Hyperpolarised gases in magnetic resonance: a new tool for functional imaging of the lung.

In magnetic resonance imaging (MRI), nuclear spins are the source of the image signal. In the lung, low-proton spin density in alveolar gas and abundant gas-tissue interfaces substantially impair conventional native 1H-MRI. Spin polarisation can be increased in two non-radioactive noble gas isotopes, 3He and 129Xe, by exposure to polarised laser light. When inhaled, such "magnetized" gases provide high-intensity MR images of the pulmonary airspaces. Thus, hyperpolarised gas (HPG) MRI opens up new routes to a) morphologic imaging of airways and alveolar spaces, and b) analysis of the intrapulmonary distribution of inhaled aliquots of these tracer gases; c) diffusion-sensitive MRI-techniques allow mapping of the "apparent diffusion coefficient" (ADC) of 3He within lung airspaces, where ADC is physically related to local bronchoalveolar dimensions; d) also, 3He magnetisation decays in an oxygen-containing atmosphere at a rate proportional to ambient PO2. This property allows image-based determination of regional broncho-alveolar PO2 and its decrease during a breathhold. Currently, these modalities of functional lung imaging are being assessed by several European and American research groups in animal models, human volunteers and patients. First results show good imaging quality with excellent spatial and unprecedented temporal resolution, and attest to the reproducibility, feasibility and safety of the technique. Regionally impaired ventilation of both structural and functional origin is detected with high sensitivity, e.g. in smokers, asthmatics, patients with COPD or after lung transplantation. Studies into regional ADC and PO2 measurement demonstrate good agreement with reference methods and physiological predictions. The present limitations of HPG-MRI include the HPG production rate and the US and EU health authorities' still pending final approval for clinical use.