Introduction to Chest X-ray Interpretation 2 Mediastinum and pleura RAH Radiology
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Introduction toChest X-ray Interpretation 2
Mediastinum and pleura
RAH Radiology
How to interpret CXR:
• Film quality
• Anatomic structures
• Basic patterns of disease
Warning:This is a big topic. Important phrases and concepts are bolded.
Anatomic structures:
• Radiologists use an “in-to-out” approach. Review the central structures first, the peripheral structures last.
• This approach best matches the patterns of pathology, which aids interpretation.
• For example, an enlarged heart suggests you should look for APO, but pneumonia is unlikely to affect the mediastinum.
It is up to you which approach to use, but we suggest:
Mediastinum -> Hilar regions -> Lungs -> Pleura -> Chest wall
Anatomic structures:
• Radiologists use an “in-to-out” approach. Review the central structures first, the peripheral structures last.
• This approach best matches the patterns of pathology, which aids interpretation.
The mediastinum and hilar regions are quite complex, but understanding the various shapes can be very useful.
Anatomic structures:
• Radiologists use an “in-to-out” approach. Review the central structures first, the peripheral structures last.
• This approach best matches the patterns of pathology, which aids interpretation.
The outer border of the aorta can often be appreciated. The root, arch, knuckle and descending portion can all be identified.
The inner aspect can be hard to see, but can often be subtly visualised.
The lateral margins of the great vessels of the neck are also noted.
Anatomic structures:
• Radiologists use an “in-to-out” approach. Review the central structures first, the peripheral structures last.
• This approach best matches the patterns of pathology, which aids interpretation.
The heart fills the lower mediastinum.
This should be less than 50% of the transverse dimension of the thoracic cavity (that is, from rib to rib at the widest part).
It is possible to identify specific regions of the heart on CXR.
We have already seen the aorta.The right atrium makes the right heart border.The inferior margin of the heart is obscured by the diaphragm, but is made by the right ventricle.
Anatomic structures:
• Radiologists use an “in-to-out” approach. Review the central structures first, the peripheral structures last.
• This approach best matches the patterns of pathology, which aids interpretation.
On the left, we can see the outer margin of the main pulmonary artery before it divides, just below the aortic knuckle.
The left heart border is formed by the left ventricle.
Where is the left atrium?
Anatomic structures:
• Radiologists use an “in-to-out” approach. Review the central structures first, the peripheral structures last.
• This approach best matches the patterns of pathology, which aids interpretation.
The left atrium is behind the heart, sitting directly below the carina (the bifurcation of the trachea).
It is important to remember the heart is three dimensional, and the axis is twisted. The anterior heart is mostly left ventricle, the inferior heart mostly right ventricle.
Anatomic structures:
• Radiologists use an “in-to-out” approach. Review the central structures first, the peripheral structures last.
• This approach best matches the patterns of pathology, which aids interpretation.
Anatomic structures:
• Radiologists use an “in-to-out” approach. Review the central structures first, the peripheral structures last.
• This approach best matches the patterns of pathology, which aids interpretation.
We can also appreciate the outer margins of the superior vena cava and inferior vena cava as the run into the right atrium.
All of these structures can be usefully assessed when the heart is abnormal.
This study shows cardiomegaly; the heart is greater than 50% of the thoracic cavity width.
Looking at this film, both the right and left heart borders are “bulging”, suggesting both left ventricular and right atrial enlargement. This could suggest severe aortic valve pathology or a generalised cardiomyopathy.
Click to see outline visible portions of the aorta, main pulmonary trunk, left ventricle, right atrium and left atrium.
The lateral chest xray also displays multiple cardiac structures.
The anterior border is formed by the right ventricle, and the posterior border by the left ventricle.
The aortic arch is demonstrated in profile. The inferior vena cava is seen crossing the diaphragm.
The trachea is well demonstrated, and divides at the carina. Between the carina and the left ventricle is the left atrium.
An enlarged heart on the lateral view can fill the retrosternal space or the retrocardiac space. The heart should be at least half a vertebral body width away from the spine.
The abnormal film demonstrates cardiomegaly.
The retrosternal space is filled; the anterior heart border reaches the mid sternum.
The retrocardiac space is filled; the posterior heart border reaches the spinal column.
This suggests biventricular enlargement. Multiple valvular pathologies or generalised cardiomyopathy should be considered.
The mediastinum can be divided up in several ways.
The superior and inferior mediastinum contain the great vessels and the heart respectively.
The boundary is called the thoracic plane, and is defined by the carina and the aortic arch.
This classification is not widely used or diagnostically very useful, because there are a lot of different organs and pathologies in the inferior mediastinum.
More useful is defining the anterior, middle and posterior mediastinum.
The middle mediastinum contains the heart, vessels and hilar regions.
The anterior mediastinum runs to the sternum, and the posterior mediastinum to the spine.
The superior mediastinum is most usefully thought to be part of the anterior mediastinum.
The anterior mediastinum contain the aortic root and the thymus.
The posterior mediastinum contains the oesophagus, descending aorta and paraspinal tissues.
These divisions are very useful diagnostically, because each area suggests a unique differential list.
Antero-superior: 5 T’s (see next slide)
Middle: Hilar lymph nodes, hiatus hernia, bronchogenic malignancy
Posterior: Oesophageal carcinoma (not seen on CXR), nerve root tumours, bone tumours, EMH
This anterior mediastinal mass is seen in the retrosternal space on the lateral view.
The differential includes the 5 T’s; thoracic aortic aneurysm, thymoma, teratoma, “terrible” lymphoma, and thyroid (retrosternal goitre or mass).
The pleural space is a potential space, surrounding the lungs.
It can fill with gas, fluid or cells. Because the space is continuous, fluid is normally seen in the dependent portion, and gas in the anti-dependent portion.
When a person is upright, the pleural space is lowest at the posterior costophrenic recesses. Small pleural collections will only be visible on a lateral film.
The anterior half of the left hemidiaphragm is obscured by the heart, but the right side reaches the sternum.
On a supine film, the dependant and antidependant areas are more difficult to understand, partially because we can’t get lateral views.
Fluid will pool at the back, and gas at the front.
This means the fluid is spread out over the supine frontal projection (“en face”).
We therefore see fluid as a general increase in density without obscuration of the lung markings (a “veiling opacity”) and gas as a general decrease in density.
This case demonstrates supine bilateral pleural effusions. There is low visibility of the hemi-diaphragms, which could be due to effusions or collapse / consolidation, but note the “veil” draped over the lower half of the lungs. This is posterior fluid.
Pneumothorax is gas in the pleural space (normally a negative space). The air can come from the lung or from the outside, with a pleural tear or penetrating injury respectively.
The two signs of pneumothorax are the visualisation of the pleural edge, and the lack of lung markings seen peripherally.
The pleural edge is unreliable in subtle cases, so it is worth carefully assessing the anti-dependant lung.
The pleural edge is unreliable in subtle cases, so it is worth carefully assessing the anti-dependant lung for the presence of lung markings to the edge.
In this example there is a moderate pneumothorax.
The pleural line is subtle, and partially hidden by ribs and an ECG dot.
Noticing the lack of lung markings at the apex is the most useful feature to identify pneumothorax.
When you do see a pneumothorax, always assess for mediastinal shift. This can be a sign of tension pneumothorax, which is a medical emergency.
Assessing shift can be difficult due to positioning and rotation, erring on the side of caution and over-calling possible tensioning is a safe approach.
Pneumomediastinum is more straight-forward than pneumothorax.
The important feature to look for is not the gas, but the displaced mediastinal connective tissue, which is much thicker than the visceral pleura.
Many juniors get confused with the optical illusion seen at the interface with the lung, which creates a dark halo around the heart. This is normal.
The gas lifts the mediastinum off the heart, so you see a stripe of tissue with gas underneath it.
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