Insights into pediatric asthma with hyperpolarized magnetic resonance imaging of the lung

Abstract

The application of sophisticated quantitative imaging techniques in the developing lung might provide insights into the underlying pathophysiology of respiratory disease beyond that we can obtain from physiologic testing alone. Hyperpolarized gas magnetic resonance imaging (MRI) can assess the functional alterations of airflowwithin the lung resulting from the structural changes occurring in the central and distal small airways and lung parenchyma of asthmatic patients. Hyperpolarized gases are used as inhaled contrast agents for MRI of the lung and provide images of the lung airspaces with high spatial resolution. Helium (He) and xenon are nonradioactive isotopes that can be polarized. The excellent safety profile of hyperpolarized gases, the lack of ionizing radiation associated with MRI, and the rapid clearance of the gases from the lungs permits repeated evaluation of subjects, now including children, as demonstrated by Cadman et al. In a young healthy subject, inhaled He gas distributes evenly throughout the airspaces, producing uniformly high signal throughout the lung on MRI, with expected gravitational dependence. When there is focal reduction in airflow, the airspaces distal to the area of obstruction do not fill with the gas and appear dark on the images, depicted as a so-called ‘‘ventilation defect.’’ Focal ventilation defects have been found in patients with a variety of obstructive lung diseases, such as asthma, chronic obstructive pulmonary disease, and cystic fibrosis, and even healthy subjects. It is likely that multiple different factors, such as mucus plugging and airway narrowing or closure from a variety of causes, can cause ventilation defects on hyperpolarized gas MRI. He MRI ventilation defects are common in asthmatic patients, and their size and extent are thought to reflect the regional severity of airway closure or narrowing. However, there