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Feasibility of Single Breath-hold Isotropic Voxel 129Xe MRI in Patients

Andrea L Barker^1,2, Andrew L Westcott^1,2, Rachel L Eddy^1,2, David G McCormack³, Grace Parraga^1,2,3, and Alexei L Ouriadov¹

¹Robarts Research Institute, London, ON, Canada, ²Medical Biophysics, Western University, London, ON, Canada, ³Division of Respirology, Department of Medicine, Western University, London, ON, Canada

Synopsis

Hyperpolarized ¹²⁹Xe gas MRI has been established as an effective tool to investigate lung function in a number of pulmonary diseases. Improved ¹²⁹Xe polarization techniques provide the opportunity to use a high-resolution 3D static ventilation image acquisition sequence (FGRE) in a 16-second breath-hold. Isotropic-voxel images were acquired in one healthy-volunteer and two patients with asthma; images were compared to coronal images acquired in two other patients with asthma. Isotropic images had sufficient SNR to analyze VDP. This proof-of-concept investigation establishes the feasibility of using an isotropic-voxel acquisition sequence in single breath-hold MRI of the lungs.

PURPOSE

Inhaled hyperpolarized gas MRI has been proven to be useful for the observation of and treatment planning for several pulmonary diseases including chronic obstructive pulmonary disease (COPD),¹ asthma² and lung cancer.³ Even after 20 years of developing hyperpolarized gas MRI,⁴ technological challenges have seriously limited its translation into clinical practice. Low SNR ¹²⁹Xe images impede the development of many novel acquisition schemes that are highly sensitive to SNR,⁵ such as isotropic voxel static-ventilation imaging, accelerated multiple b-value diffusion-weighting MRI,⁶ and accelerated simultaneous collection of ¹²⁹Xe gas and ¹²⁹Xe within lung tissue/blood. The recent improvement in xenon polarization provides a way to develop advanced imaging methods previously not possible. This is especially critical for investigations in patients with asthma and COPD. We hypothesize that the isotropic-voxel, high-resolution 3D ¹²⁹Xe static-ventilation image can be acquired in single 16 second breath-hold, with necessary and sufficient SNR to generate ventilation-defect-percent⁷ (VDP) estimates. Therefore, in this proof-of-concept evaluation, our objective was to demonstrate feasibility of this method in a small group of patients.

METHODS

A healthy-volunteer and four asthma patients provided written informed consent to an ethics-board-approved study protocol and underwent spirometry and ¹²⁹Xe MRI. Imaging was performed at 3.0T (MR750, GEHC, WI) using whole-body gradients (5G/cm maximum) and a commercial ¹²⁹Xe quadrature-flex RF coil (MR Solutions, USA).⁸ Isotropic-voxel ¹²⁹Xe static ventilation images were acquired using an axial plane 3D Fast Gradient Recall Echo (FGRE) sequence (TE/TR/initial-flip-angle=1.3ms/4.0ms/0.9^o, variable-flip-angle,⁹ reconstructed matrix size=128x128x104 [axial view], pixel bandwidth=217Hz, FOV=40x40x32cm³, voxel size=3x3x3mm³). Traditional resolution xenon static-ventilation images were acquired using a coronal plane 3D FGRE sequence (TE/TR/initial-flip-angle=1.5ms/5.1ms/1.3^o, variable flip angle, pixel bandwidth=122Hz, reconstructed matrix size=128x128x16, FOV=40x40x24cm³, voxel size=3x3x15mm³) as previously described.⁷ All images were acquired in breath-hold (<16 second) after inspiration of 1.0L of gas (¹²⁹Xe/⁴He mixture) from functional-residual-capacity. Hyperpolarized ¹²⁹Xe gas was obtained from a turn-key, spin-exchange polarizer system (Polarean 9820 ¹²⁹Xe polarizer).¹⁰ Image SNR was calculated for five central slices for each axial, coronal and sagittal view using a 10x10 region of interest inside a lung region of homogeneous signal and using the same 10x10 region of interest outside the lung where there was no lung signal.¹¹ VDP was generated using a semi-automated segmentation algorithm as previously described.⁷

RESULTS

Figures 1 and 2 show isotropic-voxel static-ventilation images in axial and coronal views acquired from one healthy-volunteer and reconstructed from the same 3D k-space dataset. Table 1 summarizes demographic information as well as imaging results including SNR and VDP for three isotropic-voxel static-ventilation measurements (healthy-volunteer and two patients with asthma) and two traditional resolution static-ventilation measurements in two other patients with asthma. VDP for all five participants was less than 10%. Overall, SNR of the isotropic-voxel images was between 10-30 and SNR of the traditional resolution images was between 30-40. A coronal view SNR was normalized (SNR_norm) to the ¹²⁹Xe volume and polarization for better understanding of the image quality limits; mean SNR_norm was 160 and 320 for the isotropic-voxel and traditional resolution images, respectively.

DISCUSSION AND CONCLUSION

In this proof-of-concept study, we showed that ¹²⁹Xe static-ventilation images can be acquired in a single 16 second breath-hold with sufficient SNR to analyze VDP. To the best of our knowledge, this is the first demonstration of ¹²⁹Xe isotropic-voxel static-ventilation imaging with a 3D FGRE sequence. Normalized SNR for the isotropic-voxel images was approximately half that of the coronal non-isotropic images, however the voxel size was five times smaller. A previous investigation of ¹²⁹Xe in patients with asthma where VDP was less than 10 reported SNR of 22⁷ and this is consistent with our results. The SNR for the healthy-volunteer here was consistent with that previously-reported in a healthy-volunteer using a 3D Steady-State Free Precession isotropic-voxel acquisition with larger voxels (4.2x4.2x4.2mm³), a larger xenon dose (1.0L, 86% enriched ¹²⁹Xe at 35% polarization) and greater flip angle (10^o).¹² Observed variation in the isotropic-voxel image SNR (healthy-volunteer vs. asthma patient-2) could be attributed to variation of the RF coil sensitivity caused by different patient sizes. A rigid and more homogenous coil¹³ combined with a phased-receive-array¹⁴ could substantially improve isotropic-voxel image quality and potentially replace the isotopically-enriched ¹²⁹Xe with the natural-abundant xenon¹⁵ and consequently, reduce the cost of ¹²⁹Xe MRI for patients. This study demonstrates the feasibility of isotropic-voxel MR imaging of the lungs within a single breath-hold scan.

Acknowledgements

No acknowledgement found.

References

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Figures

Figure 1: Coronal view isotropic ¹²⁹Xe MRI static-ventilation slices from anterior to posterior for the healthy volunteer.

Figure 2: Axial view isotropic ¹²⁹Xe MRI static-ventilation slices from superior to inferior for the healthy volunteer.

Table 1: Participant demographics and imaging results.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)

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