging conditions. Then multiple studies that examine the effect of flip angle on signal-to-noise ratio (SNR), a key indicator of image quality, in different areas of the human body (blood, liver, and brain), are analysed. The study compares the results of these studies and compares their respective optimal flip angles with the Ernst angle. The findings reveal that flip angle plays a crucial role in enhancing SNR and image quality. However, the Ernst angle only optimizes SNR under steady state conditions and when using a spoiled gradient echo (GRE) sequence. Therefore, further investigations are necessary to determine the optimal flip angle under different imaging conditions to optimize SNR and enhance overall image quality.
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Experiments have been reported in which liquid argon and xenon have been shock-compressed up to two to three times their initial densities. An inspection of the two sets of data indicates a surprisingly large compressibility in xenon at high temperatures and compression. The results of calculations for argon and xenon indicate that the energy gap between the filled valence band and empty conduction band in xenon is decreasing rapidly with increasing density. Using these results, a theoretical Hugoniot curve has been calculated that is in good agreement with experiment. On the basis of these results, we conclude that the highest pressure xenon points, which are at 500 kbar and 18 000\ifmmode^\circ\else\textdegree\fi{}K, represent a metallic-like form of xenon that is similar to cesium. This state is one in which the conduction bands are partially filled as in a metal, and it has been reached by a combination of temperature and compression.
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Separating and purifying noble gases from atmospheric air require highly efficient separation processes to achieve commercially viable quantities. Cryogenic distillation is a widely employed method for noble gas separation, which is an energy-intensive technology. This study proposes a cost-effective method for separating and purifying xenon from the gas mixture of argon, krypton, and xenon based on multistage gas hydrate technology. Experimental measurements were conducted using a 52 cm3 stainless steel equilibrium cell to obtain new hydrate dissociation data for gas mixtures containing argon, krypton, and xenon. These measurements were accompanied by a predictive thermodynamic model, which exhibited a maximum absolute relative deviation of 1.4%. Furthermore, novel experimental data were obtained for the composition of hydrate-vapor phases for the system of argon + krypton + xenon. The data included different component compositions, varying amounts of xenon, and a thorough analysis of hydrate and vapor equilibrium phases using gas chromatography. These findings offer valuable insights into the utilization of gas hydrates for separating xenon from a mixture of argon, krypton, and xenon. The experimental results show that the concentration effect of xenon in the captured hydrate lattice has the most significant increase in the first and second hydrate stages. For a mixture of 40.7 mol % argon, 33.6% krypton, and 25.7% xenon, a concentration increase from 25.7 to 80.4% of xenon was achieved using two hydrate formation and dissociation stages. Compared with cryogenic distillation for the separation of these gases to achieve Xe at high purities, the results from this work reveal that the hydrate-based separation method presents a 20% energy cost advantage over cryogenic distillation.
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Abstract In order to reduce the RF power deposition of fast spin echo sequences operated at high field strength, the flip angles of the refocusing pulse train are varied from pulse to pulse using a modulated angle refocusing train method. The technique employs high flip angle pulses prior to sampling the center of k ‐space in order to preserve T 2 contrast, low flip angles after sampling the center of k ‐space to reduce power and prolong relaxation, and a smooth transition between the high and low flip angle regimes in order to maintain the pseudosteady‐state, maximizing signal and avoiding artifact‐inducing oscillations. An analytical expression is used to predict and correct for the flip angle dependence of the signal, thus eliminating any deleterious effects of flip angle modulation on the point spread function. Analysis of resolution and SNR were performed in simulation and phantom studies. In human imaging studies, it is shown that RF energy deposition per slice in a single‐shot fast spin echo application can be reduced by up to 75%, making the sequence as practical at 3 T as it is has been at 1.5 T. Magn Reson Med 51:1031–1037, 2004. © 2004 Wiley‐Liss, Inc.
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One-dimensional (1D) fluid simulations are used to model a helium-xenon filled ac plasma display pixel. The model includes four levels for helium atomic states, seven levels for xenon atomic states and a xenon dimer state. The model also includes VUV emission including photon trapping due to collisional broadening from the resonant atomic xenon at wavelengths of 129 nm and 147 nm and from non-resonant emission by the xenon dimer molecule peaked at 173 nm. Simulations are performed for a gap width (d) of 100 microns at a pressure (P) of 400 Torr using varying xenon concentrations. At low xenon concentrations, emission is primarily in the 147 nm wavelength but shifts toward the xenon dimer above about 20% xenon in the mixture. At 2% xenon, the calculated VUV emission is about 85% from the resonant atomic xenon state at 147 nm, about 13% from the dimer and about 2% from the resonant 129 nm line. Emission from the 129 nm line is insignificant due to collisional quenching of the xenon states. The discharge efficiency, defined as the VUV photons/watt dissipated, increases with xenon content with an optimum at about 30% xenon. For opposed electrode geometry, as the xenon concentration is increased from 2% to X% xenon, the simulations show that the applied voltages scale approximately as . At a fixed Pd, a higher pressure yields more VUV emission than using a larger gap width.
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The performance for X-ray spectrometry of neon-xenon gas proportional scintillation counters using a CsI-coated microstrip plate in direct contact with the scintillation gas as a VUV photosensor is investigated for different neon-xenon mixtures. At best operation conditions, the detector gain can reach values about 50% higher than those achieved with pure xenon filling. The highest gains and the best energy resolutions are achieved for xenon contents around 40%. However, the achieved energy resolutions are similar to those achieved with pure xenon. As in pure xenon and argon-xenon mixture gas fillings, the detector performance is limited by optical positive feedback resulting from additional scintillation produced in the electron avalanche processes around the MSP anodes. The best energy resolutions are achieved for positive feedback gains in the range of 1.1 to 1.2. The performance achieved with neon-xenon mixtures is inferior to that achieved with argon-xenon mixtures.
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The local structure and xenon adsorption behavior of a metal−organic framework system [M(II)2(bza)4(pyz)]n (bza and pyz = benzoate and pyrazine, M = Rh (1a) and Cu (1b)) were investigated by using single-crystal X-ray diffraction, xenon adsorption isotherm, and 129Xe NMR measurements. Single-crystal X-ray diffraction analysis revealed that rare gas atoms were accommodated in a one-dimensional (1D) nanochannel with the dimer structure. Xenon adsorption reached saturation at the xenon uptake of 1.93 Xe per molecular unit in 1a and of 1.85 Xe in 1b, suggesting accommodation of two Xe atoms per host formula unit. Analysis of the xenon adsorption isotherm based on the Fowler−Guggenheim equation gave the xenon−xenon interaction and the isosteric heat of adsorption. The 129Xe NMR spectrum suggested that the environment of the adsorption site for xenon in 1a is very tight and anisotropic. Furthermore, the temperature dependence of the 129Xe chemical shift was explained by using xenon loading, supporting the xenon dimer as the local structure of xenon in 1a. These aspects revealed that cooperative adsorption of xenon to 1a and 1b occurs according to the xenon−xenon interaction. Xenon is stabilized in the nanochannel through formation of a dimer structure.
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