2 THE LATERAL CXR

A COMMON SCENARIO

Patient with a persistent cough. Normal clinical examination.

An equivocal frontal CXR appearance.

Physician unfamiliar with the normal appearances on a lateral CXR.

Physician goes for the easy option—“let’s get a CT”.

However:

CT = high dose of radiation. Lateral CXR = low dose.

The lateral CXR will rapidly exclude or confirm most equivocal abnormalities seen on the frontal projection.

The lateral CXR can be very, very useful provided you are familiar with the normal appearances.

“The man who is too old to learn was probably always too old to learn.”¹

The novice may feel that getting to grips with the lateral CXR is too much at this stage. Don’t worry. Keep going on the other chapters and come back to the lateral CXR later on. Eventually, you will realise its importance. Also, analysing the lateral CXR is fun—Sherlock Holmes type fun.

Reference drawings of the normal lateral appearance are provided on p. viii.

WHEN IS A LATERAL CXR USEFUL?

1. To check whether an equivocal frontal CXR shadow is actually present.

2. To position an abnormality shown on the frontal CXR.

Is it anterior or posterior?

Which lobe is it in?

Is it actually in a lobe?

3. To check the tricky areas when a patient has a particularly worrying symptom (e.g. haemoptysis). This projection is particularly good at showing:

behind the heart

behind and in front of the hila

behind the domes of the diaphragm

RADIOGRAPHIC TECHNIQUE

In a fit individual the arms are held high and well away from the thorax(Fig. 2.1). In a frail or elderly patient the arms may have to be positioned in front of the chest. The resulting upper arm shadows can mislead the unwary (Fig. 2.2). The scapulae cannot avoid intruding on the image, but they should be easy to recognise (Figs 2.3 and 2.4).

Figure 2.1 Fit person. Arms up and out of the way.

Figure 2.2 Frail person. The upper arms often lie across the thorax.

Figure 2.3 The scapula shadows are usually projected over the thorax.

Figure 2.4 Technically good CXR. The scapulae are visible, but do not cause a problem.

UNDERSTANDING THE LATERAL CXR²^-⁶

Figs 2.5-2.9 show how you can build up the main anatomical structures on a lateral CXR. Fig. 2.10 is the radiographic equivalent of the finished article.

Figure 2.5 Bare-bones lateral anatomy. Note that the lungs appear blacker inferiorly as compared with superiorly.

Figure 2.6 Add the heart, aorta and inferior vena cava…

Figure 2.7 …add the right (red) and left (blue) main pulmonary arteries (see Fig. 2.11)…

Figure 2.8 …add—in your mind’s eye—the three fissures. Knowing their approximate position is important. They separate the three lobes of the right lung and the two lobes of the left lung. In practice, it is rare for much of either oblique fissure to be visualised on a normal lateral CXR.

Figure 2.9 Finally, add the trachea and the upper lobe bronchi (i.e. the two coloured circles). All the main structures are now in place.

Figure 2.10 A normal lateral CXR.

NORMAL LATERAL CXR—SPECIFIC APPEARANCES

DIAPHRAGM

The two domes are usually easy to separate from each other.

The right dome is visualised all the way from front to back.

The shadow of the left dome only extends from the costophrenic recess posteriorly to the back of the cardiac shadow anteriorly. This is because the heart obliterates the lung/diaphragm interface anteriorly.

Figure 2.11 Normal structures as seen on the frontal CXR, for comparison with Figs 2.5-2.10. Red = right main pulmonary artery; blue = left main pulmonary artery; purple = origin of right upper lobe bronchus; yellow = origin of left upper lobe bronchus.

Very occasionally the two domes are difficult to separate, because:

The domes may overlap each other precisely…coincidence.

If the base of the heart is very narrow then obliteration of the anterior aspect of the left dome is minimal and its shadow will extend almost all the way to the sternum—i.e. it can mimic the normal “seen all the way from front to back” right dome.

Figure 2.12 Normal CXR. The right dome (arrowhead) is seen all the way from front to back. Note that the left dome (arrow) is not identified anteriorly. The outlines of both domes posterior to the heart are crisp and clean. The right dome is also crisp and clean anteriorly.

HILA

The novice will find this a difficult area to assess. Bear these points in mind:

Of the soft tissue density at each hilum, 95% is due solely to pulmonary artery and pulmonary veins.

The main pulmonary artery on the right side passes anterior to the right main bronchus, whereas the main pulmonary artery on the left side passes posteriorly and hooks over the left main bronchus.

There is summation of some of the left and right hilar densities (i.e. the vessels) on the lateral CXR.

You will be delighted to know that even the experts find this area difficult. However, the expert always evaluates the lateral view of a hilum together with the frontal projection. By taking the two views together it is much, much easier to answer a frequent question: “is this hilum enlarged or is it just prominent but normal?”.

Figure 2.13 Normal CXR. Fig. 2.14 explains the various components forming the hilar shadows.

Figure 2.14 Normal CXR. To show the location of the main pulmonary arteries (yellow). These arteries account for most of the hilar shadows. Remember—the right main pulmonary artery crosses in front of the right main bronchus and the left main pulmonary artery hooks over the left main bronchus.

LUNG FISSURES

The horizontal fissure can be identified on most lateral CXRs (Fig. 2.15).

Figure 2.15 The three fissures. Red = horizontal; green = right oblique; blue = left oblique. Visibility of the fissures varies from patient to patient—some, all, or none of a fissure may be evident. The fissures divide each lung into lobes. The right lung has three lobes—upper, middle and lower. The left lung has two lobes—upper and lower. (See p. viii.)

The oblique fissures are not aligned along a flat plane. Both have configurations similar to that of an aeroplane propeller (Fig. 2.16).

Figure 2.16 The normal configuration of an oblique fissure. Each oblique fissure resembles an aeroplane propeller.

It is often stated that the right oblique fissure lies—at its most inferior position—about 4–5 cm posterior to the sternum, and that the left oblique fissure is positioned slightly more posterior. In practice, very slight rotation of the patient can project these fissures closer to, or further away from, each other on the CXR. The good news is that absolute certainty as to which oblique fissure is which is rarely important in clinical practice.

COSTOPHRENIC ANGLES (RECESSES, SULCI)

The posterior and inferior part of each lung occupies a well-defined gully created by the pleura reaching the posterior limit of each dome of the diaphragm. These two gullies—one on each side—are the costophrenic angles or sulci (Figs 2.15 and 2.16).

On the lateral radiograph, each costophrenic recess represents the most inferior aspect of the lung and pleura on an erect CXR. This is where most pleural effusions will collect.

LUNG APICES

The lateral CXR is not much help in assessing either lung apex, due to the overlying soft tissues, shoulders and chest wall.

RETROSTERNAL LINE

A soft tissue interface with the lung is visible immediately behind the sternum (Fig. 2.17). This is the retrosternal line.

Figure 2.17 This retrosternal line (arrows) is normal.

The line is caused by the interface between the retrosternal soft tissues (mainly fat) and the anterior aspect of the right lung.

Figure 2.18 Road traffic accident. Sternal fracture. The retrosternal line (arrows) bulges posteriorly due to the adjacent haematoma.

HEART

The heart occupies most of the mediastinum (Figs 2.19 and 2.20). The cardiac borders seen on the lateral CXR are formed as follows:

Anteriorly—right ventricle

Posteriorly immediately behind the hila—left atrium

Posteriorly below the hila—left ventricle

Figure 2.19 Cardiac chambers. Frontal CXR allows correlation with the lateral CXR (Fig. 2.20). Pink = right atrium; blue = right ventricle; brown = left ventricle; green = left atrium.

Figure 2.20 The cardiac chambers. See colour coding in Fig. 2.19.

Important rule of thumb: On the lateral CXR there should not be any abrupt change in density across the shadow of the heart. If there is a change in density, you should suspect superimposed pulmonary pathology.

INFERIOR VENA CAVA

On many lateral CXRs a short (1.5–2.0 cm) well-defined vertical shadow meets the posterior and inferior aspect of the heart. This shadow is the inferior vena cava. Its posterior wall is visible because it is outlined by air in the adjacent right lung (see Figs 2.21-2.23).

Figure 2.21 Frontal view showing the shadow of the inferior vena cava (yellow) merging with the right atrium.

Figure 2.22 Inferior vena cava shadow on the lateral CXR.

Figure 2.23 Lateral CXR. The shadow of the inferior vena cava (arrow) is easily identified.

TWO BRONCHIAL RINGS

Two circular rings are often projected over the hila or peri-hilar regions(Figs 2.24-2.26). Sometimes only one ring is visible. Usually the higher ring is the right upper lobe bronchus and the lower ring is the left upper lobe bronchus.

Figure 2.24 Normal lateral CXR. Trachea = green; right upper lobe bronchus (seen end on) = purple; left upper lobe bronchus (seen end on) = brown.

Figure 2.25 The positions of each ring as shown on the frontal CXR.

Figure 2.26 The two rings (arrows) are visible in this patient. The rings represent—in almost all patients—the orifices of the upper lobe bronchi. Occasionally, they represent the main bronchi².

BONES AND SOFT TISSUES

All lateral CXRs include part of the axial and appendicular skeleton. Bones will occasionally cause an overlap shadow. Familiarity with the normal skeletal appearances will prevent misunderstandings. Note:

The vertebrae and sternum are easy to see, and are not a problem (Fig. 2.27).

The ribs are also easy to see, and are rarely a problem.

The scapulae and upper arms are often visible, and are sometimes confusing.

Figure 2.27 Normal CXR. The lower vertebral bodies are blacker because there is less soft tissue (muscle) and no scapulae superimposed. Consequently, there is less absorption of the x-ray beam inferiorly, and thus more blackening on the radiograph. Whiteness inferiorly on the CXR invariably indicates pathology (e.g. collapse, consolidation, lung mass, paravertebral mass). Also, note that assessment of a lung apex is very difficult—usually impossible.

NORMAL LATERAL CXR—THREE COMMON PITFALLS

1. Fake mass anteriorly. Sometimes the shadow of a high right dome of the diaphragm and the shadow of the posterior margin of the heart overlap and create a well-defined density that mimics an anterior mass (Fig. 2.28).

2. Age related aortic unfolding (Figs 2.29-2.31). In young people the descending aorta is situated posteriorly within the mediastinum and it is not visualised on the lateral CXR. In middle age the aorta unfolds and extends laterally to the left. Consequently its anterior and posterior walls are then outlined by the surrounding left lung. This produces a tubular opacity on the lateral CXR (Fig. 2.33).

3. Another fake mass anteriorly. The apex of the heart and adjacent epicardial fat intrudes into the left hemithorax and displaces the most infero-medial and anterior aspect of the left lung. This often produces a shadow (Fig. 2.34) which can simulate a mass lesion. This appearance is often referred to as the cardiac incisura⁶^-⁸.

Figure 2.28 Normal. Not an anterior mass.

Figure 2.29 Frontal CXR. Ascending and descending aorta, age 20.

Figure 2.30 Frontal CXR. Ascending and descending aorta, age 60.

Figure 2.31 Frontal CXR. Ascending and descending aorta, age 80.

Figure 2.33 Age 80. Age related aortic unfolding occurs in the middle-aged and elderly. The descending aorta is outlined by lung and is visible (arrows). (This is the same patient as Fig. 2.31.)

Figure 2.34 The cardiac incisura. The anterior density (arrows) is caused by the normal cardiac apex and epicardial fat displacing the most infero-medial and anterior aspect of the left lung. The shape of the cardiac incisura will show considerable variation between patients. (Incidentally, a cardiac incisura shadow, with a somewhat different contour is also shown in Fig. 2.33, above.)

Figure 2.32 Normal CXR. Age 20–40. The mid and distal part of the descending aorta is not visible.

READING THE LATERAL CXR

A lateral radiograph of the chest can be very valuable in helping to confirm, refute, position or characterise a suspected abnormality seen on the frontal projection. A checklist will help you to remember the important rules.

THE LATERAL CXR: A SIX-POINT CHECKLIST

1. Are the vertebral bodies becoming blacker from above downwards(Fig. 2.35)?

If not (i.e. they are becoming whiter), then suspect disease in a lower lobe (Figs 2.36-2.38).

2. Are both domes of the diaphragm well-defined and clearly seen?

If either dome is obscured, suspect disease in the adjacent lower lobe (Figs 2.36-2.38).

Remember—the right dome should be visible from front to back; normally the anterior aspect of the left dome disappears (Fig. 2.35).

3. Are the hila normal (Fig. 2.35)?

Figure 2.35 Normal CXR. Note that the vertebral bodies become blacker from above downwards; the domes of the diaphragm are well-defined.

Figure 2.36 Cough and fever. Vertebrae whiter inferiorly. The outline of the left dome of the diaphragm is absent posteriorly. Left lower lobe pneumonia.

Figure 2.37 Unwell, fever, chest pain. Vertebrae whiter inferiorly; the outline of the right dome of the diaphragm is absent posteriorly. Right lower lobe pneumonia.

Figure 2.38 Persistent cough and haemoptysis. The lower vertebrae are whiter than those above. Shadow of the left dome of the diaphragm is absent. The straight line of the oblique fissure is displaced posteriorly. Collapse of the left lower lobe (bronchial carcinoma).

Two questions to ask:

Does the overlapping/combination shadow of the two hila appear enlarged (Fig. 2.39)?

Is the outline of the overlapping hila well-defined (i.e. normal vessels) or do the borders appear anarchic and irregular, or lumpy/bumpy?

4. Is there any abrupt change in density across the cardiac shadow?

This is likely to be a lung abnormality (Fig. 2.40).

5. Are there any abnormal lung densities?

Figure 2.39 The hilar shadows are enlarged. Lymphadenopathy. Compare this CXR with Fig. 2.35.

Figure 2.40 Abrupt change in density over the cardiac shadow. Middle lobe pneumonia with slight loss of volume.

Check:

superimposed over the heart (Fig. 2.42)

behind the heart

posteriorly in a costophrenic recess

6. Always, always correlate the findings with the frontal CXR.

Figure 2.42 Haemoptysis in a middle-aged smoker. Frontal CXR was unremarkable. Note the solitary pulmonary nodule (arrow) projected over the heart. Bronchial carcinoma.

Figure 2.41 A common finding in the middle-aged and elderly. Hiatus hernia revealed by the air–fluid level.

REFERENCES

1. Henry S Haskins. American author (b.1875).

2. Proto AV, Speckman JM. The left lateral radiograph of the chest. Part one. Med Radiogr Photogr. 1979;55:30-74.

3. Proto AV, Speckman JM. The left lateral radiograph of the chest. Part two. Med Radiogr Photogr. 1980;56:38-64.

4. Austin JHM. The lateral chest radiograph in the assessment of non-pulmonary health and disease. Radiol Clin North Am. 1984;22:687-698.

5. Vix VA, Klatte EC. The lateral chest radiograph in the diagnosis of hilar and mediastinal masses. Radiology. 1970;96:307-316.

6. Fraser RG, Muller NL, Colman NC, Pare PD. Fraser and Pare’s Diagnosis of Diseases of the Chest, 4th ed. Philadelphia, PA: WB Saunders, 1999.

7. Heitzman ER. The Mediastinum. St Louis, MO: Mosby, 1977.

8. Hyson EA, Ravin CE. Radiographic features of mediastinal anatomy. Chest. 1979;75:609-613.