Measurement of the exponential decay of alveolar partial pressure of oxygen using hyperpolarized He MRI in animals and human subjects

Purpose: Respiratory diseases are one of the main causes of death worldwide [1] and a lot of effort is undertaken to develop new techniques to assess lung function. Using hyperpolarized (HP) He-MRI it is possible to locally measure the alveolar partial pressure of oxygen (pO2) and its temporal evolution during apnea [2]. Most of the pO2 measurement approaches currently used in humans and animals are based on a linear model of pO2 decay, pO2(t) = p0 – Rt, where p0 denotes the initial partial pressure of oxygen and R – the oxygen depletion rate. However, partial pressure of oxygen is known to decrease exponentially during apnea [3] and the linear approximation is only valid for short periods of breath-hold. The aim of this work was to demonstrate the feasibility of using the physiologically more appropriate exponential model of pO2 decay both in animals and in humans. The proposed model is given by: pO2(t) = p0exp(-t/r) [4] where p0 is the initial partial pressure of oxygen and the oxygen depletion time constant r depends on the metabolic rate of the animal. Theory: The signal decay of HP He related to the presence of oxygen can be described by: S(t) = S0exp(-∫(pO2(t)/ξ)dt, where ξ denotes the oxygen induced decay constant [2]. Thus, the integral of pO2(t) can be expressed as: ∫ pO2(t)dt = -ξln(S(t)/S0). Finally, the parameters of interest (p0, r) can be obtained by fitting the plot of