Comparison of hyperpolarised gas MRI and CT-based surrogates of ventilation

Background: Non-contrast CT-based surrogates of regional ventilation derived from pulmonary images acquired at multiple inflation levels have been proposed as alternatives to established modalities. However, their physiological accuracy has yet to be validated prior to clinical translation.
Purpose: To address the hypothesis that these surrogates can provide information comparable to a direct measure of ventilation from hyperpolarised gas MRI ventilation via:
i. development of a methodology for registering CT and gas MRI.
ii. comparison of these surrogates with gas MRI at the lobar level.
iii. evaluation of the impact of inflation levels when comparing gas MRI and ventilation CT.
iv. development of an image acquisition and analysis framework to facilitate spatial correlations of both techniques.
v. assessment of the effect of using different gases on the correlation.
Methods:
i. A method to indirectly register gas MRI to CT via same-breath 1H-structural MR images was developed and its accuracy was assessed.
ii. A ventilation model based on expansion of lobar CT segmentations was compared with gas MRI lobar ventilation measurements.
iii. The spatial overlap of ventilation CT was compared to gas MRI acquired at two different inflation levels.
iv. An image acquisition protocol was designed to minimise differences in acquisition settings between scans such as posture and breathing manoeuvre and analysis methods were developed to enable direct regional and voxel level correlations.
v. The effect of using two different noble gases, namely, 3He and 129Xe, on correlation with ventilation CT was assessed.
Results:
i. The indirect method of registration was more accurate than direct registration.
ii. Despite subtle differences, lobar ventilation measurements derived from CT and hyperpolarised gas MRI were comparable.
iii. Comparison of ventilation CT and gas MRI varied with inflation state.
iv. The spatial correlation between ventilation CT and gas MRI increased at coarser levels.
v. A marked improvement in correlation was observed for 3He and 129Xe MRI in contrast to when ventilation CT was compared with either 3He and 129Xe MRI.
Conclusion: Although CT-based surrogates of ventilation show promise for replacing established ventilation modalities such as hyperpolarised gas MRI, particularly at coarser levels, they cannot be assumed to be equivalent to the techniques they purport to replace.