Description of metrics calculated from helium-3 (3He) and 1H magnetic resonance imaging (MRI)
| Thoracic cavity volume (TCV) | Calculated from the segmentation of the 1H anatomical MRI. The TCV is the lung volume at which 3He-MRI is performed and is measured in litres. |
| Ventilated volume (VV) | Calculated from 3He ventilation image segmentation. VV represents the volume of ventilated lung and is measured in litres. |
| Ventilation defect percentage (VDP) | The percentage of the TCV that is not ventilated in the 3He MRI images. Areas of the 3He image that contribute to VDP appear black. It is calculated as VDP=100−((VV/TCV)×100). Larger VDP values are associated with increased lung disease. |
| Ventilation heterogeneity index (VHI) | A marker of the heterogeneity of the 3He signal within ventilated regions of the 3He MRI images. For each ventilated pixel, a local coefficient of variation of signal intensity in the surrounding pixels is computed. VHI is the interquartile range of the distribution of those values. Increased VHI is associated with increased ventilation heterogeneity and therefore increased lung disease. |
| Number of ventilation defects (ndefects) | The number of individual three-dimensional contiguous ventilation defects within the subject's lung. This only includes unventilated lung areas contributing to VDP. Defects were counted if the defect volume was >1% of total VDP. |
| Largest ventilation defect | This is the volume of the largest contiguous ventilation defect within the lungs. It is measured in litres and as a percentage of the TCV. |
| Reversible-volume index | This represents the relative change that occurs in VV in response to the increase in TCV when comparing EIVT to TLC images. The reversible-volume index is ≥1.0; the larger the value above 1.0 the greater the degree of EIVT ventilation defects that have resolved at TLC. In a healthy subject's lungs or in the lungs of a patient with nonreversible ventilation defects (resulting from complete obstruction), an increase in TCV due to deep inhalation will result in equal increase in VV and the reversible-volume index will be 1. In contrast, any ventilation defect present at EIVT that at least partially resolves at TLC will produce a reversible-volume index >1 (online supplementary figure S3). |
EIVT: end-inspiratory tidal volume.