The nasal sill is the soft tissue ridge forming the posterior margin of the anterior naris. It also forms the caudal margin of the nasal vestibule.
The nasal sill forms part of the nasal base, which is defined as the inferior third of the nose when viewing the nose from below with the neck exte...
The nasal vestibule forms the small mildly dilated area just inside the anterior naris. It is formed by the columella medially, the nasal ala lateral and anteriorly and the nasal sill posteriorly.
The nasion is the midline bony depression between eyes where the frontal and two nasal bones meet, just below the glabella. It is also known as the bridge of the nose.
It is one of the skull landmarks, craniometric points for radiological or anthropological skull measurement.
The nasociliary nerve is the intermediate branch of the ophthalmic division of the trigeminal nerve.
The nasociliary nerve divides off the ophthalmic division just before entering the orbit through the superior orbital fissure and through the tendinous ring between the inferior a...
Nasolabial cyst (also known as nasoalveolar cyst or Klestadt`s cyst) is a rare non-odontogenic, soft-tissue, developmental cyst occurring inferior to the nasal alar region. The cyst is derived from epithelial cells retained in the mesenchyme after fusion of the medial and lateral nasal processes...
The nasolacrimal (drainage) apparatus consists of:
Tears produced by the lacrimal gland, accessory lacrimal glands of Krause Wolfring and Zeis, and Meibomian glands track medially along the eyelid margins and collect at the lacrimal lake at ...
The nasolacrimal duct is the terminal part of the nasolacrimal apparatus.
The nasolacrimal duct is the inferior continuation of the lacrimal sac and is ~17 mm in length in total. There are two parts to the nasolacrimal duct:
intraosseous part (12 mm): lies within the nasolacrima...
Nasolacrimal tumours, in other words tumours involving the nasolacrimal drainage apparatus, are uncommon, and have a variety of histologies.
Clinical presentation of nasolacrimal tumours are typically fairly non-specific, often resulting in delayed diagnosis 1. Typical pr...
This suture forms the fissure between the frontal process of maxilla and the lateral border of the nasal bone. The nasomaxillary sutures are paried.
The naso-orbital-ethmoid (NOE) region or interorbital region simply refers to the space between the orbits.
anterior: frontal process of the maxilla, nasal process of frontal bone, nasal bone
posterior: sphenoid sinus
lateral: medial orbital wall
Naso-orbitoethmoid (NOE) fractures (also known as orbitoethmoid or nasoethmoidal complex fractures) are fractures which involve the central upper midface.
Naso-orbitoethmoid fractures are caused by a high-impact force applied anteriorly to the nose and transmitted posteriorly through...
The nasopalatine nerve (also known as the long sphenopalatine nerve) is a branch of the maxillary division of the trigeminal nerve and contributes to the pterygopalatine ganglion.
The nasopalatine nerve divides off the maxillary division just after emerging from the foramen rotun...
Nasopharyngeal carcinomas (NPC) are the most common primary malignancy of the nasopharynx. It is of squamous cell origin, some types of which are strongly associated with the Epstein-Barr virus (EBV).
Nasopharyngeal carcinoma accounts for ~70% of all primary malignancies of the na...
Nasopharyngeal carcinoma staging uses the TNM staging system with derived stage groupings.
Primary tumour (T)
Tx: primary tumour cannot be assessed
T0: no evidence of primary tumour
Tis: carcinoma in situ
T1: tumour is confined to the nasopharynx
T2: tumour extends to soft ti...
A nasopharyngeal choristoma is a rare, non-neoplastic mass (type of choristoma) typically located in the lateral aspect of the nasopharynx without intracranial extension. These lesions are composed of fibrovascular tissue and fat. Resection is curative.
A mnemonic for causes of nasopharyngeal masses is:
S: squamous cell carcinoma
A: antrachoanal polyp
I: inverted papilloma
L: lethal midline granuloma
The nasopharynx forms the superior-most part of the pharynx, in continuity with the inferior oropharynx, and the posterior continuation of the anterior nasal cavity.
anteriorly: posterior nares and posterior margin of nasal septum 1,2
inferiorly: soft palate 2
There are several types of neck dissections which can classified as follows:
Radical neck dissection
Radical neck dissection is considered to be the standard basic procedure for cervical lymphadenectomy. All other procedures represent one or more alterations of this procedure. The dissection i...
Necrotising otitis externa (NOE), also known as malignant otitis externa, is a severe invasive infection of the external auditory canal (EAC) which can spread rapidly to involve the surrounding soft tissue, adjacent neck spaces and skull base.
Predisposing conditions for NOE include...
Necrotising sialometaplasia is a benign inflammatory condition of the salivary gland remnants or elements, which is usually self-limiting. It is most commonly found on the palate.
Most patients are more than 40 years of age, with a male: female ratio of 2:1. It is rarely seen in...
Nelson syndrome is a rare disorder observed in patients with Cushing disease that have undergone bilateral adrenalectomy. It involves enlargement of an existing adrenocorticotropic hormone-secreting pituitary tumour, typically an adenoma, after surgical resection of the adrenal glands.
The nerve to medial pterygoid or medial pterygoid nerve is a branch of the mandibular division of the trigeminal nerve.
The nerve to medial pterygoid divides off the mandibular division just below the foramen ovale and just before it branches into anterior and posterior divisons....
The nerve to mylohyoid or mylohyoid nerve is a motor branch of the inferior alveolar nerve, a branch of the mandibular division of the trigeminal nerve.
The nerve to mylohyoid arises from the inferior alveolar nerve just before it enters the mandibular foramen. It does not enter ...
The nerve to stapedius arises from the facial nerve to supply the stapedius muscle. The branch is given off in the facial nerve's mastoid segment, as it passes posterior to the pyramidal process.
Damage to this branch with resulting paralysis of stapedius leads to hypersensitivity to loud noise...
Nervus intermedius neuralgia, or geniculate neuralgia, corresponds to a clinical manifestation of sudden paroxysms of excruciating otalgia which usually lasts a few seconds to a few minutes, involving the nervus intermedius (intermediate nerve of Wrisberg).
Nervus intermedius neur...
The nervus spinous (also known as the meningeal branch of the mandibular nerve) is a branch of the mandibular division of the trigeminal nerve which innervates the dura of the middle cranial fossa.
The nervus spinosus divides off the mandibular division just below the foramen ova...
Craniocerebral neuroblastoma metastases usually involve the calvaria, orbits, skull base and the dura.
Please refer to the article neuroblastoma for a general discussion on this entity.
periorbital ecchymosis " racoon eyes"
palpable calvarial masses
Orbital manifestations of neurofibromatosis type 1 (NF1) reflects a mix of CNS, musculoskeletal and ocular disease. Usually, only one orbit is affected, commonly associated with an ipsilateral extracranial disease.
NF1 manifests in the orbit in the following ways 1-5:
Neurosyphilis results from infection of the central nervous system by the spirochete Treponema pallidum, subspecies pallidum. The disease has a heterogeneous spectrum of early and late manifestations.
For a general discussion, and for links to other system specific manifestations, please refer ...
NEXUS (National Emergency X-Radiography Utilization Study) is a set of validated criteria used to decide which trauma patients do not require cervical spine imaging.
Trauma patients who do not require cervical spine imaging:
alert and stable
no focal neurologic deficit
no altered level of co...
The node of Rouviere is the most superior of the lateral group of the retropharyngeal lymph nodes. It is found at the base of the skull.
History and etymology
Henri Rouvière (1876–1952) was an anatomy professor from Le Bleymard in France. His seminal work, "Anatomy of the Human Lymphatic Syst...
Nonchromaffin paraganglion cells are cells in the neuroendocine system that make up several clusters of chemoreceptive cells. These cells are associated with a supporting maxtrix and are found in close proximity to blood vessels and nerves (especially the glossopharyngeal and vagus nerves). They...
Non-decussating retinal-fugal fibre syndrome (NRFFS), also referred to as achiasma, is the isolated congenital absence of optic chiasm with unremarkable rest of the optic pathway and midline structures. Clinically, it typically presents as seesaw nystagmus since childhood. Diagnosis is usually m...
Nonsteroidal anti-inflammatory drug (NSAID) exacerbated respiratory disease or previously known as Samter syndrome or aspirin or analgesic-induced asthma refers to the constellation of 1-2:
allergy to aspirin
nasal polyposis / rhinosinusitis
In adults, prevalence can ran...
Bone marrow signal of the clivus changes predictably with age and is well assessed with midline T1 non-fat-saturated, non-contrast images. As is seen in the rest of the body the proportion of yellow (fatty) marrow increases with age. Knowledge of these changes allows diagnosis of the abnormal cl...
This article lists examples of normal imaging of the head and neck and surrounding structures, divided by region and modality.
For normal spinal imaging, please see: normal spinal imaging
soft tissue: example 1
soft tissue: example 2
Norrie disease is a rare X-linked inherited cause of congenital bilateral blindness. It can present with a retinal mass (pseudoglioma) and cataracts. It is associated with developmental delay and hearing loss.
The nose, sometimes referred to as the external nose, is a feature of the face and is composed of soft tissues that extend externally from the skull. It is continuous posteriorly with the nasal cavity. The anterior (piriform) aperture is bounded above by the nasal bones and elsewhere by the two ...
A helpful mnemonic to remember the expected direction of nystagmus during a caloric test is:
Following ear stimulation with water (left ear):
cold water (cooler than body temperature), a right-beating nystagmus (directed toward the opposite ear) result...
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 ...
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 ...
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.
origin: branch of the external carotid artery at the level of the ...
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
The occipital bone is composed of four parts:
The occipital condyles are two large protuberances on the undersurface of the occipital bone, located besides the front half of the foramen magnum. It forms the connection between the skull and the vertebral coloumn.
atlas (C1) at the atlanto-occipital joint
Occipital nerve stimulators are a type of neuromodulation therapy aimed at treating chronic refractory headache and craniofacial pain 1-3. This form of non-pharmaceutical therapy involves an implantable device composed of an electrode and a pulse generator. The leads are placed superficially in ...
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.
It is one of the rarest craniosynostosis, a...
The occipital triangle is one of the paired triangles in the posterior triangle of the neck. The triangles of the neck are surgically focussed, first described from early dissection-based anatomical studies which predated cross-sectional anatomical description based on imaging (see deep spaces o...
The occipitotemporal/occipitomastoid suture is the obliquely oriented articulation of the anterior border of the squamous occipital bone and the mastoid portion of the temporal bone. A mastoid foramen is occasionally located near or in it.
The occipitotemporal suture and the parietotemporal sut...
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)
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)
The ocular depressors are the extra-ocular muscles responsible for depression of the globe:
inferior rectus (primary function)
superior oblique (secondary function)
The ocular elevators are the extra-ocular muscles responsible for elevation of the globe:
superior rectus (primary function)
inferior oblique (secondary function)
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)
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)
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...
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 ...
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.
Nucleus and cisternal portion
The oculomotor nu...
Oculomotor nerve palsies, or third nerve palsies, result in weakness of the muscles supplied by the oculomotor nerve, namely the superior rectus, inferior rectus, medial rectus, inferior oblique, and levator palpebrae superioris muscles.
Classically, patients present with...
Odontogenic keratocysts (OKC), previously known as keratocystic odontogenic tumours (KCOT or KOT), are benign cystic lesions involving the mandible or maxilla and are believed to arise from dental lamina. Whether these lesions are developmental or neoplastic is controversial, with the 2017 WHO c...
Odontogenic myxomas are rare tumours that involve the mandible or maxilla and account for 3-6% of odontogenic tumours 2.
Typically seen in the 2nd to 3rd decades of life (slightly earlier than ameloblastomas). They are usually not painful.
Arises from mesenchymal odont...
Odontohypophosphatasia is the mildest form of hypophosphatasia that manifests as tooth dysplasia and/or early loss of deciduous or permanent teeth.
As with all forms of hypophosphatasia, the underlying abnormality is a mutation in the ALPL gene that encodes for tissue non-specific al...
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.
The oesophagus is a muscular tube that conveys food and fluids from the pharynx to the stomach.
The oesophagus is 23-37 cm long with a diameter of 1-2 cm and is divided into three parts:
cervical: continuous with the hypopharynx, commences at the lower border of cricoid cartilag...
The olfactory fossae, grooves or recesses of the cribriform plate are shallow depressions located within the ethmoid bone in the anterior cranial fossa that contains the olfactory bulb of the olfactory nerve. The floor is formed by the medial lamella of the cribriform plate, bordered laterally b...
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 ...
Olfactory neuroblastomas (or esthesioneuroblastomas) are most frequently staged using a system proposed by Kadish et al. in 1976 1.
group a: limited to the nasal cavity
group b: limited to the nasal cavity and paranasal sinuses
group c: extends beyond the nasal cavity and paranasal sinuses:
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.
Olfactory neuroepitheliomas differ from olfactory neuroblastomas in a number of ways 1:
The omega sign can refer to a number of different anatomical structures or signs:
omega sign (epiglottitis)
omega sign (hand bump on the precentral gyrus)
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...
Ommaya reservoir, also known as Ommaya shunt, is a device consisting of an intraventricular catheter connected to a reservoir (port) implanted beneath the scalp. It is used for intrathecal administration of medication such as chemotherapy (mainly in the treatment of meningeal lymphomas, but also...
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...
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).
Oncocytomas of the salivary glands are rare benign epithelial tumours.
They typically present in older individuals (6th to 8th decades of life). There may be a slight increased female predilection.
While they can affect any of salivary glands. They most commo...
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...
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.
Various lines and measurements using the opisthion are made in the diagnosi...
The optic canal is a cylindrical canal running obliquely through the lesser wing of sphenoid bone near the base where it joins the body of sphenoid. It transmits the optic nerve and ophthalmic artery. The optic canal opens into the skull base at the optic foramen.
Optic disc drusen (ODD), or hyaline bodies, are a relatively common entity usually found incidentally on CT or on follow-up of abnormal fundoscopy.
Optic disc drusen are identified radiographically in up to 0.3-3.7% of the population and are frequently bilateral 1,4,5. They are ty...
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...
Enlargement of the optic nerves is uncommon and has a surprisingly broad differential:
optic nerve glioma
optic nerve 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, ...
Optic neuritis 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 non-infectious causes, although the latter is far more frequent. On imaging, optic neuritis is most easily identified as a ...
The optic radiation (or geniculocalcarine tract) is part of the visual pathway, forming the connection between the lateral geniculate nucleus of the pulvinar of the thalamus with the primary visual cortex of the occipital lobe.
This pathway carries third order neurones correlating to the contr...
The oral cavity is the most proximal portion of the aerodigestive tract, and is continuous posteriorly with the oropharynx.
The oral cavity includes the lips anteriorly and extends to a ring of structures (circumvallate papillae, anterior tonsillar pillars, soft palate) that di...
T staging of oral cavity tumours is as follows:
The anterior border of the oral cavity is the junction of the skin and vermilionborder of the lip. The posterior border is formed by the junction of the hard and soft palates superiorly, the circumvallate papillae inferiorly, and the ...
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.
Ora serrata cannot be seen on conventional imaging but is an inferred reference point. On the axial plane...
Orbicularis oculi are facial muscles, a ring-like muscle functioning in a number of eyelid movements.
The orbicularis oculi are made up of three parts the orbital, palpebral and lacrimal parts. Each has defined actions.
The orbicularis oculi are secured to the medial and latera...
The orbit is a feature of the face and contains the globe and it's supporting structures, as well as many nerves and vessels.
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 ...
The orbital apex refers to the posterior confluence of the orbit at the craniofacial junction, where nerves and vessels are transmitted from the intracranial compartment into the orbit via several bony apertures. It is also the point where the extra-ocular muscles derive their origins.
Orbital blow-in fractures occur when there is displacement of bone fragments towards the orbits.
blow-in fracture effectively reduces the volume of the orbit
associated intraorbital injuries include extraocular muscle entrapment and optic nerve injury
as an isolated (pure) orbital ...
Cavernous venous malformations of the orbit, also known as cavernous haemangiomas, 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 ve...
Several cystic and cyst-like orbital lesions may be encountered in imaging of the orbits:
developmental orbital cysts
dermoid: commonest benign orbital tumour in childhood
congenital cystic eye
Orbital dermoid cysts are congenital lesions representing closed sacs lined by an ectodermal epithelium and comprising the most common orbital tumour in children. They are typically divided into deep (within the orbit) and superficial (adjacent to the orbital rim).
They comprise ~...
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.