Quantitative Mapping of Specific Ventilation in the Human Lung using Proton Magnetic Resonance Imaging and Oxygen as a Contrast Agent

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 O₂ are utilized, and the high spatial resolution SV distribution determined as an average over the entire imaging period (~20 minutes). 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 was 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 minutes (80 breaths), with a spatial resolution of ~1cm³. Previously published data were then reanalyzed. The average heterogeneity of the temporally resolved maps of SV were not different to 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 minutes and ~9 minutes 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, in order to measure temporally dynamic processes.

Vaping disrupts ventilation-perfusion matching in asymptomatic users

Inhalation of e-cigarette's aerosols (vaping) has the potential to disrupt pulmonary gas exchange, but the effects in asymptomatic users are unknown. We assessed ventilation-perfusion (V̇_A/Q̇) mismatch in asymptomatic e-cigarette users, using magnetic resonance imaging (MRI). We hypothesized that vaping induces V̇_A/Q̇ mismatch through alterations in both ventilation and perfusion distributions. Nine young, asymptomatic "Vapers" with >1-yr vaping history, and no history of cardiopulmonary disease, were imaged supine using proton MRI, to assess the right lung at baseline and immediately after vaping. Seven young "Controls" were imaged at baseline only. Relative dispersion (SD/means) was used to quantify the heterogeneity of the individual ventilation and perfusion distributions. V̇_A/Q̇ mismatch was quantified using the second moments of the ventilation and perfusion versus V̇_A/Q̇ ratio distributions, log scale, LogSDV̇, and LogSDQ̇, respectively, analogous to the multiple inert gas elimination technique. Spirometry was normal in both groups. Ventilation heterogeneity was similar between groups at baseline (Vapers, 0.43 ± 0.13; Controls, 0.51 ± 0.11; P = 0.13) but increased after vaping (to 0.57 ± 0.17; P = 0.03). Perfusion heterogeneity was greater (P = 0.04) in Vapers at baseline (0.53 ± 0.06) compared with Controls (0.44 ± 0.10) but decreased after vaping (to 0.42 ± 0.07; P = 0.005). Vapers had greater (P = 0.01) V̇_A/Q̇ mismatch at baseline compared with Controls (LogSDQ̇ = 0.61 ± 0.12 vs. 0.43 ± 0.12), which was increased after vaping (LogSDQ̇ = 0.73 ± 0.16; P = 0.03). V̇_A/Q̇ mismatch is greater in Vapers and worsens after vaping. This suggests subclinical alterations in lung function not detected by spirometry.NEW & NOTEWORTHY This research provides evidence of vaping-induced disruptions in ventilation-perfusion matching in young, healthy, asymptomatic adults with normal spirometry who habitually vape. The changes in ventilation and perfusion distributions, both at baseline and acutely after vaping, and the potential implications on hypoxic vasoconstriction are particularly relevant in understanding the pathogenesis of vaping-induced dysfunction. Our imaging-based approach provides evidence of potential subclinical alterations in lung function below thresholds of detection using spirometry.