The signal detected in a Nuclear Magnetic Resonance (NMR) experiment can be made sensitive to the
position of the investigated nuclei through gradients of the main magnetic field. This started an
impressive development of techniques in biomedical Magnetic Resonance Imaging (MRI) and in
diffusion measurements with NMR in physical chemistry in the 1980’s; both techniques have later
been combined into MRI with diffusion weighting (DW) [1-2]. The spatial resolution of the technique
is mostly hampered by its low sensitivity, in medical MRI one currently obtains images with a
millimeter spatial resolution typically within seconds. Important hardware and software development
are still in progress to improve sensitivity, resolution, and reduce scan time . Another spectacular
way to increase sensitivity is the use of hyperpolarized nuclei such as 3He, 129Xe or parahydrogen, by
which a gain in magnetisation of about 105 can be exploited . With 3He, MRI of inhaled gases is a
competitive way of obtaining ventilation images with subsecond resolution, which has already proven
useful to improve the diagnosis accuracy for asthma . Moreover DW-MRI is emerging as a possible
diagnosis modality of lung diseases such as emphysema. Emphysema corresponds to an ‘‘abnormal
permanent enlargement of the airspaces distal to the terminal bronchioles, accompanied by destruction
of their walls and without obvious fibrosis”. This increase in alveolar size corresponds to less
restriction for gas diffusion for emphysematous lungs than for normal ones, as has been observed
experimentally in humans and in animal models using DW-MRI with 3He , as well as with other
After a brief introduction to the principles of NMR and MRI, the influence of molecular diffusion on
the NMR signal will be presented. Measurements performed in a simple geometry will illustrate how
the NMR signal attenuation can be related to diffusion and to geometrical constraints. Experiments 
and simulations  performed on a geometrical model of the human acinus point out some issues
concerning the quantification of diffusion by 3He-MRI and its relationship to changes in alveolar size
or acinus geometry.
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