Articles

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.

765 results found
Article

Swirl sign (intracranial haemorrhage)

The swirl sign refers to the non-contrast CT appearance of acute extravasation of blood into a haematoma, for example an extradural haematoma or subdural haematoma. It represents unclotted fresh blood which is of lower attenuation than the clotted blood which surrounds it 1. It is the corollary...
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Swischuk line

The Swischuk line is helpful in differentiating pathological anterior displacement of the cervical spine from physiological displacement, termed pseudosubluxation. Measurement the line is drawn from anterior aspect of posterior arch of C1 to anterior aspect of posterior arch of C3 the anterio...
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Swiss cheese sign (lungs)

The Swiss cheese sign has been used for the appearance on CT of fluid-containing pneumatocoeles, that typically occur following pulmonary lacerations 1. The pneumatocoeles appear as 'holes' in the lung parenchyma and hence the description with respect to Swiss-style cheeses (e.g. Emmental), whi...
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T1 black holes

T1 black holes are hypointense lesions commonly seen on T1WI in patients with multiple sclerosis, and indicates the chronic stage with white matter destruction, axonal loss and irreversible clinical outcome. There is a correlation between the number of black holes and patient positive outcome 3.
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T2 dark spot sign

T2 dark spot sign is an MRI appearance of endometriomas seen as a result of chronic haemorrhage. The sign is useful in differentiating a solitary endometrioma from a functional haemorrhagic ovarian cyst, as both might show high T1 signal with T2 shading.  The T2 dark spot, described in the sign...
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Talar beak sign (talus)

The talar beak sign is seen in cases of tarsal coalition, and refers to a superior projection of the distal aspect of the talus. It is most frequently encountered in talocalcaneal coalition. It is thought to result from abnormal biomechanic stresses at the talonavicular joint.  Differential dia...
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Tam o' shanter sign (skull)

The "tam o' shanter" is a Scottish hat, named after the character in Robert Burns' 1 poem of the same name. The appearances of advanced Paget disease of the skull are similar in appearance to the hat.  Paget involvement of the skull, with widening of the diploic space and an overall enlargement...
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Tangential calcium sign

A tangential calcium sign is a sign seen with an aortic aneurysm rupture. The calcified intimal rim is discontinuous and is seen to tangentially point away from the aneurysmal lumen. This sign is seen at the point of breach. There is associated retroperitoneal leakage.
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Target sign (choledocholithiasis)

The target sign of choledocholithiasis is a finding seen on contrast-enhanced CT and comprises: central density within the bile duct: stone surrounding low density: bile or mucosa
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Target sign (disambiguation)

There are many bull's eye signs, also referred to as target signs: red bone marrow located in the shaft of long bone with central yellow marrow on MRI peripheral plexiform neurofibromas on MRI intussusception: see target sign of intussusception choledocholithiasis: see target sign of choledo...
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Target sign (intussusception)

The target sign of intestinal intussusception, also known as the doughnut sign or bull's eye sign. The appearance is generated by concentric alternating echogenic and hypoechogenic bands. The echogenic bands are formed by mucosa and muscularis whereas the submucosa is responsible fo the hypoec...
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Target sign (pyloric stenosis)

The target sign of pyloric stenosis is a sign seen due to hypertrophied hypoechoic muscle surrounding echogenic mucosa, seen in pyloric stenosis. This is likened to that of a target. See also antral nipple sign cervix sign of pyloric stenosis shoulder sign of pyloric stenosis
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Target sign (tuberculosis)

The target sign of tuberculosis refers to the bull's eye appearance of some parenchymal tuberculomas involving the brain (see: CNS tuberculosis) and solid abdominal organs (see: hepatic and splenic tuberculosis) on cross-sectional imaging.  Radiographic features Ultrasound hypoechoic nodules ...
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Tattoo sign (mammogram)

The tattoo sign is a feature given to describe dermal calcifications seen on mammography 1. The basis of this sign is that dermal calcifications maintain fixed relationships to one another which are reproducible with similar projections at different times. This is in contrast to intramammary cal...
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Tau sign

The tau sign represents the appearance of a persistent trigeminal artery on the sagittal plane of an angiogram or on sagittal MRI images. It resembles the greek letter τ, pronounced 'tau'. Persistent trigeminal artery arises from the junction between the petrous and cavernous ICA and runs poster...
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Teardrop sign (ankle)

The ankle teardrop sign is one of the radiological signs of an ankle joint effusion. It represents the presence of excess fluid in the inferior part of the anterior compartment of the ankle. Pathology Aetiology trauma gout rheumatoid arthritis synovitis septic arthritis Radiographic feat...
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Teardrop sign (intracapsular breast implant rupture)

The teardrop sign indicates an uncollapsed intracapsular breast implant rupture and is seen as a small focal invagination of the implant shell caused by a minimal concealed leak of droplets of silicone outside the shell where the two membranes contact each other. It is best appreciated by MRI. ...
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Teardrop sign (superior mesenteric vein)

The teardrop sign of the superior mesenteric vein is one of the important signs in the local staging of pancreatic adenocarcinoma. Its importance lies in its diagnostic, as well as prognostic, significance. This sign is used in assessing the resectability of pancreatic cancer. Radiographic feat...
Article

Tectal beaking (midbrain)

Tectal beaking refers to the fusion of the midbrain colliculi into a single beak pointing posteriorly and invaginating into the cerebellum. It is seen with a Chiari type II malformation. Terminology The term bird beak sign is used in a number of other contexts: see bird beak sign (disambiguati...
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Telephone receiver deformity

A telephone receiver deformity is a characteristic bowing of the shaft of the long bones, usually the humeri or femora, seen in thanatophoric dysplasia.
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Telltale triangle sign

The telltale triangle sign, also known as the triangle sign or telltale triangle, is a radiographic sign seen on plain abdominal radiograph that signifies presence of pneumoperitoneum 1. It describes the appearance of a radiolucent triangle of gas formed between three loops of bowel or between t...
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Temporal tap manoeuvre

Temporal tap manoeuvre consists in tapping over the ipsilateral superficial temporal artery while assessing the carotid bifurcation on Doppler ultrasound aiming to produce a reflected flow in the external carotid artery (ECA) and thus helping to distinguish which vessel is being assessed: extern...
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Tensile gallbladder fundus sign

The tensile gallbladder fundus sign is positive when the gallbladder fundus is seen to bulge into, and distort, the anterior abdominal wall and is a feature of acute cholecystitis. It is particularly useful as an early sign of the condition on CT as it has around a 75% sensitivity and 95% speci...
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Tent sign (breast)

The tent sign is a term referring to a characteristic appearance of the posterior edge of the breast parenchyma when a mass (usually an infiltrating lesion) causes its retraction and forms an inverted "V" that resembles the tip of a circus tent. The detection of a "tent sign" is facilitated by ...
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Terry Thomas sign

The Terry Thomas sign refers to an increase in the scapholunate space on an AP radiograph of the wrist (or coronal CT). The increased distance indicates scapholunate dissociation (often with rotary subluxation of the scaphoid) due to ligamentous injury. There is no consensus as to what measureme...
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Thimble bladder

Thimble bladder​ is a descriptive term for extreme fibrosis and contracture of the bladder walls, resulting in a tiny bladder. The term is usually used to describe changes from advanced genitourinary tuberculosis.
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Third mogul sign

The third mogul sign can be seen on frontal chest radiograph in the presence of left atrial enlargement. It refers to an extra mogul or bump along the upper left cardiac silhouette just below the left main bronchus. The third mogul sign commonly represents the enlarged left atrial appendage, pa...
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Thoracic spine sign (ultrasound)

The thoracic spine sign on lung ultrasound is an indirect indicator of the presence of a pleural effusion or haemothorax. It represents the visualisation of the vertebral bodies in the thoracic cavity above the diaphragm which are usually not seen unless there is a fluid collection.  Radiograph...
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Thoracoabdominal sign

Thoracoabdominal sign, a variation of the silhouette sign, is a frontal chest radiograph sign which helps to localize a thoracic lesion. Since the posterior costophrenic sulcus is more caudal than the anterior lung, a thoracic lesion must be posterior if its caudal end is visible below the dome...
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Threads and streaks sign

The threads and streaks sign refers to an angiographic appearance of a vascularised tumour thrombus extending into the ipsilateral renal vein or the inferior vena cava from a renal cell carcinoma. This gives an appearance of linear, thread-like or string-like appearance of the involved vessel.  ...
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Three line sign (common bile duct)

The three line sign refers to an MRI term that describes the appearance of roundworm Ascaris lumbricoides within the CBD lumen which appears as two hypointense lines representing the worm walls and hyperintense centre which represents the worm gut. See also biliary ascariasis
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Throckmorton sign (pelvis)

Throckmorton sign, also known as John Thomas sign, refers to when the penis points in the direction of unilateral disease, typically of the pelvis or hip.  Throckmorton sign is a slang term used humorously by medical students and residents. According to the first serious study of the sign publ...
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Thrombus fissuration

Thrombus fissuration is a sign of impending rupture of an aortic aneurysm. It reflects blood dissecting into the intramural thrombus. This sign is observed on contrast-enhanced CT as linear contrast infiltrations from the aneurysm lumen through the intramural thrombus. Thrombus fissurations exte...
Article

Thumbprinting

Thumbprinting is a radiographic sign of large bowel wall thickening, usually caused by oedema, related to an infective or inflammatory process (colitis). The normal haustra become thickened at regular intervals appearing like thumbprints projecting into the aerated lumen. Pathology Aetiology ...
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Thumb sign (chordoma)

The thumb sign or thumbing of the pons is described in chordomas as is meant to be relatively specific. It is seen in midline sagittal projection as a projection of the tumour indenting the pons 1,2. See also thumb sign (disambiguation)
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Thumb sign (disambiguation)

The term thumb sign is used to refer to a number of separate radiological appearances, including:  thumb sign (Marfan disease) - a clinical sign thumb sign (epiglottitis) thumb sign (chordoma)
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Thumb sign (epiglottitis)

The thumb sign in epiglottitis is a manifestation of an oedematous and enlarged epiglottis which is seen on lateral soft-tissue radiograph of the neck, and it suggests a diagnosis of acute infectious epiglottitis. This is the radiographic corollary of the omega sign 1-3. See also Thumb sign is...
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Thurstan Holland fragment

The Thurstan Holland fragment also known as the Thurstan Holland sign is an eponymous radiological sign depicting a triangular portion of the metaphysis remaining with the epiphysis in a physeal fracture. This fragment is one of the tell-tale signs of a type 2 Salter-Harris fracture 1. History ...
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Thymic notch sign

The thymic notch sign represents the normal thymus in a newborn on a frontal chest radiograph. Interruption of the cardiac silhouette forms a notch, which may be seen on either side, but more frequently is seen on the left side. 
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Thymic sail sign

The thymic sail sign represents a triangular-shaped inferior margin of the normal thymus seen on a neonatal frontal chest radiograph. It is more commonly seen on the right side, but can also be bilateral. It is seen in 3-15% of all cases. This sign should not be confused with the spinnaker sail ...
Article

Thyroid inferno

Thyroid inferno refers to the colour Doppler appearance of the thyroid gland in active Graves disease (inclusive of variants such as Marine Lenhart syndrome), and consists of multiple small areas of colour flow seen diffusely throughout the gland representing increased vascularity and arterioven...
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Tiger stripe appearance

The tiger stripe appearance refers to the characteristic alternating hypo and hyperintense bands on MRI in Lhermitte-Duclos disease. This rare cerebellar tumour appears like the coat of a tiger. See also tigroid pattern - in brain
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Tip of the iceberg sign (ovarian dermoid cyst)

Tip of the iceberg sign refers to one of the characteristic appearances of an ovarian dermoid cyst. If there are echogenic cyst contents of sebum and hair, they cause marked posterior acoustic attenuation so that only the superficial part of the cyst is seen. Just like an iceberg, you may only b...
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Tombstone iliac wings

Tombstone iliac wings, also referred to as Mickey Mouse ears pelvis 1, are an imaging descriptor for the iliac wings of individuals with achondroplasia. These are seen to be small and squared and have been likened to the appearance of tombstones or the ears of Mickey Mouse.
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Toothpaste sign

The toothpaste sign in spinal imaging represents an extrusion of an intervertebral disc into the epidural space. It is called after the shape of extruded material relatively to the parent disc in a sagittal plane.
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Tramp-stamp oedema

Tramp-stamp oedema is a colloquial term used by some radiologists to denote posterior lumbar subcutaneous oedema. The term is used to describe oedema in the distribution seen with lower back tattoos, usually in young women, which are known pejoratively as tramp-stamps.  This oedema is thought t...
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Tram-track sign (brain)

Tram-track sign in the brain refers to the parallel calcification of the cortex in patients with Sturge-Weber syndrome 1.  It should not be confused with other tram-track signs elsewhere in the body. 
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Tram-track sign (chest)

Tram-track sign may be used in chest radiography or CT to denote the thickened non-tapering (parallel) walls of cylindrical bronchiectasis. It should not be confused with other tram-track signs elsewhere in the body. 
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Tram-track sign (disambiguation)

Tram-track sign may refer to a variety of appearances characterised by near-parallel lines in a number of contexts and regions including: tram-track sign (brain): cortical calcification in Sturge-Weber syndrome tram-track sign (orbit): e.g. optic nerve meningioma tram-track sign (chest): thic...
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Tram-track sign (orbit)

Tram-track sign refers to the parallel thickening and enhancement around the optic nerve, and is most frequently seen in the setting of optic nerve meningioma. It may however also be seen in 1: orbital pseudotumour perioptic neuritis orbital sarcoidosis oribtal leukemia orbital lymphoma or...
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Transient ischaemic dilatation

Transient ischaemic dilatation (TID) is a paradoxical phenomenon seen in myocardial perfusion SPECT imaging.   With severe balanced coronary artery disease, myocardial ischaemia may result in apparent enlargement of the left ventricular cavity during stress. The cause of this is not entirely cl...
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Transmantle sign (brain)

The transmantle sign is an MRI feature of focal cortical dysplasia (FCD), almost exclusively seen in type II focal cortical dysplasia (Taylor dysplasia - also known as transmantle cortical dysplasia for this reason). However, it is not always present, seen in ~45% (range 21-72%) of patients with...
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Tree-in-bud sign (lung)

Tree-in-bud sign or pattern describes the CT appearance of multiple areas of centrilobular nodules with a linear branching pattern. Although initially described in patients with endobronchial tuberculosis, it is now recognised in a large number of conditions. Pathology Pathogenesis The tree-i...
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Trethowan sign

The Trethowan sign is used to diagnose slipped capital femoral epiphysis (also called slipped upper femoral epiphysis). In this sign, the line of Klein passes above the femoral head. It is best done on both AP and true lateralcit projections. On the AP view of a normal hip, the Klein line inter...
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Triangular cord sign (biliary atresia)

The triangular cord sign is a triangular or tubular echogenic cord of fibrous tissue seen in the porta hepatis at ultrasonography and is relatively specific for the diagnosis of biliary atresia 1,2. This sign is useful in the evaluation of infants with cholestatic jaundice, helping in the diffe...
Article

Trident acetabulum

Trident acetabulum is an appearance which can be seen in several skeletal dysplasias 1. It is characterised by small bony spurs at the medial and lateral acetabular margins with a more subtle central spur, resembling a trident, the three-pronged spear of classical Greece 1. The trident is most ...
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Trident hand

A trident hand is a description where the hands are short with stubby fingers, with a separation between the middle and ring fingers.  The appearance mimics a trident, the three-pronged spear of classical Greece. The trident is most commonly associated with the sea god Poseidon in Greek mytholo...
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Trident sign (CNS)

The trident sign describes the typical appearances of the pons in osmotic demyelination syndrome, whereby the symmetrical high T2/FLAIR signal abnormality centrally in the pons is shaped like a trident, the three-pronged spear of classical Greece 1. The trident is most commonly associated with t...
Article

Triple bubble sign

The triple bubble sign is the classic radiographic appearance observed in jejunal atresia 1,2. The appearance is due to a proximal obstruction caused by the atretric jejunum. It is equivalent to the double bubble sign, but a third bubble is seen because of proximal jejunal distention.
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Troisier sign

Troisier sign is the clinical finding of a hard and enlarged left supraclavicular node (Virchow node), and is considered a sign of metastatic abdominal malignancy. Terminology It is sometimes referred to as the Virchow node, which is the name given by Rudolf Virchow (1821-1902) 6, a German pat...
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Trough line sign

The trough line sign is a sign of posterior shoulder dislocation on AP films.  In a posterior dislocation, the anterior aspect of the humeral head becomes impacted against the posterior glenoid rim. With sufficient force, this causes a compression fracture on the anterior aspect of the humeral ...
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Trumpeted internal acoustic meatus sign

A trumpeted internal acoustic meatus (IAM) is an indirect sign of an acoustic schwannoma and is useful in helping differentiating between one and other cerebellopontine angle entities, especially from a meningioma which typically does not extend into the meatus 1. It is characterized by widenin...
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T sign (obstetrics)

The "T sign" is really the absence of a twin-peak sign (or lambda sign) and is used in ultrasound assessment of a multifetal pregnancy. It refers to the lack of chorion extending between the layers of the intertwin membrane, denoting a monochorionic pregnancy. The intertwin membrane comes to an...
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Tubal ring sign

Tubal ring sign, also referred to as bagel sign or blob sign, one of the ultrasound signs of a tubal ectopic. It comprises of an echogenic ring which surrounds an unruptured ectopic pregnancy. It is said to have a 95% positive predictive value (PPV) for ectopic pregnancy. Differential diagnosis...
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Tuberculous dactylitis

Tuberculous dactylitis, also known as spina ventosa, is a rare skeletal manifestation of tuberculosis where the short tubular bones (i.e. phalanges, metacarpals, metatarsals) are affected. Epidemiology Tuberculous osteitis is one of the commonest bacterial osteitides. Typically the dactylitis ...
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Tulip bulb sign

Tulip bulb sign refers to the characteristic appearance of annuloaortic ectasia as seen on CT angiography. There is symmetric dilatation of the three sinuses of Valsalva, with extension into the ascending aorta and effacement of the sinotubular junction.  It is seen especially in Marfan syndro...
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Tullio phenomenon

The Tullio phenomenon describes the precipitation of vertigo and nystagmus by a loud noise. Pathology Aetiology The tympanic membrane and ossicular chain must be intact with a mobile footplate. It may be seen in several situations: congenital syphilis with a semicircular canal fistula post-...
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Turtle back sign

Turtle back sign, also referred as tortoise shell appearance, represents a very characteristic pattern of the schistosomiasis cirrhotic liver in which calcified fibrotic septa are seen aligned perpendicular to the liver capsule resembling the turtle's carapace. 
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Twin-peak sign (twin pregnancy)

The twin peak sign (also known as the lambda (λ) sign) is a triangular appearance of the chorion insinuating between the layers of the inter twin membrane and strongly suggests a dichorionic twin pregnancy. It is best seen in the first trimester (between 10-14 weeks) 5. While the presence of a t...
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Two-slice-touch rule (knee MRI)

The two-slice-touch rule is a sign on MRI of the knee used to increase the sensitivity of diagnosing meniscal tears. This rule states a meniscal tear is present if abnormal findings are present on two or more images - these do not have to be contiguous, e.g. can occur on sagittal and coronal sl...
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Unfolded aorta

The term unfolded aorta refers to the widened and 'opened up' appearance of the aortic arch on a frontal chest radiograph. It is one of the more common causes for apparent mediastinal widening and is seen with increasing age. It occurs due to the discrepancy in the growth of the ascending aorta...
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Upper lobe pulmonary venous diversion

Upper lobe pulmonary venous diversion (cephalisation) reflects elevation of left atrial pressure and can occur with pulmonary oedema. It produces stag-antler's sign on a frontal chest x-ray. The normal left atrial pressure is 5-10 mmHg. An elevation of left atrial pressure to 10-15 mmHg will re...
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Upper T sign

The upper T sign is one of the features useful in identifying the central sulcus of the cerebral cortex on cross-sectional imaging. It relies on identifying the superior frontal sulcus which intersects the precentral sulcus in a "T" junction, thus defining the precentral gyrus. The central sulc...
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Urachus sign

The urachus sign is a radiographic sign of pneumoperitoneum. It represents the outline of the median umbilical ligament with free abdominal gas in a supine patient, as seen on a plain abdominal radiograph.
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Ureteric jet

Ureteric jets (or ureteral jets) are the visualisation of the normal physiological periodic efflux of urine from the distal end of each ureter into the bladder.  Physiology When the urine passing down the ureter reaches the vesicoureteric junction it is forced out into the bladder via a strong...
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U sign (central sulcus)

The U sign denotes the characteristic "U" shaped appearance of the subcentral gyrus which surrounds the inferolateral end of the central sulcus and abuts the lateral (Sylvian) fissure. It has been found, at least in one study, to be the most reliable anatomical feature to identify the central su...
Article

Vacuum phenomenon in shoulder

Vacuum phenomenon in the shoulder refers to the presence of intra-articular gas in the shoulder joint. It is a very common occurrence, particularly in external rotation. This can cause circular or linear areas of low signal intensity on GRE MR images of the shoulder obtained with external rotati...
Article

Vegetable and plant inspired signs

There are many signs in radiology which are vegetable- and plant-inspired, they include: Unprocessed produce celery stalk anterior cruciate ligament: ACL mucoid degeneration celery stalk metaphysis: osteopathia striata and congenital rubella infection Christmas tree intestinal atresia cocon...
Article

Veiled right kidney sign

The veiled right kidney sign is a sonographic sign described in pneumoretroperitoneum, most commonly due to duodenal perforation. It refers to the appearance of the right kidney on transabdominal ultrasound 1-4. On ultrasound, there is difficulty in obtaining images of the right kidney due to i...
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Venous distension sign

The venous distension sign is a finding that may be identified on sagittal imaging of the dural venous sinuses which is said to have a diagnostic sensitivity and specificity of 94% for intracranial hypotension. The sign is positive when there is a convex inferior margin of the midportion of the...
Article

Venus necklace

A Venus necklace is a term almost never used, but for the sake of completeness is included here. It is used by some authors 1 to describe a series of T2 hyperintense lesions on the inferior surface of the corpus callosum in the midline (callososeptal interface), most frequently seen in multiple ...
Article

Vertebral body squaring (differential)

Vertebral body squaring refers to the loss of normal concavity of the anterior border. It is seen in a variety of conditions including:  Differential diagnosis Ankylosing spondylitis Ankylosing spondylitis is the most common cause of vertebral body squaring. It usually involves multiple level...
Article

Vertebral scalloping

Vertebral scalloping is a concavity to the posterior (or less commonly anterior) aspect of the vertebral body when viewed in a lateral projection. A small amount of concavity is normal, as is concavity of the anterior vertebral body (see vertebral body squaring). Posterior scalloping Causes of...
Article

Vertebra plana

Vertebra plana (a.k.a. pancake / silver dollar / coin-on-edge vertebra) is the term given when a vertebral body has lost almost its entire height anteriorly and posteriorly, representing a very advanced compression fracture. Plural is vertebrae planae. It can occur in a variety of settings, incl...
Article

Viking helmet appearance

The viking helmet appearance refers to the lateral ventricles in coronal projection in patients with dysgenesis of the corpus callosum. The cingulate gyrus is everted into narrowed and elongated frontal horns.  An alternative name is moose head appearance. 
Article

Waldenström sign (hip)

Waldenström sign is the increased distance between the pelvic teardrop and the femoral head. It is a highly specific sign of a hip joint effusion. Radiographic features Plain radiograph >11 mm total distance or >2 mm difference compared to contralateral hip 1,2 measured between the lateral a...
Article

Walking man sign (chest x-ray)

The walking man sign is seen on a lateral chest radiograph and is a sign of left atrial enlargement. It results from posterior displacement of the left main bronchus such that it no longer overlaps the right bronchus. The left and right bronchus thus appear as an inverted 'V', mimicking the legs...
Article

Wall-echo-shadow sign (ultrasound)

The wall-echo-shadow sign (also known as WES sign) is an ultrasonographic finding within the gallbladder fossa referring to the appearance of a "wall-echo-shadow": a curvilinear hyperechogenic line representing the gallbladder wall a thin hypoechoic space representing a small amount of bile a...
Article

Wasp-waist sign (spine)

The wasp-waist sign is a radiographic appearance seen in patients with Klippel Feil syndrome. It results from fusion of the vertebral bodies such that the anteroposterior diameter at the level of the affected discovertebral joint is smaller than the diameter at the superior and inferior limits o...
Article

Water bottle sign (heart)

The water bottle sign or configuration refers to the shape of the cardiac silhouette on erect frontal chest x-rays in patients who have a very large pericardial effusion. Typically the effusion has accumulated over many weeks to months (e.g. in patients with malignancy) and the pericardium has g...
Article

Water-lily sign (hydatid cyst)

The water-lily sign is seen in hydatid infections when there is detachment of the endocyst membrane which results in floating membranes within the pericyst that mimic the appearance of a water lily. It is classically described on plain radiographs (mainly chest X-ray) when the collapsed membran...
Article

Watermelon skin sign

The watermelon skin sign refers to diffuse, radiating, streaky areas of low signal intensity in prostate on T2WI in patients with prostatic tuberculosis 1.  
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Wave sign

The wave sign refers to the indentation of the normal thymus in young children by the ribs, resulting in a wavy border.

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