MR Spectroscopic Imaging of Hyperpolarized 129‐Xenon in the Dissolved‐Phase to Determine Regional Chemical Shifts of Hyperoxia in Healthy Porcine Lungs

Lung MRI with hyperpolarized xenon (¹²⁹Xe) gas reveals key characteristics of pulmonary physiology such as ventilation and alveolar-capillary gas transfer. Magnetic resonance spectroscopic imaging (MRSI) offers insights into regional oxygenation saturation (sO₂) through chemical shift changes related to xenon-hemoglobin binding. The similarity between porcine and human anatomy and physiology, particularly in terms of lung volume, airway structure, and alveolar-capillary microstructure, offers the opportunity to investigate physiological effects linked to oxygen supply using ¹²⁹Xe MRSI. We hypothesize that ¹²⁹Xe MRSI can detect regional chemical shift changes related to red blood cell oxygenation and arterial oxygen partial pressure (p_aO₂) in a porcine model. Imaging was performed on a 3-T clinical MRI scanner on four healthy pigs mechanically ventilated at fractional inspired oxygen levels (FiO₂) of 40% and 100%. Dissolved-phase images were acquired using a 3D Cartesian MRSI sequence with a spherical sampling pattern in a matrix size of 28 × 28 × 6. A spectrally tailored RF pulse excited the dissolved and gaseous phases with flip angles of 10° and 0.1°, respectively. Repetition time was 7.4 ms resulting in a total acquisition time of 18 s. In addition, ¹²⁹Xe ventilation, pulmonary anatomical scans, dynamic contrast-enhanced perfusion, and arterial blood gas were measured at each FiO₂. Pair-wise comparisons were performed between inspired oxygen levels, along with linear regression analysis of p_aO₂ and dissolved-phase chemical shift imaging. Porcine lung lobes were segmented, and two-way ANOVA were performed to evaluate regional effects of oxygen concentrations. Arterial blood gas and cardiopulmonary measures showed an increase in p_aO₂ with the increase in FiO₂. Ventilation defect percentage and perfusion metrics did not significantly change with higher oxygen concentration. Dissolved-phase ratios of red blood cells (RBC) to membrane increased with higher oxygen concentration. Increasing inspired oxygen resulted in a lower RBC chemical shift and increased linewidth, indicating RBC measures are sensitive to p_aO₂. Simple linear regression analysis of RBC chemical shift and a multiple linear regression model including linewidth were applied for regional p_aO₂ maps. Regional effects of oxygen were confirmed in the segmented lung lobes. Dissolved-phase ¹²⁹Xe chemical shift of RBC decreased linearly with p_aO₂ in healthy porcine lungs. Regional chemical shift, linewidth, and signal ratio changes were determined in dissolved-phase imaging of RBC at 40% and 100% FiO₂. Our data suggest that regional p_aO₂ prediction is possible with a multiple linear regression model including RBC chemical shift and linewidth as combined effect of oxygen across animal lung lobes affects regions differently.