Paediatric chest radiographs

Mymoona Alzouebi takes us through chest radiographs—a fundamental part of a child’s clinical assessment

Interpretation of the chest x ray is a skill that receives little emphasis in undergraduate training. Although you may have some experience of looking at adult chest x rays, remember that “children are not small adults” and that paediatric chest x rays present in different ways and with different diseases. To add to the confusion, normal anatomy changes during childhood and the most difficult questions to answer are often “is this normal or abnormal?” or “which side is abnormal?” This article aims to help you understand some of the issues involved.

A systematic analysis of the film is the best way to proceed and should include:

1. Patient identification
2. Lung volumes and positioning
3. Lungs—too lucent or too opaque?
4. Heart and mediastinum
5. Review areas

Patient identification

Do not forget:

1. Patient’s details (ideally name, date of birth, and hospital number)
2. Date of the radiograph
3. Side marker (left or right)

Lung volumes and positioning

Ideally the x ray is taken towards the end of inspiration (if you have ever had a chest x ray, you may remember being asked to “take a breath in”). An inspiratory film can be difficult to obtain with small children, particularly if they are

uncooperative or tachypnoeic.

Assessment of lung volumes

Assessing lung volume is an important part of the interpretation and may even identify the underlying problem. To do this, count down the anterior rib ends and see which one meets the middle of the hemi­diaphragm—a good inspiratory film should have the anterior end of the fifth or sixth rib meeting the middle of the diaphragm. More than six anterior ribs shows hyperinflation (figs 1 and 2), and fewer than five indicates an expiratory film (underinflated).

Fig 1: Good inspiration film.The anterior end of the seventh rib is just at the hamidiaphragm,indicating mild overinflation.The child was well and large lung volumes are attributed to good inspiratory effort

Fig 2: Same film as fig 1. The first,third,fifth and seventh ribs are highlighted to show their positions.Practise tracing along the rib to identify the anterior ends

Hyperinflation

Take a huge breath in. Your lungs will be overinflated due to the large tidal volume—compliant patients can do the same.

Sick infants have a different explanation for their hyperinflation—they breathe quickly (tachypnoea) and overinflation is due to air trapping.

Airway resistance is higher during expiration (the airways are slightly smaller), and this difference becomes critical with any pathology causing bronchial wall thickening in infants (younger than 18 months).

The result is that air goes in more easily than it comes out, and lung volumes increase air trapping. It happens with inflammation (for example, viral bronchiolitis; figs 3 and 4), heart failure (ventricular septal defect) or fluid overload (transient tachypnoea of the newborn).

Fig 3: Hyperinflation.This infant presented with symptoms of viral bronchiolitis.Note how the lungs are hyperinflated.The seventh and eighth anterior rib cross the diaphragm.Both diaphragms also appear flatter than normal

Fig 4: Same film as fig 3.The anterior ends of the seventh ribs are marked.Large lung volumes can also occur when a stable patient takes in a large breath,however, the clinical condition of the patient should make the situation clear

Underinflation

With underinflation, the third or fourth anterior ribs crosses the diaphragm. This can make normal lungs appear opaque and a normal heart can appear enlarged.

Positioning

Sick infants may not cooperate when taking a radiograph. Check for rotation by seeing if the anterior ends of the ribs are equal distance from the spine. Rotation to the right makes the heart appear ­central, and rotation to the left makes the heart look large and can make the right heart border disappear (fig 5).

Fig 5: Rotation to the left.Follow the ribs-the anterior ribs on the right are much closer to the spine than those on the left.This makes the heart look enlarged and has made the right heart border disappear

The lungs: too lucent or too opaque?

Lung density

Divide the lungs into upper, middle, and lower zones. Compare the two sides—is there any difference? Is the abnormal lung too lucent (black) or too opaque (white)?

Opacity is usually in the lung (collapse or consolidation) or pleural space (pleural effusion). The abnormality is seldom located in the chest wall (for example, body wall oedema). What do the terms consolidation and collapse mean? Is the difference important?

Consolidation

Imagine a sponge—full of airspaces and not very dense, just like the lung. Fill the airspaces with fluid and the sponge (lung) becomes dense (consolidated). In reality, that fluid could be water, blood, or pus (for example, in pneumonia). The volume of an affected segment is normally preserved. The absence of an adjacent heart border or hemidiaphragm should help indicate which lobe is affected­.

Collapse

Collapse implies loss of volume and occurs for many reasons. The lung is dense but for a different reason—this time because all the air has been removed. In children, the cause is usually in the airway (intraluminal foreign body or mucous plug; fig 7). Complete airway obstruction results in reabsorption of air in the affected lobe or segment. Collapse can also be due to extrinsic compression (for example, a mediastinal mass) or a pneumothorax.

Fig 7: Right upper lobe collapse.The horizontal fissure is raised due to volume loss due to a mucus plug in the upper lobe bronchus.Compare with fig 6. Notice the lungs are hyperinflated(bronchiolitis) and the patient is ventilated(endotracheal tube just at the top of the film-the thin radio-opaque line over the right side of C7/T1/T2 vertebral bodies)

Fig 6: Right upper lobe pneumonia.Consolidation-the lung is opaque,but there is no evidence of volume loss(the horizontal fissure is in its normal position)

Telling the difference

How do we tell the difference between collapse and consolidation? Actually, we cannot always tell, because they are both in the lung and they both look dense. Part of the answer lies in the volume of the affected lung—is there any evidence that the affected lung is of reduced volume? Has the heart been pulled across? Is the trachea deviated? Is there evidence of a disease process that might predispose to lobar or segmental collapse (such as bronchiolitis with inflamed narrow airways and excess mucous production)? Is there a history suggestive of foreign body aspiration (fig 8)? The answer is often not clear cut. Clinical examination may give further clues.

Fig 8: Foreign in left main bronchus.This 6 year old aspirated a foreign body, which occluded the left main bronchus.The left lung has collapsed pulling the mediastinum across the left(can you see the right heart border?) and pulling the left hemidiaphragm up

The implications for management are significant. For instance, with pneumonia (fig 9), empirical treatment with antibiotics is the treatment of choice, while a broncoscopy is likely to be the best solution with a foreign body (fig 8). If changes in the first instance fail to resolve, it might be necessary to plan a broncoscopy.

Fig 9: Empyema.Opacity in the left hemithorax is due to a pleural abscess(empyeme) complicating pneumococcal pneumonia.The mediastinum is displaced to the right due to the mass effect of the pleural collection.The patient is not rotated(look at the anterior ribs) and compare the right heart border with that in fig 8

Pleural effusion

Unilateral effusions are typically associated with infection (parapneumonic). As this fluid is “extra” the abnormal lung should be of normal or increased volume. The best guide is usually the position of the heart. Bilateral effusions are usually related to low albumin states (such as nephrotic syndrome).

Lucent lungs

Lucent lungs are usually due to air trapping. Some of the causes have already been described. Acquired overinflation of individual lobes or segments may be due to a foreign body (acting as a ball valve), and you should take an appropriate history. The small airways can be irreversibly damaged by inflammation (obliterative bronchiolitis) after viral infection or bone marrow transplantation. Causes of localised overinflation in the newborn include congenital lung problems (for example, congenital lobar emphysema). Lucency should trigger a search for a pneumothorax.

Bronchial wall thickening

Bronchial wall thickening is a common finding on paediatric chest radiographs. As such, its meaning alone is unclear. Look for “tram track” parallel lines around the hila—the commonest cause is viral infection or asthma. This is a common finding with cystic fibrosis (figs 10 and 11).

Fig 10:Cystic fibrosis.This adolescent with cystic fibrosis has a portacath on the right for intravenous antibiotics.Bilateral upper lobe fibrosis reflects extensive lung damage and scarring at both apices

Fig 11: Same image as in fig 10 magnified.Bronchial wall thickening is easier to see

Heart and mediastinum

The mediastinum contains structures between the lungs. The anterior mediastinum, which is located in front of the heart, contains the thymus gland (fig 12). This appears largest at about 2 years of age—it carries on growing into adolescence but not as fast as the rest of the body (and so becomes less apparent on the radiograph). The right lobe can rest on the horizontal fissure, which is often called the “sail sign.”

Fig 12: Thymus.Normal thymus fills out the contour of the superior mediastinum in this 1 year old infant.The inferior border of the right thymic lobe is a straight line.On the left,the cardiothymic notch is just visible as a slight indentation in the mediastinal outline

Fig 13: Diaphragmatic hernia.The left hemidiaphragm is raised,and the mediastinum is shifted to the right.The left lung is small(hypoplastic)

Heart

When interpreting the radiological appearance of the heart, several factors need to be assessed—size, shape, position, and pulmonary circulation. Careful interpretation of the x ray is important in cases of suspected congenital heart disease although echocardiography has become the tool of choice.

Heart size—The ratio of heart size relative to the diameter of the chest—the cardiothoracic ratio—normally measures 50% after the first year of life, but can be greater than this in the first year. It can occasionally be difficult to assess in infants because of the thymus and problems in obtaining a well positioned inspiratory film.

Heart shape and position—On a well centred film, two thirds of the heart is seen to the left of the spine and one third to the right (see figs 1, 2, 5,8, and 9). The cardiac apex, ­aortic arch (and descending aorta) and gastric air bubble should all lie on the left.

Pulmonary circulation—This is extremely important for cases of suspected congenital heart disease, as it will indicate the nature of the underlying problem. Unfortunately, the assessment can be difficult even for experienced paediatric radiologists. Decreased flow (oligaemia) can be seen with right ventricular outflow obstruction (for example, pulmonary stenosis). Increased flow results in pulmonary vessels (plethora) and examples include left to right shunts (ventricular septal defect, patent ductus arteriosus).

Mymoona Alzouebi, senior house officer in haematology, Royal Hallamshire Hospital, Sheffield
Email: mymoona123@yahoo.co.uk

Iwan Roberts, consultant radiologist, Sheffield Childrens Hospital



studentBMJ 2005;13:309-352 September ISSN 0966-6494