• Anterior and posterior ethmoidal canals:

The anterior and the posterior ethmoidal canals (AEC and PEC) ( Fig. 1 and Fig. 2 ) are lateral to either olfactory groove. The AEC transmits the anterior ethmoidal artery, vein and nerve. The PEC transmits the posterior artery, vein and nerve. [1, 2, 3, 4]

The anterior ethmoidal artery (AEA) crosses three cavities: the orbit, the ethmoid labyrinth and the anterior fossa of the skull (Fig. 1) . This artery irrigates the anterior ethmoidal cells and the frontal sinus; it gives rise to the anterior meningeal artery, and also irrigates the anterior thirds of the nasal septum and the lateral wall of the nose. In its course through the ethmoid labyrinth, the position of the AEA relative to the ethmoidal roof is very variable; the artery thus becomes vulnerable to injury during surgical procedures. [1, 2, 3]

CT landmarks used for identifying the anterior ethmoidal artery are: The bony notch on the medial wall of the orbit, which corresponds to the anterior ethmoidal foramen and the bone sulcus on the lateral wall of the olfactory fossa, which corresponds to the anterior ethmoidal sulcus. Anterior ethmoidal canal is seen, showing its course fully or partially. [1]

• Foramen venosum:

Foramen venosum or foramen of Vesalius is an inconstant foramen seen bilaterally in 48 % and unilaterally in 30% of the population. It is located anteromedially to the foramen ovale and lateral to the foramen rotundum and pterygoid canal. [5] This canal is best seen on axial slices ( Fig. 3 ) and connects the middle cranial fossa to the scaphoid fossa. It transmits an emissary vein from the cavernous sinus to the pterygoid plexus ( Fig. 4 ), and, on occasion, the accessory meningeal branch of the internal maxillary artery. This vein exits the skull base through the foramen ovale if the foramen Vesalius is absent. [6]

When present, the foramina are frequently symmetric. Asymmetry is more likely the result of a pathologic process. Unilateral enlargement of the Vesalius is seen in carotid cavernous fistulas or tumorous invasion from the nasopharynx. [7]

• Craniopharyngeal canal:

This persistent bony canal can occasionally be detected in the sphenoid bone extending from the sella turcica to the pharynx (Fig. 5 ). It is best referred to as an intrasphenoidal canal. The incidence of the craniopharyngeal canal has been calculated as 0.42% from a retrospective analysis of 8338 adult skulls. [8]

The term ―craniopharyngeal canal or ―persistent hypophyseal canal is generally used to describe a small and vertical midline defect in the skull base that measures less than 1.5 mm in diameter. The term craniopharyngeal canal is also used to describe a rarer and much larger bony canal in the same location. Currarino et al [9] provided other names to describe this skull base defect, including the large craniopharyngeal canal and transsphenoidal canal, which differ from the craniopharyngeal canal or persistent hypophyseal canal by virtue of having a greater size and association with special craniofacial anomalies. It has also been suggested that the large craniopharyngeal canals are related not to the persistent craniopharyngeal canal but rather to transsphenoidal meningoencephalocele. [6, 9]

The etiology of the craniopharyngeal canal is unknown. There are two main theories regarding to the origin of the canal. One theory proposes that this canal is the remnant of the Rathke pouch. The other theory states that the canal represents the remnant of a vascular channel formed during osteogenesis. [10, 11]

• Canalis basilaris medianus:

The Canalis basilaris medianus (CBM) is an uncommon anatomic variant or mild anomaly of the basiocciput. It consists of a well defined channel usually more than 2mm in diameter, originating on the intra cranial surface of the basiocciput, in the midline and very close to the anterior rim of the foramen magnum ( Fig. 6 ). [12] The occurrence of CBM has been estimated

102 between 2-3% in adults and 4-5% in children. [6, 12] The CBM is generally considered as vestige of notochordal canal without clinical significance. [6, 11, 12]

• Canaliculus innominatus:

Also known as petrosal foramen or Canal of Arnold, the canaliculus innominatus is a very small canal situated in the greater wing of the sphenoid between the foramen ovale and foramen spinosum and transmits the lesser petrosal nerve. [13] Ginsberg suggested a prevalence of 16%. [5]

When the Canaliculus innominatus is absent the lesser petrosal nerve is transmitted through the foramen ovale. [5, 6]

• Petromastoidal canal:

The petromastoidal canal arises from the subarcuate fossa, which is located at the posterior rim of the temporal bone, courses between the two limbs of the superior semicircular canal, and opens into the periantral mastoidal cells ( Fig. 7 ). [11, 14]

The canal is lined by dura mater and contains the subarcuate artery and vein. The subarcuate artery mostly originates directly from the anterior inferior cerebellar artery or less frequently from the basilar artery or from the internal auditory artery. [14, 15]

In surgical interventions, elevation of the dura along the posterior fossa surface of the temporal bone can cause a cerebrospinal fluid leak; however, the orifice of the canal can serve as an anatomical landmark during surgery. Assessment of the position of the subarcuate canal in relation to the other landmarks prior to surgery can be helpful for orientation during the intervention as well as in order to prevent inadvertent injury of the subarcuate artery.

Furthermore, the petromastoidal canal gains clinical importance as a pathway for transmission of infections spreading from the mastoidal cells to the dura and the intracranial cavity.

For interpretation of CT images of the temporal bone in traumatized patients knowledge of the course of the subarcuate canal is important, as this structure could be mistaken as a fracture line. [14]

• Posterior condylar canal:

The Posterior condylar canal forms a communication between the jugular foramen and the condylar fossa just posterior to the occipital condyles. It transmits an emissary vein which allows anastomosis of the jugular bulb or sigmoid sinus to the suboccipital venous plexus.[16]

The Posterior condylar canal is the largest of the emissary foramina; its appearance is variable and may depend on its position relative to a particular CT cut ( Fig. 8 ). [16]

• Palatovaginal canal:

This canal found inferomedial to the pterygoid canal ( Fig. 9 ) and extends from the pterygopalatine fossa to the roof of the pharynx ( Fig. 10 ). It is narrower than the pterygoid canal and foramen rotundum, and hence is the most likely to be confused with a fracture on axial images. It transmits the pterygovaginal artery which is a branch of the maxillary artery, and the pharyngeal nerve, which passes from the pterygopalatine ganglion to the pharyngeal orifice of the Eustachian tube. [11]

Identification of this anatomic structure may play a 147 role in detecting canal enlargement due to spread of malignant nasopharyngeal tumors or canal fracture in case of intractable post traumatic epistaxis with injury of ptérygo vaginal artery. [17, 18]

• Inferior tympanic and mastoid canaliculi:

The mastoid canaliculus and the inferior tympanic canaliculus are seen respectively in 28% and 6% of CT scans of the temporal bone. [19]

The inferior tympanic canaliculus is a canal that extends from the jugular foramen to the medial wall of the middle ear seen in only 6% of CT scans of the temporal bone.( Fig. 11 ) This canal transmits the inferior tympanic branch of the glossopharyngeal nerve (Jacobson’s nerve) from the inferior glosso pharyngeal ganglion. [19]

The mastoid canaliculus is a canal that extends from the posterolateral aspect of the jugular foramen to the descending portion of the facial nerve canal seen in 28% of CT scans of the temporal bone.( Fig. 12 ) It transmits the auricular branch of the vagus nerve (Arnold’s nerve) from the superior vagal ganglion. [19]

Both canaliculi form important surgical landmarks for the identification of the cranial nerves within the jugular foramen. Their clinical importance also pertains to the presence of glomus formations along the nerves of Arnold and Jacobsen. The earliest signs of jugulotympanic glomus tumors may therefore be destruction of these canals. [11]

In addition, the inferior tympanic canaliculus is enlarged in the setting of an aberrant internal carotid artery, since this represents an enlarged inferior tympanic artery which anastamoses with the caroticotympanic artery after embryonic regression of the cervical internal carotid artery ( Fig. 13 ). [11]