Establishing a hemoglobin adjustment for <scp>¹²⁹Xe</scp> gas exchange <scp>MRI</scp> and <scp>MRS</scp>

Abstract

Purpose 129 Xe MRI and MRS signals from airspaces, membrane tissues (M), and red blood cells (RBCs) provide measurements of pulmonary gas exchange. However, 129 Xe MRI/MRS studies have yet to account for hemoglobin concentration (Hb), which is expected to affect the uptake of 129 Xe in the membrane and RBC compartments. We propose a framework to adjust the membrane and RBC signals for Hb and use this to assess sex‐specific differences in RBC/M and establish a Hb‐adjusted healthy reference range for the RBC/M ratio. Methods We combined the 1D model of xenon gas exchange (MOXE) with the principle of TR‐flip angle equivalence to establish scaling factors that normalize the dissolved‐phase signals with respect to a standard (14 g/dL). 129 Xe MRI/MRS data from a healthy, young cohort ( n = 18, age = 25.0 3.4 years) were used to validate this model and assess the impact of Hb adjustment on M/gas and RBC/gas images and RBC/M. Results Adjusting for Hb caused RBC/M to change by up to 20% in healthy individuals with normal Hb and had marked impacts on M/gas and RBC/gas distributions in 3D gas‐exchange maps. RBC/M was higher in males than females both before and after Hb adjustment ( p &lt; 0.001). After Hb adjustment, the healthy reference value for RBC/M for a consortium‐recommended acquisition of TR = 15 ms and flip = 20° was 0.589 0.083 (mean SD). Conclusion MOXE provides a useful framework for evaluating the Hb dependence of the membrane and RBC signals. This work indicates that adjusting for Hb is essential for accurately assessing 129 Xe gas‐exchange MRI/MRS metrics.

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