Feasibility and SNR Performance of Hyperpolarized ¹²⁹Xe Gas Exchange Imaging Using a Balanced SSFP Sequence

To develop a balanced SSFP (bSSFP) sequence for pulmonary ¹²⁹Xe dissolved-phase imaging based on a previously described double-echo sequence facilitating separation into red blood cell and membrane signals, to test its feasibility and compare its SNR performance and imaging results to an analogous FLASH sequence.

12 healthy volunteers underwent hyperpolarized ¹²⁹Xe MRI with both sequences with two different choices of flip angle for FLASH. SNR was quantified using a pseudo multiple replica method, normalized by ¹²⁹Xe polarization and ventilated volume. Chemical shift separation was performed in k-space taking results from time-domain fits of signal evolution as input, either approximating T_E as constant or taking the finite readout duration into account. Results were compared between sequences and methods by linear mixed models, Bland-Altman, and scatter plots.

Normalized SNR of the first echo was significantly greater for bSSFP compared to FLASH with the same flip angle, p = 0.003, but not with lower flip angle resulting in equivalent depolarization, p = 0.98. The effective number of signal averages in signal decomposition was significantly higher compared to FLASH with both flip angles and both dissolved resonances, p < 0.008. Neglecting the finite readout duration was found to introduce bias of derived RBC-M ratios but prevent noise amplification.

Hyperpolarized ¹²⁹Xe dissolved-phase imaging using dual-echo bSSFP is feasible and provides superior effective number of signal averages resulting in improved SNR efficiency of signal decomposition. Future work should concentrate on the application in patients with lung disease.