Chest X-ray Interpretation: A System for Every Film

Introduction

The chest radiograph is the most frequently performed imaging investigation in the UK, and the ability to interpret one systematically is expected of every doctor from day one. The purpose of a system is not that it finds abnormalities faster - it usually does not - but that it stops you stopping at the first abnormality you see, which is the commonest reason a significant finding is missed.

Before looking at the lungs at all, confirm three things: the right patient, the date and time, and whether there are previous films to compare with. A comparison film answers more questions than any amount of scrutiny of a single image, and a nodule that has been unchanged for five years needs nothing further.

Assessing the film - RIPE

Assess quality before content, because a technically poor film changes how the findings are interpreted rather than making them uninterpretable.

The ABCDE approach

A - Airway

• Trachea - should be central, or minimally deviated to the right at the aortic arch. It is pulled towards volume loss (collapse, pneumonectomy, fibrosis) and pushed away by volume gain (large effusion, tension pneumothorax, large mass).
• Carina - normally at the level of T4 to T5, with a bifurcation angle of about 60 to 100 degrees. Splaying of the carina suggests left atrial enlargement or subcarinal lymphadenopathy.
• Main bronchi - the right is wider, shorter and more vertical, which is why aspirated material and misplaced tubes go right
• Hila - composed mainly of pulmonary arteries and veins. The left hilum is normally higher than the right, by up to 2 cm. A left hilum lower than the right is always abnormal, and any hilum that is enlarged, denser or abnormally positioned needs explaining - the differential is lymphadenopathy, a mass, or pulmonary arterial enlargement.
• Paratracheal stripe - widening suggests lymphadenopathy or a mass

B - Breathing

• Compare the two lungs zone by zone - upper, mid and lower. The zones are radiographic divisions and do not correspond to lobes, so describe findings by zone rather than guessing at lobes.
• Lung markings should extend to the periphery. An area with no markings beyond a fine white line is a pneumothorax.
• Look for asymmetry of density, and remember that an apparently white lung may be an abnormally black one on the other side
• Trace the pleural edges and the fissures - the horizontal fissure on the right runs from the hilum to the sixth rib in the axillary line, and displacement of a fissure indicates volume loss
• Costophrenic and cardiophrenic angles - blunting indicates fluid, and around 200 ml is needed before it shows on a PA film

C - Circulation

• Heart size - the cardiothoracic ratio should be under 50% on a PA film. Measure the widest cardiac diameter against the widest internal thoracic diameter. Do not assess this on an AP film.
• Heart borders - the right border is the right atrium, the left border the left ventricle. Loss of a border is the silhouette sign and localises adjacent pathology.
• Aortic knuckle - widening suggests aneurysm, unfolding or dissection
• Mediastinal width - a widened mediastinum on a PA film raises the possibility of dissection, haematoma, lymphadenopathy or a mass. Beware that AP and supine films widen it artefactually.
• Pulmonary vasculature - upper lobe vessels are normally smaller than lower lobe ones when upright. Upper lobe venous diversion is an early sign of pulmonary venous hypertension.

D - Diaphragm

• The right hemidiaphragm is normally higher than the left by up to 3 cm, because of the liver. A raised right or an elevated left hemidiaphragm needs explaining: phrenic nerve palsy, lobar collapse, subphrenic collection, hepatomegaly, or eventration.
• Both should be smooth and dome-shaped. Flattening indicates hyperinflation.
• Look beneath the diaphragm - free gas (pneumoperitoneum) appears as a lucent crescent under the right hemidiaphragm on an erect film and indicates a perforated viscus. Do not confuse it with gas in the stomach or colon (Chilaiditi sign).
• The gastric bubble should be under the left hemidiaphragm

E - Everything else

• Bones - ribs (fractures, metastatic lesions, notching), clavicles, scapulae, humeri and the visible spine. Look at every rib in turn - rib fractures and lytic lesions are among the commonest missed findings.
• Soft tissues - surgical emphysema, breast shadows (and an absent breast shadow after mastectomy, which makes that lung look darker), and neck soft tissues
• Lines and tubes - see below
• Artefacts - ECG leads, jewellery, clothing and hair plaits, all of which mimic pathology
• Below the diaphragm - upper abdominal gas pattern, and any visible calcification

The silhouette sign

A border is visible on a radiograph only where two structures of different density lie in contact. The heart border is visible because aerated lung sits against it. If that lung becomes airless - consolidated, collapsed or replaced by fluid or tumour - the two densities become the same and the border disappears.

The consequence is powerful: the border that is lost tells you where the disease is, even when the opacity itself is ill-defined.

Patterns worth recognising

Consolidation

• Alveoli filled with fluid, pus, blood or cells, with preserved lung volume
• Air bronchograms - branching black airways silhouetted against white consolidated lung - are the defining sign and confirm the process is alveolar rather than pleural
• Ill-defined margins except where limited by a fissure
• Causes: pneumonia (commonest), pulmonary oedema, haemorrhage, aspiration, infarction, and malignancy including lymphoma and adenocarcinoma

Collapse (atelectasis)

• Loss of aeration with volume loss, which is the essential distinction from consolidation
• Signs of volume loss: displacement of a fissure, mediastinal and tracheal shift towards the collapse, an elevated hemidiaphragm, crowded ribs, and compensatory hyperinflation of the remaining lung
• Causes: obstruction by tumour, mucus plug or a foreign body; compression by effusion or pneumothorax; hypoventilation postoperatively
• Any lobar collapse in an adult smoker requires exclusion of an obstructing bronchial carcinoma - a CT and bronchoscopy, not simply physiotherapy

Pleural effusion

• Blunting of the costophrenic angle first (about 200 ml on PA, 50 ml on lateral), then a meniscus curving upwards laterally
• A large effusion produces a dense white hemithorax with mediastinal shift away. If the mediastinum is shifted towards a white hemithorax, the cause is collapse or pneumonectomy, not effusion.
• On a supine film, fluid layers posteriorly and produces a diffuse veil-like increase in density over the whole lung with preserved vascular markings - easily missed

Pneumothorax

• A visible visceral pleural line with no lung markings beyond it
• Tension is suggested by mediastinal shift away, diaphragmatic depression and a distressed patient - but tension pneumothorax is a clinical diagnosis and should not await a film
• On a supine film air collects anteriorly and basally, producing the deep sulcus sign - an abnormally deep, lucent costophrenic angle
• Beware the mimics: skin folds (a line with markings visible beyond it), the medial border of the scapula, and bullae

Pulmonary oedema

• The classical mnemonic is ABCDE: Alveolar oedema (perihilar bat-wing shadowing), B lines (Kerley B lines - short horizontal septal lines at the periphery), Cardiomegaly, Dilated upper lobe vessels (upper lobe diversion), Effusions
• Changes evolve in order as pulmonary venous pressure rises - diversion first, then interstitial oedema and septal lines, then alveolar flooding
• Non-cardiogenic oedema (ARDS) shows bilateral shadowing with a normal heart size and typically no effusions or septal lines

Other patterns

Lines, tubes and devices

On an acute medical or intensive care unit, checking device position is often the reason the film was requested, and getting it wrong causes direct harm.

Review areas

Most missed abnormalities on a chest radiograph are not subtle - they are in places people forget to look. Go back and check each of these deliberately, after your main review.

1. Lung apices - overlapped by the clavicles and ribs. Pancoast tumours and apical pneumothoraces hide here.
2. Behind the heart (retrocardiac) - a left lower lobe collapse or a hiatus hernia is easily missed. This is why adequate penetration matters.
3. The hila - compare size, density and height, and remember the left should be higher
4. Below the diaphragm - free gas, and the upper abdominal organs
5. The lung periphery and the pleural edges - for a thin pneumothorax line and small peripheral nodules
6. Behind the diaphragm - the lung bases extend well below the level of the dome posteriorly
7. Bones - every rib, both clavicles, the scapulae, the shoulders and the visible spine
8. Soft tissues - surgical emphysema, breast shadows, the neck

Presenting a film

A structured presentation is expected in examinations and genuinely useful on a ward round.

1. Identify - "This is a chest radiograph of [patient], taken on [date]. I would want to check the details and any previous films."
2. Projection and quality - "It is a PA film with adequate inspiration and penetration, and no significant rotation."
3. A brief statement of the obvious, if there is a life-threatening abnormality - "There is a large left-sided tension pneumothorax and this patient needs immediate decompression."
4. Systematic review - work through ABCDE, describing positive and relevant negative findings
5. Review areas - state that you have checked them
6. Summary - one or two sentences bringing the findings together into a diagnosis or differential
7. Next steps - "I would assess the patient, and my differential is... I would like to request...". A film is never interpreted in isolation from the patient.

Limitations and when to go further

The chest radiograph is a two-dimensional projection of a three-dimensional structure, and its limitations are substantial and worth stating explicitly.

• Around 20% of lung cancers are missed on the initial film, especially in the apices, behind the heart and around the hila. A normal chest radiograph never excludes lung cancer in a symptomatic patient - refer or arrange CT anyway.
• It cannot exclude pulmonary embolism - the film is usually normal, and a clear film in a breathless hypoxic patient should raise rather than lower the suspicion
• It is insensitive to early interstitial lung disease - up to 10% of established cases have a normal film
• It cannot reliably distinguish an empyema from a lung abscess, or a bulla from a pneumothorax - both distinctions need CT and both have led to serious harm when got wrong
• It cannot assess heart size on an AP film
• Small pneumothoraces and small effusions may be invisible, particularly on supine films
• Radiological improvement lags behind clinical recovery - pneumonia may take 6 weeks or more to clear, which is why the follow-up film is timed as it is

Move on to further imaging where the film is normal but suspicion persists, where a lesion needs characterising, or where the clinical question cannot be answered by a projection image: CT for masses, interstitial disease, pleural disease, cavitation and staging; CT pulmonary angiography for suspected embolism; thoracic ultrasound for pleural fluid and to guide every pleural procedure; and echocardiography where the heart rather than the lung is the likely problem.