Regional differences of lung parenchyma attenuation after contrast administration in quantitative dual energy chest CT

Abstract Purpose The administration of contrast material increases the density of lunge parenchyma on computed tomography. Lung pathologies which are characterized by an only slight alteration of lung attenuation (e.g. ground glass opacities or emphysema) should therefore be evaluated on non-enhanced scans in order to avoid misinterpretation. However, contrast administration is very helpful in some clinical scenarios like suspected pulmonary embolism or malignancy. This study aimed to quantify the amount of increase of lung parenchymal density after intravenous contrast administration compared to non-enhanced images and whether this increase exhibits regional variation similar to or different from the attenuation on non-enhanced scans. Materials and methods This retrospective, IRB-approved, bi-center study included patients who underwent both contrast-enhanced dual-energy and non-enhanced chest CT scans within a year between 04/2018 and 12/2022. Scans were co-registered and semi-manually segmented into the whole lung and isovolumetric segmentations of the ventral/dorsal halves and upper/middle/lower thirds. Mean lung density for each region was calculated from contrast-enhanced (CE), virtual non-contrast (VNC), and true non-contrast (TNC) scans. Bland-Altman analyses with non-parametric limits of agreement assessed the mean difference in attenuation values, with differences between lung regions tested by the Mann-Whitney U test. Subgroup differences were also analyzed by Mann-Whitney U tests. Correlation analyses were performed to investigate the correlation between the increase in density from contrast media and the non-enhanced density. Results Fourty eight patients (26 females, median age: 63y) fulfilled the inclusion criteria. The majority suffered from emphysema and/or overinflation. The mean increase in lung density between TNC and CE scans was 11.65 HU 95% Confidence Interval: 6.10, 17.19 for the whole lung. Amount of increase ranged from 7.79 HU 0.65, 14.94 to 16.35 HU 11.22, 21.53 for different lung regions. Comparing VNC with CE scans showed an average increase in lung density of 11.21 HU 8.63, 13.79. Amount of increase ranged from 2.02 HU -2.06, 6.10 to 16.58 HU 13.47, 19.70 for different lung regions. A significant difference between contrast enhancement based on TNC versus VNC images was found for the upper third only. For the whole lung and all other lung regions no differences in density increase were seen. Regression analyses and Spearman’s Rho showed that increase in density tended to be more pronounced in lung regions with higher baseline density in non-enhanced scans. Conclusions The mean increase in density of lung parenchyma after contrast administration was approx. 11 HU, for both TNC and VNC baseline images. There were substantial variations across individuals and lung regions with a tendency of higher increases in areas with higher baseline density on non-enhanced images. Knowledge of these phenomena avoid misinterpretation of contrast enhanced scans.