Frontal CXR—lung markings

Normal lung markings are solely due to vessels and to nothing else. The normal bronchial walls, normal interstitium, and normal lymphatics are not visible.

There is a complete absence of any lung markings in the lung immediately adjacent to each costophrenic angle.

In this peripheral part of the lung the normal vessels are simply too small to be visualised.

If this area shows detectable markings, then interstitial disease, or oedema, is likely¹.

Frontal CXR—hila

The hilum is the site at which the bronchi and vessels enter or leave the lung.

The two hila are approximately the same size and of very similar density.

The hilar shadows are due to pulmonary arteries and pulmonary veins. X marks the main pulmonary trunk.

Blue = pulmonary trunk and pulmonary arteries;

Red = pulmonary veins;

Lilac = part of left atrium.

Frontal CXR—cardiothoracic ratio

Most normal hearts have a cardiothoracic ratio (CTR) of less than 50% when assessed on a PA chest radiograph obtained in full inspiration².

Measuring the CTR.

Two lines are drawn tangential to the outermost aspects of the right and left sides of the heart and the maximum cardiac width is measured.

The transverse diameter of the thorax is measured as the maximum internal width of the thoracic cage (ie inner cortex of rib to inner cortex of rib).

a/b < 50%.

Lateral CXR—the lobes of the lung

The two fissures in the right lung divide the lung into three lobes.

The single fissure in the left lung divides the lung into two lobes.

The oblique fissure on each side is propeller shaped and consequently we only, and very occasionally, see a part of a fissure on a normal lateral CXR.

On the other hand, we will see most of the horizontal fissure on the lateral CXR because it is straight. We will often see this fissure on the frontal CXR for the same reason.

Lateral CXR—skeletal shadows

The scapula shadows (blue) are usually evident … but rarely create any difficulties.

The oblique shadows of the ribs are always evident.

The vertebrae are always seen.

Lateral CXR—three normal appearances

A normal lateral CXR will show three normal features relating to the two lungs (which project one over the other):

(1) The vertebral bodies gradually appear blacker as you look from T3 down to T12 …“grey at top; black at bottom”.

(2) Both domes of the diaphragm are clearly visualised. Note that the anterior aspect of one dome (the left dome) fades away because the heart is positioned on this part of the dome.

(3) There is no abrupt change in density over the cardiac shadow. (Of course, any overlying rib shadows need to be discounted).

The lateral CXR

Four questions underpin accurate analysis:

Normal lateral CXR.

▪ Vertebral bodies are “blacker” at the T9–T12 levels. Remember the rule: “grey at top; black at bottom”.

▪ Both domes of the diaphragm are sharply defined.

▪ No abrupt change in density over the cardiac shadow.

Female patient with cough and fever of recent onset. The lateral CXR is abnormal: (1) The vertebral bodies appear whiter at the T9–T12 level; (2) only one dome of the diaphragm is sharply defined. Conclusion: lower lobe pneumonia. The frontal CXR confirms that the pneumonia is situated behind the heart in the left lower lobe.

Patient with a fever and right sided chest pain. The lateral CXR is abnormal. The main finding is that there is an “abrupt change in density over the cardiac shadow”. This density is white, extends inferiorly from the hilum, and has sharp margins superiorly and inferiorly. The density represents collapse within the middle lobe of the right lung. This is shown in the explanatory drawing. (The dotted line indicates the position of the oblique fissure in the right lung.)

Question 1: Is there pneumonia (consolidation)?

Physical examination is not always sufficiently accurate on its own to confirm or exclude the diagnosis of pneumonia³. Many pneumonias will be obvious on the frontal CXR. Some are much more difficult to detect—the hidden pneumonias.

Detecting a hidden pneumonia

▪ Search for any evidence of a silhouette sign^1,2,4 on the CXR.

▪ Assess the mediastinal and diaphragmatic boundaries/borders.

▪ When a border is ill-defined, the precise site of a concealed area of consolidation is revealed. Sometimes the loss of a border is much more obvious than the actual air space density that occurs with a pneumonia.

The silhouette sign. An intrathoracic lesion or density touching a border of the heart, aorta, or diaphragm will obliterate part of the border on the CXR^1,2.

Explanation: the borders of the heart and both domes of the diaphragm are visible on a normal CXR because the air in the lung contrasts with the water density of the heart and diaphragm. If lung air is replaced by pus (pneumonia) the immediately adjacent border will disappear or be ill-defined. This obliteration is termed the silhouette sign.

What does the word “consolidation” mean?

When lung alveoli fill with fluid (pus, water, or blood) there will be a shadow on the radiograph. Although the word “consolidation” is often used synonymously to imply that the shadow is an area of pneumonia, this is not strictly correct. There are other causes for air space shadowing (ie consolidation) and these include: pulmonary haemorrhage, pulmonary oedema, and fluid aspiration.

Hidden pneumonia: checking the borders

Partly ill-defined or absent	Suspect consolidation and/or collapse of the …
Right heart border	Middle lobe (a)
Left heart border	Left upper lobe (b)
Left dome of diaphragm	Left lower lobe (c)
Right dome of diaphragm	Right lower lobe (d)

Silhouette sign—Pitfall. Mediastinal fat¹.

Some middle aged people accumulate a large collection of fat adjacent to the heart causing the heart border to be ill defined. The fat is of lower density than the water density of lung consolidation and this difference will help you to avoid this pitfall. The patient on the left has lost the sharp silhouette of both the right heart border and of the right dome of the diaphragm. The patient on the right has lost the silhouette of part of the right dome of the diaphragm. In both cases the cause is a large collection of fat.

Silhouette sign—Pitfall. Depressed sternum.

The right heart border is effaced, and there is added density suggesting middle lobe consolidation. The lateral CXR shows the depressed sternum which is the cause of the grossly abnormal appearance on the frontal CXR¹. The lungs are clear.

Question 2: Is there a pneumothorax?

An erect CXR obtained in full expiration is recommended. The normal lungs are more opaque (or slightly whiter) on an expiration CXR. Consequently, when a pneumothorax is present the air (black) in the pleural space contrasts with the adjacent (whiter) lung. This accentuation of the difference in contrast, as compared with an inspiration film, sometimes makes it slightly easier to detect a pneumothorax.

A pneumothorax shows three features on the erect CXR:

▪ A clearly defined line (ie the visceral pleura). This line parallels the chest wall.

▪ The upper part of this line will be curved at the lung apex.

▪ An absence of lung markings (ie vessels) between the lung edge and chest wall.

Shallow pneumothorax. The visceral pleural line is visible.

Pneumothorax. Note the well-defined line that represents the lung edge (ie the visceral pleura). Also, there are no vessels lateral to this line.

Supine CXR. In a severely injured patient the CXR will inevitably be obtained with the patient supine. On a supine CXR the pleural air will rise to the highest point in the pleural space—ie to the anterior aspect of the thorax. Consequently the lung base and the area around the heart need careful evaluation⁴.

Question 3: Are there signs of left ventricular failure (LVF)?

Look for cardiac enlargement and for lung and pleural changes.

Cardiac enlargement.

Almost all patients with LVF have an enlarged heart. The occasional exception is a patient with an acute myocardial infarction.

Most enlarged hearts have a cardiothoracic ratio (CTR) over 50% when assessed on a PA CXR (p. 309).

The CTR is well over 50% in this patient.

Early LVF. Septal lines (Kerley B lines) caused by fluid in the interstitium. These short, straight lines reach the pleural surface and have this characteristic appearance at the lung base.

Early LVF. Small pleural effusions. Slight blunting of the costophrenic angles. Effusions in LVF are usually bilateral.

Interstitial oedema. The fluid lies mainly in the interstitium of the lung (ie oedema within the alveoli is less evident). Consequently, the shadowing is predominantly nodular and linear.

Extensive alveolar oedema. The fluid lies mainly within the alveolar air spaces (ie oedema lying in the interstitium is much less evident). Consequently the shadowing has a fluffy or cotton wool-like appearance.

Question 4: Severe asthmatic attack—is there a complication?

The complications to look for are lung consolidation, lobar collapse, pneumothorax and pneumomediastinum.

Lobar collapse. Collapse of the right upper lobe.

Pneumothorax. Air can dissect through the lung and in the region of the hilum it may enter the pleural space. The arrow indicates the visceral pleural line.

Pneumomediastinum. Streaks of air in the mediastinal soft tissues.The streaks may extend up into the neck. In this patient the air has dissected into the soft tissues around the left cardiac margin.

Pneumomediastinum explained. Air has dissected through the lung to the hilum and up into the neck. It can appear in the soft tissues of the chest wall. In addition, the dissecting air may outline a margin of the heart.

Question 5: Is there a pleural effusion?

Fluid in the pleural space can adopt several different appearances.

On the erect frontal CXR¹

The commonest appearance is a meniscus at a costophrenic angle. It requires 200–300 ml of pleural fluid to efface the normal sharp sulcus between the diaphragm and the ribs. If the effusion is large, the entire hemithorax is opaque and the heart is pushed towards the normal side.

Other patterns also occur:

Top: Puddling in a subpulmonary position is relatively common. A subpulmonary effusion is usually easier to detect on the left side, where the puddle in the pleural space can cause the gastric air bubble to appear widely separate from the (apparent) superior margin of the diaphragm.

Bottom left: A linear (lamellar) effusion.

Bottom right: Fluid loculated in the pleural space. In this example—within the horizontal fissure.

Small effusion on the patient’s right side (a meniscus is evident).

A larger effusion is present on the left side … again, a meniscus appearance is evident. Note that the lung shadows are consistent with alveolar and interstitial oedema.

Encysted pleural fluid.

(1) In the horizontal fissure.

(2) Posteriorly in the pleural space. The latter has produced a “blob like” appearance (without a sharp superior margin) overlying the lower zone of the right lung.

On the supine CXR¹

Pleural fluid on a supine CXR¹.

The fluid spreads to the most dependent site (ie to the posterior aspect of the pleural space). This causes the right hemithorax to appear greyer/whiter than the opposite side.

In this patient almost all of the shadowing over the right hemithorax is due to a large pleural effusion.

Question 6:

When assessing the mediastinum there are no absolute measurements that indicate abnormal widening on a frontal CXR.

Traumatic rupture^8–10

▪ 30% of patients with an aortic rupture have a normal CXR on presentation. If the CXR is normal, the level of clinical concern will determine whether an additional definitive radiographic examination (for example CT) is needed.

▪ A widened mediastinum does not necessarily indicate aortic rupture following blunt trauma to the chest. Tearing of small mediastinal veins is the most common cause of mediastinal widening on the CXR.

Pitfall.

In middle-aged and elderly patients, the aorta will often show age-related unfolding. Furthermore, on an AP projection an unfolded but otherwise normal aorta can be considerably magnified.

This patient has an unfolded aorta.

Question 7: Is there a rib fracture?

Oblique views of the ribs are not indicated following a relatively minor injury to the thorax. Clinical management is rarely altered by demonstration of a simple rib fracture. A frontal CXR is obtained solely to exclude an important complication such as a pneumothorax.

Question 8: Is there a cause for non-specific chest pain?

In many of these patients the pain remains unexplained and the CXR is normal.

Systematic analysis of the CXR (p. 312) will sometimes reveal an unexpected cause of the pain (eg pneumothorax, spontaneous pneumomediastinum¹¹, rib fracture).

Question 9: Is there evidence of a pulmonary embolus?^1,4

90% of emboli occur without pulmonary infarction and the CXR often appears normal.

Sometimes non-specific CXR findings are present, including: small areas of linear collapse; a small pleural effusion; slight elevation of a dome of the diaphragm.

Whenever a pulmonary embolus remains a clinical possibility then—whatever the CXR findings—the patient requires definitive imaging, usually a CT pulmonary angiogram.

Question 10: Is there evidence of an inhaled foreign body?

This patient had inhaled a peanut and attended the Emergency Department. This CXR is abnormal. It shows evidence of an obstructed bronchus. An additional CXR was obtained, following a particular manouvre, and that CXR provided confirmation of the obstruction. The peanut was removed.

The CXR analysis is provided on p. 30.