4. Lung Ultrasound Signs and Findings

Pleural Effusions

Pleural effusions occur when transudate or exudate builds up in the base of the lungs. They are best viewed from the PLAPS-point where the lower lobes are viewed. Point of Care Ultrasound (POCUS) is over 90% sensitive and specific for the detection of pleural effusions.

In this section you will learn how to recognize pleural effusions, calculate pleural effusion size, look for all of the common ultrasound signs.

Simple Pleural Effusion on Ultrasound
Pleural Effusion on Ultrasound

Calculating Pleural Effusion Size

The simplest way to estimate pleural effusion volume is to use the formula by Balik et al. It involves measuring the maximal effusion diameter ( in millimeters) between the diaphragm and base of the lung in a supine patient (see Figure Below) and multiplying that number by 20.

Pleural volume (mL) = (measured distance in mm) x 20.

Balik method for quantifying pleural effusion fluid volume

Ultrasound Signs of Pleural Effusions

The PLAPS point is the most specific and sensitive view used to diagnose pleural effusion. If you have a patient with a suspected pleural effusion, the following signs/findings that can help you arrive at a proper diagnosis:

  • Spine Sign
  • Jellyfish sign
  • Sinusoid Sign
  • Quad Sign
  • Plankton Sign
  • Hematocrit Sign
  • Loculated Pleural Effusions

Spine Sign

You will find a Spine sign at the PLAPS point along with a pleural effusion.

In a normal lung, you should be able to see the spine up until the edge of the diaphragm, but never passing the diaphragm. This is because the lung’s air above the diaphragm prevents any sound waves from passing.

However, in pleural effusions, sound waves can pass through the pleural fluid allowing the spine to be seen above the diaphragm.

Spine Sign Ultrasound Pleural Effusion Consolidation
Spine Sign

Jellyfish Sign

You will find the Jellyfish sign at the PLAPS-point, located slightly above the diaphragm, in the RUQ. Recall that the PLAPS-point is the key location to find pleural effusions. The “Jellyfish Sign” occurs when a consolidated lung is seen floating in the pleural effusion.

Jellyfish Sign Lung Ultrasound
Jellyfish Sign on Ultrasound

Sinusoid Sign

  • You can turn on M-mode to look for the Sinusoid sign (looks like a sine wave). It is caused by the parietal and visceral pleura moving closer and further apart while the patient breathes. (White arrows point to the lung line/visceral pleura while black arrows point to the pleural line/parietal pleura). It is equivalent to an M-mode view of the jellyfish sign.
Sinusoid Sign on Lung Ultrasound
Sinusoid Sign

Quad Sign

  • A pleural effusion has an anechoic appearance often delineated by the pleural line, the rib shadows, and the lung line, called the “Quad Sign.”
Quad Sign Lung Ultrasound Unlabeled
Quad Sign Lung Ultrasound Labeled

Plankton Sign

Now that you know how to detect a pleural effusion on point of care ultrasound (POCUS), you can further differentiate pleural effusions into transudative or exudative. Though most effusions are transudative, exudative effusions can have the plankton sign, as seen below. The plankton sign shows an effusion with swirlinghyperechoic debris.

Plankton Sign Pleural Effusion Lung Ultrasound
Plankton Sign on Lung Ultrasound

Hematocrit Sign

The hematocrit sign refers to the echogenic layering of material in a pleural effusion. This can be due to exudative effusions or a hemothorax (Chichra A).

Hematocrit Sign Pleural Effusion Lung Ultrasound
Hematocrit Sign on Lung Ultrasound

Transudative vs Exudative Pleural Effusion

Here are some differences between transudative and exudative pleural effusions:

Transudative Pleural EffusionExudative Pleural Effusion
Ultrasound Findings– Absent plankton sign/hematocrit sign
– Typical effusion signs
– Present plankton sign/hematocrit sign
-Typical effusion signs
Fluid Quality– Transudate with low protein and cell count
– Normal glucose levels
– Cloudy transudate with high protein and cell count
– Light’s Criteria: Pleural fluid protein >0.5 and LDH >0.6 or LDH
– Low glucose levels
Mechanism– Increased capillary hydrostatic pressure and/or decreased capillary oncotic pressure (low albumin).– Increased vascular permeability from inflammation/infection/malignancy
Common Etiologies– Heart failure, nephrotic syndrome, liver cirrhosis– Trauma (hemothorax), bacterial infections, lymphatic obstructions, malignancies, pulmonary embolism

Loculated Pleural Effusion

  • Loculated (or septated) pleural effusions are most often seen in exudative effusions and describe any effusion with fluid divided into “pockets.” They can be caused by infections, abscesses, scarring, or fibrosis in the pleural cavity that complicates proper fluid drainage.

The image below shows an image of a loculated pleural effusion that lies above the diaphragm.

Loculated Pleural Effusion Lung Ultrasound
Loculated Pleural Effusion
4. Lung Ultrasound Signs and Findings

Air Bronchograms

Consolidations can also present with static and/or dynamic air bronchograms. On ultrasound, this trapped air appears as small hyperechoic specks.

  • Dynamic Air Bronchograms tend to occur in pneumonia and move as the patient inhales and exhales.
  • Static Air Bronchograms tend to occur when air bubbles are trapped behind an obstruction, as occurs in atelectasis, and don’t move with respiration.
Dynamic Air bronchogram Lung Ultrasound
Dynamic air bronchograms at PLAPS point
4. Lung Ultrasound Signs and Findings


As even more fluid builds up in the lung, parts of the lung can become completely fluid-filled, leading to consolidation. This is commonly seen in pneumonia. Note that consolidation can also be due to atelectasis from airway obstruction (i.e. mucous plug) or extrinsic compression (i.e. large pleural effusion).

As fluid build-up progresses, your ultrasound findings will progress from multiple B-lines, confluent B-lines, subpleural consolidation, the shred sign, to a dense consolidation. Once the air is completely gone from the lung and replaced with fluid this will result in an echogenic structure on ultrasound similar to echogenicity of the liver. This is termed “hepatization of the lung.”

Here is an example of subpleural consolidation:

Subpleural Consolidation
Subpleural Consolidation

The Shred sign

The border at which the fluid or pus-filled lung tissue meets air looks like a jagged echoic edge which is known as the “Shred sign” shown below (Lichtenstein 2009).

The shred sign lung ultrasound thoracic pneumonia
This image shows a subpleural consolidation abutting the pleural line (arrowheads) with a tissue-like pattern. This figure highlights the shredded lower border, limited by the arrows, which is the shred sign. (Lichtenstein 2009)

Dense Lung Consolidation

When lung is completely filled with fluid or is completely collapsed (atelectasis) you will have a dense appearing lung consolidation (Hepatization of the lung)

Lung Consolidation Ultrasound
Dense Lung Consolidation
4. Lung Ultrasound Signs and Findings

Interstitial Edema/Syndrome and B-lines

The lung interstitium, or the network of tissue that supports the alveoli and blood vessels, can thicken or fill with fluid.

B-lines form when interlobular septa and lung tissue thicken or fill with fluid. Thus, many clinicians have equated B-lines with “wet lung.” Remember, A-lines are horizontal while B-lines are vertical.

In order to be sure you are seeing B-lines, make sure they have these following qualities (Lichtenstein, et al):

  • Appear ray-like, hyperechoic, and vertical.
  • Emanate from the pleural line.
  • Move with lung sliding.
  • Extend to the periphery of the far-field.
  • Can be associated with a thickened pleural membrane.
Few Scattered B-Lines
Few Scattered B-Lines

It is worth noting that patients with chronic scarring of their septa from pulmonary fibrosis, old infections, or interstitial lung disease have chronically widened/thickened septa that also create B-lines even without a “wet lung” etiology. Furthermore, lung fissures between the lobes can produce single B-lines, so less than 3 B-lines in a single field of view is generally considered normal.

Confluent B-lines

  • As more fluid builds ups, it will become increasingly difficult to differentiate between singular B-lines. Thus, as more and more B-lines converge, they can create an appearance of “Confluent B-lines.”
Confluent B-Lines Lung Ultrasound
Confluent B-lines
4. Lung Ultrasound Signs and Findings

Absent Lung Sliding

Lung sliding may be reduced or absent in a variety of circumstances. There is no lung sliding present when the parietal and visceral pleura become separated by air (pneumothorax) or fluid (pleural effusion). Furthermore, severe COPD or anything that hyperinflates the lungs can markedly reduce pleural sliding.

Absence of lung sliding can be seen using B-mode or using M-mode (stratosphere sign and barcode sign).

Absent Lung Sliding Ultrasound
Absence of Lung Sliding – B Mode
Stratosphere and Bar Code Sign Lung Ultrasound Pneumothorax
Absence of Lung Sliding – M Mode

The most clinically relevant fact about lung sliding is that if lung sliding is present, you can rule out pneumothorax with 100% sensitivity. On the flip side, the absence of lung sliding can have many causes with pneumothorax being one of them.

Thus, if your patient does not have lung sliding, you must use your patient’s clinical features to consider other causes such as severe consolidation, chemical pleurodesis, acute infectious or inflammatory states, fibrotic lung diseases, acute respiratory distress syndrome, or mainstem intubation.

4. Lung Ultrasound Signs and Findings

Lung Ultrasound signs and Findings

Before we look at specific lung diseases with ultrasound, you will first need to understand the pathological signs and findings of lung ultrasound.

Lung ultrasound pathology profiles such as pneumothorax, pneumonia, cardiogenic pulmonary edema, etc will have a different combination and distribution of these pathological lung ultrasound findings/signs. Once you understand these basic lung ultrasound findings you will be able to interpret just about any lung ultrasound images.