Rationale and Objectives
Ventilation heterogeneity is impossible to detect with spirometry. Alternatively, pulmonary ventilation can be imaged three-dimensionally using inhaled 129 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 129 Xe MRI scans such that the underlying ventilation distribution and its heterogeneity can be quantified.
Materials and Methods
Quantitative 129 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 129 Xe MRI also revealed altered ventilation heterogeneity in response to albuterol in two asthmatics with normal spirometry.
Conclusions
Quantitative 129 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.
Introduction
The distribution of ventilation is known to be nonuniform in healthy lungs , and this heterogeneity increases with age and disease. Ventilation heterogeneity is impossible to quantify using spirometry because it measures the lung as a single unit and is insensitive to pathology in the small airways—the so-called silent zone. Alternative approaches include using the multiple-breath nitrogen washout (MBNW) test to determine the distribution of specific ventilation (SV) , the lung clearance index (LCI) , or the multiple inert gas elimination technique (MIGET) to quantify the ventilation–perfusion relationship ; however, none of these tests provides spatial information. Alternatively, imaging methods such as computed tomography (CT) delineate spatial changes in lung structures that may allow ventilation abnormalities to be inferred. However, CT does not directly measure ventilation and its radiation dose limits some longitudinal studies.
Recently, magnetic resonance imaging (MRI) techniques have emerged that enable direct detection of inhaled gases, such as oxygen , perfluorinated gases , and hyperpolarized (HP) 3 He . These techniques enable visualization of ventilation defects that have been shown to correlate with airway tone and airway abnormalities . HP 3 He MRI readily depicts regional ventilation heterogeneity in patients with pulmonary obstructive diseases . More recently, 129 Xe gas has emerged as the most promising alternative to address dwindling supplies of 3 He . 129 Xe MRI appears to more readily detect ventilation defects than 3 He MRI and has been used to visualize elimination of ventilation defects after bronchodilator administration .
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Methods
Subjects
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Image Acquisition
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Image Analysis
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Statistical Methods
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Results
Study Population
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Distribution of Ventilation in Young Healthy Subjects
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Effects of Age
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Ventilation Distribution in Older and Younger Asthmatics
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Differences between FEV1 and 129 Xe MRI Ventilation Distribution
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Ventilation Distribution Before and After Albuterol Treatment
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Discussion
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