MARGIN at a glance Project homepage

MARGIN (2020 – 2022) – Project summary

In modern nanoporous materials, gas diffusivity correlates with their efficiency for gas separation and storage, or for catalysis. It also correlates with their relevance for investigations of helium superfluid phases and phase transitions with perturbing solid impurities, or confining boundaries since these systems essentially behave as low-density gases of quasi-particles at very low temperatures. More generally, gas and liquid diffusion measurement techniques are widely used for the characterization of porous media. Nuclear magnetic resonance (NMR) provides a wealth of non-invasive methods for measuring fluid transport in various kinds of porous systems. Gas diffusion NMR is a mandatory tool to address specific problems, such as finding mean free paths in aerogels and characterizing pore sizes and structure.

A precise understanding of all physical processes involved during diffusion measurements is essential to correctly interpret NMR diffusion results obtained in porous media. Gas diffusion in porous media near room temperature is frequently described by diffusion models which take into account the possible presence of adsorbed layers and the spatial structure of pores. However, usual phenomenological models of gas diffusion fail to account for observations made with strongly adsorbed gases or at low temperatures, because the influence of the attractive wall potential on molecular gas dynamics is overlooked in standard diffusion models. Preliminary results obtained for ³He gas diffusion in an ordered aerogel by the MRS Lab at 4.2 K revealed a significant deviation from the expected behavior, attributed to the increased effect of the aerogel adsorption potential on ³He gas diffusion at low temperatures. This effect of the adsorption potential is believed to manifest itself in an increase of the gas density and in changes of atomic trajectories near the walls in the gas phase.

Theoretical background and experimental data lack for gases in porous systems at low temperatures or low gas densities, and the MARGIN project is designed to address both issues. It aims at discriminating between confinement and wall adsorption effects through NMR investigations of gas diffusion in several well-characterized model porous media over a wide range of temperatures and gas densities for which different adsorption regimes are expected, from no adsorption to multilayer adsorption through Henry’s, Langmuir’s and BET’s regimes. Experimentally, ³He will be mainly used as a probe gas and apparent diffusion will be measured in porous systems such as ordered aerogels, nanopowders, etc., at low temperatures in MRS Lab (Kazan) and at high temperatures in LKB (Paris) using hyperpolarisation methods with laser optical pumping for high sensitivity. Complementary xenon gas diffusion experiments above 170 K are planned in the same porous media to probe diffusion of atoms with stronger adsorption potentials than for ³He. Numerical simulations of diffusion and nuclear spin dynamics will be also intensely used to reliably interpret experimental results and draw strong conclusions. MARGIN aims at demonstrating that NMR could become one of the accepted characterization tools for gas diffusion, instead of being limited to assessing diffusion and relaxation in liquids, inside porous media.

Besides the achievement of its main objectives, namely the detailed characterization of gas diffusion mechanisms in porous media, the MARGIN project is expected to yield data and results which might be relevant for different open questions in fundamental physics. One of them is related to recently revisited theories of motion-induced magnetic relaxation and frequency shifts in fluids, beyond the standard Redfield approach. On this topic, the planned NMR measurements in samples with near-planar confinement might provide a benchmark for these theories, which have been triggered by new generations of EDM search experiments.

Prior work:

Probing high-porosity silica aerogels with laser-polarised ³He gas

References:

“NMR measurements of hyperpolarized ³He gas diffusion in high porosity silica aerogels” (Full text, open access copy), TASTEVIN G. and NACHER P.-J., J. Chem. Phys. 123, 064506 [1−13] (2005). DOI : 10.1063/1.1997130

“NMR diffusion of hyperpolarised helium-3 in aerogel : a systematic pressure study“, GUILLOT G., NACHER P.-J., TASTEVIN G., Magn. Res. Im. 19, 391−394 (2001).

“Probing silica aerogel structure with spin-polarised helium-3“, TASTEVIN G., NACHER P.J., GUILLOT G., J. Low Temp. Phys. 121, 773−778 (2000).

The “Polarised Helium and Quantum Fluids” team, in collaboration with Prof. Chris. BIDINOSTI (U. Winnipeg, Canada), explores the feasibility of an innovative gradient-free magnetic resonance imaging (MRI) technique for low-field MRI of hyperpolarised ³He gas.

TRASE images of a human wrist, obtained at 0.2 T
[J.C. Sharp et al., NMR Biomed (2013) 26: 1602]

TRansmit Array Spatial Encoding (TRASE) MRI uses trains of radiofrequency  180°-tipping pulses produced by ”phase-gradient” coils which generate transverse B₁ fields of uniform magnitude and spatially varying directions.^1-3 If two such coils (A and B ), characterized by phase-gradients with different wave vectors (k_A and k_B),  are used for tipping, the building block of a TRASE imaging sequence may be a pair of 180°-pulses successively applied with each coil: a pulse pair (180_A, 180_B) shifts the transverse magnetisation in k-space  by an amount 2(k_A – k_B) as would do an applied gradient pulse of static field amplitude, B₀, in standard NMR.
¹ JC Sharp et al., Magn Reson Med 63:151 (2010);   ² JC Sharp et al.,  NMR Biomed, 26: 1602 (2013);
³ Q Deng et al., Magn Reson Imaging 31:891 (2013)

We are currently exploring the potential benefits and limitations of TRASE MRI at low magnetic fields.

In the millitesla field range, hyperpolarisation can be employed to obtain images with high signal-to-noise ration but the adverse effect of concomitant B₀ gradients puts a limit to the resolution achievable in standard MRI: this issue is automatically solved in TRASE. However, to accommodate for changes in B₁-field orientation, concomitant RF field components necessarily exist. Moreover, oscillating radiofrequency fields are used, consisting of equal rotating and counter-rotating components. In contrast with high-field NMR for which the concomitant and counter-rotative components simply contribute to radifrequency power deposition, time evolution in low field may be influenced by both since B₁ magnitudes are not much smaller than B₀.

We perform experiments in a 15-cm bore resistive magnet (B₀: 1‒5 mT), equipped with a usual radiofrequency coil (generating a field with uniform amplitude and phase), standard imaging gradient coils, and additional phase-gradient coils (generating a field with uniform amplitude and a phase that exhibits a linear variation along one direction) – see figure.

The efficiency of RF phase-encoding has been compared to that of encoding with B₀-gradient pulses both in 1D and 2D imaging of water samples and sealed cells of ³He gas (hyperpolarized by optical pumping). Our initial results  [Ref. 1]  demonstrate the potential interest of the TRASE method. However, they display unexpected artefacts that are currently being investigated.

Systematic theoretical, numerical, and experimental investigations are underway to analyse the potentially adverse effects of the radiofrequency counter-rotating field [Re. 3] and concomitant field gradient [Ref. 4]. In particular, their joint effect leads to significant image artefacts when short 180°-pulses are used. We have designed and tested robust pulse sequences which are fairly immune to such effects. Generalisation of these studies to 2D and 3D gradient-free MRI is under way.

This page is under construction

Very low field MRI

Benefits for MR imaging of the lung
The major interest of operation at very low field is the drastic reduction the local field inhomogeneities due to the magnetic susceptibility of the lung tissues. This allows monitoring the evolution of the transverse magnetisation over very long time scales.
Pioneering work at LKB [1],  [2], has established the increase of performances (in terms of signal-to-noise ratio and of probed length scales) and the potential interest of very low field MRI for:
– local measurement of gas diffusion inside the lungs, that characterises the size and the structure of the distal air spaces. The decrease of atomic mean free path measured in these air spaces is due to their highly divided topology, that can be significantly altered by lung pathologies such emphysema or some kinds of asthma.
– mapping of the transverse relaxation rates to image the variations of oxygen partial pressure in the air content, that provides information on the local or regional efficiency of functional gas exchanges.
– ventilation imaging, through optimal use of the available magnetisation for direct visualisation of the gas distribution inside the lungs.

Construction of a dedicated MRI system
We have designed and built a compact system (15 cm i.d., 29 cm o.d.) for low field MRI (up to 5mT).
This is a 1:4 model of a whole-body imaging system developed for methodological investigations and demonstrations both in vitro and in small animals. It is equipped with a 3D gradient set and optimised radiofrequency coils. The system can be PC-controlled (using drivers developped at LKB) or run by a commercial NMR console.
We adjust the rf frequency or the magnetic field strength to achieve resonant excitation/detection of ³He nuclei and protons with the same hardware equipment.

Samples
³He samples are laser polarised on site, by metastability exchange optical pumping, which usually allows one-shot data acquisition. Optical pumping is performed either in situ (in a sealed gas cell that sits inside the MRI system, as shown in the above picture) or remotely (in a homebuilt gas polariser; in this case ³He is transferred into a valved measurement cell, together with a neutral buffer gas if needed). For proton NMR/MRI we use thermally polarised liquid samples (doped water enclosed in homebuilt sealed or valved containers). Signal averaging is thus mandatory to achieve reasonable signal-to-noise ratios.

Measurements of restricted diffusion

Diagram for diffusion regimes in lungs (green symbols) and tubes (other symbols). A systematic evaluation of restricted gas diffusion (K. Safiullin, PhD work, 2011) has been carried out in a crossover regime where various length scales are of the same order (diffusion length, container size, gradient dephasing length). This situation is met during gas MRI and diffusion measurements inside the lung airways which have relevant dimensions spanning a wide range (0.1 – 30 mm). Combined with numerical lattice simulations, these results will help to reliably correlate apparent diffusion coefficient (ADC) measurements performed over different time scales to the characteristic length scales of the airways that restrict gas diffusion.

 

Innovative NMR sequences
Another study has been devoted to the development of an innovative MRI scheme for improved signal to noise ratio (SNR) and/or spatial resolution [3] (See below an in vitro demonstration — K. Safiullin, PhD work, 2011 — JMR article, 2013 [3]).
MRI and ADC studies have also been performed in commercial preserved cat lungs, demonstrating the potential of our set-up in spite of its current limitations. These studies, beyond their intrinsic interest, are expected to find applications in the preclinical MRI work that is pursued by various groups worldwide.

Centre: photographs of cells (5 cm long, 11 and 7 mm i.d.) filled with polarised ³He  gas. Sides: k-space sampling schemes and resulting 2D MRI projection images obtained at 2.7 mT with our new Slow Low-Angle Shot (left) and the usual Fast Low-Angle Shot (right) sequences. A 1×1 mm² resolution (instead of 1.8×2.5 mm²) is obtained with the SLASH sequence for the same SNR.