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 (Xe) 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 (sO) 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 Xe MRSI. We hypothesize that Xe MRSI can detect regional chemical shift changes related to red blood cell oxygenation and arterial oxygen partial pressure (pO) 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 (FiO) 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, Xe ventilation, pulmonary anatomical scans, dynamic contrast-enhanced perfusion, and arterial blood gas were measured at each FiO. Pair-wise comparisons were performed between inspired oxygen levels, along with linear regression analysis of pO 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 pO with the increase in FiO. 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 pO. Simple linear regression analysis of RBC chemical shift and a multiple linear regression model including linewidth were applied for regional pO maps. Regional effects of oxygen were confirmed in the segmented lung lobes. Dissolved-phase Xe chemical shift of RBC decreased linearly with pO in healthy porcine lungs. Regional chemical shift, linewidth, and signal ratio changes were determined in dissolved-phase imaging of RBC at 40% and 100% FiO. Our data suggest that regional pO 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.