Neurological
Molar Tooth
In Joubert Syndrome,
a rare autosomal-recessive disorder of brain development,
the main imaging findings are partial or complete absence of the cerebellar vermis,
hypoplastic cerebellar peduncles and fourth ventricle deformity.
The hypoplasia of the cerebellar peduncles results in a molar tooth sign of the mid-brain as seen in Figure 1 (3).
The molar tooth sign however is not specific to JR and has been described in Varadi-Papp syndrome,
Malta Syndrome,
Senior-Loken syndrome and COACH syndrome (4).
Dawsons Fingers
Multiple sclerosis is a chronic relapsing-remitting disease of the central nervous system disease caused by demyelinating plaques which are best seen on MRI as T2/FLAIR hyperintese and T1 hypointense lesions.
Location of the plaques can be infratentorial,
in deep white matter,
periventricular,
pericallosal,
juxtacortical or mixed white matter-grey matter lesions.
Periventricular demyelinating plaques distributed along the axis of medullary veins,
perpendicular to the body of the lateral ventricles and/or callosal junction give an appearance described as ‘Dawson fingers’ (Figure 2) after the histopathologist to describe them.
This is thought to reflect perivenular inflammation and is relatively specific for multiple sclerosis (5).
Musculoskeletal
Mini Brain
Mini brain sign (Figure 3) refers to appearance of vertebral plasmacytoma on MRI imaging.
Plasmacytoma appears predominantly as expansile lytic bony lesion which when occurs in vertebral body results in replacing the normal bone marrow signal (high T1/low T2) by low T1/high T2 signal with curvilinear low signal intensity areas resembling cortical sulci.
These represent thickened cortical struts and are presumably caused by compensatory hypertrophy of the remaining trabecular bone.
It was suggested that this is pathognomonic appearance of vertebral plasmacytoma which can obviate biopsy for confirmation of the diagnosis.
Black Eyebrow
The black eyebrow sign describe curvilinear lucency in the superior aspect of the orbit on conventional frontal facial bone radiography Figure 4.
This indicates presence of intraorbital emphysema which is usually associated with orbital blowout fracture.
The air within the orbit tracks from the maxillary sinus through the inferior orbital floor or from the ethmoid sinuses through the medial wall of the orbit (6).
Teardrop sign
Seen on frontal facial radiographs in patients with traumatic orbit floor fractures.
Orbital fat herniates through the defect in the orbital floor into the maxillary sinus.
The herniated fat is outlined by air in the sinus and appears as a ‘teardrop’ shaped opacity immediately inferior to the orbit on radiographs (Figure 4).
This is often seen in combination with the ‘Black Eyebrow’ sign described above (7).
Double PCL sign
In bucket handle tear of the medial meniscus,
the flipped meniscal fragment is oriented parallel and inferolateral to the posterior cruciate ligament giving the appearance of double PCL in the mid sagittal magnetic resonance images of the knee,
Figure 5.
The meniscal fragment is displaced into the intercondylar fossa and prevented from further lateral displacement by intact ACL.
This sign has high specificity in the range of 98-100% (8).
Patellar ‘Tooth Sign’
Tooth sign (Figure 6) describes vertical ridging of enthesophyte at the site of patellar insertion of the quadriceps tendon secondary to chronic traction enthesopathy.
It is best appreciated on the axial or ‘skyline’ view of the knee.
There is no correlation between sign of enthesophyte and severity of patient symptoms (9).
Gaptooth SignThis sign describes the appearance of a frontal wrist radiograph with widening of the scapholunate interval in the setting of scapholunate ligament rupture.
Usually associated with shortening of scaphoid on frontal view due to rotary subluxation (Figure 7).
It has been named after several celebrities known for frontal dental diastema (a visible gap between the two maxillary central incisors) as the separation of the scaphoid and lunate bones resembles the ‘gap’ between the central incisors.
A space of >3mm and >4mm have been described as cut-offs for widening of the interval (10).
Bone within a bone
As the name indicates,
in bone within a bone sign,
a bone appears to arise within another bone (Figure 8).
There is a wide differential for this appearance.
Occasionally,
this could be a normal variant in neonates and infant up to 2 months old in the thoracolumbar vertebrae.
It can also result in paediatric patients from abnormal periosteal reaction such as in Caffey’s disease (infantile cortical hyperostosis),
and Prostaglandin treatment for duct dependent congenital heart disease.
Similar appearance is also seen in conditions that causes cortical split or double cortical line as in sickle cell disease and Gaucher disease secondary to bone infarction,
in scurvy due to subperiosteal hemorrhage,
and in osteomyelitis.
Altered bone growth in childhood is another reason for bone within a bone appearance as in rickets,
leukemia,
infection,
metabolic disorders (e.g.
hypervitaminosis D) and heavy metal intoxication.
In osteopetrosis,
a rare primary hereditary bone disorder,
failure or inhibition of osteoclast also result in bone within bone appearance (11).
GI/GU
Bowel Thumbprinting
First described in 1961 by Avnet et al.
on the appearance of colon on barium enema following reversal of sigmoid volvulus (12).
This sign refers to the appearance of thickened and separated colonic haustra projecting into the bowel lumen due to severe mucosal and submucosal oedema (13).
It is most commonly seen on plain film of abdomen although it can also be seen on CT(14).
This has a wide differential as any cause of edema can result in this appearance (Figure 9).
Cerebriform appearances of CAH
The characteristic sign of CAH on ultrasound is a cerebriform appearance as shown in Figure 10.
This autosomal recessive inborn error of metabolism due to a deficiency in enzymes (most commonly 21-alpha-hydroxylase) required for the production of cortisol and aldosterone that result in a variety of possible phenotypical and biochemical presentations.
US of the adrenal gland is the modality of choice for the workup of CAH in neonates and infants (15).
A combination of limb width greater than 4 mm,
lobulated or cerebriform surface,
and stippled echogenicity is highly sensitive and specific for the diagnosis of CAH (16).
The brain-like appearance of gyriform infoldings of the adrenal cortex was first described by Marie et al.
in 1952 (17).
Pelvic Tooth
Mature cystic teratomas (also known as a dermoid cyst) are the most common benign tumours of the ovary.
They contain at least 2 of 3 types of mature tissues; ectodermal (skin or brain),
mesodermal (muscle and fat) and endodermal (mucinous epithelium) origin.
Mesodermal tissue (fat,
bone,
cartilage,
muscle) is present in over 90% leading the identification of fat within a cyst to be diagnostic (18).
Teeth are seen within the 31% of cases,
resulting in the pelvic tooth sign seen on pelvic and abdominal radiographs as seen in Figure 11 with axial CT correlate.
Pseudokidney sign of intussusception
Intussusception seen on ultrasound can give a reniform appearance,
with a centrally hyperechoic region and surrounding hypoechoic areas as seen in Figure 12,
which gives the appearance of a kidney.
Intussusception is the telescoping of a segment of bowel into another portion,
usually more distal,
and causes obstruction.
Children are most commonly affected and the imaging of choice in the first instance is ultrasound.
Other common signs that can be seen on ultrasound include target sign on axial imaging of the affected area and donut sign.
Ultrasound has a high accuracy with sensitivity of 98-100% and specificity of 88-100%.
First line treatment is air enema with average success rates of 74% and recurrence rates of 10% (19).
Caecal Embryo
Caecal volvulus is a torsion of a mobile caecum and ascending colon around its mesentery.
This often results in a large bowel obstruction which can lead to colonic ischaemia from venous occlusion and perforation with faeculuent peritonitis.
On abdominal radiograph,
there is a grossly dilated obstructed loop of right colon with the long axis of the loop extending from the right lower quadrant to the epigastrium or left upper quadrant.
This creates a characteristic shape that resembles a mammalian embryo as seen in Figure 13.
Thorax
Finger in Glove
In bonchial obstruction as a result of mucoid impaction,
larger airways that are involved manifest by tubular or branching opacities that resemble fingers,
giving the classical ‘finger in a glove appearance’.
This is a common finding on chest imaging and has many causes,
both genetic and acquired.
One of the rarest is the congenitally acquired Segmental Bronchial Atresia (Figure 14),
which results from the interruption of the origin of the segmental bronchus.
Distal to the atretic segment,
central bronchi become dilated and accumulate mucoid.
There is a male predominance (2:1) with an average age of detection at 17. Most are asymptomatic and detected incidentally (20).
Breast within breast
Breast Hamartomas (or fibroadenolipomas) are benign breast lesions made of a proliferation of fibrous,
glandular,
and fatty tissues surrounded by a thin capsule of connective tissue (Figure 15).
They typically occur in females over 35 years of age,
presenting as a lump or an incidental finding on mammography.
The appearance on mammography is a well circumscribed rounded mass with mixed internal soft tissue and fat density.
The internal matrix of the hamartoma resembles the mammographic appearance of normal breast tissue,
resulting in the “breast within a breast” sign (21).