Standardized Spectral Mapping With ¹²⁹Xe Chemical Shift Imaging: Investigating Red Blood Cell Shift Heterogeneity in...

Hyperpolarized ¹²⁹Xe chemical shift imaging (CSI) is emerging as an alternative approach to Dixon-based methods for assessing pulmonary gas exchange. CSI additionally permits mapping spectral parameters like linewidths, enabling more accurate, regional estimation of $T_2^{*}$ . Moreover, regionally mapping the ¹²⁹Xe chemical shift in red blood cells (RBCs) offers a novel means to assess arteriolar pathology through local measurements of blood oxygenation. However, such CSI-based mapping requires standardized acquisition and processing workflows along with well-defined healthy reference distributions to enable robust quantification of patterns reflecting normal physiology. To this end, we implemented a fast, elliptically sampled Cartesian ¹²⁹Xe CSI acquisition at 3 T, exciting the dissolved resonances at 208 ppm. We used simulations to determine the optimal k-space filter and minimum SNR for reliable spectral fitting. This approach was applied in a cohort of healthy 18-30-year-olds (N = 11) to establish healthy reference distributions for quantitative mapping of RBC and membrane $T_2^{*}$ , RBC chemical shift, and the RBC:Membrane signal ratio. CSI reference mean values ( $\pm 1$ standard deviation after Box-Cox transform) for membrane and RBC $T_2^{*}$ were 1.07 ms (0.97-1.17 ms) and 1.13 ms (1.05-1.22 ms), respectively, with an RBC shift of 217.9 ppm (217.2-218.5 ppm), and RBC:Membrane of 0.44 (0.37-0.54). CSI-derived mean values were all significantly different from those measured by whole-lung spectroscopy (p  $\le$  0.02). Notably, RBC:Membrane and membrane $T_2^{*}$ increased significantly from the gravitationally non-dependent to dependent lung regions, while isogravitational heterogeneity of RBC $T_2^{*}$ and RBC shift decreased. Conversely, in a preliminary cohort of pulmonary hypertension (PH) patients (N = 4), RBC shift heterogeneity was elevated, even in gravitationally dependent lung, although significance was not reached in this small sample (p = 0.07). These results demonstrate how standardized quantitative ¹²⁹Xe CSI facilitates rigorous characterization of cardiopulmonary disease signatures and, specifically, highlight RBC shift heterogeneity as a promising marker of PH.