Fig. 1.

Illustration of ground truth segmentation. Full lung masks were segmented on ¹H MRI and subsequently applied to ³He MRI. To determine appropriate thresholds for full lung masks and ventilation segmentations, ROIs were manually drawn inside the lungs on ¹H MRI, and inside the central airways and the heart on ³He MRI. SD = standard deviation; SI = signal intensity; ROI = region of interest. Colour coding: Orange = central airway ROI; blue = heart ROI; red = non-ventilated lung regions; yellow = hypo-ventilated lung regions; green = normal-ventilated lung regions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)