Learning Radiology - atelectasis, left, lower, lobe, lll

Atelectasis of the Left Lower Lobe

General Considerations

All types of atelectasis involve loss of volume in some or
all of a lung with resultant increased density of the involved lung
The atelectasis referred to here is that caused by bronchial obstruction, usually a tumor (i.e. a bronchogenic carcinoma), a foreign body or a mucus plug

Signs of atelectasis

Increase in density of the affected lung
Displacement of the fissures or the mediastinum towards the atelectasis
Crowding of the vessels and bronchial tree in the area of
volume loss
Elevation of the hemidiaphragm
Overaeration of the opposite lung

Lower Lobe Atelectasis

Lower lobe is usually tethered to the hemidiaphragm and mediastinum by the inferior pulmonary ligament (IPL)
- The IPL attaches the lower lobe to the mediastinum
  and may be seen normally on CT scans of the chest

The inferior pulmonary ligament is actually not a
ligament but reflections of the pleura (visceral and parietal) inferior to the hilum extending downward to the
diaphragmatic pleura

It is on the medial border of the left lower lobe
There is no superior pulmonary ligament as a
counterpart

With collapse, there is rotation of the major fissure
posteriorly and medially

Imaging signs of Left Lower Lobe Atelectasis

Wedge-like density behind heart with base of triangle on the left hemidiaphragm and the apex at the hilum on frontal view
Silhouetting of all or just the medial portion of the left hemidiaphragm
Posterior displacement of the major fissure on the lateral
view
Hilum may be displaced inferiorly
Descending branch of the pulmonary artery may disappear
There may be a shift of the heart towards the left which
means less of the right heart border or none of the right heart
border may project to the right of the spine
There may be a a spine sign due to superimposition of
the non-aerated lower lobe and the spine on the
lateral view
In severe atelectasis of the lower lobe, the atelectatic lobe
is so small it may not be recognizable on a conventional
radiograph
- The hemidiaphragm may be seen throughout its
  length. i.e. there is no silhouette sign

The normal contour at the top left of the aortic knob may become invisible

Appearance of left lower lobe atelectasis is similar to right
lower lobe atelectasis

Associations

Following cardiac surgery, especially in which there is topical hypothermia of the heart and phrenic nerve

Left lower lobe atelectasis. The blue arrows point to the edge of a triangular region of increased density
in the left cardiophrenic sulcus representing the major fissure with the atelectatic left lower
lobe medial to it. The red arrows on the axial CT of another patient show the atelectatic left lower
lobe bounded by the displaced major fissure.
For this same photo without arrows, click here

Left lower lobe atelectasis. The frontal view demonstrates a sharp edge which is the major fissure and a triangular density at the left base behind the heart (white arrow). On the lateral view, the area of increased density is seen posteriorly (red arrow) while only the right hemidiaphragm is visualized since the left is being silhouetted by the atelectasis.

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