Subtraction lung image for evaluating pulmonary ventilation in xenon-enhanced CT.

PURPOSE: The goal of this work was to develop a method of transforming a xenon-enhanced CT (Xe-CT) image of the lung, so as to overlap well with its baseline CT image, and creating a subtraction image (enhanced image minus baseline image), and to demonstrate the possibility of evaluating pulmonary ventilation using the subtraction image.

METHODS: Eight healthy men (37.1 +/- 10.1 yr) underwent Xe-CT lung studies. In protocol 1 for five subjects, 30% nonradioactive xenon (Xe) was inhaled for 2 min (washin) followed by air breathing for 4 min (washout). In protocol 2 for three subjects, only washin (30% Xe) for 2 min was applied. In each study, a specific range of the thorax (30 mm) in the supine position was scanned cranio-caudally three times in the helical mode: At the start and end of washin and at the end of washout in protocol 1 and at 1 min intervals from the start to end of washin in protocol 2. After each study, 10-mm-thick CT images were reconstructed to have similar anatomical structures throughout the study. Two-dimensional geometrical warping was performed on enhanced CT images so that they could geometrically overlap with the baseline CT image. Second to eighth degree polynomials were applied to the warping functions.

RESULTS: It was derived from the Kety model that subtraction images during washin would directly reflect pulmonary ventilation. Geometrical warping achieved an increase of 0.3%-22.0% in the area, for which ventilation could be evaluated in the subtraction image. In the cases in protocol 2 where the initial lung volume was well retained throughout the study, the ratios of the specific ventilation from the subtraction image to that from the specific ventilation map were 0.88 +/- 0.06 and 0.96 +/- 0.10 for the right and left lungs, respectively.

CONCLUSIONS: A subtraction lung image during washin would provide quantitative information on pulmonary ventilation when the baseline and enhanced images could have close lung volume and these two images could overlap well. Image subtraction requires only two scans and therefore less radiation exposure compared to ordinary protocols in Xe-CT. The proposed Xe-CT subtraction method with the geometrical warping technique could be clinically utilized for evaluating pulmonary ventilation.