Chest & Cardiac Imaging

The chest radiograph is the most frequently performed imaging study in the world and the most frequently misread, because it is a two-dimensional shadow of a three-dimensional structure containing overlapping tissues.

One physical fact generates almost every sign.

Radiographs show five densities in ascending order: air, fat, soft tissue and fluid, bone, and metal. Soft tissue and fluid have essentially the same density, which is why a radiograph cannot distinguish pus from blood from tumour.

Two structures of the same density in direct contact lose the line between them. That is the silhouette sign, and it localises a lesion in the front-to-back axis using only a frontal film.

A second idea prevents the commonest errors of interpretation. Ask whether the lung is filled or whether it has lost volume. Consolidation fills alveoli while preserving lung volume, so the trachea and mediastinum stay central and air bronchograms appear. Collapse loses volume, so structures are pulled toward the abnormality.

A third habit prevents missed findings: read the film systematically and read the review areas last, because the eye is drawn to the obvious abnormality and stops.

1. Assessing the Film Before Reading It

A film that is not adequate cannot be interpreted safely, and three checks take seconds.

Rotation: the medial ends of the clavicles should be equidistant from the spinous processes. Rotation distorts the mediastinum and creates apparent hilar and cardiac abnormalities.

Inspiration: five to seven anterior ribs, or nine to ten posterior ribs, should be visible above the diaphragm. A poorly inspired film crowds the lung markings and fakes cardiomegaly and basal shadowing, and this is the single commonest reason a normal film is reported as abnormal.

Penetration: the vertebral bodies should be just visible behind the heart.

Projection matters enormously. A posteroanterior film is taken with the patient upright and the beam passing back to front, so the heart, which lies anteriorly, is close to the detector and is magnified minimally. An anteroposterior film magnifies the heart, so cardiomegaly cannot be assessed on a portable AP film, and free gas under the diaphragm cannot be excluded on a supine one.

2. The Silhouette Sign

Loss of a normal border tells you which structure the abnormality is touching, and therefore where it lies.

The logic is worth holding rather than the table. A dense opacity that obscures the heart border must be in contact with the heart, which is anterior. An opacity that overlies the heart on the frontal film but leaves its border crisp must be behind it, in a lower lobe.

3. Consolidation Versus Collapse

This distinction determines the differential entirely.

Consolidation means alveoli filled with something other than air, whether pus, blood, fluid or cells. Volume is preserved, so the mediastinum does not shift, and air bronchograms appear because air-filled bronchi become visible against opacified surrounding lung.

Collapse means loss of aerated volume, so everything moves toward the abnormality: the trachea and mediastinum shift ipsilaterally, the hemidiaphragm rises, ribs crowd, and the fissure displaces.

An air bronchogram effectively excludes a completely collapsed lobe and indicates that the alveoli are filled rather than empty, which is a genuinely useful discriminator.

The classic lobar collapse appearances are worth recognising: left lower lobe collapse produces a sail sign behind the heart, and right middle lobe collapse obscures the right heart border with a wedge on the lateral film.

4. Pleural Disease

Pleural effusion blunts the costophrenic angle, and around 200 to 300 mL is needed before this is visible on a frontal film, while a lateral film detects less and a lateral decubitus film or ultrasound detects far less still.

A large effusion produces a meniscus and can push the mediastinum away. An effusion with the mediastinum shifted toward the effusion means there is also collapse, usually from an obstructing lesion, and that combination should never be attributed to the effusion alone.

Pneumothorax shows a visible visceral pleural line with absent lung markings beyond it.

Tension pneumothorax is a clinical diagnosis, not a radiological one. A patient who is hypotensive with tracheal deviation and absent breath sounds requires immediate decompression, and waiting for a film to confirm it is a recognised cause of avoidable death.

On a supine film, air collects anteriorly rather than apically, producing the deep sulcus sign, an abnormally deep and lucent costophrenic angle, which is easily missed.

5. The Mediastinum and the Heart

The mediastinum is divided for diagnostic convenience, and the compartment narrows the differential efficiently.

Cardiomegaly is a cardiothoracic ratio above 0.5 on a properly taken posteroanterior film. The qualification matters, because AP and poorly inspired films both exaggerate it.

Pulmonary venous hypertension has a recognisable sequence as pressure rises: upper lobe blood diversion, then interstitial oedema with Kerley B lines and peribronchial cuffing, then alveolar oedema with perihilar bat-wing opacification, and finally pleural effusions.

Kerley B lines are short horizontal lines reaching the pleural surface, representing thickened interlobular septa, and they indicate interstitial fluid.

6. Tuberculosis and the Indian Chest Film

Tuberculosis dominates Indian chest radiology, and its appearances shift with immune status.

Primary tuberculosis typically shows lower or middle zone consolidation with hilar lymphadenopathy, and lymphadenopathy is the more reliable feature, particularly in children.

Post-primary or reactivation tuberculosis favours the apical and posterior segments of the upper lobes and the superior segment of the lower lobes, producing cavitation, fibrosis and volume loss.

The apical predilection reflects higher oxygen tension and poorer lymphatic clearance in the upper zones.

Miliary tuberculosis produces innumerable uniform 1 to 3 mm nodules distributed evenly throughout both lungs, and the uniformity of both size and distribution is what distinguishes it from metastases, which vary in size and favour the lung bases.

In advanced HIV, the classical appearances break down. Cavitation becomes less common, lymphadenopathy more common, and a normal-looking chest radiograph does not exclude active pulmonary tuberculosis, which is why sputum testing is performed regardless of the film.

7. CT of the Chest

High-resolution CT is the study for interstitial lung disease, where the pattern and distribution carry the diagnosis. Basal subpleural reticulation with honeycombing suggests usual interstitial pneumonia, while upper zone predominance suggests hypersensitivity pneumonitis or sarcoidosis.

CT pulmonary angiography is the study for pulmonary embolism, showing filling defects within contrast-opacified arteries.

The pulmonary nodule is assessed on size, margins, growth and calcification. Benign calcification patterns are central, diffuse, laminated or popcorn, the last indicating a hamartoma. Spiculated margins, upper lobe location, growth over time and a size above 8 mm raise concern.

A nodule stable in size over two years is usually benign, which is the basis of surveillance protocols, though this rule is less reliable for subsolid nodules.

8. Cardiac Imaging Beyond the Radiograph

Each modality answers a different cardiac question, and choosing between them follows the same logic as elsewhere.

Echocardiography is the workhorse. It assesses chamber size, wall motion, valve structure and function, pericardial fluid and ejection fraction, at the bedside and without radiation. It is the first-line investigation for almost every structural cardiac question.

CT coronary angiography has a specific strength worth understanding. Its negative predictive value is very high, so a normal study effectively excludes significant coronary disease, which makes it most useful in patients at low to intermediate probability where the aim is to rule disease out rather than confirm it.

The coronary calcium score quantifies calcified plaque burden and refines risk estimation in asymptomatic people, since calcium is a marker of atherosclerosis rather than of stenosis.

Cardiac MRI is the reference standard for ventricular volumes and for tissue characterisation. Late gadolinium enhancement distinguishes the pattern of scar: subendocardial or transmural enhancement in an arterial territory indicates infarction, while mid-wall or patchy enhancement not respecting a territory indicates a non-ischaemic cardiomyopathy or myocarditis.

That distinction matters because it separates a cause that may be revascularised from one that will not be.

Recognisable cardiac silhouettes

A few configurations remain examinable on the plain film.

A boot-shaped heart with a concave pulmonary bay and reduced pulmonary vascularity suggests tetralogy of Fallot. An egg on a string with a narrow mediastinum suggests transposition of the great arteries. A figure of three sign on the aortic contour and rib notching suggest coarctation.

A globular, water-bottle heart with clear lung fields suggests a large pericardial effusion, and the discrepancy between a very large heart shadow and normal pulmonary vasculature is what distinguishes it from cardiac failure.

9. The Review Areas and the Systematic Read

The eye finds the obvious abnormality and then stops looking, which is why a system matters more than knowledge in day-to-day reporting.

A workable order is airway, breathing, circulation, diaphragm, everything else, then the review areas.

The review areas are the places where findings are habitually missed, and they are examined deliberately at the end rather than in passing.

The lung apices, obscured by clavicles and first ribs, where a Pancoast tumour or apical tuberculosis hides.

Behind the heart, where a left lower lobe collapse or a hiatus hernia sits, which is why the retrocardiac region is inspected specifically for a double contour or lost hemidiaphragm.

Below the diaphragm, where free gas, a gastric bubble in the wrong place or a distended stomach appears.

The bones, where a rib metastasis, a fracture or a lytic lesion is easily overlooked because attention is on the lungs.

The soft tissues and the periphery of the film, where surgical emphysema, a missing breast shadow after mastectomy or a mastectomy-related lucent hemithorax appears.

Lines and tubes

A film taken after any intervention is read for position first.

An endotracheal tube tip should sit a few centimetres above the carina, since it advances with neck flexion and withdraws with extension. Too deep means right main bronchus intubation with left lung collapse.

A central venous catheter tip should lie at the cavoatrial junction, and the film also excludes pneumothorax after insertion.

A nasogastric tube must be seen to pass below the diaphragm and deviate to the left, and a tube that follows the bronchial tree is in the airway. Feeding through a misplaced tube is a recognised never event, and pH testing plus radiography are used precisely because auscultation is unreliable.

10. Worked Examples

Example 1. A film shows an opacity that obscures the right heart border but leaves the right hemidiaphragm sharply defined. Where is the lesion, and what is the reasoning?

The right middle lobe.

The silhouette sign depends on the fact that two structures of the same radiographic density in direct contact lose the interface between them. The right heart border is visible on a normal film because aerated right middle lobe lies against it, and air and soft tissue differ enough in density to produce a line.

When the right middle lobe fills with fluid, pus or cells, it becomes the same density as the heart, and the border disappears. The right hemidiaphragm remains sharp because the right lower lobe, which lies against it, is still aerated.

The value of this reasoning is that it localises the lesion in the anteroposterior axis using only a frontal film, since the middle lobe lies anteriorly against the heart while the lower lobe lies posteriorly against the diaphragm.

Example 2. A breathless patient has a homogeneous opacity occupying the left hemithorax with the trachea deviated toward the opacity. What does this tell you?

The trachea moving toward the abnormality indicates volume loss, so this is collapse rather than a simple effusion.

A large pleural effusion adds volume and pushes the mediastinum away from the opacity. Collapse removes aerated volume, so everything is pulled toward it: the trachea and mediastinum shift ipsilaterally, the hemidiaphragm rises and the ribs crowd.

The important inference is causal. Complete collapse of a lung in an adult implies obstruction of the main bronchus, and the commonest causes are a bronchogenic carcinoma, a mucus plug or an inhaled foreign body.

A combination is also possible and is a specific trap. An effusion with the mediastinum shifted toward it means there is collapse as well, usually from an obstructing tumour, and attributing everything to the effusion would miss the lesion causing it. CT and bronchoscopy follow.

Example 3. A hypotensive trauma patient has absent breath sounds on the right with tracheal deviation to the left. A junior doctor requests a chest radiograph. Comment.

The film should not be waited for. This is tension pneumothorax, which is a clinical diagnosis, and the immediate treatment is decompression followed by intercostal drain insertion.

The pathophysiology explains the urgency. A one-way valve allows air into the pleural space without escape, so intrapleural pressure rises progressively, collapsing the lung, shifting the mediastinum, kinking the great veins and reducing venous return. Death results from obstructed cardiac filling rather than from hypoxia alone.

Waiting for radiographic confirmation is a recognised cause of avoidable death, and delay in a hypotensive patient can be measured in a few minutes before arrest.

The corollary is worth noting: on a supine film, as most trauma films are, pleural air collects anteriorly rather than apically and may show only a deep sulcus sign, so the film can be falsely reassuring even when it is obtained.

Example 4. A portable anteroposterior film in a ventilated patient is reported as showing cardiomegaly. Comment.

Cardiomegaly cannot be diagnosed on an anteroposterior film, and this report should not be acted on.

The cardiothoracic ratio threshold of 0.5 assumes a posteroanterior projection taken with the patient upright at a standard distance. In that arrangement the heart, which lies anteriorly, sits close to the detector, so magnification is minimal.

On an anteroposterior film the beam enters from the front and the heart lies further from the detector, so it is magnified. Portable films also use a shorter focus-to-film distance, which magnifies further, and supine positioning increases venous return and pulmonary blood volume, broadening the mediastinum.

The practical rule is that an AP film can be used to exclude a large heart if the cardiothoracic ratio is normal, but an apparently enlarged heart requires either a proper posteroanterior film or echocardiography before the finding is accepted.

Example 5. A patient with advanced HIV has cough and fever with a normal-looking chest radiograph. Can pulmonary tuberculosis be excluded?

No. In advanced HIV with significant immunosuppression, the classical radiographic appearances of tuberculosis break down.

Cavitation depends on a vigorous cell-mediated immune response that liquefies caseous material, and as CD4 counts fall that response is lost, so cavities become less common. Instead the pattern shifts toward lymphadenopathy, lower zone involvement, miliary spread and, importantly, a chest radiograph that may look entirely normal despite culture-positive pulmonary disease.

The clinical consequence is that a normal film does not exclude tuberculosis in this population, and sputum testing is performed regardless. Nucleic acid amplification testing with rifampicin resistance detection is the appropriate first-line investigation, since it is rapid, sensitive in this setting and simultaneously identifies drug resistance.

The wider lesson is that radiographic patterns describe the host response as much as the organism, so any condition altering that response alters the picture.

Summary

Radiographs show five densities, and soft tissue and fluid are indistinguishable.

Two structures of the same density in contact lose the line between them.

The silhouette sign localises a lesion in the anteroposterior axis on a frontal film.

Check rotation, inspiration and penetration before interpreting.

A poorly inspired film fakes cardiomegaly and basal shadowing.

An anteroposterior film magnifies the heart, so cardiomegaly cannot be assessed on it.

Consolidation preserves volume and produces air bronchograms.

Collapse loses volume, so structures shift toward the abnormality.

An air bronchogram effectively excludes complete lobar collapse.

Around 200 to 300 mL of fluid is needed before an effusion blunts the costophrenic angle.

An effusion with mediastinal shift toward it means collapse as well.

Tension pneumothorax is a clinical diagnosis and must not await a film.

A supine pneumothorax may show only a deep sulcus sign.

Anterior mediastinal masses are the four Ts.

Cardiomegaly is a cardiothoracic ratio above 0.5 on a posteroanterior film.

Kerley B lines indicate interstitial fluid from thickened septa.

Primary tuberculosis shows lower zone consolidation with lymphadenopathy.

Reactivation tuberculosis favours upper lobe apical and posterior segments with cavitation.

In advanced HIV, cavitation is lost and a normal film does not exclude tuberculosis.

A nodule stable over two years is usually benign, and popcorn calcification indicates a hamartoma.