The concentration of 133Xe gas in the lungs is increasing in which phase of a xenon ventilation study?

In a xenon ventilation study, the "wash in" phase refers to the period where the inhaled 133Xe gas is being absorbed by the lung tissues, and its concentration in the lungs is actively increasing. During this phase, the xenon gas moves from the airways into the alveoli and then into the surrounding lung parenchyma, which leads to an accumulation of the radioactive gas in the lung tissue over time.

This rise in concentration continues until the system reaches an equilibrium, at which point the amount of gas in the lungs stabilizes, and the rate of gas entering the lungs equals the rate of gas leaving. Understanding this dynamic is essential for interpreting the results of a xenon ventilation scan, as it distinguishes between the uptake of the gas and subsequent phases like washout, where the gas begins to leave the lungs after cessation of inhalation.