Establishing a hemoglobin adjustment for 129 Xe gas exchange MRI and MRS.

Abstract

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. 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. 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 < 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). 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.