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Lung MRI with hyperpolarised 3He gas

Magnetic resonance imaging (MRI) is a well-established and widely used diagnostic method relying on the detection of hydrogen nuclei (protons) in the (weakly) magnetised tissues of various body organs placed in a high magnetic field (routinely 1.5T or higher). Because of the low proton density in lung tissues and of the significant field inhomogeneity associated with their magnetic susceptibility and microscopic structure, MRI of lungs is normally very challenging. However, innovative MRI of the lung using hyperpolarised gases has been developed since 1996. It involves helium-3 or xenon-129 gas, first polarised by laser optical pumping, then inhaled and imaged by NMR techniques.

Images de thorax
Chest images obtained using He3 MRI (left) or proton MRI (right). Courtesy of Jim Wild, Academic Radiology Dept., U. of Sheffield, UK.

An introduction to the history, principles, and operation of NMR and MRI, as well as more examples of lung images obtained using polarised gases, can be found in this book chapter

Our group has been involved in this activity since its beginning ; we coordinated the European research project PHIL (2000-2004), took part in the Phelinet program (2007-2011) that carried on this collaborative work. We are now involved in a broader network on the use of hyperpolarisation methods : Eurohyperpol (2011— ).

Low field MRI : interest for imaging of the lung
Our current research is mainly focused on very low magnetic field NMR and MRI (a few milliteslas). 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
Système d'IRM à bas champ
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 optomised radiofrequency coils. The system can be PC-controlled (using drivers developped at LKB) or run by a commercial NMR console.




Measurements of restricted diffusion

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Diagramme des régimes de diffusion, dans les poumons (en vert) et nos cellules (autres symboles).

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.

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Centre : photographs of cells (5 cm long, 11 and 7 mm i.d.) filled with polarised He3 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 1x1 mm² resolution (instead of 1.8x2.5 mm²) is obtained with the SLASH sequence for the same SNR.

[1] Bidinosti, C.B. et al., J. Magn. Reson. 162 (2003) 122-132 : “In-vivo NMR of hyperpolarized ³He in the human lung at very low magnetic fields”.

[2] Bidinosti, C.B. et al., Magn. Reson. Mat. Phy. 16, (2004) 255-258 : “MRI of the lung using hyperpolarized ³He at very low magnetic field (3 mT)”.

[3] K. Safiullin, C. Talbot and P.J. Nacher, J. Magn. Reson., "Achieving high spatial resolution and high SNR in low—field MRI of hyperpolarised gases with Slow Low Angle SHot" , J. Magn. Res. 227 (2013) 72–86.

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