Methacholine-Induced Temporal Changes in Airway Geometry and Lung Density by CT: Lung Density Measurements
To look for potential radial regional differences in the lung, a similar analysis was performed separating the selected ROIs into peripheral and central locations. Peripheral ROIs were located within 5 to 7 mm of the lung border, while the rest of the lung field was defined as central. Because of concern that using the sharp (high resolution) reconstruction algorithm might influence our density measurements, we reconstructed the images from a slice of one of the pigs a second time using a normal (standard) reconstruction algorithm. Densities of 100 identical regions for both the normal and sharp reconstructions were compared. Pulmonary Fungal Infection
Air Content Correction: To convert HU in the lung field into a percentage of air content value, we used a formula previously described by Hoffman. The lung is considered to be composed only of structures equal in CT density to air or water (lung tissue and blood density of 1.055 g/mL is very close to the density of water). Assuming a linear scale between air and water, it is possible to use a linear transformation to calculate the relative air content of any selected region of the lung. To transform a specific ROI density to percentage of air content, the following equation was used:
where CTx is the density value (HU) of the lung voxel (x) under study; CTair is the density value of voxels within a region of known 100% air content (lumen of trachea or mainstem bronchus); and CTwater is the density value of voxels in a region of 100% “water” (chamber of heart, lumen of descending aorta, pulmonary trunk, etc). For each time point (baseline, 30 s, 2 min, and 4 min) in every experiment, the mean (n = 150) and coefficient of variance (CV) of the percentage of air content were computed (CV = SDs/mean X 100).
Gravitational Gradient of Lung Density: Since there is a gravitational gradient of lung CT density under normal physiologic, supine-position conditions in numerous species, including dogs, rabbits, sloths, horses, and humans, and because density gradient changes have been shown to relate to regional indexes of lung function, we investigated whether this gradient is affected by bronchoconstriction. In each of the experimental conditions, the air content values of the 150 regions that were analyzed were plotted against their corresponding lung height. The latter was defined as the anteroposterior distance from the dependent (posterior in the supine animal) aspect of the lung. In these pigs, the relationship (air content vs lung height) was found to be best described by a logarithmic function of the type: Air content = a log (lung height) + b
Use of a logarithmic scale for the lung height axis converted this relationship into a linear function (Y = aX+b) from which the following indexes were determined for each experimental condition: slope of the relationship (a), its intercept (b), and correlation coefficient (r). The slope (a) represents the magnitude of the gravitational gradient (ie, the greater the slope, the greater is the gradient), the intercept (b) represents the air content at the most dependent region of the lung, and the correlation coefficient (r) represents the degree by which the actual air content measurements agree with the fitted curve.
Statistical Analysis: Statistical evaluation included the mean values for percentage of air content, CV, slope, intercept, and r of the percentage of air content to lung height relationship, SEMs, and comparison of means using Student’s two-tailed paired t test, with a significance level of p < 0.05.