Pitfalls in lung subtraction CT: delayed contrast acquisition causes underestimation of pulmonary perfusion in a...

Lung subtraction computed tomography (LSCT), assessing pulmonary perfusion by subtracting non-contrast-enhanced images from contrast-enhanced images, may underestimate pulmonary perfusion when contrast-enhanced image acquisition is delayed. This study aimed to evaluate the effects of delayed acquisition of contrast-enhanced images on the LSCT assessment of pulmonary perfusion and identify indicators of sufficient contrast enhancement for LSCT generation using healthy beagles. Five beagles underwent continuous cine computed tomography (CT) for 30 s. After the second scan, iodixanol was intravenously administered. LSCT images were generated by subtracting pre-contrast images from contrast-enhanced images. On LSCT images, lung CT values and the percentage of areas with CT values of 0-20 HU were measured at the pulmonary trunk and tracheal bifurcation levels when the lung CT value on LSCT images was the highest and at 1.5 and 3.0 s thereafter. Pulmonary trunk CT values and pulmonary trunk-to-aorta (Pa/Ao) CT value ratios were recorded. Lung CT values decreased significantly over time, whereas the percentages of 0-20 HU areas increased. Pulmonary trunk CT values were associated positively with lung CT values and negatively with the percentages of 0-20 HU areas at both slice levels. Pa/Ao ratios showed similar significant associations at the pulmonary trunk level but not at the tracheal bifurcation level. In conclusion, delayed acquisition resulted in underestimation of pulmonary perfusion on LSCT images. These findings highlight the importance of appropriate acquisition timing. Pa/Ao ratios might indicate whether contrast enhancement is sufficient for LSCT generation.