The authors performed a literature review on the topic of radiological signs,
chose a number of signs,
some of which well-known,
others more obscure,
and then carried out a search through their case archive to find illustrative examples in the various modalities used in radiology.
The authors present each sign next to an annotated image with the specific item that gave the inspiration for its name and discuss the scientific basis behind the characteristic appearance of each case as well as the differential diagnosis.
Angle wing sign
The angle wing sign,
known also as bat’s wing,
can be seen on a frontal chest radiograph and refers to symmetrical,
perihilar ground-glass opacities (Fig. 1).
It is classically described in rapidly developing pulmonary edema (especially with cardiogenic genesis),
but could also be found in patients with pneumonia (including aspiration pneumonia),
pulmonary alveolar proteinosis,
bronchoalveolar carcinoma and others .
The “butterfly” vertebra (Fig. 2) is a relatively rare congenital vertebral anomaly,
that results from a failure in the fusion of the lateral halves of the vertebral body giving it a distinctive shape.
It appears most commonly in the lumbar spine and can occur as an isolated anomaly or in association with other musculoskeletal deformities or anomalies in other systems.
It could lead to recurrent back pain or kyphoscoliosis.
It should not be confused with a compression fracture,
metastatic disease or other pathologic conditions in order to save the patient unnecessary stress and medical procedures [1,
The comb sign is seen on contrast-enhanced CT (or MRI) images as multiple linear densities on the mesenteric side of the small bowel,
aliened as the teeth of a comb (Fig. 3).
It is due to hypervascularity,
dilatation and fibrofatty proliferation of the mesenteric artery.
Although it can also be observed in other inflammatory conditions of the bowel it is most commonly seen in the ileum of patients with Crohn disease and it suggests that the disease is advanced and clinically active .
Dural tail sign
The dural tail sign is seen as a thickening and contrast enhancement of the dura mater adjacent to a tumor.
It is believed to be due to vascular congestion and edema,
though in some cases direct tumor invasion of the dura was reported .
It is most commonly observed in cases of meningiomas (Fig. 4),
although it could also occur in some glial tumors,
sarcoidosis and other conditions.
The earth-heart sign could be seen on chest radiographs of patients with tension pneumomediastinum (Fig. 5).
It refers to the round,
Earth-like shape of the heart created by the external pressure leading to impairment of the central venous return and restriction of the diastolic filling.
Together with the classical signs of pneumomediastinum it could help in the diagnosis of this rare and potentially lethal condition .
The “fish vertebrae” sign is most commonly used to describe the biconcave deformity of the vertebral bodies in women with postmenopausal osteoporosis.
It is named due to the similarity of the shape of the affected vertebrae to the normal appearance of the fish vertebrae,
which have depressions in the superior and inferior surfaces of their bodies (Fig. 6) .
This sign can be seen in other conditions leading to osteopenia incl.
renal osteodystrophy and prolonged corticosteroids use as well as some blood disorders most notably in sickle cell disease.
The gull-wing appearance is a characteristic finding seen in the interphalangeal joints of patients with erosive osteoarthritis on posteroanterior hand radiographs [4,
It is due to the combination of joint space narrowing,
central subchondral erosions and marginal osteophytes creating an appearance similar to that of the wings of a flying seagull (Fig. 7).
The term “hamburger sign” could be used to describe the normal appearance of the facet joints in the cervical spine on axial CT images,
with the articular processes of the two articulating vertebra representing the two buns (Fig. 8).
When a facet joint is dislocated and especially in cases of locked facet,
a “reverse hamburger sign” could be seen .
Another famous “hamburger” can be found in cases of confluent lymphadenopathy,
developing both in front and behind the mesenteric vessels,
creating a “sandwich-like” appearance.
It is specific for lymphoproliferative disease (most commonly non-Hodgkin lymphoma).
Ice cream cone sign
The ice cream cone like appearance in acoustic schwannomas is formed by the tumor component in the internal acoustic canal representing the “cone” and the tumor component in the cerebellopontine angle – “the ice cream”.
The schwannoma of the vestibulocochlear nerve (Fig. 9) accounts for most of the cerebellopontine angle masses and this characteristic radiological sign could be helpful in its differentiation from a meningioma with the same location .
Bilateral acoustic schwannomas or a combination of a schwannoma with multiple meningiomas are diagnostic of neurofibromatosis type 2.
Though it should be mentioned that uncommonly metastatic lesions could mimic bilateral schwannomas.
The J-shaped sella (Fig. 10) can be seen on lateral radiographs or sagittal CT/MRI images of the sella turcica.
It is a variant in the morphology of the pituitary fossa in which the tuberculum sellae is flattened (the stem of the J) and the dorsum sellae has its normal round shape (the loop of the J).
It could be seen as a normal variant or in cases of optic glioma,
achondroplasia and other pathological conditions.
The Knuckle sign (Fig. 11) in pulmonary embolism refers to the abrupt tapering of a pulmonary artery due to a large embolus.
It could be seen best on CT pulmonary angiography scan but in some cases could also be visible on a plain chest X-ray.
When this sign is detected,
other signs of pulmonary embolism should also be considered.
It is often associated with the Fleischner sign (enlarged pulmonary artery) .
The central sulcus,
separating the frontal from the parietal lobe,
is an important landmark in the brain anatomy,
which could be surprisingly difficult to identify on cross-sectional imaging.
Many sings have been created in order to aid its correct localization.
The L sign (Fig. 12) is one of these features and it is based on the characteristic shape,
similar to the letter “L”,
that the superior frontal sulcus and the precentral sulcus create at their intersection.
After identifying the precentral sulcus,
the central one is easy to find as it is the next posterior sulcus.
In some cases,
the junction between the superior frontal sulcus and precentral sulcus might not have an "L" configuration,
but a "T" one (the upper T sign – see below) .
The Matterhorn sign (Fig. 13),
named after one of the world's most famous peaks,
could be used to describe a peak-shaped,
disc herniation that compresses the dura or even the cord,
typically seen in the thoracic spine.
Nutmeg liver is a term used both in pathology and radiology to describe the characteristic appearance of the liver in chronic passive congestion (Fig. 14).
In contrast enhanced CT/MRI images this appearance is created by the decreased portal flow and hepatic congestion leading to a specific heterogeneous pattern of contrast enhancement,
best seen in the arterial and early portal venous phase.
On delayed images the enhancement of the liver usually becomes more homogenous.
This sign is associated with right heart failure,
hepatic veno-occlusive disease and Budd-Chiari syndrome.
Omental cake refers to a diffuse abnormal thickening of the omentum creating an appearance similar to that of a cake icing (Fig. 15).
It is usually created by the infiltration of the omental fat by metastatic tumors most commonly arising from the stomach,
Ascites is often present and in general,
thick omental caking is a sign of an advanced disease.
It can also although rarely result from lymphoma or an infectious disease.
The polka-dot sign refers to the characteristic appearance of a vertebral hemangioma on axial CT images,
similar to the polka dot design pattern of equally sized and spaced circles .
The vertebral hemangiomas owe their look to the replacement of the normal cancellous bone by thickened vertical trabeculae surrounded by fat marrow - white dots on a black background (Fig. 16).
The quad sign can be observed on ultrasound in patients with pleural effusion.
The fluid itself is anechoic and is being framed between four borders – two rib shadows,
the pleural line and the lung line,
thus forming the shape of a rectangle (quad) (Fig. 17).
Rat tail sign
The rat-tail sign could be seen on fluoroscopy barium studies of the esophagus.
It refers to the acute narrowing of the dilated esophagus at the level of the lower esophageal sphincter and the gastro-esophageal junction producing a “rat tail” appearance (Fig. 18).
In some very rare instances similar changes could be seen in esophageal cancer or cases of extrinsic compression,
making additional diagnostic evaluation such as endoscopy or CT necessary .
The Shmoo sign can be seen on frontal radiographs in cases of enlarged and rounded left ventricle and dilated aorta.
These changes give the mediastinum an outline similar to that of a famous comic book character,
called the Shmoo,
shaped like a bowling pin with legs (Fig. 19).
The thumb sign (known also as thumbing of the pons) is seen in chordomas of the clival region.
It can be observed on midline sagittal cross-sectional images,
when dorsally the tumor reaches and compresses the pons (Fig. 20).
This sign is relatively specific and can help in differentiating a chordoma from a meningioma of the skull base .
This sign should not be mistaken with the other famous Thumb sign,
seen on a lateral radiograph of the neck as a manifestation of epiglottitis.
Upper T sign
The upper T sign is another sign that could be useful in the identification of the central sulcus on cross-sectional imaging.
Due to the convex surface of the brain it could be difficult to identify specific sulci and gyri especially in case of the presence of a lesion distorting the normal anatomical pattern of the brain.
recognizing the central sulcus and its area,
including the primary motor and somatosensory cortex,
could be extremely important for the correct diagnosis and treatment of the patient and in order to avoid neurologic deficits in case of surgery.
The upper T sign relies on the fact that the well-identifiable superior frontal sulcus intersects the precentral sulcus in a junction,
forming a T shape (Fig. 21).
This helps define the central sulcus which can be found immediately posterior to the T shape .
This sigh should not be confused with the lower T sign which is the confluence of the inferior frontal sulcus with the precentral sulcus.
The vacuum phenomenon (Fig. 22) is a common radiological finding in degenerative intervertebral disc.
It is due to interdiscal gas accumulation (usually nitrogen),
often accompanied by peripheral sclerosis and other degenerative changes.
Although rarely it could also be seen in osteoporotic vertebral fractures,
spinal malignancies and pyogenic discitis .
Besides in the intervertebral discs,
vacuum phenomena could also be seen in joints.
Whiskering is a term,
used to describe the short and linear spiculations (enthesophytes) that can be seen on the edges of bones at sites of muscle insertion (Fig. 23).
It is most commonly seen in the iliac,
ischial and calcaneus bones of patients with spondyloarthropathies or degenerative joint disease.
The X-marks-the-spot could be observed in complete bowel volvulus.
The “X” shape is created at the place of the twisting by the crossing bowel loops (Fig. 24).
The recognition of this sign could lead to a faster and more confident diagnosis of a cecal and sigmoid volvulus – potentially life threatening conditions,
where time could be of the essence [10,
Yin and young sign in aortic aneurysms
The yin-yang sign is named after the characteristic appearance of the aneurysms on Doppler sonography due to the bidirectional flow the blood swirling within the aneurysm .
On contrast-enhanced CT it could be seen when the patent portion of the lumen of the aneurysmal vessel appears as a high density area,
whereas the mural thrombus appears as a zone of lower attenuation (Fig. 25).
On axial slices it creates an image similar to that of the ancient Chinese symbol of yin and young or as some authors see it – the Pepsi logo.
Zebra spleen refers to the normal,
transient inhomogeneous splenic enhancement during the arterial phase (Fig. 26).
It could be seen in CT,
or ultrasound imaging and is due to the differing blood flow rates between red pulp (enhancing early) and white pulp (enhancing later) .
It should be recognized and not be mistaken with a splenic laceration,
hematoma or infarction.
Usually on a portal venous or delayed phase images the normal spleen should show homogenous attenuation.