In moderate to large pleural effusions, atelectatic lung is frequently noted on the ultrasound images. Remember, when the lung is not collapsed, you would only see the artifacts (A-lines or B-lines) and not the tissue architecture. This collapsed lung within the effusion moves with respiration and appears like a jellyfish. Also known as the ‘whale tail’ sign.

Points to note:

Jellyfish sign favors atelectasis over pneumonia as the collapsed lung can change its shape with respiration whereas pneumonia is more firm.

It also favors transudative etiology of the effusion as the viscosity of exudative effusions tend to hamper mobility of the lung tissue. This is not definitive though. Think of exudate if you notice any echogenic structures within the effusion such as septations or bright particles.

Here are more examples

You may also see prominent B-lines in cases of pleural effusion, especially when there is not a lot of atelectasis. Examples: (screen indicator to the right = cardiac preset = towards head in this case)