Purpose

We sought to compare ¹²⁹Xe MR image quality across sites and scanner platforms with a rigorous protocol compatible for multi-center studies. MRI signal from non-renewable hyperpolarized ¹²⁹Xe magnetization is challenging to objectively quantify across major MR platforms since hardware for transmit and receive coils, field gradient-corrections, and multiple levels of data filtering are all proprietary for vendors. However, for multi-site studies involving hyperpolarized ¹²⁹Xe MRI, which are currently underway¹, quantitative comparisons across major vendors are essential. Here, we compared ¹²⁹Xe MR imaging across vendors and sites with a near-identical protocol intended to harmonize performance across sites. Building on an initial study recently published², the approach here advances these methods by implementing harmonized scan parameters, and near-identical phantoms, chest coils, and rf-pulse characteristics.

Methods

Two large high-pressure ¹²⁹Xe phantoms (Phantom_1: 171.5 and Phantom_2: 174.2 psia; ~ 4 Liters each; Polarean LLC) were created by filling thick-walled high-density polyethylene cylinders with 58% naturally abundant Xe (26.4% of which is ¹²⁹Xe), 5% He, and 37% O₂ to reduce the intrinsic T₁. The phantoms were imaged at Cincinnati Children’s Hospital Medical Center and at four collaborating imaging research centers (University of Virginia, University of Wisconsin, University of Iowa, and Duke University), spanning three major MRI scanner vendors. Near-identical whole-chest quadrature ¹²⁹Xe adult-sized wrap coils were used at each site, from one manufacturer (Clinical MR Solutions), operating at approximately 34 MHz at 3T. At each site the chest coil was wrapped around an annular loader shell filled with a standard saline solution[designed to emulate the shape and loading of human lungs. The shell opening was sized to hold the high-pressure ¹²⁹Xe phantoms. The protocol used for the scans was a simple, non-slice-selective, fully-sampled gradient echo sequence¹, with parameters listed in Table 1. The location of each phantom in the coil (left/right) was recorded, with 3 sites imaging both phantoms simultaneously in the coil and 2 sites imaging phantoms in the left and right positions separately.

Utilizing MATLAB and Python, the raw k-space data from each site were reconstructed into images via 2D Fourier transform with no additional post-processing or filtering. Images were segmented to measure the mean signal of the phantoms, avoiding edges. The standard deviation of the noise was measured using a central region of interest to avoid the edges of the field of view, which can be distorted from magnet and gradient imperfections (see Figure 2).

Results

The signal-to-noise values listed in Table 2 reveal very similar results between the MRI research sites and major vendors. SNR was 18.4±1.6, averaged across all scans. The variations in SNR between sites (13%), and vendors (10%), were both reasonably low.

Discussion and Conclusion

We were able to obtain very similar quantitative results for ¹²⁹Xe MRI across 5 sites and 3 major MRI vendors by following a simple, rigorous MRI protocol using near-identical phantoms and coils. This is what we expect when no hardware issues or additional sources of noise are present. There are some limitations: no resolution or distortion assessment is available with this phantom design, and scans were performed with TR and TE that are easily matched across platforms, which may not be the case in settings with more rapid scanning. In the future, however, we can perform quantitative imaging comparisons across sites in vivo with a similarly reliable algorithm for quality control. Multi-site studies using hyperpolarized ¹²⁹Xe gas MRI are currently underway². With multi-site results and clinical application of this technology ongoing, simple cross-vendor quality control is essential.

Acknowledgements

CF Foundation Grant: WOODS19A0

References

1. Bier EA, Nouls JC, Wang Z, et al. A thermally polarized ¹²⁹ Xe phantom for quality assurance in multi‐center hyperpolarized gas MRI studies. Magnetic Resonance in Medicine. 2019;82(5):1961-1968. doi:10.1002/mrm.27836

2. Niedbalski PJ, Hall CS, Castro M, et al. Protocols for multi‐site trials using hyperpolarized 129 Xe MRI for imaging of ventilation, alveolar‐airspace size, and gas exchange: A position paper from the 129 Xe MRI clinical trials consortium. Magnetic Resonance in Medicine. 2021;86(6):2966-2986. doi:10.1002/mrm.28985