Overview
An introduction to chest imaging: how x-rays and CT physically produce an image, the lobar anatomy of the lungs as seen on chest x-ray (CXR), and the language used to describe abnormal lung opacities. Five clinical cases then work through that vocabulary against the common pathologies it is used to describe: pulmonary embolism, pulmonary contusion, pneumonia with effusion, lung cancer, and interstitial lung disease/idiopathic pulmonary fibrosis (IPF).
Lung anatomy
- Right lung: 3 lobes — upper (RUL), middle (RML), lower (RLL).
- Left lung: 2 lobes — upper (LUL), lower (LLL). It has no middle lobe; the lingula is a tongue-shaped part of the LUL that is the left lung’s structural analogue of the RML.
- On a lateral CXR the fissures separating the lobes become visible: the right lateral view shows all three right lobes; the left lateral view shows both left lobes.
- Secondary pulmonary lobule: the basic structural unit referenced throughout the lecture. A pulmonary artery and a terminal bronchiole travel together through the centre of the lobule, supplying a cluster of alveoli (an acinus). Pulmonary veins instead drain at the lobule’s periphery, via the interlobular septa. What fills the alveoli within this unit (pus, fluid, blood or cells) is what later distinguishes pneumonia, oedema, contusion and adenocarcinoma on imaging.
How x-ray and CT work
- X-ray: an x-ray machine emits a beam through the patient onto a photographic plate or digital detector. Denser tissue (e.g. bone) absorbs more x-rays and appears white on the resulting radiograph; less dense tissue absorbs less and appears grey.
- CT: an x-ray source and a ring of detectors rotate around the patient, taking measurements from many angles that are reconstructed into cross-sectional (axial) images.
- Choosing between them:
- CXR — pros: quick, mobile, low radiation. Cons: less sensitive for some pathologies.
- CT — pros: highly sensitive for assessing all chest structures. Cons: less accessible, slower, requires IV contrast, higher radiation.
- Quantitative comparison: effective dose is ~0.02 mSv for CXR versus ~6.6 mSv for CT (equivalent to about 3.5 days versus 3.1 years of background radiation exposure). Sensitivity for pulmonary nodule detection is 45% for CXR versus 85% for CT.
Describing abnormal lungs: opacity
- “Opacity” is the general term for any area of increased whiteness on a CXR. Named subtypes:
- Consolidation — fluffy/patchy opacity.
- Interstitial — fine reticular (net-like) pattern.
- Nodule — small round opacity.
- Mass — larger round opacity.
- Atelectasis — wedge-shaped area of collapse.
- Consolidation and air bronchograms: in consolidation, the alveoli fill with material (pus, fluid, blood or cells) while the larger bronchi running through them stay air-filled. This creates air bronchograms — branching lucent (dark) airways visible against the surrounding opacified lung — a sign that confirms an opacity is alveolar/airspace consolidation rather than another process.
- Causes of consolidation, by what fills the alveoli:
- Pus — pulmonary infection (pneumonia).
- Fluid — pulmonary oedema.
- Blood — pulmonary contusion or haemorrhage.
- Cells — cancer (adenocarcinoma).
Case 1: pulmonary embolism
- 45-year-old woman, sudden chest pain and shortness of breath; recently hospitalised for surgical fixation of a fibular fracture (a VTE risk factor). HR 110, BP 125/80, T 36.9, FBC normal.
- Differential: Yours/Medwiki/Block/CVS/Reference/Disease/Pulmonary embolism (Well’s score 6, moderate risk), infection, mass. Imaging aims to confirm the diagnosis and guide treatment.
- CXR: subtle abnormality in the left lower lateral chest/costophrenic region.
- CT pulmonary angiogram: confirmed bilateral pulmonary emboli — filling defects within the pulmonary arteries bilaterally, with a peripheral wedge-shaped area (pulmonary infarct) corresponding to the CXR finding.
Case 2: pulmonary contusion
- 22-year-old male, motor vehicle rollover, trauma call activated. CXR should be performed as soon as possible in suspected chest trauma.
- Supine portable AP CXR: patchy increased opacity in the right and left mid/lower lung fields, not confined to a single lobe.
- CT: bilateral patchy ground-glass/consolidative opacities, again non-lobar in distribution — consistent with pulmonary contusion (bruising of the lung parenchyma from blunt trauma).
- Mechanism: on the secondary-pulmonary-lobule model, the alveoli fill with blood (haemorrhage) rather than pus or fluid, producing a consolidation-like opacity.
Case 3: pneumonia with parapneumonic effusion
- 3-year-old male, one week of fevers and poor oral intake. T 38.2, tachycardic, tachypnoeic, reduced breath sounds at the left lower zone.
- CXR (frontal and lateral): left lower lobe (LLL) consolidation with an adjacent pleural effusion.
- Diagnosis: LLL pneumonia with a para-pneumonic effusion (a pleural effusion arising adjacent to the pneumonia).
Case 4: lung cancer (adenocarcinoma)
- 60-year-old female, 6 months of cough with intermittent sputum, no weight loss, smoker.
- CXR: rounded opacity/mass in the left upper zone, unchanged on a follow-up film 2 months later.
- CT: a spiculated (irregular, radiating-edge) soft-tissue mass in the left upper lobe — a spiculated margin is a feature suggestive of malignancy.
- CT-guided biopsy confirmed adenocarcinoma.
- Mechanism: in adenocarcinoma the alveoli fill with tumour cells (a lepidic/alveolar growth pattern), which is why it can produce a consolidation-like opacity — distinct from pus (pneumonia) or blood (contusion).
- Staging: FDG PET-CT detects metabolically active (FDG-avid) tissue and is used for staging with the TNM framework — T = primary tumour, N = lymph nodes, M = metastases.
- Treatment: left upper lobectomy; the post-operative CXR shows the expected post-surgical changes in the left hemithorax.
Case 5: interstitial lung disease and idiopathic pulmonary fibrosis
- 55-year-old man, more than a year of progressive shortness of breath and reduced exercise tolerance.
- CXR: diffusely increased reticular/interstitial markings throughout both lungs, most pronounced at the bases, with reduced lung volumes.
- Serial CXRs from 2019, 2021 and 2023 show progressive worsening of the reticular change and volume loss over time.
- CT: bilateral peripheral and basal reticular change with honeycombing (clusters of small cystic airspaces), most marked at the bases — a usual interstitial pneumonia (UIP) pattern.
- Diagnosis: a high-confidence CT diagnosis of a UIP pattern is usually sufficient to diagnose idiopathic pulmonary fibrosis, obviating the need for lung biopsy.
- Pathophysiology: alveolar remodelling and fibrosis lead to dilated, fibrotic bronchi within a scarred interstitium, which impairs gas exchange (blocks O2 in and CO2 out). Gross pathology shows honeycombed, cystic, destroyed lung tissue at the bases with relative sparing of the apices.
- CXR can also be used to track radiographic response to treatment (pre- versus post-treatment comparison).
Self-test
- Name the lobes of the right lung and of the left lung, and identify the structure in the left lung that is the structural analogue of the right middle lobe.
- Describe the arrangement of structures within a secondary pulmonary lobule: which two structures travel together centrally, and where do the pulmonary veins drain?
- Explain why dense tissue appears white and less dense tissue appears grey on a plain radiograph.
- Describe how a CT image is acquired, from source/detector motion to the final image.
- Compare chest x-ray and CT chest in terms of effective dose, equivalent background exposure, and sensitivity for pulmonary nodule detection.
- List the five named patterns of lung opacity described on chest x-ray.
- Explain the mechanism that produces air bronchograms, and state what finding this sign confirms.
- List the four things that can fill the alveoli to cause consolidation, with the disease process each corresponds to.
- A 45-year-old woman with recent lower-limb fracture surgery develops sudden chest pain and shortness of breath. What is the leading differential diagnosis, and what did the CT pulmonary angiogram show in this case?
- Distinguish the alveolar filling process in pulmonary contusion from that in pneumonia, and describe the distribution of contusion on CT.
- A 3-year-old with a week of fever, poor oral intake and reduced left lower zone breath sounds is imaged. What did the chest x-ray show, and what is the diagnosis?
- A 60-year-old smoker has a persistent left upper zone mass on CXR that proves spiculated on CT. What did biopsy show, what staging investigation was used, and what does each letter of the TNM framework stand for?
- Explain why adenocarcinoma can produce a consolidation-like opacity on imaging.
- What CT pattern was seen in the 55-year-old man with progressive dyspnoea, and what diagnostic significance does a high-confidence finding of this pattern have?
- Describe the pathophysiological chain by which pulmonary fibrosis impairs gas exchange.
- For each of the five cases in this lecture (PE, contusion, pneumonia, cancer, interstitial lung disease), give the imaging finding that distinguished it from the others.
Answers
Reveal answers
- Right lung: upper, middle and lower lobes. Left lung: upper and lower lobes only. The lingula, a tongue-shaped part of the left upper lobe, is the structural analogue of the right middle lobe.
- The pulmonary artery and a terminal bronchiole travel together through the centre of the lobule, supplying a cluster of alveoli (acinus). The pulmonary veins drain at the periphery of the lobule, via the interlobular septa.
- X-rays are absorbed more by dense tissue (e.g. bone), so less radiation reaches the detector there, producing a white area; less dense tissue absorbs fewer x-rays, so more radiation reaches the detector, producing a grey area.
- An x-ray source and a ring of detectors rotate around the patient, taking x-ray measurements from many angles; these are reconstructed into cross-sectional (axial) images.
- CXR: ~0.02 mSv, about 3.5 days of background exposure, 45% sensitivity for nodule detection. CT: ~6.6 mSv, about 3.1 years of background exposure, 85% sensitivity for nodule detection — CT is far more sensitive but delivers substantially more radiation.
- Consolidation, interstitial, nodule, mass, atelectasis.
- The alveoli fill with material (pus, fluid, blood or cells) while the larger bronchi running through them remain air-filled, so the air-filled airways appear as branching lucent structures against the surrounding opacified lung. It confirms the opacity is alveolar/airspace consolidation rather than another process.
- Pus — infection/pneumonia; fluid — pulmonary oedema; blood — contusion/haemorrhage; cells — cancer (adenocarcinoma).
- Pulmonary embolism (Well’s score 6, moderate risk) was the leading differential. CT pulmonary angiogram confirmed bilateral pulmonary emboli, with filling defects in the pulmonary arteries bilaterally and a peripheral wedge-shaped area corresponding to a pulmonary infarct.
- In contusion the alveoli fill with blood (haemorrhage) from blunt trauma, rather than pus as in pneumonia. On CT, contusion appears as bilateral patchy ground-glass/consolidative opacity that does not follow a lobar distribution.
- CXR showed left lower lobe consolidation with an adjacent pleural effusion. Diagnosis: left lower lobe pneumonia with a para-pneumonic effusion.
- Biopsy showed adenocarcinoma. Staging used FDG PET-CT. TNM: T = primary tumour, N = lymph nodes, M = metastases.
- Because in adenocarcinoma the alveolar spaces fill with tumour cells (a lepidic/alveolar growth pattern), which produces the same kind of alveolar opacity as pus, fluid or blood do in other conditions.
- A usual interstitial pneumonia (UIP) pattern — bilateral peripheral and basal reticular change with honeycombing, worst at the bases. A high-confidence CT diagnosis of UIP is usually sufficient to diagnose idiopathic pulmonary fibrosis, avoiding the need for lung biopsy.
- Alveolar remodelling and fibrosis lead to dilated, fibrotic bronchi within a scarred interstitium; the scarred interstitium blocks the movement of O2 in and CO2 out, impairing gas exchange.
- PE: filling defects in the pulmonary arteries on CTPA. Contusion: non-lobar bilateral ground-glass/consolidative change following trauma. Pneumonia: lobar consolidation (LLL) with an adjacent effusion. Cancer: a spiculated left upper lobe mass, biopsy-proven adenocarcinoma. Interstitial lung disease: bilateral basal reticular change with honeycombing (UIP pattern) on CT.