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A radiofrequency (RF) power amplifier is an essential component of any magnetic resonance imaging (MRI) system. Unfortunately, no commercial amplifier exists to fulfill the needs of the transmit array spatial encoding (TRASE) MRI technique, requiring high duty cycle, high RF output power and independently controlled multi-channel capability. Thus, an RF amplifier for TRASE MRI is needed. A dual-channel RF power amplifier dedicated for TRASE at 0.22 T (9.27 MHz) was designed and constructed using commercially available components. The amplifier was tested on the bench and used a 0.22 T MRI system with a twisted solenoid and saddle RF coil combination capable of a single-axis TRASE. The amplifier is capable of sequential, dual-channel operation up to 50% duty cycle, 1 kW peak output and highly stable 100 μs RF pulse trains. High spatial resolution one-dimensional TRASE was obtained with the power amplifier to demonstrate its capability. The constructed amplifier is the first prototype that meets the requirements of TRASE rectifying limitations of duty cycle and timing presented by commercial RF amplifiers. The amplifier makes possible future high resolution in vivo TRASE MRI.

Numerous designs of optical gas polarizers have been proposed, broadening possible applications of the hyperpolarized gases as contrast agents in magnetic resonance imaging. We present a home–made 129Xe polarizer based on the spin exchange optical pumping method. The polarizer operates under 1 bar of the gas mixture (at the maximum temperature of 160 °C) in a high volume optical cell (5025 cm3). Approximately 100 cm3 of 129Xe polarized at 1.50±0.37% is produced in a single cycle of polarization. Operation under standard pressure imposes polarization transfer mainly via van der Waals molecules, resulting in the efficient spin exchange between rubidium and 129Xe atoms. The design, construction and operation of the polarizer are described in details.

International audience; Systematic studies of the metastability exchange optical pumping (MEOP) process in He-3 gas at high magnetic fields (0.45, 0.9, 1.5, and 2 T) and pressures ( 32 and 64 mbar) are performed. The impact of experimental parameters such as laser power, beam pro. le, and shape of the pumping cell is evaluated. By varying the discharge intensity in the cell, the density of metastable state atoms and the plasma-induced nuclear relaxation rate are also controlled, and their effect on the MEOP efficiency can be investigated. Very accurate experimental results are obtained, opening the way to quantitative tests of a recently proposed model of the MEOP process at high magnetic field. We report selected MEOP results with nuclear polarizations exceeding 50 % at 64 mbar and 2 T, which represents a dramatic improvement in performances over MEOP at low magnetic field. The present findings suggest that still higher polarizations can be achieved in higher magnetic fields, and motivate investigations at higher gas pressures. New ways of producing hyperpolarized 3He for magnetic resonance imaging and medical applications can be envisaged, as most clinical whole-body scanners operate at 1.5 T.

The Magnetic Resonance Imaging (MRI) of human lungs for diagnostic purposes became possible by using nuclear spin hyperpolarized noble gases, such as 3He. One of the methods to polarize 3He is the Metastability Exchange Optical Pumping (MEOP), which up to now has been performed at low pressure of about 1 mbar and in low magnetic field below 0.1 T (standard conditions). The equilibrium nuclear polarization can reach up to 80%, but it is dramatically reduced during the subsequent gas compression to the atmospheric pressure that is necessary for the lungs examination. Further polarization losses occur during the transportation of the gas to the hospital scanner. It was shown recently that up to 50% polarization can be obtained at elevated pressure exceeding 20 mbar, by using magnetic field higher than 0.1 T (nonstandard conditions). Therefore, following the construction of the low-field MEOP polarizer located in the lab, a dedicated portable unit was developed, which uses the magnetic field of the 1.5 T MR medical scanner and works in the continuous-flow regime. The first in Poland MRI images of human lungs in vivo were obtained on the upgraded to 3He resonance frequency Siemens Sonata medical scanner. An evident improvement in the image quality was achieved when using the new technique. The paper shows how spectroscopic measurements of 3He carried out in various experimental conditions led both to useful practical results and to significant progress in understanding fundamental processes taking place during MEOP.

After being hyperpolarized using the technique of Metastability Exchange Optical Pumping (MEOP), 3He can be used as a contrast agent for lung magnetic resonance imaging (MRI). MEOP is usually performed at low magnetic field (∼1 mT) and low pressure (∼1 mbar), which results in a low magnetization production rate. Polarization preserving compression with a compression ratio of order 1000 is also required. It was demonstrated in sealed cells that high nuclear polarization values can be obtained at higher pressures with MEOP, if performed at high magnetic field (non-standard conditions). In this work, the feasibility of building a high-field polarizer that operates within a commercial 1.5 T scanner was evaluated. Preliminary measurements of nuclear polarization with sealed cells filled at different 3He gas pressures (1.33 to 267 mbar) were performed. The use of an annular shape for the laser beam increased by 25% the achievable nuclear polarization equilibrium value (Meq) at 32 and 67 mbar as compared to a Gaussian beam shape. Meq values of 66.4% and 31% were obtained at 32 and 267 mbar, respectively, and the magnetization production rate was increased by a factor of 10 compared to the best results obtained under standard conditions. To study the reproducibility of the method in a polarizing system, the same experiments were performed with small cells connected to a gas handling system. Despite careful cleaning procedure, the purity of the 3He gas could not be matched to that of the sealed cells. Consequently, the polarization build-up times were approximately 3 times longer in the 20–30 mbar range of pressure than those obtained for the 32 mbar sealed cell. However, reasonable Meq values of 40%–60% were achieved in a 90 ml open cell. Based on these findings, a novel compact polarizing system was designed and built. Its typical output is a 3He gas flow rate of 15 sccm with a polarization of 33%. In-vivo lung MRI ventilation images (Signal to Noise Ratio (SNR) of approximately 55 for a voxel size of 50 mm × 3 mm × 3 mm) were acquired to demonstrate the polarizer's application.

Version 2 (final one) includes a new figure and a new table. Typos of Version 1 have been corrected, in particular in Fig. 3 and in Table 2 (now Table3). The final publication is available at link.springer.com (with open access).; International audience; Metastability exchange optical pumping (MEOP) is experimentally investigated in 3He at 4.7 T, at room temperature and for gas pressures ranging from 1 to 267 mbar. The 23S - 23P transition at 1083 nm is used for optical pumping and for detection of the laser-induced orientation of 3He atoms in the rf discharge plasmas. The collisional broadening rate is measured (12.0 ± 0.4 MHz.mbar-1 FHWM) and taken into account for accurate absorption-based measurements of both nuclear polarization in the ground state and atom number density in the metastable state. The results lay the ground for a comprehensive assessment of the efficiency of this method, by comparison with achievements at lower magnetic field strengths (1 mT - 2 T) over an extended range of operating conditions. Stronger hyperfine decoupling in the optically pumped metastable state is observed to systematically lead to slower build-up of nuclear orientation in the ground state, as expected. The nuclear polarizations obtained at 4.7 T still decrease at high gas pressures but in a less dramatic way than observed at 2 T in the same sealed glass cells. To date, thanks to the linear increase in gas densities, they correspond to the highest nuclear magnetizations achieved by MEOP in pure 3He gas. The improved efficiency puts less demanding requirements for compression stages in polarized gas production systems and makes high-field operation particularly attractive for magnetic resonance imaging of the lungs, for instance.

Inverse-electron demand Diels–Alder cycloadditions of sterically hindered cycloalkylidene derivatives of benzoylacetonitrile and N,N′-dimethylbarbituric acid with enol ethers, cyclic enol ethers and also sterically hindered cycloalkylidenecycloalkanes were investigated. New spiro, dispirodihydropyrans, spirouracils, and dispirouracils were obtained. To confirm the experimental results, frontier orbital HOMO and LUMO energies of heterodienes and dienophiles were calculated by semi-empirical AM1, PM3 methods and ab initio Hartree–Fock calculations.

Inverse-electron demand Diels–Alder cycloadditions of sterically hindered cycloalkylidene derivatives of benzoylacetonitrile and N,N′-dimethylbarbituric acid with enol ethers, cyclic enol ethers and also sterically hindered cycloalkylidenecycloalkanes were investigated. New spiro, dispirodihydropyrans, spirouracils, and dispirouracils were obtained. To confirm the experimental results, frontier orbital HOMO and LUMO energies of heterodienes and dienophiles were calculated by semi-empirical AM1, PM3 methods and ab initio Hartree–Fock calculations.

International audience; An accurate optical method to measure the nuclear polarization of He-3 atoms in the 1(1)S ground state is described. The absorption of a weak, probe laser beam is used to measure the relative populations of two hyper. ne sublevels of the 2(3)S metastable state that are not addressed by the pumping laser beam. Since a common spin temperature between the ground and metastable states is established by metastable exchange collisions, the nuclear polarization can be derived from these absorption measurements. The method is highly sensitive, robust, and can be used to monitor the dynamics of optical pumping and relaxation processes without interfering with them. It was successfully implemented and tested in the 0.45-2.0 T magnetic field range at the He-3 gas pressure up to 67 mbar.

Magnetic resonance imaging (MRI) using optically polarized 3 He is a new and very promising diagnostic method for studying structure and function of lungs. A novel low field MRI system based on permanent magnet and a home-built 3 He polarizing unit is presented. The system is designed to perform experiments on small animals. First 'H and 3 He images of phantoms, small biological objects, and rat lungs in vivo are shown.

A low magnetic field magnetic resonance imaging system for small animal lung imaging using hyperpolarized ^{3}He gas is presented. The hyperpolarized ^{3}He gas at 1 mbar pressure and 30% polarization is obtained by the metastability exchange optical pumping technique. The magnetic resonance imaging unit is based on a permanent magnet of open geometry, built from a new generation Nd-B-Fe magnetic material. It produces the magnetic field of 88 mT with homogeneity better than 50 ppm in the 10 cm diameter sphere, after application of passive shimming. The magnetic field gradients of 30 mT/m are generated by a set of biplanar, actively shielded gradient coils. The first ^{1}H images of various biological objects, as well as ^{3}He images of the rat lung in vivo obtained in the described system are shown. In terms of sensitivity and resolution, the technique is superior to conventional ^{1}H magnetic resonance imaging, and offers great possibilities in early diagnosis of lung diseases.

The construction of a single mode semiconductor diode laser spectrometer that can be tuned electronically by temperature as well as current is presented. The laser was tested by tuning to the rubidium resonant line (780.2 nm):5s2S1/2 - 5p2P3/2. By using a lithium iodate crystal the primary infrared light (830 nm) emitted by a Sharp LTO- 15PT diode laser was successfully converted into the secondary light wave.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The efficiency of the metastability exchange optical pumping (MEOP) performed at elevated 3 He gas pressures in high magnetic field was improved by modifying the spatial profile of the pumping laser beam in such a way that it matched the nonuniform distribution of plasma in the optical pumping cell. Comparing to a conventional Gaussian profile, at the 3 He gas pressure equal to 267 mbar, both the achieved nuclear polarization (equal to 26%) and the total magnetization in the cell (equal to 1.37 sccfp) are 60% higher when the annular laser beam profile is applied.