Fig. 1. [13N]nitrogen (13N2) tracer kinetics and positron emission tomography (PET) images for one animal. Tracer kinetics are shown separately for nondependent (ND; triangles ), middle (M; circles ), and dependent (D; squares ) regions of lung of equal height. In each PET image, the left lung is on the left and the right lung is on the right. (A ) After intravenous injection of a bolus of 13N2–saline over 3 s, PET images were acquired during 60 s of apnea and 3 min of ensuing ventilation. During apnea (left of dashed vertical line ), the D region had both a higher peak concentration of 13N2, indicative of higher perfusion per voxel, and a greater clearance of 13N2, indicative of higher shunt, than the M and ND regions. This topographical information is readily visualized from one PET slice of lung, which shows that the D region had greater activity than the M and ND regions during the early (5 s < t < 10 s) phase of apnea (left PET image ). The activity in the D region decreased substantially by the end of apnea, whereas this decrease was much less pronounced in the M and ND regions (right PET image ). (B ) After inhalation of 13N2through a closed-circuit ventilator system, sequential PET images were acquired during wash-in and equilibration of tracer. Because 13N2remains confined to the alveolar airspace, its concentration after equilibration is proportional to regional gas content. As both the tracer kinetics and the equilibration PET image show, there was a vertical gradient in gas content, which was lowest in the D region.