An asymmetrical whole-body birdcage RF coil without RF shield for hyperpolarized ¹²⁹ Xe lung MR imaging at 1.5 T

Purpose: This study describes the development and testing of an asymmetrical xenon-129 (¹²⁹ Xe) birdcage radiofrequency (RF) coil for ¹²⁹ Xe lung ventilation imaging at 1.5 Tesla, which allows proton (¹ H) system body coil transmit-receive functionality.

Methods: The ¹²⁹ Xe RF coil is a whole-body asymmetrical elliptical birdcage constructed without an outer RF shield to enable ¹ H imaging. $B_1^{+}$ field homogeneity and flip angle mapping of the ¹²⁹ Xe birdcage RF coil and ¹ H system body RF coil with the ¹²⁹ Xe RF coil in situ were evaluated in the MR scanner. The functionality of the ¹²⁹ Xe birdcage RF coil was demonstrated through hyperpolarized ¹²⁹ Xe lung ventilation imaging with the birdcage in both transceiver configuration and transmit-only configuration when combined with an 8-channel ¹²⁹ Xe receive-only RF coil array. The functionality of ¹ H system body coil with the ¹²⁹ Xe RF coil in situ was demonstrated by acquiring coregistered ¹ H lung anatomical MR images.

Results: The asymmetrical birdcage produced a homogeneous $B_1^{+}$ field (±10%) in agreement with electromagnetic simulations. Simulations indicated an optimal detuning configuration with 4 diodes. The obtained g-factor of 1.4 for acceleration factor of R = 2 indicates optimal array configuration. Coregistered ¹ H anatomical images from the system body coil along with ¹²⁹ Xe lung images demonstrated concurrent and compatible arrangement of the RF coils.

Conclusion: A large asymmetrical birdcage for homogenous $B_1^{+}$ transmission with high sensitivity reception for ¹²⁹ Xe lung MRI at 1.5 Tesla has been demonstrated. The unshielded asymmetrical birdcage design enables ¹ H structural lung MR imaging in the same exam.