Introduction
Inhaled hyperpolarized noble gases emit a signal detectable by MR which maps the distribution of gas in the trachea-bronchial tree and regions of gas exchange 1-2. In patients with asthma, hyperpolarized gases distribute unevenly in the lungs. Regions with dark, attenuated, He-3 signal are commonly referred to as “ventilation defects” 3-6. Ventilation defects are attributed to eosinophilic inflammatory debris and narrowing of the airways. Ventilation defects increase in size with exercise and methacholine-induced bronchoconstriction 7-8, and decrease in size post-albuterol inhalation 7,9. Lung regions with ventilation defects localize to regions of air trapping mapped with multidetector CT in adults with asthma 10. We previously reported that the ventilation defect % (Vdef %) was greater in children with severe compared to mild asthma, and correlated with poor symptom control and the magnitude of airflow limitation measured with spirometry 11. In adults with severe asthma, the Vdef % positively predicted the number of past exacerbations 12.
Although regional variations in the pattern of inflammation are likely in asthma, little is known as to whether inflammed regions have greater VH. Fain and colleagues 10 found increased neutrophils in BAL from lung regions with ventilation defects and air trapping. We previously reported an adolescent with severe asthma wherein the proximal bronchial segment to a lobe with ventilation defefects had greater epithelial injury and infiltration of eosinophils compared to a segment to a ventilated lobe 13. The Vdef% decreases following acute albuterol treatment in adults with asthma, but in the sub-group with sputum eosinophilia, the improvement in VH is relatively less 14. Mucus plugs identified by multidetector CT in adults with asthma correspond to sputum eosinophilia and production of eosinophil peroxidase-mediated oxidants 15. However, studies based on sputum analysis are limited in so far as the precise regional source of expectorated sputum cannot be identified.
To address these questions, a sample of children with problematic wheeze/asthma underwent hyperpolarized He-3 (HHe-3) MRI. The investigations had two purposes: 1) to study in cross-section whether the magnitude of VH could differentiate phenotypic characteristics, and 2) whether areas of regional hypoventilation map to segments with greater granulocyte infiltration.