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