First and second year — the floor first, then every step
Ultrasound sizes and characterises the fluid and guides the needle; CT separates empyema from lung abscess and finds pleural thickening that suggests malignancy.
Orient first
Simple-appearing fluid can be transudate or exudate; septated or complex fluid is almost always an exudate.
Empyema has enhancing, thickened parietal and visceral pleura separated by fluid — the split pleura sign — and a lens shape that compresses lung; a lung abscess is round with an irregular wall and makes acute angles with the chest wall.
Nodular, circumferential or mediastinal pleural thickening over 1 cm suggests malignancy.
Acquire the study
Patient sitting; intercostal scanning from the posterior axillary line; mark the deepest pocket for drainage.
Septations and loculations; floating echoes that swirl.
Maximal depth in mm at the site you would drain, the depth of the diaphragm and lung, and intercostal vessels on Doppler.
Pleural nodules and diaphragmatic thickening.
What confirms it
Empyema: split pleura sign with a complex collection; confirmed by aspiration.
What licenses you to exclude it
Imaging cannot separate transudate from uncomplicated exudate; the fluid analysis does.
The classic misread
Calling consolidated lung (hepatisation with air bronchograms) fluid.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Normal limits
Pleural fluid (US detectability) · How ultrasound finds fluid versus the already-registered CXR entry
ultrasound detects small dependent collections that a frontal radiograph can miss — the already-registered pleural-effusion-detectable entry is the parent; this row is the US sentence, not a fake mL
A tiny dependent sliver is not an empyema. Septations are the drain-planning look-for.
USG
Diagnostic criteria
Pleural fluid · Attenuation of pleural fluid
simple transudative fluid near water (roughly 0–20 HU); higher attenuation raises exudate, haemorrhage or proteinaceous fluid but overlaps widely
HU cannot reliably separate transudate from exudate. Acute haemothorax is denser (see the abdominal haematoma HU entry for the same physics) and may show a fluid–fluid level.
CT
Loculated pleural collection · Loculation versus free fluid
a named non-dependent, lenticular or septated collection; free fluid layers — the already-registered pleural-fluid-HU and US-detectability entries are the companions
Loculation changes the drain plan. Ultrasound septations are the look-for.
CT · USG · X-ray
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.