We discuss whether position measurements in quantum mechanics can be contradictory with Bohmian trajectories, leading to what has been called “surrealistic trajectories” in theliterature. Previous work has considered that a single Bohmian position can be ascribed to the pointer. Nevertheless, a correct treatment of a macroscopic pointer requires that many particle positions should be included in the dynamics of the system, and that statistical averages should be made over their random initial values. Using numerical as well as analytical calculations, we show that these surrealistic trajectories exist only if the pointer contains a small number of particles; they completely disappear with macroscopic pointers.With microscopic pointers, non-local effects of quantum entanglement can indeed takeplace and introduce unexpected trajectories, as in Bell experiments; moreover, the initialvalues of the Bohmian positions associated with the measurement apparatus may influencethe trajectory of the test particle, and determine the result of measurement. Nevertheless,a detailed observation of the trajectories of the particles of the pointer reveals the nature ofthe trajectory of the test particle; nothing looks surrealistic if all trajectories are properlyinterpreted.

MRI of hyperpolarised gases is usually performed with fast data acquisition to achieve high spatial resolutions despite rapid diffusion-induced signal attenuation.We describe a double-cross k-space sampling scheme suitable for Slow Low Angle SHot (SLASH) acquisition and yielding an increased SNR. It consists of a series of anisotropic partial acquisitions with a reduced resolution in the read direction, which alleviates signal attenuation and still provides a high isotropic resolution. The advantages of SLASH imaging over conventional FLASH imaging are evaluated analytically, using numerical lattice calculations, and experimentally in phantom cells filled with hyperpolarised 3He–N2 gas mixtures. Low-field MRI is performed (here 2.7 mT), a necessary condition to obtain long T2* values in lungs for slow acquisition. Two additional benefits of the SLASH scheme over FLASH imaging have been demonstrated: it is less sensitive to the artefacts due to concomitant gradients and it allows measuring apparent diffusion coefficients for an extended range of times.

In low field nuclear magnetic resonance (NMR) it is desirable to combine proper characteristics of the detection scheme with a good signal to noise ratio. For example, a reduced coupling between the sample and the detection coil is needed for NMR with highly magnetized samples and a large bandwidth is required in magnetic resonance imaging (MRI). We discuss a solution based on a simple active feedback circuit that preserves the signal to noise ratio as opposed to traditional solutions which do not. We give illustrations of its use in experiments on low temperature hyperpolarized liquid ³He-4He mixtures and in hyperpolarized ³He gas MRI.

We report on multiple echo measurements in hyperpolarized liquids using optically pumped spin-1/2 noble gas atoms: either 129Xe dissolved in cyclohexane or ³He dissolved in superfluid ⁴He. An NMR pulse sequence 90◦ − τ − 90◦ (with slice-selective flipping pulses for 129Xe experiments) was used and long echo trains have been observed in the presence of applied gradients due to average dipolar fields typically one order of magnitude larger than those of bulk water in high magnetic fields. We show that a mean field description is valid for explaining the multiple echoes observed in these liquids, even for spin temperatures as low as 10 mK for 129Xe or 10 μK for ³He, and the echoes originate from the distant dipolar fields within the samples. Numerical lattice simulations have been used to assess the effects of slice selection and of finite sample size in addition to those of atomic diffusion. They account for the observed echo widths and amplitudes much better than previously published models, which disregard finite size effects that appear to be of key importance. This opens the way to using multiple echoes resulting from distant dipolar fields for the determination of the absolute magnetization in hyperpolarized liquids without signal calibration.

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 plasma. 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 2³S state. The results lay the ground for a comprehensive assessment of the efficiency of MEOP, by comparison with achievements at lower field (1 mT–2 T) over an extended range of operating conditions.Stronger hyperfine decoupling in the optically pumped 2³S state is observed to systematically lead to slower build-up of ³He orientation in the ground state, as expected. The nuclear polarizations obtained at 4.7 T still decrease at high pressure 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 density, 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 MEOP particularly attractive for magnetic resonance imaging of the lungs, for instance.