A thermally polarized ¹²⁹ Xe phantom for quality assurance in multi-center hyperpolarized gas MRI studies

Hyperpolarized ¹²⁹ Xe MR is increasingly being adopted worldwide, but no standards exist for assessing or comparing performance at different ¹²⁹ Xe imaging centers. Therefore, we sought to develop a thermally polarized xenon phantom assembly, approximating the size of a human torso, along with an associated imaging protocol to enable rapid quality-assurance imaging.

MR-compatible pressure vessels, with an internal volume of 5.85 L, were constructed from pressure-rated, engineering grade PE4710 high-density polyethylene. They were filled with a mixture of 61% natural xenon and 39% oxygen to approximately 11.6 bar and placed in a loader shell filled with a 0.56% saline solution to mimic the human chest. Imaging employed a 2D spoiled gradient-echo sequence using non-slice-selective excitation (TR/TE = 750/6.13 ms, flip angle = 74°, FOV = 40 × 440 mm, matrix = 64 × 32, bandwidth = 30 Hz/pixel, averages = 4), resulting in a 1.6 min acquisition. System characterization and imaging were performed at 8 different MRI centers.

At 3 Telsa, ¹²⁹ Xe in the pressure vessels was characterized by T₁ = 580.5 ± 8.3 ms, linewidth = 0.21 ppm, and chemical shift = +10.2 ppm. The phantom assembly was used to obtain transmit voltage calibrations and 2D and 3D images across multiple coil and scanner configurations at 8 sites. Across the 5 sites that employed a standard flexible chest coil, the SNR was 12.4 ± 1.8.

The high-density polyethylene pressure vessels filled with thermally polarized xenon and associated loader shell combine to form a phantom assembly that enables spectroscopic and imaging acquisitions that can be used for testing, quality assurance, and performance tracking-capabilities essential for standardizing hyperpolarized ¹²⁹ Xe MRI within and across institutions.