CT scan and MRI:
The classic tell-tale sign of the LPA wrapping around the back of the trachea is easily seen on contrasted CT (figure 5, 6 ) or MRI; however these image modalities play  a more significant role in the assessment and extent of airway stenosis. MRI provides a good definition of vascular and intracardiac anatomy which takes much longer which is often an issue in an unstable patient. Multidetector CT is much quicker and demonstrates the position and extent of tracheobronchial compression, anatomy of the pulmonary arteries and the spatial relationships among the pulmonary arteries, trachea, bronchi and oesophagus. This is important for surgical planning.  Inspiratory and expiratory phase CT scans are also useful for the detection of air trapping and fix tracheal stenosis with a diameter of the trachea remaining the same in both respiratory phases.31 Complete rings are not visible on CT scan; however there may be strong suspicion if the trachea is round and not D- shaped with no surrounding air around the endotracheal tube beyond the subglottis (Figue 7). Tracheobronchial airway abnormalities may be quite complex especially with Type II LPAS where there may be a bridging bronchus or blind ending diverticula with lung agenesis. There is a suggestion that CT findings may determine operability and that a conservative approach may be feasible if the trachea calibre is more than 40% compared to reference values.32-34 CT may overestimate the degree of obstruction in the presence of mucus plugs or where diameters are only 1 to 2 mm.35 Bronchoscopy and bronchograms may be very useful in this setting to clearly delineate the extent of the stenosis(figure 8).7,35,36