Articles are a collaborative effort to provide a single canonical page on all topics relevant to the practice of radiology. As such, articles are written and edited by countless contributing members over a period of time. A global group of dedicated editors oversee accuracy, consulting with expert advisers, and constantly reviewing additions.

11 results found

Obliteration of the retrosternal airspace

Obliteration of the retrosternal airspace is seen in any cause of an anterior mediastinal mass. 

Occult fracture

Occult fractures are those that are not visible on imaging, most commonly plain radiographs and sometimes CT, either due to lack of displacement or limitations of the imaging study. There may be signs of a fracture without one actually being seen. MRI or nuclear medicine studies are sometimes re...

O'Donoghue unhappy triad

O'Donoghue unhappy triad or terrible triad often occurs in contact sports, such as basketball, football, or rugby, when there is a lateral force applied to the knee while the foot is fixated on the ground. This produces the "pivot shift" mechanism. The O'Donoghue unhappy triad comprises three t...

Omega sign of epiglottitis

The omega sign refers to the thickened aryepiglottic folds and epiglottis seen in epiglottitis, when the larynx is seen endoscopically or via laryngoscope, and not to the appearance on lateral plain films. CT would show the finding, but placing a child with epiglottitis supine to CT their neck i...


An oncocalyx is a dilated tumour filled renal calyx, typically seen in patients with transitional cell carcinomas of the renal pelvis.

Onion bulb formation

Onion bulb formation is seen in hypertrophic neuropathies such as Charcot-Marie-Tooth type 1A (CMT1A) and chronic inflammatory demyelinating polyneuropathy (CIDP). It is the result of proliferation of Schwann cells and deposition of collagen 1.  Radiographic features MRI The nerve roots appea...

Onion peel sign

The onion peel sign (also called the cumbo sign) is a feature seen with complicated pulmonary hydatid cyst in which air lining between the endocyst and pericyst has the appearance of an onion peel. 

Open ring sign

The open ring sign is a relatively specific sign for demyelination, most commonly multiple sclerosis (MS), and is helpful in distinguishing between the causes of ring enhancing lesions. Interestingly open ring enhancement is not seen in neuromyelitis optica (NMO).  Radiographic features The en...

Orbital emphysema

Orbital emphysema is the presence of gas within the orbital soft tissues. It is usually due to orbital fractures communicating with the paranasal sinuses but can be caused by penetrating trauma and infection. It is a common finding also after orbital or ocular surgery.  Location preseptal pos...

Oreo cookie sign (heart)

The Oreo cookie sign refers to the appearance of a pericardial effusion on lateral radiographs of the chest. A vertical opaque line (pericardial fluid) separating a vertical lucent line directly behind sternum (pericardial fat) anteriorly from a similar lucent vertical lucent line (epicardial fa...

Owl's eye sign

The owl's eye sign represents bilaterally symmetric circular to ovoid foci of high T2-weighted signals in the anterior horn cells of the spinal cord and is seen on axial MR imaging. The sagittal corollary is a "pencil-like" vertical linear high T2-weighted signal extending usually over a number ...

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