Hyperpolarized 129Xe MRI/MRS enables quantitative mapping of function in lung airspaces, membrane tissue, and red blood cells (RBCs) within the pulmonary capillaries. The RBC signal also exhibits cardiogenic oscillations that are reduced in pre-capillary pulmonary hypertension (PH). This effect is obscured in patients with concomitant defects in transfer from airspaces to RBCs, which increase RBC oscillation amplitudes. Here, we provide a framework for interpreting RBC oscillations and show their relationship to pulsatile blood flow, capillary blood volume, capillary compliance, and impedance of the capillary and venous circulation. This framework was first applied to characterize RBC oscillations in a cohort of subjects with pulmonary disease but no known PH (n=129). 129Xe MRI of RBC transfer was used to estimate capillary blood volume, and as it decreased, RBC oscillations sharply increased (r2adj=0.53), consistent with model predictions. Model-derived fit parameters were then used to estimate the distribution of pulmonary vascular resistance (PVR) across arterial, capillary, and venous circulation, and to correct oscillations for RBC transfer defects. 70% of PVR was estimated to arise from pulmonary arteries, 11% from capillaries, and 19% from veins. When tested in a second cohort of subjects who underwent 129Xe MRI/MRS and right heart catheterization (n=40), oscillations corrected for capillary blood volume correlated moderately with PVR (r2=0.27, p=0.0014). For every 1.96 WU increase in PVR, corrected oscillations decreased by 1 absolute percentage point. This work demonstrates that, although 129Xe-RBC oscillations are only indirectly sensitive to pre-capillary obstruction, corrected oscillations below 7.5% were 100% specific for elevated PVR.
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