1. A method of characterising lung function in a subject in need of such characterisation comprising: performing, with an imaging system having a computer and a processor, an Oxygen-Enhanced Magnetic Resonance Imaging (OE-MRI) technique, on a voxel defined within a lung space of interest, generating image data, with the imaging system, over a time period during which the subject inhales gases with at least two different partial pressures of oxygen; and applying a compartmental model algorithm, with the processor, to the image data generated for the voxel over said time period to measure rates at which oxygen was removed from an alveoli gaseous space into a fluid of an alveolar membrane, interstitial spaces and alveolar capillaries and removed from the alveolar capillaries when taken into the body via a blood stream to provide information on ventilation, diffusion across the alveolar membrane and perfusion of a lung.