[Functional imaging of the lung using a gaseous contrast agent: (3)helium-magnetic resonance imaging]

Current imaging methods of the lung concentrate on morphology as well as on the depiction of the pulmonary parenchyma. The need of an advanced and more subtle imaging technology compared to conventional radiography is met by computed topography as the method of choice. Nevertheless, computed tomography yields very limited functional information. This is to be derived from arterial blood gas analysis, spirometry and body plethysmography. These methods, however, lack the scope for regional allocation of any pathology. Magnetic resonance imaging of the lung has been advanced by the use of hyperpolarised (3)Helium as an inhaled gaseous contrast agent. The inhalation of the gas provides functional data by distribution, diffusion and relaxation of its hyperpolarised state. Because anatomical landmarks of the lung can be visualised as well, functional information can be linked with regional information. Furthermore, the method provides high spatial and temporal resolution and lacks the potential side-effects of ionising radiation. Four different modalities have been established: 1. Spin density imaging studies the distribution of gas, normally after a single inhalation of contrast gas in inspiratory breath hold. 2. Dynamic cine imaging studies the distribution of gas with respect to regional time constants of pulmonary gas inflow. 3. Diffusion weighted imaging can exhibit the presence and severity of pulmonary airspace enlargement, as in pulmonary emphysema. 4. Oxygen sensitive imaging displays intrapulmonary oxygen partial pressure and its distribution. Currently, the method is limited by comparably high costs and limited availability. As there have been recent developments which might bring this modality closer to clinical use, this review article will comprise the methodology as well as the current state of the art and standard of knowledge of magnetic resonance imaging of the lung using hyperpolarised (3)Helium.