Probing the regional distribution of pulmonary gas exchange through single-breath gas- and dissolved-phase 129Xe MR imaging

摘要

Although some central aspects of pulmonary function (ventilation and perfusion) are known to be heterogeneous, the distribution of diffusive gas exchange remains poorly characterized. A solution is offered by hyperpolarized ¹²⁹Xe magnetic resonance (MR) imaging, because this gas can be separately detected in the lung's air spaces and dissolved in its tissues. Early dissolved-phase ¹²⁹Xe images exhibited intensity gradients that favored the dependent lung. To quantitatively corroborate this finding, we developed an interleaved, three-dimensional radial sequence to image the gaseous and dissolved ¹²⁹Xe distributions in the same breath. These images were normalized and divided to calculate “¹²⁹Xe gas-transfer” maps. We hypothesized that, for healthy volunteers, ¹²⁹Xe gas-transfer maps would retain the previously observed posture-dependent gradients. This was tested in nine subjects: when the subjects were supine, ¹²⁹Xe gas transfer exhibited a posterior-anterior gradient of −2.00 ± 0.74%/cm; when the subjects were prone, the gradient reversed to 1.94 ± 1.14%/cm (P < 0.001). The ¹²⁹Xe gas-transfer maps also exhibited significant heterogeneity, as measured by the coefficient of variation, that correlated with subject total lung capacity (r = 0.77, P = 0.015). Gas-transfer intensity varied nonmonotonically with slice position and increased in slices proximal to the main pulmonary arteries. Despite substantial heterogeneity, the mean gas transfer for all subjects was 1.00 ± 0.01 while supine and 1.01 ± 0.01 while prone (P = 0.25), indicating good “matching” between gas- and dissolved-phase distributions. This study demonstrates that single-breath gas- and dissolved-phase ¹²⁹Xe MR imaging yields ¹²⁹Xe gas-transfer maps that are sensitive to altered gas exchange caused by differences in lung inflation and posture.