We will expose some cases from our institution according to body segments, explaining these signs and their interpretation.

Musculoskeletal system

Polka dot: It consists of a pattern of small punctate areas of high attenuation seen in cross sectional CT studies, which represents trabecular thickening surrounded by fat marrow or vascular channels in vertebral hemangiomas.¹

Fig 1: Polka dot. Vertebral hemangioma involving the body of a thoracic vertebra.

Scaphoid signet ring: It refers to the dense rounded cortex of the scaphoid tubercle on a posteroanterior radiograph due to subluxation from flexion of the scaphoid, that is seen in injuries of the scapholunate ligament or perilunate/lunate dislocations.²

Fig 2: Scaphoid signet ring. On posteroanterior radiograph (a,c) there is disruption of the alignment of the proximal carpal row due to a lunate dislocation, which is confirmed on the lateral radiograph (b).

Corduroy: This sign mimics the appearance of corduroy clothing and describes the appearance of thickened vertically oriented trabeculae seen in vertebral hemangiomas on lateral radiographs or sagittal CT scans.³

Fig 3: Corduroy sign. Coronal CT image of thoracic spine demonstrates corduroy appearance of a vertebral hemangioma.

Fig 4: Inverted Napoleon hat. Coronal radiograph of spine shows L5 vertebral body projected on the sacrum forming the dome of the inverted Napoleon hat (arrowheads).

Rugger jersey spine: It refers to high attenuation densities endplates at multiple contiguous vertebral levels producing alternating sclerotic-lucent parallel bands that simulates the stripes on a rugby jersey. It’s distinctive for secondary hyperparathyroidism.³

Fig 5: Rugger jersey spine. Sagittal CT in a patient with terminal renal failure. Consecutive sclerotic bands at vertebral endplates with central lucency (arrowheads) simulate the stripes on a rugby jersey.

Bow tie (discoid meniscus): It describes the body of a normal meniscus in continuity with the anterior and posterior horns on sagittal MRI of the knee. A discoid meniscus can be suggested on MRI when three or more bow tie shapes are identified on contiguous sagittal images (4-mm-thick). An absent bow tie sign is also suggestive of a bucket-handle tear.⁵

Fig 6: Bow tie sign. Sagittal T2-weighted (a) and coronal PD-weighted fat sat (b) MR images in a 10-year-old male. A normal bow tie appearance of the medial meniscus is represented in the sagittal image (a), on the other hand the lateral meniscus has a discoid morphology (arrowhead). MRI of the knee in a 20-year-old female (c-d) shows an absent bow tie sign of the medial meniscus (curved arrow). A meniscal tear (straight arrow) and a displaced segment in the intercondylar area suggest a bucket-handle tear.

Ring sign (hook of the hamate fracture): On posteroanterior radiographs of the wrist the hook of the hamate normally overlaps with the midportion of hamate body and projects as an ovoid density. Signs of fracture of hook of the hamate include absence, discontinuity, displacement or sclerosis of the hook.⁶

Fig 7: Ring sign (hook of the hamate fracture). Normal appearance of the hook of the hamate (a-b). Posteroanterior radiograph of the wrist (c), where a displacement and loss of ring shape of the hook is identified (arrowhead). A displaced fracture of the hook is later confirmed on CT scan (straight arrow in d).

Boutonnière deformity: It refers to pathologic flexion of the proximal interphalangeal joint (PIP) and hyperextension of the distal interphalangeal joint (DIP) of a finger. A disruption of the central aspect of the extensor tendon causes PIP flexion passing between the lateral bands, just as a button passes through a buttonhole. It is seen in inflammatory arthritis and traumatic tendon tears.⁷

Fig 8: Boutonnière deformity. Plain radiograph in a female patient with rheumatoid arthritis, where a flexion of PIP and hyperextension of DIP of some fingers are identified (arrowheads).

Button sequestrum: it consists of a bone opacity surrounded by a well-defined lucent area, resembling a button. It commonly represents eosinophilic granuloma or osteomyelitis.⁸

Fig 9: Button sequestrum. Plain radiographs of a finger in a male with a previous cat bite demonstrates a well-defined lucent lesion with sclerotic margins and a central opacity resembling a button (straight arrow).

Neurological system

Stiletto footprint: It’s a useful sign to understand the skull base anatomy. The anterior aspect represents foramen ovale and the heel the foramen spinosum.⁹

Fig 10: Stilleto footprint. The foramen ovale represents the anterior aspect of the footprint (arrowhead) and the foramen spinosum the heel itself (straight arrow).

String of pearls (fibromuscular dysplasia): It refers to the appearance of alternating dilation and multifocal stenoses in fibromuscular dysplasia.¹⁰

Fig 11: String of pearls (fibromuscular dysplasia). Sagittal CT (a) shows consecutive stenoses and dilations of internal carotid artery (arrowheads). Angiography (b) represents in a better way the string of pearls appearance (straight arrows).

String of pearls (watershed cerebral infarction): It refers to the pattern used to describe the linear orientation of multiple internal border zone infarcts located parallel to the lateral ventricles.¹¹

Fig 12: DWI (a) and ADC map (b) show multifocal lesions with restricted diffusion parallel to lateral ventricles (arrowheads and straight arrows) resembling a string of pearls.

Invisibility cloak sign: This sign is a new proposal by our work team. On MRI, spinal schwannomas can be hyperintense on T2 and have a similar signal compared to the cerebrospinal fluid,¹² and therefore not be evidently visible, demonstrating only indirect signs such as root displacement. This is reminiscent of a person hidden under a cloak of invisibility, something classically seen in famous fantasy novels.

Fig 13: Invisibility cloak sign. Axial T2 MRI in a 68-year-old male shows radicular displacement but no a clear lesion (arrowhead), that becomes visible as an avid nodular mass on axial T1 FSE fat sat +Gd (straight arrow).

Thoracic segment

Boot-shaped heart: It’s the appearance of the heart in patients with tetralogy of Fallot. The boot shape is produced by upturning of the cardiac apex because of right ventricular hypertrophy and by the concavity of the main pulmonary artery.¹³

Fig 14: Boot-shaped heart. Chest radiograph in a patient with tetralogy of Fallot, where a boot-shaped heart is identified.

Finger in glove: It refers to the description of bronchiectasis filled with respiratory secretions, resembling a finger in a glove. It’s classically associated with allergic bronchopulmonary aspergillosis. It can also be recognized in cystic fibrosis, asthma and obstructive diseases.³

Fig 15: Finger in glove. Coronal CT chest in a 16-year-old male with cystic fibrosis. Multiple dilated bronchi filled with respiratory secretions are identified (arrowheads), resembling some fingers in a glove.

Signet ring (bronchiectasis): It’s seen on chest CT and refers to the appearance of a dilated bronchus, which exceeds the diameter of the adjacent pulmonary artery. The dilated bronchus represents the hollow of the ring, and the pulmonary artery the signet.³

Fig 16: Signet ring (bronchiectasis). Axial CT chest in a 16-year-old male with cystic fibrosis. Dilated bronchi diameter exceeds adjacent pulmonary arteries, resembling a signet ring.

Abdomen and pelvis

Beaded collar (primary sclerosing cholangitis): On MR Cholangiopancreatography (MRCP), primary sclerosing cholangitis can manifest as multi-focal segmental strictures of intra and extrahepatic bile ducts, producing a beaded appearance.³

Fig 17: Beaded collar (primary sclerosing cholangitis). MRCP in a female patient with ulcerative colitis identifying dilation of the extrahepatic bile duct (arrowhead) and multifocal stricture of intra and extrahepatic bile duct.

String of pearls (small bowel obstruction): In small bowel obstruction when there’s a lot of accumulated fluid, small bubbles appear separated by valvulae conniventes and can be seen as a chain of radiolucent images, which constitutes the string of pearls.¹⁴

Fig 18: String of pearls (small bowel obstruction). Abdominal radiograph (a) in a 74-year-old woman with abdominal pain and vomiting. Multiple small contiguous bubbles in small bowel look like a string of pearls (arrowheads). Abdominal CT (b) confirms the presence of pathological dilation of small bowel filled with fluid and small trapped gas bubbles (arrowheads).

Phrygian cap: It’s a common variant of the gallbladder which occurs when there’s folding of the fundus upon the gallbladder body. It remembers a hat worn by inhabitants of Phrygia.³

Fig 19: Phrygian Cap. Abdominal US where there’s a folding of the fundus (arrowhead) upon the gallbladder body.

Comb sign (Crohn’s disease): It refers to fibrofatty proliferation and vascular dilation in active Crohn’s disease, giving the appearance of the teeth of a comb.³

Fig 20: Comb sign (Crohn’s disease). Coronal CT shows fibrofatty proliferation and vascular dilation in a patient with Crohn’s disease.