The difference in ventilation heterogeneity between asthmatic and healthy subjects quantified using hyperpolarized 3He MRI

Tzeng YS, Lutchen K, Albert M. The difference in ventilation heterogeneity between asthmatic and healthy subjects quantified using hyperpolarized He-3 MRI. J Appl Physiol 106: 813-822, 2009. First published November 20, 2008; doi:10.1152/japplphysiol. 01133.2007.-In this pilot study, algorithms for quantitatively evaluating the distribution and heterogeneity of human ventilation imaged with hyperpolarized (HP) He-3 MRI were developed for the goal of examining structure-function relationships within the asthmatic lung. Ten asthmatic and six healthy human subjects were imaged with HP He-3 MRI before bronchial challenge (pre-MCh), after bronchial challenge (post-MCh), and after a series of deep inspirations (post-DI) following challenge. The acquired images were rigidly coregistered. Local voxel fractional ventilation was computed by setting the sum of the pixel intensity within the lung region in each image to 1 liter of inhaled He-3 mixture. Local ventilation heterogeneity was quantified by computing regional signal coefficient of variation. Voxel fractional ventilation histograms and overall heterogeneity scores were then calculated. Asthmatic subjects had a higher ventilation heterogeneity to begin with (P = 0.025). A methacholine challenge elevated ventilation heterogeneity for all subjects (difference: P = 0.08). After a DI postchallenge, this heterogeneity reversed substantially toward the baseline state for healthy subjects but only minimally in asthmatic subjects. This difference was significant in absolute quantity (difference: P = 0.007) as well as relative to the initial increase (difference: P = 0.03). These findings suggest that constriction heterogeneity is not a characteristic unique to asthmatic airway trees but rather a behavior intrinsic to all airway trees when provoked. Once ventilation heterogeneity is established, it is the lack of reversal following DIs that distinguishes asthmatics from non-asthmatics.