Page 274 - Atlas of Small Animal CT and MRI

2.10

Cranial nerves

For an in‐depth description of the imaging anatomy of reformatted imaging planes that parallel the path of a
the skull and cranial nerves, the reader is referred to a nerve (Figure 2.10.1).
number of studies detailing the CT and MR appearance The normal optic nerve may have a striated “tram‐
of the normal cranial nerves and the skull foramina from track” appearance following contrast medium adminis-
which they emerge. This chapter is limited to a discus- tration on both imaging modalities as a result of the
1–4
sion of the most common clinical disorders involving relatively greater enhancement of the surrounding
the cranial nerves. dural sheath compared to the nerve. Fat‐suppression
techniques are particularly useful for increasing the
Cranial nerves conspicuity of the optic nerve on contrast‐enhanced
MR sequences.
Cranial nerve II
The optic nerve, or cranial nerve II, is a tract of the brain Cranial nerve V
and is unique among the cranial nerves in that it has a
meningeal covering and a subarachnoid space. Axons The trigeminal nerves arise from either side of the pons
arising from retinal ganglion cells form the optic nerve and exit the cranium through the trigeminal canal of the
after collecting and exiting at the optic disc of the eye. temporal bone. Within the temporal bone, sensory com-
The nerve extends caudally in the retrobulbar space and ponents of the nerve form the large trigeminal ganglion.
enters the cranium through the optic canal. The paired The nerve divides to form three major peripheral
nerves partially decussate at the optic chiasm, with the branches, the ophthalmic, maxillary, and mandibular
resulting optic tracts terminating in the lateral genicu- nerves, that exit through the orbital fissure, round fora-
late nucleus and other nuclei with vision functions. The men, and oval foramen, respectively.
normal canine optic nerve has been reported to be In a review of MR contrast enhancement patterns of
between 1.2 and 2.4 mm in diameter. 5 cranial nerve V in 42 dogs without clinical signs referable
The normal optic nerve is isoattenuating to brain to trigeminal nerve dysfunction, the entire nerve enhanced
parenchyma on unenhanced CT images and is T1 and in over 90% of dogs, and enhancement was limited to the
T2 isointense to normal white matter on MR images. region of the trigeminal ganglion in the remaining dogs.
The margins of the nerve are usually well delineated Intensity of enhancement was subjectively determined to
because of orbital fat within the retrobulbar space on be less than that of the pituitary gland. 6
both modalities and as a result of surrounding CSF on
MR images. The normal optic nerve can be followed Cranial nerves VII and VIII
from the optic chiasm, through the optic canal, and into Cranial nerve VII, the facial nerve, arises in the
the retrobulbar space. Thin‐collimation CT imaging and medulla oblongata and emerges from the trapezoid
volume‐acquisition MR techniques can be used to define body. The nerve exits the cranial cavity through the

Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
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