Micro-CT imaging of rat lung ventilation using continuous image acquisition during xenon gas contrast enhancement

We measured ventilation (V) in seven anesthetized, mechanically ventilated, supine Wistar rats. Images of the whole lung were continuously acquired using a dynamic, flat-panel volumetric micro-computed tomography (micro-CT) scanner during ventilation with a xenon/oxygen (Xe-O(2)) gas mixture. Forty time-resolved volumes consisting of eighty 0.45-mm-thick slices (covering the entire lung) were acquired in 40 s, using a gantry rotation rate of one rotation per second. The animals were ventilated at a respiratory rate of 60 breaths/min, matching the gantry rotation rate, and imaged without suspending ventilation. A previously published theoretical model was modified slightly and used to calculate the whole lung ventilation from volumes of interest generated by seeded region growing. Linear regression of calculated whole lung ventilation volumes vs. expected tidal volumes yielded a slope of 1.12 +/- 0.11 (slope +/- SE) and a y-intercept of -1.56 +/- 0.42 ml (y-intercept +/- SE) with 95% confidence intervals of 0.83 to 1.40 and -2.6 to -0.5 ml, respectively. The same model was used to calculate the regional ventilation in axial slices for each animal. Voxels were fit to the model to yield a map of V, which displayed an anterior/posterior gravitational gradient of (-3.9 +/- 1.8) x 10(-6) mlxs(-1)xcm(-1) for slices immediately superior to the diaphragm and (-6.0 +/- 2.4) x 10(-6) mlxs(-1)xcm(-1) for slices at the midlevel of the heart (mean +/- SD). Thus continuous Xe-enhanced computed tomography enables the noninvasive determination of regional V with the temporal and spatial resolution necessary for rats.