Comparison of 3He and 129Xe MRI for Evaluation of Lung Microstructure and Ventilation in Healthy Volunteers and COPD Patients at 1.5 T
Neil James Stewart1, Ho-Fung Chan1, Guilhem Jean Collier1, Felix Clemens Horn1, Graham Norquay1, Juan Parra-Robles1, Denise Yates2, Paul Collini3, Rod Lawson4, Helen Marshall1, and Jim Michael Wild1

1Academic Unit of Radiology, University of Sheffield, Sheffield, United Kingdom, 2Novartis Institutes for Biomedical Research, Cambridge, MA, United States, 3Academic Unit of Immunology and Infectious Diseases, University of Sheffield, Sheffield, United Kingdom, 4Sheffield Teaching Hospitals NHS Foundation Trust, Sheffield, United Kingdom

Synopsis

3He and 129Xe ventilation and diffusion-weighted MR images were acquired at 1.5T in healthy volunteers and, at multiple time-points, in COPD patients in order to compare the functional sensitivity and assess the repeatability of MR-derived measures of ventilated volume (VV%) and apparent diffusion coefficient (ADC) from each of the two gases. ADC values from both nuclei exhibited excellent agreement and significant correlations with pulmonary function tests (PFTs) (p<0.001), whilst VV% values were less comparable. ADC and VV% metrics derived from both nuclei were also shown to be repeatable, with coefficient of variation values similar to those of PFTs.

Purpose

Hyperpolarised gas MRI with 3He and 129Xe has been established as a functional tool to image lung ventilation and detect lung microstructural changes [1]. A recent comparison of ventilation imaging and apparent diffusion coefficient (ADC) mapping with 3He and 129Xe at 3T [2] demonstrated that comparable ventilation and microstructural information can be obtained from the two gases. In addition, 3He ADC MRI has been shown to be highly repeatable in healthy volunteers and COPD patients [3]. However, comparison of the functional sensitivity of 3He and 129Xe lung MRI has not been performed to date at 1.5T, and the repeatability of ADC mapping MRI with 129Xe is yet to be adequately assessed. In this work, 3He and 129Xe ventilation and diffusion-weighted MR images were acquired at 1.5T in healthy volunteers and, at multiple time-points, in COPD patients in order to compare the functional sensitivity and assess the repeatability of MR-derived measures of ventilated volume and lung microstructure from each of the two gases.

Methods

Nine healthy volunteers (with no history of lung disease) and five COPD patients were recruited for hyperpolarised 3He and 129Xe gas MR imaging. 2D 3He and 129Xe diffusion-weighted MR images (see e.g. [4]) were acquired for all subjects and ADC maps were calculated from the first (b=0s/cm2) and second diffusion interleaves (3He b=1.6s/cm2, 129Xe b=8.0s/cm2). In COPD patients, 3D steady-state free-precession (SSFP) images of lung ventilation were acquired with both gases using the parameters described in [5,6], and ventilation volume percentages (VV%) were calculated as described previously [5]. The whole imaging protocol was repeated in the COPD patients on three additional scan sessions (same-day, next-day and after 2 weeks) to assess the repeatability of 3He and 129Xe MR-derived measures of ventilated volume and lung microstructure. Measurements of VV% and ADC derived from 3He and 129Xe MRI were compared to evaluate the relative functional sensitivity of the two gases to obstructive lung disease and emphysematous microstructural changes. These metrics were also compared to pulmonary function tests (PFTs), including the diffusing capacity of carbon monoxide (TLCO), forced expiratory volume in 1 second (FEV1), and the ratio of FEV1 to forced vital capacity (FVC) (FEV1/FVC), acquired on the same day as each scan. The coefficient of variation (CoV) of each MR and PFT parameter was calculated to assess repeatability. For COPD patients, the mean values and standard deviations over all scan sessions were used for comparisons.

Results and Discussion

As expected, MR images from both nuclei highlighted significant ventilation abnormalities in COPD subjects and higher global mean ADC values compared to volunteers (two-tailed t-test, p<0.001, Figure 1). VV% values were observed to be larger on average for 3He than 129Xe (p<0.001), similar to previous observations [7], reflecting the ability of 3He to penetrate less well-ventilated lung by virtue of its higher diffusion coefficient. A strong positive correlation between 3He and 129Xe ADC values was observed in healthy volunteers and COPD patients (Spearman’s correlation coefficient 0.982, p<0.001, Figure 2a). ADC values derived from both gases exhibited significant correlations with PFTs (Table 1), most notably TLCO, which is in good agreement with previous findings [2,3]. Comparing VV% measurements and PFTs, only 3He VV% exhibited a significant correlation with FEV1/FVC (p<0.05), and the relationship between 3He and 129Xe derived VV% values was not significant (Figure 2b). The lack of observed significant correlation between VV% values of the two gases and other PFT measures can be attributed to a low number of data-points (n=5) and the absence of comparative healthy volunteer data. ADC metrics of both gases were found to be highly repeatable, with mean coefficient of variation (CoV) values (<3%) comparable to or less than those of the PFT results (Table 2). CoV values were slightly higher for VV% measures – in particular for 129Xe, likely due to difficulty in manual segmentation of ventilation images and/or variability in 129Xe polarisation levels. However the mean 3He and 129Xe VV% CoV values over all subjects were still <5% and <15%, respectively, indicating good intra-subject repeatability.

Conclusion

Preliminary experimental validation of hyperpolarised 3He and 129Xe MR ventilation imaging and ADC mapping has been demonstrated at 1.5T in COPD patients and healthy volunteers. ADC values from both nuclei exhibited excellent agreement and significant correlations with PFT results, whilst corresponding VV% values were not as comparable due to small sample sizes. ADC and VV% metrics derived from both nuclei were repeatable, paving the way to increased use of 129Xe as a more readily-available, cost-effective alternative to 3He for clinical lung imaging studies.

Acknowledgements

No acknowledgement found.

References

[1] Fain, S. B. et al., J Magn Reson Imaging, 25(5):910-23 (2007). [2] Kirby, M. et al., Radiology, 265:600-10 (2012). [3] Diaz, S. et al., J Magn Reson Imaging, 27(4):763-70 (2008). [4] Parra-Robles, J. et al., Magn Reson Med, 67:322-5 (2012). [5] Horn, F. C. et al., NMR in Biomed, 27(12):1461-7 (2014). [6] Stewart, N. J. et al., Magn Reson Med, 74:346-352 (2015). [7] Kirby, M. et al., J Appl Physiol, 114:707-15 (2013).

Figures

Table 1: Summary of Spearman’s correlation coefficients and p-values for MRI-derived measurements and PFTs in a population of healthy volunteers and COPD patients.

Table 2: Summary of inter-subject mean and range of coefficient of variation (CoV) values for MRI-derived measurements and PFTs in COPD patients.

Figure 1: Representative apparent diffusion coefficient (ADC) maps (left) and ventilation images (right) acquired from 3He and 129Xe MRI in the same COPD subject.

Figure 2: Correlation between 3He and 129Xe ADC values from healthy volunteers and COPD patients (a) and VV% values for COPD patients (b), with associated Spearman’s correlation coefficients. Solid lines represent linear fits to the data and error bars represent the standard deviation of all repeated scans for each subject.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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