1. A method of characterising function of tissues of a body that receive oxygen from a vascular supply in a subject in need of such characterization, the method comprising: performing, with an imaging system having a computer and a processor, an Oxygen-Enhanced Magnetic Resonance Imaging (OE-MRI) on a tissue space associated with the tissue 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, the image data providing information in respect of delivery of oxygen to a tissue and metabolic consumption of oxygen within the tissue; and applying, with the processor, a compartmental model algorithm to the image data generated to provide information on metabolic rate of oxygen consumption of the tissue, wherein the compartmental model algorithm includes a factor associated with metabolic consumption of oxygen in the tissue.