Elucidation of structure–function relationships in the lung: contributions from hyperpolarized 3 helium MRI

Summary Magnetic resonance imaging (MRI) using hyperpolarized 3 helium (He) gas as the source of signal provides new physiological insights into the structure–function relationships of the lung. Traditionally, lung morphology has been visualized by chest radiography and computed tomography, whereas lung function was assessed by using nuclear medicine. As all these techniques rely on ionizing radiation, MRI has some inherent advantages. 3 He MRI is based on ‘optical pumping’ of the 3 He gas which increases the nuclear spin polarization by four to five orders of magnitude translating into a massive gain in signal. Hyperpolarized 3 He gas is administered as an inhaled ‘contrast agent’ and allows for selective visualization of airways and airspaces. Straightforward gas density images demonstrate the homogeneity of ventilation with high spatial resolution. In patients with lung diseases 3 He MRI has shown a high sensitivity to depict ventilation defects. As 3 He has some more exciting properties, a comprehensive four‐step functional imaging protocol has been established. The dynamic distribution of ventilation during continuous breathing can be visualized after inhalation of a single breath of 3 He gas using magnetic resonance (MR) sequences with high temporal resolution. Diffusion weighted 3 He MRI provides a new measure for pulmonary microstructure because the degree of restriction of the Brownian motion of the 3 He atoms reflects lung structure. Since the decay of 3 He hyperpolarization is dependent on the ambient oxygen concentration, regional and temporal analysis of intrapulmonary pO 2 becomes feasible. Thus, pulmonary perfusion, ventilation /perfusion ratio and oxygen uptake can be indirectly assessed. Further research will determine the significance of the functional information with regard to physiology and patient management.