Download Cardiac silhouette findings and mediastinal lines and stripes: X

Cardiac silhouette findings and mediastinal lines and
stripes: X-ray and computed tomography correlation
Poster No.:
C-222
Congress:
ECR 2009
Type:
Educational Exhibit
Topic:
Chest
Authors:
R. Marano, G. Savino, C. Liguori, A. Meduri, L. Natale, L. Bonomo;
Rome/IT
Keywords:
Digital Chest Radiography, Chest Computed Tomography,
Radiographical Mediastinum Anatomy
DOI:
10.1594/ecr2009/C-222
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Page 1 of 40
Learning objectives
Although the increasingly use of the Computed Tomography (CT), the Chest X-Ray
(CXR) remains a very important modality for the evaluation of parenchymal and
mediastinal disease providing a vast amount of useful informations. These informations
are derived from the relationship of the normal anatomic structures of the mediastinum,
pleura, and lungs and form the basis of the "cardiac silhouette" and "mediastinal lines
and stripes" concepts, which may play an important role in establishing a diagnosis
or a spectrum of diagnosis before proceeding to CT. The capability to recognize an
abnormal CXR on the base of displacement/distortion or suppression of one of these
lines is often mandatory to request a potentially useful CT examination. So, Radiologists
and in particular the trainees must be familiar with the anatomic basis of these findings
to be able to recognize their normal and abnormal appearance, developing appropriate
differential diagnosis prior a subsequent chest CT.
Background
Mediastinal Lines and Stripes:
The Mediastinal Lines correspond to the contours of the upper and middle mediastinum,
and represent the edges of a dense, pleural covered structure marginated by the air
within the lung. These lines are typically thin, with a thickness of 1 mm.
Lines present on CXR: Anterior & Posterior Junction Lines (Figure 1) on page
.
Page 2 of 40
Fig.
The Mediastinal Stripes are thicker lines formed by air outlining thicker intervening soft
tissue.
Stripes present on CXR: Right & Left Paratracheal Stripes, Aortic-pulmonary Stripe
(Figure 2) on page
.
Page 3 of 40
Fig.
The Interfaces are another concept of Mediastinal Lines and Stripes. They are formed
by the contiguity of two structures with different density.
Interfaces present on CXR: Right & Left Paraspinal Lines, Azygosesophageal Recess,
Paraortic line, Paracaval Line, Pararterial line, Aortopulmonary Window, and Cardiac
Borders (Figure 3) on page
.
Page 4 of 40
Fig.
The absence of one of the Mediastinal Lines or Stripes is sometimes insignificant, since it
may be caused by technical conditions of the CXR or anatomical variations and different
is the frequency of their visualization on the CXR (Figure 4) on page
.
Page 5 of 40
Fig.
Anyway, CT shows easier the anatomy and formation's mechanism of the Mediastinal
Lines and Stripes explaining the anatomical and pathological reasons for their possible
displacement, absence, or suppression.
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Images linked within the text of this section:
Fig.
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Fig.
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Fig.
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Fig.
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Imaging findings OR Procedure details
The Anterior Junction Line
It is formed by the apposition of the visceral and parietal pleura of the antero-medial
portion of the anterior segments of the upper lobes, with a small amount of intervening
anterior mediastinal fat. It normally shows an oblique course from upper right to lower left
crossing the superior two-thirds of the sternum (Figure 5) on page
.
Fig.
Normally, it appears as a thin line, but sometimes it may appear as a stripe due to an
increased amounts of intervening mediastinal fat (Figure 6) on page
or possibly
thymus in younger patients.
Page 11 of 40
Fig.
On the frontal CXR, the line opens superiorly along the innominate veins and it is
not typically appreciable above the clavicles, while inferiorly it ends at the level of the
pericardium. The anterior junction line may be seen on 24.5%-57% of frontal CXR. The
heart, great vessels, sternum, or the thoracic spine can obscure the line on the frontal
CXR. The line can be also absent if its course is not tangential to the X-ray beam. Being
located in the anterior mediastinum, its deformation or obliteration can be due to the
presence of anterior mediastinal disease (thyroid masses, lymphadenopathy, neoplasms,
thymic masses, or lipomatosis). On the other hand, lung volume loss or hyperinflation
can also displace the line.
The Posterior Junction Line
It is formed by the apposition of the visceral and parietal pleura of the postero-medial
rd
th
portion of the upper lobes posterior to the esophagus and anterior to 3 -5 thoracic
vertebrae. On the frontal CXR it normally appears as a thin line, typically projecting
through the trachea, with a vertical course that superiorly opens along the pleural dome
being so appreciable above the clavicles, while inferiorly it ends over the aortic and
azygos vein arches (Figure 7) on page
.
Page 12 of 40
Fig.
Sometimes, it may also appear as a stripe with varying amounts of intervening
posterior mediastinal fat. The posterior junction line may be seen on 32% of P-A CXR.
Deformation of this line may be due to posterior mediastinum disease (esophageal
masses,lymphoadenopaties, aortic disease, or neurogenic tumors). Similarly to the
anterior junction line, lung volume loss or hyperinflation can also displace the line.
The Right Paratracheal Stripe
It is formed by contact between the right upper lobe and the right lateral wall of the trachea
with the presence of the intervening mediastinal fat. This stripe has generally a normal
thickness of 4-mm, and begins superiorly at the level of the clavicles and extends inferiorly
to the right tracheo-bronchial angle at the level of the arch of the azygos vein (Figure
8) on page
.
Page 13 of 40
Fig.
It is the most commonly seen mediastinal line or stripe with a visualization frequency
on the frontal CXR of 83-97%. Abnormal contour or widening of this stripe could be due
to lipoma (Figure 9) on page
, lymphoadenopathies (Figure 10) on page
,
thyroid or parathyroid neoplasms, and tracheal carcinoma or stenosis. Pleural effusion
or thickening can also determine widening of the stripe.
Page 14 of 40
Fig.
Fig.
The Left Paratracheal Stripe
It is formed by contact between the left upper lobe and the left lateral wall of the trachea
with the presence of the intervening mediastinal fat, extending superiorly from the aortic
arch to join with the reflection from the left subclavian artery (Figure 11) on page
.
Page 15 of 40
Fig.
This stripe is seen less frequently than the right stripe, being visible on 21%-31% of
the frontal CXR, and may be obscured by contact between the left lung and either the
proximal left common carotid artery anteriorly or the left subclavian artery posteriorly.
Deformation of the stripe can be due to lymphadenopathy, neoplasms, or also mediastinal
hematoma Abnormal contour or widening is commonly seen in large left-sided pleural
effusion.
The Aortic Pulmonary Stripe
It represents the interface formed by the pleura of the anterior segment of the left upper
lobe coming in contact with and tangentially reflecting over the mediastinal fat anterolateral to the main pulmonary trunk/left pulmonary artery and aortic arch. Normally, the
stripe is straight or slightly convex (Figure 12) on page
.
Page 16 of 40
Fig.
Its normal appearance may be altered by anterior mediastinal disease such as thyroid,
thymic masses or prevascular lymphadenopathy often causing increased convexity
laterally.
The Aortopulmonary Window
It represents a mediastinal space seen as an interface on frontal CXR, lying posterior to
the aortic-pulmonary stripe (Figure 13) on page
.
Page 17 of 40
Fig.
This space is limited cranially by the inferior wall of the aortic arch, inferiorly by the
superior wall of the left pulmonary artery, anteriorly by the posterior wall of the ascending
thoracic aorta, and posteriorly by the anterior wall of the descending thoracic aorta. The
left tracheal wall, anteriorly and the anterior wall of the left main bronchus posteriorly
form the medial border. The contact between the left lung and aortic arch form the lateral
border, extended inferiorly to contact the left pulmonary artery with a slight concave
interface. A straight contour of the AP window may be normal, but is abnormal when
previous CXR have shown the normal concave contour. The lost of normal concave
aspect with a convexity is considerate abnormal and related to different mediastinal
diseases (prominent mediastinal fat, lymphonodes, bronchial artery aneurysms, nerve
sheath tumors, or bronchopulmonary-foregut malformations) (Figure 14) on page
.
Page 18 of 40
Fig.
Paralysis of the left vocal cord or diaphragm could suggest a search for disease in the
AP window. Disease in structures that form the borders of the AP window (eg, aortic
aneurysms) (Figure 15) on page
can also determine abnormal appearance of the
window.
Fig.
Page 19 of 40
The Right Paraspinal Line
It is formed by the tangential contact between the right lung and pleura with the posterior
mediastinal soft tissues (Figure 16) on page
.
Fig.
Despite its name, it is not a true line, but an interface between the lung and the posterior
th
th
mediastinal fat and soft tissues. It is normally straight, along the 8 -12 thoracic vertebral
body and appreciated on 23% of frontal CXR. A lateral displaced of the line can be due
to osteophytes or prominent mediastinal fat, but it can suggest a posterior mediastinal
abnormality (mediastinal hematoma, mass, or extramedullary hematopoiesis).
The Left Paraspinal Line
It is formed by tangential contact of the left lung and pleura with the posterior mediastinal
fat and adjacent soft tissues, extending vertically from the aortic arch to the diaphragm
medially to the lateral wall of the descending thoracic aorta (paraortic line) (Figure 17)
on page
.
Page 20 of 40
Fig.
Sometimes, it may lie lateral to the paraortic line along the lower descending thoracic
aorta. It occurs on 41% of frontal CXR, more frequently than the right paraspinal line
due to the presence of the descending thoracic aorta on the left, which endorse the
tangential contact of lung-mediastinum interface. Similarly to the right, the abnormal
contour of the line can be due to osteophytes or prominent mediastinal fat while a
displacement may suggest posterior mediastinal abnormalities (mediastinal hematoma,
mass, extramedullary hematopoiesis, or esophageal varices).
The Azygoesophageal Recess
It is not a typical mediastinal line or stripe, being an interface caused by differences in
density between the mediastinum and the postero-medial portion of the right lower lobe. It
is a space within the mediastinum lying lateral or posterior to the intra-thoracic esophagus
and anterior to the spine, extending from the level of the arch of the azygos vein to the
aortic hiatus and right diaphragm inferiorly (Figure 18) on page
,
Page 21 of 40
Fig.
being bordered anteriorly and medially by esophagus, the azygos vein, and left atrium.
In its upper third the recess is continuous with the subcarinal space and may either be
straight or concave relative to the right lung; a right superior convexity may be seen
in children and younger adults but is abnormal in the elderly. In its middle third, the
recess is usually straight, but may be slightly convex relative to the right lung near the
right pulmonary veins or shows mild leftward convexity, while the lower third is usually
straight. Abnormal contour/convexity may be due to different conditions affecting the
middle/posterior mediastinal compartments: lymphonodes, bronchopulmonary-foregut
malformations, esophageal neoplasms, pleural abnormalities, left atrial enlargement,
hiatal hernias (Figure 19) on page
or gastroesophageal varices (Figure 20). on
page
Page 22 of 40
Fig.
Page 23 of 40
Fig.
The Cardiac Silhouette Sign
When examining the lung fields of a normal frontal CXR, the silhouettes of the
heart borders, the ascending and descending thoracic aorta, the aortic knob and the
hemidiaphragms should be clear. Borders, outlines and edges seen on plain CXR depend
on the presence of two adjacent areas of different density (soft tissue and air). More or
less, only 4 natural different densities are detectable on plain films: air, fat, soft tissue
and calcium. If two soft tissue densities lie adjacent, they will not be visible separately. If,
however, they are separated by air, the borders of both will be seen. Obliteration of any
of these silhouettes can be caused by different condition (pneumonia, pleural effusion,
neoplasms, collapse, etc.) and is known as the 'silhouette sign'. All of these structures
are in contact with a specific portion of the lung, therefore by determining exactly which
structure is obliterated, it is possible to determine the side of the pathology (Figure 21)
on page
.
Page 24 of 40
Fig.
This is a useful sign for detecting/localizing abnormalities in the P-A CXR without a lateral
view: if an opacity were present in the left hemitorax overlapping the left ventricle with
the LV edge still present, the lesion lies posterior (absence of the silhouette sign)(Figure
22) on page
.
Page 25 of 40
Fig.
Otherwise, if the right heart border were lost due to the presence of an opacity in the right
cardio-frenic angle, this finding lies anterior (presence of the silhouette sign) (Figure 23)
on page
.
Page 26 of 40
Fig.
Anyway, if the right heart border were still present the opacity in the right cardio-frenic
could have a lower density (Figure 24) on page
.
Fig.
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Images linked within the text of this section:
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Conclusion
Despite the increased dependence on CT in the evaluation of chest disease, traditional
chest radiography remains a precious tool in the routinely setting. Radiologists and in
particular the trainees must be familiar with the anatomic basis of the mediastinal lines,
mediastinal stripes, and cardiac silhouette seen on CXR and able to recognize their
normal and abnormal appearances to develop an suitable differential diagnosis prior to
get additional information with chest CT.
Personal Information
Fig.
References
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