Using Hyperpolarized Xe MRI to Quantify the Pulmonary Ventilation Distribution.

RATIONALE AND OBJECTIVES

Ventilation heterogeneity is impossible to detect with spirometry. Alternatively, pulmonary ventilation can be imaged three-dimensionally using inhaled Xe magnetic resonance imaging (MRI). To date, such images have been quantified primarily based on ventilation defects. Here, we introduce a robust means to transform Xe MRI scans such that the underlying ventilation distribution and its heterogeneity can be quantified.

MATERIALS AND METHODS

Quantitative Xe ventilation MRI was conducted in 12 younger (24.7 ± 5.2 years) and 10 older (62.2 ± 7.2 years) healthy individuals, as well as in 9 younger (25.9 ± 6.4 yrs) and 10 older (63.2 ± 6.1 years) asthmatics. The younger healthy population was used to establish a reference ventilation distribution and thresholds for six intensity bins. These bins were used to display and quantify the ventilation defect region (VDR), the low ventilation region (LVR), and the high ventilation region (HVR).

RESULTS

The ventilation distribution in young subjects was roughly Gaussian with a mean and standard deviation of 0.52 ± 0.18, resulting in VDR = 2.1 ± 1.3%, LVR = 15.6 ± 5.4%, and HVR = 17.4 ± 3.1%. Older healthy volunteers exhibited a significantly right-skewed distribution (0.46 ± 0.20, P = 0.034), resulting in significantly increased VDR (7.0 ± 4.8%, P = 0.008) and LVR (24.5 ± 11.5%, P = 0.025). In the asthmatics, VDR and LVR increased in the older population, and HVR was significantly reduced (13.5 ± 4.6% vs 18.9 ± 4.5%, P = 0.009). Quantitative Xe MRI also revealed altered ventilation heterogeneity in response to albuterol in two asthmatics with normal spirometry.

CONCLUSIONS

Quantitative Xe MRI provides a robust and objective means to display and quantify the pulmonary ventilation distribution, even in subjects who have airway function impairment not appreciated by spirometry.