Atelectasis on CT scan: plain English explanation

For patients with atelectasis on CT or chest X-ray  ·  7 min read

If your CT or chest X-ray report mentions "atelectasis," "bibasilar atelectasis," "subsegmental atelectasis," or "linear atelectasis" — this guide explains what each means and when (if ever) it is something to be concerned about.

What is atelectasis?

Atelectasis (pronounced at-eh-LEK-tah-sis) comes from Greek words meaning "incomplete expansion." It refers to the collapse or partial collapse of lung tissue — specifically, the deflation of the small air sacs (alveoli) that make up lung tissue. When these air sacs collapse, the affected lung area can no longer participate in gas exchange.

On CT, atelectatic lung appears denser (whiter) than normally aerated lung, often in wedge-shaped or linear bands, and is usually associated with volume loss in that area.

Types of atelectasis

Subsegmental / linear atelectasis

The mildest form. Appears as thin linear bands, usually at the lung bases, often running horizontally. This is very commonly seen on CT and is usually clinically insignificant — particularly in outpatients or those who were lying flat during the scan. The common phrase in reports is "bibasilar subsegmental atelectasis" (small bands at the bottom of both lungs).

Segmental atelectasis

A larger area of collapse affecting one segment of a lung lobe. More significant than linear atelectasis but still often treatable with breathing exercises, physiotherapy, or addressing the underlying cause.

Lobar atelectasis

Collapse of an entire lobe of the lung. This is clinically significant and typically requires investigation to find the cause — most commonly an obstructing mucus plug, foreign body, or mass in the airway, or extrinsic compression by fluid or a mass.

Compressive atelectasis

Lung compressed by external pressure — most often from a pleural effusion (fluid around the lung) or a pneumothorax pushing on adjacent lung tissue.

Why bibasilar atelectasis is so common and often benign

The most frequently mentioned atelectasis finding — bibasilar atelectasis — refers to small areas of collapsed tissue at the bottom portions of both lungs. It is extremely common and rarely clinically significant for the following reasons:

• Gravity: When lying flat (as during any imaging scan), the lower lung zones receive less airflow and the weight of the chest wall encourages partial collapse
• Shallow breathing: Patients asked to hold still or breathe shallowly during scans don't fully expand their lower lung bases
• Post-operative: After any surgery involving general anaesthesia, reduced deep breathing leads to dependent atelectasis — it is expected and usually resolves with ambulation and breathing exercises
• Pain: Patients with rib fractures, abdominal pain, or pleurisy breathe shallowly, causing dependent atelectasis

When atelectasis is more significant

Atelectasis deserves closer attention when:

• It involves an entire lobe (lobar atelectasis)
• It is causing significant oxygen desaturation or respiratory symptoms
• There is an obstructing mass or foreign body in the airway causing it
• It is new and unexplained in a patient without an obvious cause (surgery, immobility)
• It is associated with a post-obstructive pneumonia (infection behind a blocked airway)

Treatment and resolution

Minor atelectasis usually resolves on its own with deep breathing, ambulation, and incentive spirometry (a device used after surgery to encourage deep breaths). More significant atelectasis may require bronchoscopy to clear obstruction, treatment of the underlying pleural effusion, or physiotherapy.

Frequently asked questions

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