Measuring short-term changes in specific ventilation using dynamic specific ventilation imaging.

Specific ventilation imaging (SVI) measures the spatial distribution of specific ventilation (SV) in the lung with MRI by using inhaled oxygen as a contrast agent. Because of the inherently low signal-to-noise ratio in the technique, multiple switches between inspiring air and O are utilized, and the high spatial resolution SV distribution is determined as an average over the entire imaging period (∼20 min). We hypothesized that a trade-off between spatial and temporal resolution could allow imaging at a higher temporal resolution, at the cost of a coarser, yet acceptable, spatial resolution. The appropriate window length and spatial resolution compromise were determined by generating synthetic data with signal- and contrast-to-noise characteristics reflective of that in previously published experimental data, with a known and unchanging distribution of SV, and showed that acceptable results could be obtained in an imaging period of ∼7 min (80 breaths), with a spatial resolution of ∼1 cm. Previously published data were then reanalyzed. The average heterogeneity of the temporally resolved maps of SV was not different from the previous overall analysis, however, the temporally resolved maps were less effective at detecting the amount of bronchoconstriction resulting from methacholine administration. The results further indicated that the initial response to inhaled methacholine and subsequent inhalation of albuterol were largely complete within ∼22 min and ∼9 min, respectively, although there was a tendency for an ongoing developing effect in both cases. These results suggest that it is feasible to use a shortened SVI protocol, with a modest sacrifice in spatial resolution, to measure temporally dynamic processes. Dynamic imaging providing maps of specific ventilation with a temporal resolution of ∼7 min with a spatial resolution of ∼1 cm using MRI was shown to be practical. The technique provides an ionizing radiation free means of temporally following the spatial pattern of specific ventilation. Reanalysis of previously published data showed that the effects of inhaled methacholine and albuterol were largely complete at ∼22 min and ∼9 min, respectively after administration.