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.

71 results found
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Obelion

The obelion refers to the skull landmark along the sagittal suture intersected by an imaginary line connecting the two parietal foramina, where both are present. It is said to represent the site of commencement of sagittal suture closure 1. It is one of the skull landmarks, craniometric points ...
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Obstruction of nasolacrimal drainage apparatus

Obstruction of nasolacrimal drainage apparatus results in epiphora, and can be primary or secondary, congenital or acquired. Obstruction can occur in canaliculi, lacrimal sac, or nasolacrimal duct (post saccular) levels. Congenital obstruction is usually secondary to persistence of the membrane...
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Occipital artery

The occipital artery, a posterior branch of the external carotid artery,opposite the external maxillary, near the lower margin of the posterior belly of the digastic muscle, and ends in the posterior part of the scalp. Summary origin : branch of the external carotid artery at the level of the ...
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Occipital bone

The occipital bone is a trapezoid skull bone that contributes to the posteroinferior part of the cranial vault. It is pierced by the foramen magnum, permitting communication from the cranial cavity to the vertebral canal Gross anatomy The occipital bone is composed of four parts: squamous par...
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Occipital plagiocephaly

Occipital plagiocephaly is a type of plagiocephaly. It is used to describe the shape of the skull which is a result of an early fusion of the lambdoid suture. The premature fusion may occur either on one side or both sides of the suture. Epidemiology It is one of the rarest craniosynostosis, a...
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Occipitomental (Waters) view (skull AP axial view)

The occipitomental (OM) or Waters view is an angled AP radiograph of the skull, and can be used to assess for facial fractures as well as the ethmoid and maxillary sinuses. 
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Occipitotemporal suture

The occipitotemporal/occipitomastoid suture is the obliquely oriented continuation of lambdoid sutural syndesmosis is seen between the anterior border of the occipital bone and the mastoid portion of the temporal bone. The mastoid foramen can be located in the occipitomastoid suture.
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Ocular abductors

The ocular abductors are the extra-ocular muscles responsible for abduction of the globe: lateral rectus (primary function) superior oblique (tertiary function) inferior oblique (tertiary function)
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Ocular adductors

The ocular adductors are the extra-ocular muscles responsible of adduction of the globe: medial rectus (primary function) inferior rectus (tertiary function) superior rectus (tertiary function)
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Ocular depressors

The ocular depressors are the extra-ocular muscles responsible for depression of the globe: inferior rectus (primary function) superior oblique (secondary function)
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Ocular elevators

The ocular elevators are the extra-ocular muscles responsible for elevation of the globe: superior rectus (primary function) inferior oblique (secondary function)
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Ocular external rotators

The ocular external (lateral) rotators are the extra-ocular muscles responsible for external rotation (extorsion) of the globe: inferior oblique (primary function) inferior rectus (secondary function)
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Ocular internal rotators

The ocular internal (medial) rotators are the extra-ocular muscles responsible for internal rotation (intorsion) of the globe: superior oblique (primary function) superior rectus (secondary function)
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Ocular metastasis

Ocular metastases, also termed uveal metastases, account for over 80% of all ocular pathology, and need to be distinguished from extraocular metastasis, which are a quite different group of tumours. This article will discuss metastatic lesions affecting the orbits. For other intracranial metast...
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Ocular pathology

Ocular pathology covers a wide range of conditions and therefore represents the cause of a wide range of symptoms, signs and radiographic features. Ocular metastases account for over 80% of all ocular pathology. With regard to the remainder of ocular lesions, the primary differentiating factor ...
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Oculomotor nerve

The oculomotor nerve is the third of the cranial nerves and arises from the midbrain. It is responsible for the movements of four of the six extra-ocular muscles, the other two being innervated by the trochlear and abducens nerves. Gross anatomy Nucleus and cisternal portion The oculomotor nu...
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Oculomotor nerve palsy

An oculomotor nerve palsy results in weakness of the superior rectus, inferior rectus, medial rectus, inferior oblique, and levator palpebrae, leading to an eye that is "down and out". It has numerous possible aetiologies which can be divided according to which portion of the nerve is ...
Article

Odontogenic myxoma

Odontogenic myxomas are rare tumour that involve the mandible or maxilla and account for 3-6% of odontogenic tumours 2. Epidemiology Typically seen in the 2nd to 3rd decades of life (slightly earlier than ameloblastomas). They are usually not painful. Pathology Arises from mesenchymal odonto...
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Odontoid process fracture: Roy-Camille classification

The Roy-Camille classification of fractures of the odontoid process of C2 depends on the direction of the fracture line 1. The level of fracture line as described by the Anderson and D’Alonzo classification, is not predictive of the degree of instability or the risk of non-union. This classific...
Article

Odontoma

Odontomas are one of the most common of mandibular lesions encountered, and the most common odontogenic tumours of the mandible. They account for up to two-thirds of all such tumours, the next most common are ameloblastomas making up the majority of the remaining one-third. Epidemiology They c...
Article

Oesophagus

The oesophagus is a muscular tube that conveys food and fluids from the oropharynx to the stomach. Gross anatomy The oesophagus is 23-37 cm long with a diameter of 1-2 cm and is divided into three parts: cervical: continuous with the oropharynx, commences at the lower border of cricoid cartil...
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Olfactory neuroblastoma

Olfactory neuroblastomas, also known as esthesioneuroblastomas, are tumours arising from the basal layer of the olfactory epithelium in the superior recess of the nasal cavity. They usually present as a soft tissue mass in the superior olfactory recess involving the anterior and middle ethmoid ...
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Olfactory neuroblastoma (staging)

Olfactory neuroblastomas (or esthesioneuroblastomas) are most frequently staged using a system proposed by Kadish et al in 1976 1. group a: tumour is limited to the nasal cavity group b: tumour is limited to the nasal cavity and paranasal sinuses group c: tumour extends beyond the nasal cavit...
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Olfactory neuroepithelioma

Olfactory neuroepithelioma (9523/3) is a rare tumour which mimics olfactory neuroblastomas (9500/3) in presentation and imaging features and is often used synonymously with the latter 1-2. Pathology Olfactory neuroepitheliomas differ from olfactory neuroblastomas in a number of ways 1: neura...
Article

Omega sign

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...
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Omohyoid muscle

The omohyoid muscle is an infrahyoid muscle of the neck that is innervated by the ansa cervicalis of the cervical plexus receiving fibres from the ventral rami of C1–C3 spinal nerves. The omohyoid is a paired, flat strap of muscle that is made up of superior and inferior bellies joined by an int...
Article

Oncocytic papillary cystadenoma of the upper respiratory tract

An oncocytic papillary cystadenoma is a rare lesion that can occur as a result of oncocytic cell proliferation. They typically occur in the upper respiratory tract (predominantly larynx but occasionally the nasopharynx and very rarely in the parotid glands 2).  
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Oncocytoma of salivary glands

Oncocytomas of the salivary glands are rare benign epithelial tumours. Epidemiology They typically present in older individuals (6th to 8th decades of life). There may be a slight increased female predilection. Pathology Location While they can affect any of salivary glands. They most commo...
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Ophthalmoplegia

Ophthalmoplegia describes the abnormal eye movement that occurs because of paralysis of one or more of the six extraocular muscles involved in eye movements. Classification can be based on the cause of the ophthalmoplegia or the directions of the affected movements. There are numerous causes of...
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Opisthion

The opisthion is the median (midline) point of the posterior margin of the foramen magnum. It is one of the skull landmarks, craniometric points for radiological or anthropological skull measurement. Related pathology Various lines and measurements using the opisthion are made in the diagnosi...
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Optic canal

The optic canal is a rounded canal, located in the lesser wing of sphenoid near the base where it joins the body of sphenoid. It transmits the optic nerve and ophthalmic artery.
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Optic disc drusen

Optic disc drusen (ODD) are also known as hyaline bodies and are a relatively common entity. They are usually found incidentally on CT or on follow-up of abnormal fundoscopy. Epidemiology Optic disc drusen are identified radiographically in up to 0.3-3.7% of the population and are frequently b...
Article

Optic nerve

The optic nerve is the second cranial nerve which along with the olfactory nerve (CN I) is really an extension of the central nervous system, not surrounded by Schwann cells with first sensory bipolar cell body located peripherally in the retina. Their central processes synapse on ganglion cells...
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Optic nerve enlargement

Enlargement of the optic nerves is uncommon and has a surprisingly broad differential: optic nerve glioma optic nerve meningioma orbital pseudotumour optic neuritis sarcoidosis leukemia orbital lymphoma metastases perioptic haemorrhage Erdheim-Chester disease juvenile xanthogranuloma ...
Article

Optic nerve sheath meningioma

Optic nerve meningiomas are benign tumours arising from the arachnoid cap cells of the optic nerve sheath, and represent ~20% of all orbital meningiomas, the majority of which are direct extensions from intracranial meningiomas.  These tumours typically appear as masses within the optic nerve, ...
Article

Optic neuritis

Optic neuritis merely denotes inflammation of the optic nerve, and is one of the more common causes of optic neuropathy.  It can be thought of as broadly divided into infectious and noninfectious causes, although the latter is far more frequent.  On imaging, optic neuritis is most easily identif...
Article

Ora serrata

The ora serrata is the most anterior extent of the retina. This serrated transitional zone lies between the anterior margin of the retina and the ciliary bodies. Radiographic appearance MRI Ora serrata cannot be seen on conventional imaging but is an inferred reference point. On the axial pla...
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Oral cavity

The oral cavity is the most proximal portion of the aerodigestive tract, and is continuous posteriorly with the oropharynx.   Gross anatomy The oral cavity includes the lips anteriorly and extends to a ring of structures (circumvallate papillae, anterior tonsillar pillars, soft palate) that di...
Article

Orbit

The orbit is a feature of the face and contains the globe. Gross anatomy In the adult, the orbit has a volume of approximately 30 mL, of which the globe occupies 6.5 mL. It has a roof, floor, medial and lateral wall. The orbit is open anteriorly where it is bound by the orbital septum, which f...
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Orbital cavernous venous malformation

Cavernous venous malformations of the orbit (aka cavernous haemangioma)  are the most common vascular lesion of the orbit in adults. It is important to note that according to newer nomenclature (ISSVA classification of vascular anomalies) these lesions are merely known as slow flow venous malfo...
Article

Orbital cystic lesions

Several cystic and cyst-like orbital lesions may be encountered in imaging of the orbits: developmental orbital cysts choristoma: dermoid: commonest benign orbital tumour in childhood  epidermoid teratoma  congenital cystic eye colobomatous cyst acquired abscess haematoma lacrimal gla...
Article

Orbital dermoid cyst

Orbital dermoid cysts are congenital lesions representing closed sacs lined by an ectodermal epithelium and corresponding to the most common orbital tumour in children. They are typically divided according to whether or not they are deep (within the orbit) or superficial (adjacent to the orbital...
Article

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...
Article

Orbital infection

Orbital infection can be a commonly encountered entity. It is important to differentiate between orbital and periorbital cellulitis, as this has therapeutic and prognostic implications: periorbital cellulitis (preseptal cellulitis) is limited to the soft tissues anterior to the orbital septum ...
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Orbital inflammatory disease: differential diagnosis

The differential diagnosis of orbital inflammatory diseases (including orbital pseudotumours) can be divided based on their location into: dacryoadenitis of lacrimal glands myositis of extraocular muscles perineuritis of optic nerve orbital cellulitis preseptal postseptal orbital apiciti...
Article

Orbital lymphoma

Primary lymphoma of the orbit corresponds to one of the commonest orbital tumours and accounts for as much as half of all orbital malignancies. It is a B-cell non-Hodgkin lymphoma, and in most cases arises from mucosa-associated lymphoid tissue.  Epidemiology Orbital lymphomas account for only...
Article

Orbital lymphoproliferative lesions

Orbital lymphoproliferative lesions compromise a wide spectrum of diseases ranging from benign to malignant. Clinical presentation Patient's may present with a palpable mass or proptosis. Pain is an uncommon symptom unlike orbital pseudotumour, which manifests with acute pain. Pathology Thes...
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Orbital mass

An orbital mass carries a relatively wide differential: tumours: lymphoma metastasis lacrimal gland or duct tumours rhabdomyosarcoma of the orbit retinoblastoma optic nerve meningioma optic nerve glioma optic nerve schwannoma  neurofibroma developmental orbital cysts 3: choristoma e...
Article

Orbital meningioma

Meningiomas of the orbit are uncommon. They can be divided into two broad categories: primary orbital meningioma: almost all are optic nerve meningiomas rarely ectopic rests of arachnoid cells give rise to orbital meningiomas separate from the optic nerve sheath 1 secondary orbital meningiom...
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Orbital metastasis

Orbital metastases are relatively uncommon, but some primary tumours do have a predilection to metastasise to the orbit. This article concerns itself with extraocular metastases, rather than intraocular tumours or direct extension of tumours from neighbouring regions. For a discussion of intrao...
Article

Orbital pathology

Orbital pathology covers a variety of diverse diseases that affect the orbit. The complicating factor is that the orbit is composed of a large number of different tissues which each have a plethora of pathologies that can affect them.  Classification For simplification, they can be separated i...
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Orbital pseudotumour

Orbital pseudotumour is an idiopathic inflammatory condition that most commonly involves the extraocular muscles. Less commonly there is inflammatory change involving the uvea, sclera, lacrimal gland and retrobulbar soft tissues. The exact aetiology is not known. Clinical presentation Patients...
Article

Orbital septum

The orbital septum is a thin sheet of fibrous tissue that originates from the orbital rim periosteum and blends with the tendon of the levator palpebrae superioris superiorly and inserts into the tarsal plate inferiorly. The orbital septum separates the intra-orbital fat from eyelid fat and orb...
Article

Orbital spaces

The orbital spaces are important when considering different pathologies: globe subdivided into anterior and posterior chambers by the lens optic nerve-sheath complex optic nerve  ophthalmic artery central retinal artery and vein surrounding sheath of meninges as an extension of the cerebr...
Article

Orbital vascular lesions

Orbital vascular lesions may be difficult to distinguish on imaging. However, the following conditions have been described: arteriovenous malformation capillary haemangioma cavernous haemangioma lymphangioma / lymphangiovenous malformation / venolymphatic malformation orbital venous malform...
Article

Orbital venous varix

Orbital venous varix (OVV) is an uncommon vascular malformation which is composed of enlarged single or multiple tubular venous channels with direct communication to the systemic venous system.  Terminology Orbital venous varices are divided into primary and secondary. Primary orbital varices ...
Article

Oro-antral fistula

Oro-antral fistula (OAF) is a pathological communication between the oral cavity and the maxillary sinus (antrum). Epidemiology There is a slightly greater male predilection 2. Pathology Subtypes Oro-antral fistulas may be subtyped by location into: alveolo-sinusal fistula palatal-sinusal...
Article

Oropharynx

The oropharynx forms part the pharynx, being the continuation of oral cavity and nasopharynx superiorly, and the larynx and hypopharynx inferiorly.  Gross anatomy The oropharynx is the posterior continuation of the oral cavity and in the inferior continuation of the nasopharynx.  Boundaries ...
Article

Oscillopsia

Oscillopsia (also known as Dandy syndrome or jumbling of the panorama) is the inability to maintain horizon while walking. Since our heads bob up and down while walking, the otolithic system controls eye movement to maintain a constant horizon when walking. When there is bilateral absent vestib...
Article

Ossicular chain disruption

Ossicular chain disruption (or ossicular discontinuity) is loss of normal alignment between the three inner ear ossicles. The condition is a cause of conductive hearing loss. Epidemiology Exact incidence and prevalence are not known. Hearing loss associated with temporal bone fractures in chil...
Article

Osteoarthritis of the temporomandibular joint

Osteoarthritis of the temporomandibular joint (TMJ) is the end point of long-standing TMJ dysfunction. It is a common finding incidentally on base of skull imaging, and it should be remembered that TMJ pain does not correlate well with OA changes. Indeed pain from TMJ dysfunction is often self l...
Article

Osteoma

Osteomas are benign mature bony growths, seen almost exclusively in bones preformed in membrane (e.g. the skull).  Terminology When they arise from bone they may be referred to as a "homoplastic osteoma", and when they arise in soft tissue they may be referred to as a "heteropla...
Article

Ostiomeatal complex

The ostiomeatal complex (or unit), sometimes less correctly spelled as osteomeatal complex, is a common channel that links the frontal sinus, anterior and middle ethmoid sinuses and the maxillary sinus to the middle meatus that allows air flow and mucociliary drainage.  Gross anatomy The ostio...
Article

Ostiomeatal complex parts (mnemonic)

A useful mnemonic to remember the five parts of the ostiomeatal complex is: HI EMU Mnemonic H: Hiatus semilunaris I: Infundibulum E: Ethmoidal bulla M: Maxillary ostium U: Uncinate process Emus have large beaks, and hence could have large ostiomeatal complexes (if they had paranasal sinu...
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Ostiomeatal narrowing due to nasal anatomical variations

Some ostiomeatal complex anatomical variations, which do not characterise disease by itself, can promote narrowing and even obstruction 1: concha bullosa ~10% (range 4-15%) - aerated middle turbinate  intralamellar cell: air cell within vertical portion of middle turbinate  oversized ethmoid ...
Article

Ostmann fat pad

The Ostmann fat pads (also somewhat lewdly known as Ostmann fatty bodies) are located inferomedial to the Eustachian tubes and is thought to be important in normal closure, preventing transmission of nasopharyngeal pressure to the middle ear. Failure to visualize this thin triangular fat pad is ...
Article

Otic capsule

The otic capsule refers to the dense osseous labyrinth of the inner ear that surrounds the cochlea, the vestibule and the semicircular canals. It is surrounded by the less dense / pneumatised petrous apex and mastoid part of the temporal bone.  Related pathology petrous temporal bone fractures...
Article

Otic ganglion

The otic ganglion is a flat disc-shaped parasympathetic ganglion situated in the infratemporal fossa. It lies immediately below the foramen ovale, medial to the mandibular nerve and supplies secretomotor fibres to the parotid gland. Its preganglionic parasympathetic fibres originate from the in...
Article

Otitis externa

Otitis externa refers to inflammation of the external ear. Epidemiology It is quite a common condition and may affect up to 10% of people during their lifetime. Pathology It can present in several forms: acute otitis externa chronic otitis externa necrotising otitis externa Aetiology Th...
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Otosclerosis

Otosclerosis (also known as otospongiosis) is an idiopathic slowly progressive primary disorder of the bony labyrinth. It is one of the leading causes of deafness in adults.  Terminology The term otosclerosis is somewhat of a misnomer. Much of the clinical course is characterised by lucent rat...
Article

Oxalosis

Oxalosis results in supersaturation of calcium oxalate in the urine (hyperoxaluria), which in turn results in nephrolithiasis and cortical nephrocalcinosis.  This article focus on the secondary oxalosis, please refer to primary oxalosis for a specific discussion on this entity.  Pathology Cal...

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