Effect of T 1 relaxation on ventilation mapping using hyperpolarized 129 Xe multiple breath wash‐out imaging

Purpose To investigate the effect of incorporating T 1 as a function of wash‐out breath number (T 1 ( n )) on estimation of fractional ventilation ( r ) using hyperpolarized 129 Xe multiple breath wash‐out (MBWO) imaging in rats. Methods MBWO imaging was performed in 8 healthy mechanically ventilated rats at several inter‐image delay times (τ) and tidal volumes ( TV ). r maps were calculated from the imaging data using a model of T 1 ( n ) (assuming that the longitudinal relaxation rate of 129 Xe in the lung is directly proportional to p A O 2 ) and compared to r maps obtained by assuming a fixed T 1 measured before wash‐out breaths ( r ′). Results Fractional ventilation was overestimated by up to 19.3% when T 1 was fixed. An inverse relationship between bias (Δ r ) and ventilation was observed at all τ and TV . Additionally, Δ r significantly increased when TV was decreased ( F statistic F (2,7) = 48.97, P  < 10 −4 ). Histograms from r ′ maps were significantly more skewed toward lower values as compared to r histograms at all τ and TV ( P  < 0.05) except TV  =  V dose – 1 mL. Conclusion Analysis of hyperpolarized 129 Xe MBWO imaging using a model incorporating T 1 ( n ) corrects for an overestimating bias in the mapping of fractional ventilation in mechanically ventilated rats introduced by assuming a fixed T 1 .