2. INTERLOBAR FISSURES
Invaginations of the visceral pleura that separate or partially
separate the lobes of the lung
The Major (Oblique) Fissure
The major fissures originate posteriorly, above the level of the aortic
arch, near the level of the fifth thoracic vertebra, and angle
anteriorly and inferiorly, nearly parallel to the sixth rib.
They terminate along the anterior diaphragmatic pleural surface of
each hemithorax, several cm posterior to the anterior chest wall.
3. On CT, the major fissures are oriented obliquely to the scan plane,
and because of volume averaging, their appearance is variable and
depends on slice thickness.
Thick slices- An avascular band, several centimeters thick, within the
lung parenchyma.
Thin slices- Thin, well-defined lines surrounded by a plane of
relatively avascular lung measuring about 1 cm thick.
Right major fissure is incomplete in 70%.
Left major fissure is incomplete in 40% to 70%.
4. Double fissure sign- The fissure is visible in two locations
on the same scan separated by a few mm - cardiac motion
during the scan results in an artifact
5. The Minor (Horizontal) Fissure
It separates the superior aspect of the right middle lobe from
the right upper lobe.
The minor fissure is incomplete in more than 80% of cases
On a frontal radiograph- roughly horizontal line, generally at
or near the level of the anterior fourth rib.
6. HRCT
A linear opacity, directed from anterior to posterior
A linear opacity extending from medial to lateral, paralleling and
anterior to the major fissure
A circle or ring
An ill-defined opacity or avascular region resulting from the
fissure lying in the plane of scan
7. Azygos fissure
Defines the presence of an azygos lobe- apical or posterior segments
of the right upper lobe.
when the azygos vein invaginates the right upper lobe during
gestation.
It consists of four layers of pleura (two parietal and two visceral) and
contains the arch of the azygos vein.
Left “azygos” fissure- with the left superior intercostal vein
8. CXR
Fissure- a characteristic curvilinear appearance adjacent to the right
mediastinum, convex laterally.
Azygos vein- teardrop appearance at the inferior extent of the fissure.
CT
Thin, curved line extending from the right brachiocephalic vein,
anteriorly, to a position adjacent to the right
posterolateral aspect of the T4 or T5 vertebral body
12. Inferior accessory fissure
Separates the medial basal segment of either lower lobe from the
remaining basal segments.
Seen in 30% to 45% cases.
CXR - extends superiorly and medially from the medial third of the
hemidiaphragm.
CT- extends laterally and anteriorly from the region of the inferior
pulmonary ligament to join the major fissure.
14. Superior accessory fissure
Demarcates the superior segment from the remainder of the
lower lobe.
More common in Right side.
Left minor fissure
Separates the lingula from the remainder of the left upper
lobe.
Seen in 10% of population.
16. Inferior pulmonary ligaments each represent a double layer of
visceral pleura that anchors the lung to the mediastinum.
Extend inferiorly and posteriorly from just below the pulmonary hila
to the diaphragm.
The ligaments may also contain systemic vessels supplying the lung.
They may divide the medial pleural space below the hila into anterior
and posterior compartments.
CXR- Invisible.
17. CT
Very short, thin, linear or triangular opacities below the level
of the right and left inferior pulmonary veins and often
adjacent to the esophagus.
Visible in 40% to 70% of cases.
Often appear contiguous with intersublobar septum - A
connective tissue septum in the medial lung, termed the
18. Inferior pulmonary ligament-
triangular opacity lateral to the
esophagus
Intersublobar septum
Thin triangular opacity
arising in the region of
the esophagus
21. Intercostal stripe
1- to 2-mm-thick soft tissue stripe is visible in the
anterolateral and posterolateral intercostal spaces, representing
the innermost intercostal muscle but also combined
thicknesses of visceral and parietal pleura, the fluid-filled
pleural space, the endothoracic fascia, and fat layers.
22. Intercostal stripe is visible as a thin white line
In the paravertebral regions, the innermost intercostal
muscle is anatomically absent
23. Paravertebral line
In the paravertebral regions, the innermost intercostal muscle
is anatomically absent.
Represents the combined thicknesses of the visceral and
parietal pleura and the endothoracic fascia
24. Parietal Pleural Thickening
CXR
1. Blunting of the lateral or posterior costophrenic angle*
2. Soft tissue density stripe, separating the lung from the adjacent ribs
and chest wall
3. Thickened soft tissue visible internal to the ribs in patients with
pneumothorax
4. Pleural calcification
5. Low attenuating asymmetrical increase in extrapleural fat
25. Thickened pleura separating aerated lung from the
adjacent ribs with Pleural calcification
Parietal pleural thickening and calcification in asbestos exposure
26. Parietal Pleural Thickening
CT
1. Soft tissue density stripe internal to the ribs > 1 mm
2. Soft tissue density stripe internal to the innermost intercostal muscle
and separated from it by a thin layer of extrapleural fat >1 mm
3. Soft tissue density stripe in paravertebral region > 1 mm
4. Pleural calcification
5. Thickening of the normal extrapleural fat layer
27. Soft tissue density stripe in
the paravertebral regions
Thickened parietal pleura
internal to the ribs &
intercostal stripe
28. Calcified pleura on the surface of the
hemidiaphragms
Parietal pleural thickening and calcification in asbestos exposure
29. Visceral Pleural Thickening
CXR
1. Thickening of a fissure
2. Separation of the lung from the adjacent ribs and chest wall, occurring
in association with contiguous lung disease
3. Thick pleura at the lung surface in patients with pneumothorax
CT
1. Soft tissue density stripe at the lung surface in patients with pleural
effusion & normal lung parenchyma
2. An enhancing stripe at the lung surface in patients with abnormal lung
30. Normal Fat Pads
Most abundant over the posterolateral fourth to eighth ribs
CXR –
Soft tissue stripe passing internal to the ribs, separating the lung from
the chest wall.
Symmetrical and smooth in contour, appears low in attenuation,
and is unassociated with costophrenic angle blunting.
31. The Subcostalis and Transversus Thoracis Muscles
CXR – Invisible.
CT
Subcostalis muscle- 1- to 2-mm-thick line seen internal to one or
more ribs, along the posterior chest wall at the level of the heart.
Transversus thoracis muscles- Anteriorly, at the level of the heart
and adjacent to the lower sternum or xiphoid process, internal to the
anterior ends of ribs or costal cartilages.
Smooth, uniform in thickness & symmetric bilaterally.
32. Paravertebral Intercostal Veins
CXR – Invisible.
CT
Continuity of these opacities with the azygos or hemiazygos
veins is sometimes visible.
Lung Window - intercostal vein segments do not indent
the lung surface.
34. Plain Radiographs
In upright subjects, pleural fluid first accumulates in costophrenic
angles and subpulmonic regions.
Costophrenic angle blunting is usually the first finding
Blunting of the lateral costophrenic angle on the frontal view requires
175 mL
Blunting of the posterior costophrenic angle on the lateral view
requires 75 mL
On lateral decubitus film, as little as 10 mL of fluid may be seen.
35. Elevation of the lung base by a
subpulmonic collection &
blunting of the posterior
Costophrenic angle
Blunting of the right
costophrenic angle
Thorn sign
37. Plain Radiographs
With increasing effusion, fluid may be seen lateral to the lower lobes.
Medial pleural fluid collections are usually smaller - lung is fixed
medially by the hilum and inferior pulmonary ligament.
Larger effusions result in a typical meniscus appearance.
38. Plain Radiographs
Supine Patients
A. Increased density of a hemithorax due to layering of the effusion
posteriorly
B. Blunting of the lateral costophrenic angle
C. Obscuration of the hemidiaphragm
D. Thickening of the paravertebral stripe- accumulation of fluid in the
posterior gutters.
39. Frontal View
Lateral displacement of diaphragmatic dome or peak, with the lung lateral
to the peak angling down sharply
2 cm of separation of the lung base from the top of the stomach bubble.
Lateral view
Elevates the posterior lung, flattening its undersurface
Insinuates itself into the major fissure, resulting in focal triangular
thickening of the fissure
Flattens the inferior contour of the lung anterior to the fissure, which then
angles sharply downward.
41. Thorn Sign
Fluid extending into the lateral aspect of the minor fissure and
slightly separating the lobes often results in an opacity
resembling a rose thorn.
42. Large right pleural effusion with costophrenic angle
blunting and a thorn sign
43. Unenhanced CT- Crescent shaped low attenuation
CECT- Both airless lung and thickened pleura enhance, and
this difference is accentuated.
The thickness of a free pleural effusion usually decreases in
less dependent parts of the thorax, anteriorly and superiorly.
Atelectasis is common in patients with large effusions, and
atelectatic lung may be seen floating within the fluid.
44. A large effusion with collapsed lower lobe
Pleural effusion with liver disease
45. Displaced crus sign
Pleural fluid collections in the posterior costophrenic angle lie medial
and posterior to the diaphragm and cause lateral displacement of the
crus.
Peritoneal fluid collections are anterior and lateral to the diaphragm
Interface sign
The interface of the pleural fluid with the liver and spleen is hazy.
In Acsites, it is sharp.
46. Bare Area Sign
Fluid seen posterior to the liver is within the pleural space; the peritoneal
space does not extend into this region.
Diaphragm Sign
In patients with both pleural and peritoneal fluid, the diaphragm can be seen
as a uniform, curvilinear structure of muscle density with relatively low
density fluid both anterior and posterior to it.
Pseudodiaphragm
On CT, the posterior edge of the lower lobe, when surrounded by fluid both
anteriorly and posteriorly, can appear to represent the diaphragm
47. Abscess cavity in a subphrenic
location.
Costophrenic angle
blunting due to right
pleural effusion & an air-
fluid level due to
subphrenic abscess
Pseudodiaphragm with pleural effusion and lower
lobe collapse
48. Pleural fluid collections limited in extent by pleural adhesions.
They often occur in association with exudative pleural
effusions.
Often they are elliptical or lenticular in shape.
Radiographic Findings
Varies with its location and the radiographic projection.
Sharply marginated when its surface is parallel to the x-ray
beam and ill defined when viewed en face.
50. Phantom Tumor Or Pseudotumor
Localized fluid collections in the fissures may be loculated and
mimic the presence of a focal lung lesion.
Typically, fluid collections in a fissure are rounded or lenticular and
may show a tapering triangular opacity or “beak” at the point at
which they merge with the fissure itself
Fluid collections in the minor fissure often appear sharply
marginated and lenticular on both PA and lateral radiographs.
Collections in the major fissure may be ill defined on the frontal
view.
51. Round collection
localized to the
major fissure
Pseudotumor in congestive heart failure
Lenticular collecton
with a beak at its
junction with the
major fissure
52. CT Findings
A. Localized collection
B. An elliptical or lenticular shape (rather than crescentic)
C. Nondependent location
Loculated effusions are often associated with pleural thickening,
best seen with contrast enhancement
Split Pleura Sign - Both the visceral and parietal pleural surfaces
are thickened, embracing the fluid collection
54. Lenticular fluid collection
within the left major fissure
& loculated collection in the
periphery
Rounded collection
within the fissure with
a beak visible medially.
55. Split Pleura Sign
Loculated, lenticular fluid collection at the right base, associated
with thickening of both the parietal & visceral pleural layers
57. Empyema containing air due to thoracentesis
The parietal pleura is thickened
DD for air-containing fluid collection also includes
BPF and gas-forming organisms.
58. Most pleural effusions are classified as exudates or transudates
based on their composition.
This distinction is usually made at thoracentesis.
Other specific causes of pleura effusion are chylothorax and
hemothorax.
59. Reflects the presence of a pleural abnormality associated with
increased permeability of pleural capillaries.
Exudative effusions have a high protein content.
1. A ratio of pleural fluid protein to serum protein higher than 0.5
2. A ratio of pleural fluid lactate dehydrogenase (LDH) to serum LDH
that exceeds 0.6
3. A pleural fluid LDH level more than two thirds the upper limits of
normal for serum.
60.
61. It usually results from systemic abnormalities causing an
imbalance in the hydrostatic and osmotic forces governing
pleural fluid formation.
It is low in protein.
62.
63. The presence of thickened parietal pleura in association with a
pleural effusion suggests that the fluid collection is an exudate.
If pleural thickening is not visible on CT, an associated effusion may
be an exudate or a transudate.
On USG, the presence of septation, complex nonseptation, or
homogeneous echogenicity suggest exudate.
Anechoic effusions may be either transudative or exudative.
64. It contains intestinal lymph, which is high in protein and fatty
acid, and is low in cholesterol & appears milky.
It results from disruption of the thoracic duct or thoracic lymphatic
obstruction by tumor.
The thoracic duct originates at the cisterna chyli in the upper
abdomen and enters the thorax along the right anterior aspect of the
spine; it crosses to the left near the level of T6, lying along the left
lateral wall of the esophagus, posterior to the descending aorta, and
drains into the left brachiocephalic or subclavian vein.
65. The thoracic duct originates at the cisterna chyli in the upper
abdomen and enters the thorax along the right anterior aspect of the
spine; it crosses to the left near the level of T6, lying along the left
lateral wall of the esophagus, posterior to the descending aorta, and
drains into the left brachiocephalic or subclavian vein.
Chylous effusions may be small or massive and unilateral or bilateral.
69. Pleural fluid collection having a hematocrit over 50% of blood
hematocrit.
On CT, high attenuating fluid (> +50HU), with dense clot or a fluid-
fluid level and the densest fluid being in the dependent portion of the
pleural space.
Causes – Traumatic, rupture of an aneurysm or dissection, pulmonary
or pleural neoplasm, pneumothorax, coagulopathy, rupture of a
pulmonary arteriovenous malformation & pulmonary or pleural
endometriosis.
