Towards an evaluation of the accuracy of ventilatory rate constants derived from dynamic computer-assisted tomographic measurements.

Dynamic computer-assisted tomographic (CT) measurements using non-radioactive xenon gas have recently been employed along with a simple model of pulmonary transport to derive ventilation rate constants (K) in small tissue volumes. Inherent in the model description is the assumption that instantaneous diffusion equilibrium occurs between inhaled Xe gas and lung tissue. The validity of this assumption as it relates to the accuracy of the derived values of K has been considered for both normal and oedematous tissue, the latter characterised by a severe widening of the interstitial fluid space. It was found that only a small error in the predicted values for K results, when the simple model is applied to the dynamic CT measurements of xenon enhancement.