Comparison of 3 He and 129 Xe MRI for evaluation of lung microstructure and ventilation at 1.5T

Abstract

Background: To support translational lung MRI research with hyperpolarized ¹²⁹ Xe gas, comprehensive evaluation of derived quantitative lung function measures against established measures from ³ He MRI is required. Few comparative studies have been performed to date, only at 3T, and multisession repeatability of ¹²⁹ Xe functional metrics have not been reported.

Purpose/hypothesis: To compare hyperpolarized ¹²⁹ Xe and ³ He MRI-derived quantitative metrics of lung ventilation and microstructure, and their repeatability, at 1.5T.

Study type: Retrospective.

Population: Fourteen healthy nonsmokers (HN), five exsmokers (ES), five patients with chronic obstructive pulmonary disease (COPD), and 16 patients with nonsmall-cell lung cancer (NSCLC).

Field strength/sequence: 1.5T. NSCLC, COPD patients and selected HN subjects underwent 3D balanced steady-state free-precession lung ventilation MRI using both ³ He and ¹²⁹ Xe. Selected HN, all ES, and COPD patients underwent 2D multislice spoiled gradient-echo diffusion-weighted lung MRI using both hyperpolarized gas nuclei.

Assessment: Ventilated volume percentages (VV%) and mean apparent diffusion coefficients (ADC) were derived from imaging. COPD patients performed the whole MR protocol in four separate scan sessions to assess repeatability. Same-day pulmonary function tests were performed.

Statistical tests: Intermetric correlations: Spearman's coefficient. Intergroup/internuclei differences: analysis of variance / Wilcoxon's signed rank. Repeatability: coefficient of variation (CV), intraclass correlation (ICC) coefficient.

Results: A significant positive correlation between ³ He and ¹²⁹ Xe VV% was observed (r = 0.860, P < 0.001). VV% was larger for ³ He than ¹²⁹ Xe (P = 0.001); average bias, 8.79%. A strong correlation between mean ³ He and ¹²⁹ Xe ADC was obtained (r = 0.922, P < 0.001). MR parameters exhibited good correlations with pulmonary function tests. In COPD patients, mean CV of ³ He and ¹²⁹ Xe VV% was 4.08% and 13.01%, respectively, with ICC coefficients of 0.541 (P = 0.061) and 0.458 (P = 0.095). Mean ³ He and ¹²⁹ Xe ADC values were highly repeatable (mean CV: 2.98%, 2.77%, respectively; ICC: 0.995, P < 0.001; 0.936, P < 0.001). DATA CONCLUSION: ¹²⁹ Xe lung MRI provides near-equivalent information to ³ He for quantitative lung ventilation and microstructural MRI at 1.5T.

Level of evidence: 3 Technical Efficacy Stage 2 J. Magn. Reson. Imaging 2018.

Keywords: chronic obstructive pulmonary disease; helium-3; hyperpolarized gas; lung MRI; lung cancer; repeatability; xenon-129.

Figure 1

Left: Comparison of ³He and ¹²⁹Xe MR ventilation images of i. a healthy nonsmoker (group b), ii. a patient with NSCLC (white arrows indicate the location of a lesion), and iii. a patient with COPD. (Note: The slice thickness of ³He images is half that of ¹²⁹Xe images; see Table 2). Right: Corresponding breath‐hold scheme for ventilation imaging scans. Scans were acquired in the order shown, with a change of RF coil and repositioning of the patient between ³He and ¹²⁹Xe scans.

Figure 2

Bland–Altman analysis of ³He and ¹²⁹Xe VV% values in patients with NSCLC and COPD and healthy nonsmokers group b. The solid gray line indicates the mean difference (bias) between ³He and ¹²⁹Xe VV%, and dashed lines represent ±1.96 standard deviations from the mean. Datapoints and error bars for COPD patients denote intrasubject means and standard deviations of repeated acquisitions, respectively.

Figure 3

Representative dataset of ³He and ¹²⁹Xe ADC maps obtained from a healthy nonsmoker (group a) and a patient with COPD. The mean ADC over all slices is quoted underneath each dataset.

Figure 4

Correlation between mean ³He and ¹²⁹Xe ADC_glob in healthy nonsmokers (group a), ex‐smokers, and patients with COPD, with associated Spearman's correlation coefficient (r) and P value of statistical significance. The dashed line represents a linear fit to the data and error bars represent the standard deviation of repeated scans for each COPD patient.

Figure 5

Selected ³He and ¹²⁹Xe ventilation image slices acquired at each of the four scan timepoints of the repeatability study from a COPD patient. Calculated ventilated volume percentages (VV%) are quoted underneath each respective set of images.

Figure 6

Representative ³He and ¹²⁹Xe ADC maps acquired at each of the four scan timepoints of the repeatability study from a COPD patient. Global mean ADC values (ADC_glob) are quoted underneath each respective set of maps.