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.