Vertical distribution of specific ventilation in normal supine humans measured by oxygen-enhanced proton MRI

Abstract

Specific ventilation (SV) is the ratio of fresh gas entering a lung region divided by its end-expiratory volume. To quantify the vertical (gravitationally dependent) gradient of SV in eight healthy supine subjects, we implemented a novel proton magnetic resonance imaging (MRI) method. Oxygen is used as a contrast agent, which in solution changes the longitudinal relaxation time (T1) in lung tissue. Thus alterations in the MR signal resulting from the regional rise in O(2) concentration following a sudden change in inspired O(2) reflect SV-lung units with higher SV reach a new equilibrium faster than those with lower SV. We acquired T1-weighted inversion recovery images of a sagittal slice of the supine right lung with a 1.5-T MRI system. Images were voluntarily respiratory gated at functional residual capacity; 20 images were acquired with the subject breathing air and 20 breathing 100% O(2), and this cycle was repeated five times. Expired tidal volume was measured simultaneously. The SV maps presented an average spatial fractal dimension of 1.13 ± 0.03. There was a vertical gradient in SV of 0.029 ± 0.012 cm(-1), with SV being highest in the dependent lung. Dividing the lung vertically into thirds showed a statistically significant difference in SV, with SV of 0.42 ± 0.14 (mean ± SD), 0.29 ± 0.10, and 0.24 ± 0.08 in the dependent, intermediate, and nondependent regions, respectively (all differences, P < 0.05). This vertical gradient in SV is consistent with the known gravitationally induced deformation of the lung resulting in greater lung expansion in the dependent lung with inspiration. This SV imaging technique can be used to quantify regional SV in the lung with proton MRI.

In silico modeling of oxygen-enhanced MRI of specific ventilation

“…2010, 2014), was applied to the model. The virtual lung cycled through 20 tidal volume breaths of room air and 20 breaths of 100% O 2 repeated five times.…”

“…Specific ventilation (SV) is a measure of the efficiency of lung ventilation, defined as the ratio of the volume of fresh gas entering a region to its end‐expiratory volume (Sa et al. 2010). In this study, we assess a functional imaging technique that involves the use of oxygen‐enhanced proton MRI to quantitatively measure SV in the lungs (Sa et al.…”

“…In this study, we assess a functional imaging technique that involves the use of oxygen‐enhanced proton MRI to quantitatively measure SV in the lungs (Sa et al. 2010, 2014). While MR‐based measurements of ventilation distribution using hyperpolarized gases are now becoming common, for the most part these do not produce quantitative results, the exception being work that has used short multiple‐breath washouts of hyperpolarized gases (Horn et al.…”

“…1985, 2010, 2014) leading to the obvious question: Are the measurements of SV in a single sagittal slice representative of SV (gravitational distribution and heterogeneity) in the whole lung? Secondly, an important assumption of the SVI technique is that local lung tissues are in equilibrium with the surrounding P A O 2 .…”

“…Image registration has not been systematically applied to SVI studies to date (Sa et al. 2010, 2014), and although images that are significantly displaced are discarded, an examination of how in‐plane image misalignment affects the resulting SVI measurement is required.…”