Hyperpolarized helium-3 gas magnetic resonance imaging of the lung

3He magnetic resonance imaging (MRI) is capable of producing new and regional information on normal and abnormal lung ventilation. The basis of 3He MRI involves "optical pumping" to hyperpolarize the 3He nuclei by photon angular momentum transfer. The hyperpolarized gas is administered via inhalation. 3He is an inert, nontoxic noble gas and absorbed in less than 0.1%. Imaging consists of a four-step protocol. 1) Gas density 3He MRI with high spatial resolution displays the distribution of a 3He bolus in a 10-second breath-hold. An almost homogeneous distribution is regarded as normal. Patients with lung diseases show multiple ventilation defects. 3He MRI has been shown to be more sensitive than proton MRI, computed tomography, nuclear medicine or pulmonary function testing for detection of ventilation defects. 2) Dynamic imaging 3He MRI with high temporal resolution shows the dynamic distribution of ventilation during continuous breathing after inhalation of a single breath of 3He gas. Homogeneous and fast distribution is regarded as normal, whereas patients show irregular and delayed patterns with redistribution and air trapping. 3) Diffusion-weighted 3He MRI provides a new measure for pulmonary microstructure because the apparent diffusion coefficient (ADC) reflects lung structure. Normal ADC values are less than 0.25 cm2/s and are increased in fibrosis and emphysema (0.3-0.9 cm2/s). 4) Oxygen-sensitive 3He MRI allows for regional and temporal analysis of intrapulmonary Pao2, which reflects regional pulmonary perfusion, ventilation-perfusion ratio, and oxygen uptake. In patients, an inhomogeneous Po2 distribution indicates alterations of ventilation-perfusion matching. Based on increased experience, 3He MRI can be regarded as a highly promising tool for functional analysis of ventilation. The clinical significance of the increase in sensitivity and sensitivity associated with 3He MRI is yet to be determined.