Lung Ultrasound: Evaluation for Pleural Fluid and Pneumothorax

The Basics

Think about gravity: fluid will collect in most dependent region (down); air tends to collect towards the least dependent regions (up)

Air does not reflect sound waves well. Lungs are filled with air. Rather than getting most of our information from visualizing the anatomy (as in a RUQ ultrasound, for example), much of our information comes from “artifact” or ultrasound waves being affected by phase changes.   

Pleural Fluid

Lung ultrasound has comparable accuracy to CT (sensitivity 94%, specificity 97%) & has been found to be more accurate than supine CXR in diagnosis of pleural effusion. (2)

How to:

  • Lower frequency probe (phased array)
  • Patient positioned supine
  • Probe marker towards head
  • Posterior axillary line
  • Start using liver or spleen as acoustic window (similar to FAST RUQ and LUQ views), locate hyperechoic diaphragm. Look for effusion superior to diaphragm.

What to look for:

  • Anechoic (dark) space between visceral pleura and parietal pleura.
  • “Sinusoid sign” – in M mode you can see a sinusoidal pattern of dynamic changes as the visceral pleural position changes during respiration.
  • “Spine sign” – The thoracic spine is not typically visualized as air in lungs obscures view. In the case of an effusion you are able to visualize this spine as the fluid provides a window through which the ultrasound waves are better able to travel and reflect the image of the spine to the transducer.
  • “Quad sign” - 4 boundaries forming “quad” of anechoic fluid – the 2 rib shadows on sides, parietal pleura, and visceral pleura

In the above video you can visualize anechoic (dark) pleural fluid superiorly to (up) the hyperechoic (white) diaphragm. You can see the dynamic changes with respiration. The lung appears echoic and is able to be visualized rather than having artifacts as a well-aerated lung would.  The fluid is compressing the lung.  If you look closely to the aspect of the fluid close to the transducer you can see echoic septations or proteinacous material sometimes referred to as “plankton”.

The above video demonstrates the “spine sign” where the thoracic spine is visualized deep to the pleural fluid. In a normal lung, the spine is not visualized as the air in the lung does not allow penetrance of the ultrasound waves to the depth of the spine.


Ultrasound has sensitivity around 88% and specificity near 100% for pneumothorax compared to supine chest x-ray with sensitivity around 52% and sensitivity around 99%.  (1)

How To:

M-mode appearance of pleural interface in normal lung.

M-mode appearance of pleural interface in normal lung.

  • Linear array probe (higher frequency)
  • Patient positioned supine
  • Probe marker towards head
  • Midcalvicular line 2nd-4th rib spaces

What to look for:

  • Normal lung-pleura interface:
  • Lung sliding as parietal and visceral pleura sliding against each other during respiration.
  • In M-Mode you can see a granular appearing “sandy beach” or “seashore sign”.

The pathognomonic “lung point sign” where the normal plural interface boarders the pneumothorax where there is a loss of lung sliding.
In M-mode there will be a loss of the granular “sandy beach” and you will see a “barcode sign or “stratosphere sign” with straight lines below the pleura.

In the above video of a lung without a pneumothorax you can see lung sliding as the visceral pleura slides along the parietal pleura during respiration. You can also see vertical “comet tail” lines, an artifact formed as a beam bounces between the visceral and parietal pleura.

In the above video you see a “lung point” at the border of a pneumothorax where there is parietal and visceral pleural sliding on the left and none on the right.


  1. Ding W, Shen Y, Yang J, He X, Zhang M.  Diagnosis of pneumothorax by radiography and ultrasonography: a meta-analysis.  Chest. 2011 Oct;140(4):859-66. doi: 10.1378/chest.10-2946. Epub 2011 May 5.
  2. Lichtenstein DA. BLUE-protocol and FALLS-protocol: two applications of lung ultrasound in the critically ill. Chest. 2015 Jun;147(6):1659-70. doi: 10.1378/chest.14-1313.
  3. Volpicelli G, Elbarbary M, Blaivas M, Lichtenstein DA, Mathis G, Kirkpatrick AW, Melniker L, Gargani L, Noble VE, Via G, Dean A, Tsung JW, Soldati G, Copetti R, Bouhemad B, Reissig A, Agricola E, Rouby JJ, Arbelot C, Liteplo A, Sargsyan A, Silva F, Hoppmann R, Breitkreutz R, Seibel A, Neri L, Storti E, Petrovic T, International Liaison Committee on Lung Ultrasound (ILC-LUS) for International Consensus Conference on Lung Ultrasound (ICC-LUS): International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med 2012, 38:577–591.