X‑ray artefacts and mimics

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 24 October 2026 · Level: Intermediate

An artefact is something on the image that is not part of the patient's anatomy or disease: a button, a hair braid, a processing halo. A mimic is a real structure or normal variant that looks like pathology: a skin fold that looks like a pneumothorax, a nipple that looks like a nodule. Both lead to unnecessary chest drains, CT scans and anxiety, and the reverse error is just as dangerous: dismissing real disease as "probably artefact". This page groups the common traps by the finding they imitate, and gives you a test for each one.

1. Things that mimic a pneumothorax

A true pneumothorax shows a thin, white visceral pleural line, roughly parallel to the chest wall, with no lung markings beyond it. See pneumothorax.

Mimic	How to tell it apart
Skin fold	Common on AP and supine films where the patient lies on the detector. The edge often extends beyond the rib cage or runs off the chest wall; lung markings continue on both sides of it; and it is usually a broad edge where grey changes to darker grey, rather than a thin white line.
Medial border of the scapula	Trace the line: it continues into the rest of the scapula. Lung markings are visible beyond it.
Bulla	In emphysema. A large bulla has a concave inner margin facing the chest wall and may have septations crossing it. A pneumothorax edge is convex and parallel to the chest wall. Draining a bulla by mistake is dangerous; CT helps when unsure.
Clothing, tubing, sheets, gown edges	Straight or curved lines that continue outside the chest, with markings beyond them.
Mach band	An optical illusion: a dark band seen next to a bright edge (such as the heart border). It has no white line and no edge on the lung side.

2. Things that mimic a nodule or mass

• Nipple shadows. Usually bilateral and symmetrical, in the lower zones, often with a sharp lateral edge and a fading medial one. If a single one looks like a nodule, repeat with nipple markers or compare with the other side.
• Skin lesions (moles, neurofibromas, seborrhoeic keratoses): the edge may look "too sharp", because air outlines the lesion on the skin surface.
• Healing rib fracture callus: check the rib it sits on, and look for other fractures.
• Bone islands and overlapping costal cartilage calcification.
• End-on vessels and overlapping vessels where they cross.
• Fluid in a fissure (a "pseudotumour" or "vanishing tumour") in heart failure: lens-shaped, in the line of a fissure, and it resolves with diuresis. See heart failure.
• Azygos fissure: a normal variant, a curved line in the right upper zone with a teardrop-shaped azygos vein at its lower end.
• Round pneumonia in children, which can look like a mass.

A truly new nodule needs CT, because the plain film cannot characterise it. See lung nodules and masses.

3. Unilateral hyperlucency and opacity

When one side of the chest looks darker or whiter than the other, run through technical causes before diagnosing disease.

Darker side: think of	Whiter side: think of
Rotation (check the clavicles)	Rotation
Mastectomy (no breast shadow, axillary clips)	Breast tissue or a breast prosthesis on one side
Poland syndrome (absent pectoralis major)	Overlying soft tissue, a sling or a dressing
Pneumothorax, large bulla, inhaled foreign body with air trapping (in children)	Supine effusion, consolidation, collapse

Pectus excavatum deserves a special mention. The depressed sternum blurs the right heart border, mimicking right middle lobe disease, and displaces the heart to the left. Look for very horizontal posterior ribs and steeply sloping anterior ribs, and check the lateral view if there is one.

4. Gas mimics: free air and pneumomediastinum

• Chilaiditi sign: colon interposed between the liver and the right hemidiaphragm. The key test is to look inside the gas: haustral folds crossing it mean bowel, not free gas.
• Gastric bubble under the left hemidiaphragm: it has a thick wall, rugal folds and often a fluid level.
• Subphrenic abscess: an air–fluid level, often a single loculus, in an unwell patient.
• Pseudo-Rigler sign on the abdominal film: two overlapping gas-filled loops can make a bowel wall look outlined on both sides.
• Mach band along the heart border can mimic a thin rim of mediastinal gas. True pneumomediastinum usually shows a white line (the mediastinal pleura) with gas beside it and streaks extending into the neck.

Recent abdominal surgery and peritoneal dialysis are common benign causes of genuine free gas. See pneumoperitoneum.

5. External objects and technique

• Hair braids and ponytails: dense streaks over the apices that can mimic upper-zone shadowing.
• Clothing, buttons, gown snaps, jewellery, piercings: usually sharply defined and often outside the body outline.
• ECG electrodes and leads, oxygen tubing, blankets: common on ICU films. Trace every line to see where it goes.
• Companion shadows: thin soft-tissue stripes running along the upper ribs and clavicles, which can mimic pleural thickening or a pneumothorax edge.
• Motion blur: blurred heart and diaphragm outlines.
• Digital processing artefacts: edge enhancement can create a dark halo next to bright metal, which can mimic loosening around a prosthesis. Detector defects can create lines or blank areas. If a finding has perfectly straight edges or repeats on other patients' films, suspect the equipment.
• Exposure: an over-penetrated film mimics emphysema; an under-penetrated one mimics consolidation. See image quality: RIPE.

6. MSK and abdominal mimics

• Growth plates and unfused ossification centres in children mimic fractures. Know the expected ossification order at the elbow (see paediatric elbow) and use an atlas of normal variants. Comparison views of the other side should be used selectively, not routinely.
• Accessory ossicles and sesamoids (for example the os trigonum or os peroneum) are smooth, rounded and corticated all the way round, whereas an acute fracture fragment has a sharp, uncorticated edge.
• Nutrient canals: thin lucent lines running obliquely through the cortex of long bones.
• Mach effect and overlapping cortices can create apparent lucent lines through bone. Check whether the line continues beyond the bone outline.
• Phleboliths: small round pelvic calcifications, often with a lucent centre, that mimic distal ureteric stones. See calcification and stones.
• Costal cartilage calcification and tablets or contrast in the bowel can mimic stones or foreign bodies.

7. A safe approach to "is it real?"

1. Does it stay inside the patient? Lines that run outside the body or the rib cage are usually external.
2. Does it obey anatomy? Real pathology respects fissures, lobes, pleura and bone. Artefacts often ignore them.
3. Is it on the comparison film? Old films are the best artefact detector.
4. Is it on the second view? A nipple or skin lesion will not be in the lung on the lateral.
5. Can you repeat or examine? Nipple markers, removing clothing, or simply looking at the patient often settle the question.
6. If doubt remains and it matters, escalate: senior review, ultrasound or CT. Do not "explain away" a finding in an unwell patient.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	Skin fold versus pneumothorax, nipple shadows, and the Chilaiditi sign: all exam favourites.
Nurse	Remove avoidable external objects before imaging where it is safe, and know that ICU films often contain leads and tubing that are not inside the patient.
Junior doctor	Never place a chest drain on the strength of a possible skin fold or bulla. Get senior review or further imaging.
Radiographer	Prevent artefacts at source (clothing, jewellery, hair, positioning), use nipple markers when asked, and recognise equipment artefacts.
Advanced / FRCR	Processing artefacts, normal variants, and confident "normal" calls in rapid reporting.

Unfamiliar terms are explained in the glossary.

Test yourself on mimics →

References

1. Radiology Masterclass. Chest X-ray anatomy. radiologymasterclass.co.uk
2. Radiology Masterclass. Pneumoperitoneum. radiologymasterclass.co.uk
3. Roberts ME, et al. British Thoracic Society Guideline for pleural disease. Thorax 2023;78(Suppl 3):s1–s42. thorax.bmj.com
4. Radiopaedia. Pneumomediastinum. radiopaedia.org
5. Life in the Fast Lane (LITFL). Chest X-ray interpretation. litfl.com

For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.