Thoracic cage and pleura
Thoracic cage, diaphragm and pleura on imaging
The chest radiograph is framed by ribs, diaphragm and pleura, and many abnormalities show first at their edges. CPSP asks about rib and diaphragm anatomy, the diaphragmatic openings, the pleural reflections, and how air, fluid and pleural disease look on imaging.
Ribs and sternum
- Ribs 1–7 are true; 8–12 false; 11–12 floating. The head of a typical rib meets its own vertebra and the one above (6th rib: T5 and T6 bodies); ribs 11 and 12 have no tubercle joint.
- A rib usually breaks just in front of its angle. A first rib fracture means high energy (check subclavian vessels and plexus). Posterior rib fractures in an infant suggest non-accidental squeezing.
- Cervical rib from C7 (its transverse processes slope down; T1's slope up) compresses the lower trunk and subclavian artery.
- Rib notching of the inferior borders of ribs 3–8 in coarctation is from dilated intercostal collaterals; ribs 1–2 are spared because their arteries come from the costocervical trunk.
- Sternal angle: 2nd costal cartilage, level of the T4–T5 disc, where the azygos vein enters the SVC. Xiphisternum at T9. Costal margin = 7th–10th cartilages.
- Pectus excavatum blurs the right heart border and shifts the heart left. A dark hemithorax with normal lung markings suggests absent pectoralis major or mastectomy.
- Adequate inspiration: the right dome meets about the 6th anterior rib midclavicular; flat domes suggest emphysema.
Intercostal space
- Layers: external, internal and innermost intercostals, then endothoracic fascia, then parietal pleura. The vein, artery and nerve run in the costal groove between internal and innermost layers.
- Needles go just above the lower rib; near the spine the artery lies in mid-space, so posterior taps stay well lateral. A block at the rib angle catches the lateral cutaneous branch.
- The internal thoracic artery runs about 1 cm from the sternal edge and ends in the 6th space as superior epigastric and musculophrenic arteries. The first posterior intercostal vein drains to the brachiocephalic vein.
- A chest-wall lesion has obtuse angles with the wall and a fading outer margin; check the ribs.
Diaphragm
| Opening | Level | Contents |
|---|---|---|
| Caval (in the central tendon) | T8 | IVC, right phrenic nerve; tendon pulls it open in inspiration |
| Oesophageal (muscular, right crus) | T10 | Oesophagus, vagal trunks, left gastric branches; the right crus acts as an external sphincter |
| Aortic (behind the median arcuate ligament) | T12 | Aorta, thoracic duct, azygos vein; not compressed in inspiration |
- The right crus (L1–L3) is longer than the left (L1–L2). Splanchnic nerves pierce the crura.
- Central tendon develops from the septum transversum. Motor supply: phrenic nerve C3, C4, C5 (accessory phrenic from C5 may persist). Sensory: centre by the phrenic (shoulder-tip pain), periphery by lower intercostal nerves (lower lobe pneumonia can mimic appendicitis).
- On a lateral film the left dome is lost anteriorly where the heart sits on it.
- Phrenic palsy (cardiac surgery, interscalene block): raised dome moving paradoxically up on sniffing. Eventration: smooth bulge of a thin intact diaphragm, often anteromedial right.
- Pregnancy raises the dome about 4 cm but increases its excursion.
| Hernia | Site and clue |
|---|---|
| Bochdalek | Posterolateral, usually left; newborn with bowel in the chest, scaphoid abdomen; death from lung hypoplasia |
| Morgagni | Anterior, right, retrosternal; colon and omentum in an adult; mimics a cardiophrenic fat pad |
| Traumatic rupture | Usually left; nasogastric tube curled above the diaphragm |
| Sliding hiatus hernia | Retrocardiac mass with an air–fluid level |
Pleura: reflections
| Line | Lung lower border | Pleura lower border |
|---|---|---|
| Midclavicular | 6th rib | 8th rib |
| Midaxillary | 8th rib | 10th rib |
| Paravertebral | T10 | T12 (below the 12th rib medially) |
- Pleura reaches below the 12th rib beside the spine, so renal surgery or a low posterior approach can cause pneumothorax. The cervical pleura rises above the clavicle under the suprapleural membrane: supraclavicular blocks and subclavian lines can puncture it.
- The costodiaphragmatic recess is the lowest pleural space: fluid collects there first, and a low intercostal puncture can cross it.
- The pulmonary ligament runs down from the hilum with the inferior pulmonary vein at its upper end.
- Parietal pleura has somatic nerves and is pain-sensitive; visceral pleura has only autonomic supply, so lung biopsy needles hurt only at the parietal layer.
Air, fluid and pleural disease on imaging
| Finding | Interpretation |
|---|---|
| Blunted posterior costophrenic angle on the lateral film | Earliest sign of a small effusion |
| Lateral angle blunted on the PA film | About 200 mL of fluid |
| Veil-like haze with vessels visible, supine film | Effusion layering posteriorly |
| "Raised" dome with its peak lateral and a wide gap to the gastric bubble | Subpulmonic effusion |
| Oval fissural opacity clearing with diuretics | Fluid in a fissure |
| Deep lucent costophrenic sulcus, supine | Pneumothorax in the anteroinferior pleural space |
| Straight horizontal line on an erect film | Hydropneumothorax (no meniscus) |
| No lung sliding with a "lung point" on ultrasound | Pneumothorax |
| Septated fluid on ultrasound | Exudate (anechoic fluid may be either) |
| Dense dependent fluid after rib fractures (about 50 HU) | Haemothorax |
| Lens-shaped collection, obtuse angles, enhancing split pleural layers | Empyema (an abscess is round with acute angles) |
| Calcified plaques on domes and posterolateral walls, apices spared | Asbestos plaques (parietal pleura) |
| Nodular rind over 1 cm including mediastinal pleura and fissures | Malignant mesothelioma |
- On ultrasound, the liver's mirror image above the diaphragm vanishes with an effusion, and the spine becomes visible.
Remember Pleura goes two ribs lower than lung: 8, 10 and 12 for pleura against 6, 8 and 10 for lung.
Exam trap The phrenic nerve passes through the caval opening only on the right; the vagal trunks use the oesophageal hiatus.
Exam trap A supine film can hide both a large effusion and a pneumothorax; look for the haze and the deep sulcus.
Test yourself on this topic
5 of the 82 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A ventilated trauma patient has a supine chest radiograph showing an unusually deep, lucent left lateral costophrenic sulcus and a sharp left heart border, but no visible pleural line at the apex. Where does free pleural air collect first when a patient lies supine?
Answer: Anteroinferior pleural space
Air rises to the least dependent part of the pleural cavity. In the supine patient this is the anterior and inferior pleural space over the front of the lung base and the anterior costophrenic recess, not the apex. Air here deepens and lucentises the costophrenic sulcus (the deep sulcus sign) and sharpens the heart border and diaphragm. Supine films can therefore miss a sizeable pneumothorax, which CT or ultrasound will show.
Supine pneumothorax
- Air rises anteriorly when supine
- Collects anteroinferiorly
- Deep sulcus sign
- Apex may look normal
Remember Supine trauma film: look for a deep sulcus.
Exam trap Positive pressure ventilation can rapidly turn an occult supine pneumothorax into a tension pneumothorax.
Why the other options are wrong
- Apex of the pleural cavity The site in an erect patient, not a supine one.
- Posterior costophrenic recess Dependent when supine; fluid, not air, gathers there.
- Within the horizontal fissure Air may enter a fissure but not as the first site.
- Posterior paravertebral gutter Dependent posteriorly when supine.
On a normal lateral chest radiograph, one hemidiaphragm can be traced from the front of the chest to the back, while the other is lost from view anteriorly. Which hemidiaphragm is lost anteriorly, and why?
Answer: Left, where the heart sits on it
A border is seen only where tissues of different density meet. The heart rests on the anterior part of the left hemidiaphragm, so soft tissue of the heart abuts soft tissue of the diaphragm and the outline disappears (the silhouette sign). The right hemidiaphragm has aerated lung above it along its whole length, so it can be followed from front to back. The gastric bubble under the left side also helps to identify it.
Hemidiaphragms on lateral CXR
- Right: seen front to back
- Left: lost anteriorly
- Heart silhouettes the left dome
- Gastric bubble under the left
Remember Heart hides the front of the left dome.
Exam trap Loss of the posterior part of the left dome on the lateral view suggests left lower lobe disease.
Why the other options are wrong
- Right, where the liver lies under it The liver lies below; lung above keeps the right outline visible.
- Left, where the stomach lies under it Gas in the stomach actually helps to outline the left side.
- Right, where the IVC passes through it The IVC does not hide the whole anterior right hemidiaphragm.
- Left, because it lies lower Height does not cause loss of the outline.
A 70-year-old man admitted with heart failure has a chest radiograph showing a sharply marginated oval opacity in the right mid zone, lying along the line of the horizontal fissure. After three days of diuretics it has disappeared. What did the opacity represent?
Answer: Fluid within a fissure
In heart failure, pleural fluid can collect between the layers of visceral pleura lining a fissure, most often the horizontal fissure. Seen in profile it forms a lens-shaped or oval density with tapered ends along the fissure line that can look like a mass. It disappears quickly when the heart failure is treated, which is why it has been called a vanishing or phantom tumour.
Vanishing tumour
- Fluid in a fissure in heart failure
- Usually the horizontal fissure
- Lens-shaped, tapered ends
- Disappears with diuretics
Remember Mass along a fissure in heart failure: fluid.
Exam trap Repeat the film after treating heart failure before investigating a fissural 'mass' further.
Why the other options are wrong
- Round pneumonia Would not lie neatly along the fissure or resolve in days with diuretics.
- Pulmonary infarction A wedge-shaped pleural-based density that resolves slowly.
- Bronchogenic carcinoma Would not vanish with diuretics.
- Pulmonary hamartoma A stable coin lesion that does not resolve.
Before coronary bypass surgery, the left internal thoracic artery is harvested as it runs down behind the costal cartilages about 1 cm from the sternal edge. In which intercostal space does it end, and into which two branches does it divide?
Answer: 6th space; superior epigastric and musculophrenic
The internal thoracic artery arises from the first part of the subclavian artery, descends behind the upper six costal cartilages about 1 cm from the sternal edge, and gives anterior intercostal, perforating and pericardiacophrenic branches. In the 6th intercostal space it divides into the superior epigastric artery, which enters the rectus sheath, and the musculophrenic artery, which follows the costal margin. Its length and patency make it the preferred bypass graft.
Internal thoracic artery
- From the subclavian artery
- About 1 cm lateral to the sternum
- Ends in the 6th space
- Into superior epigastric + musculophrenic
Remember Sixth space: superior epigastric and musculophrenic.
Exam trap Superior and inferior epigastric arteries anastomose in the rectus sheath, a collateral route in coarctation.
Why the other options are wrong
- 4th space; superior epigastric and pericardiacophrenic Too high; the pericardiacophrenic branch arises near the top.
- 8th space; inferior epigastric and musculophrenic The inferior epigastric is a branch of the external iliac.
- 2nd space; anterior intercostal and perforating These are its branches along the way, not its termination.
- 10th space; superior epigastric and subcostal Too low, and the subcostal is a branch of the aorta.
During CT-guided biopsy of a peripheral lung nodule, the patient feels sharp pain as the needle passes through the chest wall into the pleural space, but no further pain as it advances through the lung surface into the nodule. Why is the lung surface painless?
Answer: Visceral pleura has only autonomic supply
The parietal pleura is supplied by somatic nerves: the intercostal nerves over the costal pleura and periphery of the diaphragm, and the phrenic nerve over the mediastinal and central diaphragmatic pleura. It is sensitive to pain, touch and temperature. The visceral pleura, like the lung, receives only autonomic fibres from the pulmonary plexus and is insensitive to cutting and piercing, which is why lung lesions are often painless until they reach the parietal pleura.
Pleural nerve supply
- Parietal pleura: somatic, pain-sensitive
- Costal: intercostal nerves
- Mediastinal, central diaphragm: phrenic
- Visceral pleura: autonomic, painless
Remember Pleuritic pain comes from the parietal pleura.
Exam trap Pleuritic chest pain means the parietal pleura is involved, even if the disease started in the lung.
Why the other options are wrong
- Visceral pleura is supplied by phrenic nerves The phrenic supplies parietal pleura over the mediastinum and central diaphragm.
- Lung tissue is supplied by intercostal nerves Intercostal nerves supply the costal parietal pleura, not the lung.
- Local anaesthetic spreads into the lung Not the reason; the visceral pleura has no somatic pain fibres.
- The parietal pleura has no nerve supply It has a rich somatic supply and is pain-sensitive.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Thorax, Thoracic cage, diaphragm and pleura