MRI of the Cranial Nerves—More than Meets the Eye: Technical Considerations and Advanced Anatomy




Magnetic resonance (MR) imaging is the method of choice to evaluate the cranial nerves. Although the skull base foramina can be seen on CT, the nerves themselves can only be visualized in detail on MR. To see the different segments of nerves I to XII, the right sequences must be used. Detailed clinical information is needed by the radiologist so that a tailored MR study can be performed. In this article, MR principles for imaging of the cranial nerves are discussed. The basic anatomy of the cranial nerves and the cranial nerve nuclei as well as their central connections are discussed and illustrated briefly. The emphasis is on less known or more advanced extra-axial anatomy, illustrated with high-resolution MR images.


Magnetic resonance (MR) imaging is the method of choice to evaluate the cranial nerves. Although the skull base foramina can be seen on CT, the nerves themselves can only be visualized in detail on MR. To see the different segments of nerves I to XII, the right sequences must be used. It goes without saying that detailed clinical information is needed by the radiologist so that a tailored MR study can be performed. In this article, the MR principles for imaging of the cranial nerves are discussed. The anatomy of the 12 cranial nerves will be addressed but it is not possible to describe all of the anatomy in one article. Therefore, the basic anatomy of the cranial nerves and the cranial nerve nuclei as well as their central connections are discussed and illustrated briefly while the emphasis is on less known or more advanced extra-axial anatomy, illustrated with high-resolution MR images. For more complete anatomic descriptions and anatomic and/or MR illustrations the reader is referred to dedicated already existing complete works.


MR technique


The MR technique must be pushed to its limits to see all 12 cranial nerves and especially some of the segments or branches of these nerves that are more difficult to depict. Imaging plane, coil and sequence choice, slice thickness and in-plane resolution, use of special techniques like parallel imaging, asymmetric k-space, fat suppression, and so forth will all influence the final image quality.


Imaging Plane


All cranial nerves are paired and therefore it is wise to compare size and signal intensity of both nerves. The coronal plane is best suited to study the cranial nerves I to VI, as they have a dominant postero-anterior course. Cranial nerves VII to XII run in an antero-lateral-caudal direction but the lateral component is dominant. Hence the best plane will be the axial plane. Nevertheless it is always safe to perform for all cranial nerves at least one sequence in a second plane (additional axial or coronal plane).


Coil Choice


A phased array (synergy) head coil can be used for all cranial nerves. The advantage of a head coil is that it can cover all 12 cranial nerves and that it provides images with good signal-to-noise ratio (S/N) even on the midline and especially in the deepest regions: the cavernous sinus and pre-pontine cistern. A head coil also allows excellent evaluation of the brainstem (cranial nerve nuclei) and brain (eg, olfactory cortex, auditory cortex). Some very small nerves or branches can only be seen when very high resolution is available. This high resolution in combination with enough S/N is especially at 1.5 Tesla often only achievable when synergy surface coils are used. As mentioned above, these coils will only improve the results when one is imaging the more peripherally located nerves (I, II, VII, VIII). These coils can be put inside the head coil, the “concentric coil technique,” so that the head coil can still be used to visualize the deeper structures and complete brain ( Fig. 1 ). At 3 Tesla, the higher S/N often allows production of better images with higher resolution without the need of these surface coils. This also allows the use of different sequences and on 1.5 Tesla, two-dimensional (2D) sequences are still frequently used, at 3 Tesla today nearly all sequences are three-dimensional (3D) sequences and cover all cranial nerves. Finally, microscopic coils can be used ( Fig. 2 ) when very small superficial nerve branches must be visualized. These coils can produce images with extremely high resolution; however, this is only possible when the MR unit is equipped with very strong gradients.




Fig. 1


Concentric coil technique applied for anterior skull base ( A ) and temporal bone ( B ) imaging. ( A ) The two coil elements of the synergy Flex S coil ( black arrows ) are put inside the eight-element synergy head-coil. To visualize the anterior skull base and olfactory nerves in detail, the small Flex S coils are positioned high and toward the midline. The coils must keep a more or less parallel position if a significant gain from parallel imaging is expected. The cables of the Flex S coils should not cross on the body of the patient. ( B ) The Flex S coils are now centered on the ears ( black arrows ). A headphone ( white arrow ) is put between the head-coil and the Flex S coils to push the surface coils even closer to the skin and to the inner ear.



Fig. 2


Microscopic coils. A microscopic coil with a diameter of 4.7 mm (1) can be used to visualize very small superficial nerves. This coil will reach a depth of 2.35 mm and therefore nerves beyond this depth cannot be imaged with this coil. This coil will only provide higher resolution than a routine coil when strong gradients are available.


Sequences


The choice of the sequence will depend on the tissue or fluid that is surrounding the nerve.


In the brain stem, the cranial nerve nuclei and fascicular segment (nerve segment inside the brain stem) of the nerve cannot be visualized but their location can be deduced when the surrounding myelinated structures are recognized. These are best seen on T2-weighted (T2W), proton-density and especially multi-echo fast field echo (m-FFE) ( Fig. 3 ) or T2∗W 2D spoiled gradient echo multiecho sequence (MEDIC) images. In case of pathology the Flair sequences seem to be the most sensitive and a diffusion sequence is needed to exclude acute infarctions.




Fig. 3


Patient with right fourth nerve palsy and internuclear ophthalmoplegia, axial m-FFE ( A ) and b-1000 DWI ( B ) images at the level of the inferior colliculus–medullary velum. ( A ) The myelinated structures like the MLF ( black arrow ) and superior cerebellar peduncle ( white arrowhead ) are black on this image. The trochlear nucleus ( gray arrow ) must be located between the periaqueductal gray ( white arrow ) and the MLF. The signal intensity of the left MLF is increased. ( B ) Diffusion restriction can be seen on the diffusion image indicating acute infarction in the area of the left trochlear nucleus and MLF ( white arrowhead ), causing a right superior oblique muscle palsy.


Heavily T2W sequences are used once the cisternal segment of the nerve, the segment that is surrounded by cerebrospinal fluid (CSF), is examined. The sequences that are available or used depend highly on the type of MR unit. Typically CISS, 3D-TSE, b-FFE, DRIVE, 3D-FSE, FIESTA, 3D FSE XETA, and so forth sequences are used. All these sequences are heavily T2W 3D-sequences and provide images with very high resolution. However, these sequences should be carefully chosen. Some of these sequences are based on “steady state” (eg, CISS, b-FFF) and produce artifacts at the periphery of the image, especially when a higher spatial resolution is used. Hence, these sequences should be reserved for imaging around the brainstem, in the center of the image. These sequences are better replaced by other type of sequences (3D-TSE, DRIVE) for high resolution imaging of more superficially located nerves (eg, nerves I, VII, VIII) ( Fig. 4 ).




Fig. 4


DRIVE turbo spin-echo ( A ) versus balanced-FFE gradient-echo ( B ) heavily T2W coronal image through the olfactory bulbs. ( A ) The olfactory bulbs are surrounded by CSF and no artifacts are seen. Further posteriorly the olfactory tract can always be found under the olfactory sulcus ( black arrow ) situated between the medial orbital gyrus ( white arrow ) and rectus gyrus ( gray arrow ). ( B ) Image made in the same patient at the same level showing the typical phase artifacts, seen as black curved lines crossing the CSF ( gray arrows ), on this gradient-echo image.


Once the nerves are surrounded by a venous plexus (III to VI in the cavernous sinus, VI behind the clivus in the basilar plexus, IX to XI in the jugular foramen, XII in the hypoglossal canal) they are best seen on high resolution contrast-enhanced Time-Of-Flight MRA images or high resolution 2D (SE or TSE) or 3D (TSE or FFE) T1W images. On these images, the cranial nerves are seen as black structures surrounded by high signal intensity gadolinium-filled venous structures ( Fig. 5 ).




Fig. 5


Axial high resolution contrast-enhanced 3D-FFE image through the jugular foramina. This image has a resolution of 0.71 × 0.71 mm and a thickness of 0.6 mm. The hypoglossal nerves are seen as gray structures surrounded by contrast-enhanced surrounding venous structures ( white arrows ). This technique allows visualization of the glossopharyngeal nerves ( black arrows ) located close to the internal carotid artery and of the vagus nerves ( black arrowheads ) and accessory nerves ( gray arrows ), which at this level are crossing the center of the jugular foramen.


The peripheral segments and branches of the cranial nerves are surrounded by soft tissues and especially fat in the neck and face. High-resolution T1W SE and TSE sequences are in this region best suited to visualize the nerves. The use of fat saturation will make the fat disappear and makes visualization of normal nerves difficult or impossible. Fat saturation has additional value only when an abnormal enhancement of the nerve is expected or must be excluded.


Slice Thickness, In-Plane Resolution


The T2W and proton density SE images through the brainstem have a thickness of 4 mm, to avoid partial volume effect. The in-plane resolution of these images is 0.9 × 0.9 mm.


Coronal 2D T1 SE images can be thicker as there is no risk for partial volume problems as the nerves run antero-posterior and will be visualized anyway. Therefore, for this sequence, all energy is put in in-plane resolution, 0.58 × 0.64 mm while the slice thickness is 4 mm.


Selective contrast-enhanced T1-weighted FFE images through the cerebellopontine angle (CPA) and jugular foramen (JF) have a slice thickness of 0.625 mm and an in-plane resolution of 0.71 × 0.71 mm. This very thin slice thickness is needed to distinguish the different nerves.


The heavily T2W b-FFE (imaging around brainstem) and DRIVE (imaging more peripherally) have the highest resolution. The b-FFE images are 0.5 mm thick and have an in-plane resolution of 0.5 × 0.5. At 3 Tesla they have a 0.4 3 or even 0.35 3 mm isotropic voxel size. The DRIVE images are 0.35 mm thick and have an in-plane resolution of 0.33 × 0.30, but the 3D slab has only 48 partitions (covers a distance of 17 mm in cranio-caudal direction) while the b-FFE has a somewhat lower spatial resolution but covers a total cranio-caudal distance of up to 12 cm.


Special Techniques/Software


The nerves can be visualized only when the spatial resolution is very high or the voxel size is very small. The time needed to acquire such images is very long with the use of routine sequences. Therefore, any MR technique that speeds up the sequence, without or with minimal loss of S/N, can be used to shorten the examination time and to reduce movement/swallowing/breathing artifacts or can be invested to get even higher resolution in the same time.


Parallel imaging (eg, SENSE) is therefore used in most of the above-mentioned sequences and also helps to reduce susceptibility artifacts in sequences sensitive to this type of artifact (eg, DRIVE, b-FFE). However the maximal SENSE factor that can be used is equal to the number of coil elements used in the phased array coil (if there are only two coil elements, like for the flex S surface coils used for temporal bone imaging, then the acquisition time can maximal be reduced to ½ (1/N° coil elements) with a S/N reduction of the square root of 2 (N° coil elements). This maximal SENSE factor is already causing too much S/N loss when very high resolution imaging of the CPA nerves is performed and that is why routinely a parallel imaging factor of 1.7 is preferred. SENSE is avoided for midline imaging, even when the head coil is used, as the resulting signal drop will immediately degrade the contrast resolution quality of these very high resolution images and consequently make it more difficult to visualize the nerves.


The use of an asymmetric k-space, only possible when TSE types of sequences are used, can reduce the examination time easily by 30%, without the penalty of S/N loss. Hence, this is an ideal technique to increase resolution in the same imaging time or to stick to the same resolution in a shorter examination time.




Cranial nerve anatomy


The Olfactory Nerve or Cranial Nerve I


Olfactory epithelium


The olfactory epithelium is located in the upper one fifth of the nasal cavity and covers the septal and lateral surface of this cavity, including the upper part of the superior turbinate ( Fig. 6 ).




Fig. 6


Olfactory bulbs—coronal contrast-enhanced high-resolution T1W image. The olfactory bulbs ( white arrow ) are isointense with brain and are located just above the cribriform plate. Notice the enhancing olfactory mucosa located high in the nasal cavity on the medial surface of the superior turbinates and in the mucosa of the upper part of the septum ( gray arrows ).


Transethmoidal segment


Bipolar olfactory neurons connect the olfactory epithelium with the olfactory bulbs. These neurons experience a continuous cycle of growth, degeneration, and replacement, which make them “unique” neurons. The dendrites of these olfactory neurons reach the surface of the olfactory epithelium while the unmyelinated axons, about 3 million on each side, run upwards through the openings of the cribriform plate. These axons are grouped in bundles, called filia, which are invested by Schwann cells. This explains why olfactory schwannomas can develop. These filia enter the olfactory bulb and these filia together constitute the real “olfactory nerve.” Some of these filia between the cribriform plate and olfactory bulb can sometimes be seen on high-resolution T2W images ( Fig. 7 ).




Fig. 7


Olfactory filia (nerves)—high-resolution para-sagittal reformatted b-FFE image. The olfactory bulb ( black arrow ) can be seen as an ovoid dark structure surrounded by white CSF. The olfactory tract ( white arrow ) is emerging from the bulb. Some of the filia can be followed through the cribriform plate into the inferior surface of the bulb ( gray arrows ).


Olfactory bulb and tract—intracranial


The olfactory bulb and tract (intracranial) are extensions of the brain and are not a real cranial nerve. The olfactory nerve fibers are received in about 50,000 mitral cells inside the olfactory bulbs. The olfactory bulbs have a convex ovoid shape and can easily be recognized on coronal T2W or T1W images (see Figs. 4, 6, and 7 ). The axons of the olfactory bulb mitral cells are located centrally in the olfactory tract and run from their paramedian origin in the olfactory bulbs posterolaterally toward the anterior border of the anterior perforated substance. Along this course, the olfactory tract follows the inferior border of the olfactory sulcus, located between the gyrus rectus and medial orbital gyrus. The olfactory tract is flatter and more difficult to visualize but its constant position under the olfactory sulcus helps to track this structure ( Fig. 8 ). The olfactory tract divides in lateral, medial, and intermediate stria in front of the anterior perforated substance. This division can be depicted on (para)-axial high-resolution T2W images ( Fig. 8 B).




Fig. 8


Olfactory bulb-tract-striae—axial high-resolution (0.35 × 0.35 × 0.35 isotropic voxels) b-FFE image through the olfactory bulb-tract-striae ( A ) and para-axial reformatted image through the tract and striae ( B ). ( A ) The left olfactory bulb ( black arrow ), tract ( white arrows ), and division in lateral and medial striae ( gray arrows ) just anterior to the anterior perforated substance can be seen in and under the olfactory sulcus. Notice the oblique course of the olfactory tract from anteromedial to posterolateral. ( B ) The olfactory tract ( white arrows ) can be followed and splits in lateral, intermediate, and medial striae ( gray arrows ). Notice the acute medial angulation of the medial stria.


Olfactory pathways—intracranial


Damage to the olfactory pathway on one side will cause ipsilateral anosmia. Olfaction, and taste are actually the only uncrossed sensations. Some axons in the medial stria reach the septal area via the diagonal band, others cross the midline via the anterior commissure to reach the contralateral olfactory tract. The lateral stria terminates in the piriform lobe (uncus, anterior part of the parahippocampal gyrus, cortical part of the amygdala) and connects through the thalamus (mediodorsal nucleus) with the orbital frontal cortex, which is the highest center for olfactory discrimination. The intermediate stria reach the intermediate cortical olfactory area, a small zone of gray matter at the level of the anterior perforated substance.


The Optic Nerve or Cranial Nerve II


The optic nerve is just like the olfactory bulb and tract; not a true cranial nerve but rather an extension of the brain. The optic nerve can be divided in several segments: intraocular, intraorbital, intracanalicular, and intracranial. The optic pathway then continues in the optic chiasm and optic tracts. The optic radiation and visual cortex are beyond the scope of this chapter.


Intraocular segment


The axons of the retinal ganglion cells form the intraocular optic nerve. This intraocular segment as well as the retina, where the ganglion cells are located, is difficult to visualize. Some of this anatomy can, however, be visualized when microscopic coils and very high resolution are used ( Fig. 9 ). These axons get myelinated by oligodendrocytes as they leave the optic disc.




Fig. 9


Intraocular (+ retina) and orbital segment of the optic nerve—axial high-resolution contrast-enhanced T1W image ( A ) and TSE T2W image ( B ) acquired with a 4.7-mm-diameter microscopic coil using strong gradients. ( A ) The intraorbital segment ( white arrowhead ) and intraocular segment ( white arrow ) can be seen at the level of the lamina cribrosa of the sclera. The enhancing choroid ( gray arrows ) and dark sclera ( black arrow ) can be distinguished. CSF is in the subarachnoid space around the intraorbital segment ( black arrowhead ). ( B ) The choroid is now seen as a white area ( gray arrow ) between the dark thick sclera ( black arrow ) and a thin black line inside the globe, the retina ( gray arrowheads ). Intraocular ( white arrow ) and intraorbital ( white arrowhead ) optic nerve, CSF in the subarachnoid space ( black arrowhead ).


Intraorbital segment


The intraorbital segment runs through the intraconal space of the orbit, from the globe to the orbital apex and is at that level already surrounded by meninges, explaining the development of many intraorbital meningiomas. The subarachnoid CSF space, located between the pia mater and arachnoid/dura, is surrounding the intraorbital optic nerve and this CSF space is actually in continuity with the CSF of the suprasellar cistern. The nerve and surrounding CSF are best seen on heavily T2W or STIR images. This segment should be imaged with minimal water-fat chemical shift as the nerve-CSF-fat interfaces are otherwise artifacted and difficult to delineate ( Fig. 10 ). The central retinal artery, a branch of the ophthalmic artery, enters the optic nerve with its accompanying vein only in the distal part of the intraorbital segment over a nerve length of 1 cm in the area just behind the globe.




Fig. 10


Intraorbital optic nerve—para-coronal STIR image perpendicular on the course of the nerve. The CSF in the subarachnoid space ( black arrow ) around the intraorbital optic nerve segment is seen as a symmetric hypointense ring, showing that there are no deformations or chemical shift artifacts on this image. The optic nerve in the center of the CSF has a high signal intensity, isointense with white matter.


Intracanalicular segment


This segment of the optic nerve inside the optic canal is best seen on MR images ( Fig. 11 ); the bone fragments that can damage the nerve at this site caused by a trauma are of course better seen on CT. Inside this canal the nerve is positioned above the ophthalmic artery.




Fig. 11


Intracanalicular optic nerve—axial contrast-enhanced high resolution T1W image. The intracanalicular segment of the optic nerve can be seen in the optic canal ( white arrows ) which is far more difficult on CT because of the artifacts produced by the bony walls of the optic canal on CT. Intraorbital segment ( black arrows ) and intracranial segment ( gray arrows ) are shown.


Intracranial segment


The intracranial segment bridges the gap between the optic canal anterolaterally and optic chiasm posteromedially over a distance of about 10 mm. This nerve segment is covered only by pia mater.


Optic chiasm


At the optic chiasm, fibers from the temporal hemiretina remain uncrossed and continue in the ipsilateral optic tract. Fibers from the nasal hemiretina cross and continue in the contralateral optic tract. This explains the X-shaped morphology of the chiasm, located just anterior to the pituitary stalk, which is best appreciated on reformatted 3D-T1W images (eg, 3D-FFE, 3D-MPRAGE) or 3D-T2W images (eg, DRIVE, b-FFE) ( Fig. 12 ).




Fig. 12


Optic chiasm—axial reformatted high-resolution T2W balanced-FFE image. Para-axial reformation allows visualization of the intracranial segments of the optic nerves ( white arrows ), “X” shaped optic chiasm (∗), optic tracts ( gray arrows ) and the medial and lateral root of the optic tract on the left side ( black arrows ).


Optic tracts


The continuation of the chiasm in the left and right optic tract can also be visualized. These tracts divide in a smaller medial (containing only 10% of the fibers) and larger lateral root of the optic tract terminating in the medial geniculate body (see Fig. 12 ). These tracts are best distinguished on high-resolution T2W or FLAIR images.


The Oculomotor Nerve or Cranial Nerve III


The oculomotor nerve has a motor function (innervation of five of the extraocular muscles) and a parasympathetic function (innervation of the ciliaris and sphincter pupillae).


Oculomotor nuclear complex


The oculomotor nuclear complex is composed of five individual motor nuclei supplying the extraocular muscles and a more dorsal parasympathetic nucleus of Edinger-Westphal and is located at the level of the superior colliculus. The complex lies between the aqueduct and the red nucleus and is partially embedded in the periaqueductal gray. The structures are best visualized on multi-echo FFE (m-FFE) images ( Fig. 13 ) and the location of the nuclear complex can be deduced from it. The fascicular segment of the oculomotor nerve has an anterolateral course through the midbrain and crosses the medial longitudinal fasciculus (MLF), red nucleus, substantia nigra, and the medial part of the cerebral peduncle.




Fig. 13


The oculomotor nuclear complex—axial m-FFE image at the level of the superior colliculus. The oculomotor nuclear complex ( dot ) can be found at this level close to the midline, embedded in the periaqueductal gray anterolateral to the aqueduct and posterior to the red nucleus. The fascicular segment courses through the MLF, red nucleus, substantia nigra, and medial part of the cerebral peduncle ( line ).


Cisternal segment


The cisternal segment starts once the nerve leaves the brainstem and enters the interpeduncular cistern. This segment is best seen on high-resolution heavily T2W images ( Fig. 14 ); however, this nerve segment is also large enough to be depicted on T1W images. The nerve passes under the posterior cerebral and above the superior cerebellar artery, continues anteriorly below the posterior communicating artery, and pierces the dural roof of the cavernous sinus.




Fig. 14


Cisternal segment of the oculomotor nerve—high-resolution (0.4 mm 3 ) axial ( A ) and sagittal ( B ) and moderate resolution (0.6 mm 3 ) coronal ( C, D ) T2W FFE images through the cisternal segment of the oculomotor nerve. ( A ) The oculomotor nerve enters the prepontine cistern once it leaves the brainstem. The cisternal segment ( white arrows ) courses anterolaterally and passes through the space between the posterior cerebral artery (PCA) ( gray arrow ) above and superior cerebellar artery (SCeA) below ( black arrow ). ( B ) The third nerve can be followed from the interpeduncular cistern ( white arrow ) to the parasellar area ( gray arrow ) and passes between the PCA ( black arrowhead ) and SCeA ( black arrow ). ( C ) In the coronal plane the third nerves ( gray arrows ) can be seen inside the interpeduncular cistern and are in contact with the medial wall of the cerebral peduncles. Cisternal or preganglionic segment of the trigeminal nerves ( white arrows ) are shown. ( D ) More anteriorly in the prepontine cistern both third nerves ( gray arrows ) are seen in the space between the PCA ( black arrowhead ) and SCeA ( black arrow ).


Cavernous segment


This segment of the third nerve runs in the lateral wall of the cavernous sinus, superolateral to the cavernous internal carotid artery. This segment can best be seen in detail on coronal gadolinium-enhanced high-resolution T1W images through the cavernous sinus but nerve visualization on contrast-enhanced heavily T2W images has also be reported. The third nerve is the nerve with the highest position in the wall of the cavernous sinus. However, just before entering the superior orbital fissure (SOF), the trochlear nerve ascends along the lateral wall of the third nerve and eventually even lies superolateral to the third nerve as they enter the SOF ( Fig. 15 ).




Fig. 15


Cavernous segment of the third nerve—coronal contrast-enhanced T1W images through the posterior ( A ), middle ( B ), and anterior ( C ) part of the cavernous sinus. ( A ) The third nerve is the largest nerve coursing in the wall of the cavernous sinus ( black arrow ); the trochlear nerve ( gray arrow ) is the smallest nerve, located just below the third nerve. The abducens nerve ( white arrow ) is located deep in the cavernous sinus, not in the wall, and enters the cavernous sinus just above Meckel’s cave. ( B ) More anteriorly the ophthalmic nerve emerges from the Gasserian ganglion and is seen as an ovoid hypointensity ( white arrowhead ). The trochlear nerve ( gray arrow ) already ascended up to the superolateral border of the third nerve ( black arrow ). Abducens nerve ( white arrow ) is seen. ( C ) At the level of the entrance to the superior orbital fissure, the trochlear nerve ( gray arrow ) is seen above the oculomotor nerve ( black arrow ). The optic nerve ( black arrowhead ), ophthalmic nerve ( white arrowhead ), abducens nerve ( white arrow ), and maxillary nerve ( gray arrowhead ) are located just above the vidian canal and nerve.


Extracranial segment


The third cranial nerve divides in a superior and inferior branch within the SOF and enters the orbit through this SOF and the annulus of Zinn. The superior branch innervates the superior rectus muscles and levator palpebrae superioris muscles, while the inferior branch innervates the inferior rectus, medial rectus, and inferior oblique muscles. These branches can again be depicted on high-resolution coronal T1W images ( Fig. 16 ). Preganglionic parasympathetic fibers follow the cranial nerve III into the orbit and then exit the branch to the inferior oblique muscle to synapse in the ciliary ganglion. Postganglionic parasympathetic fibers continue as short ciliary nerves and enter the globe by piercing the lamina cribrosa of the sclera. They reach the ciliary body and iris, controlling the papillary sphincter and ciliary muscle.




Fig. 16


Extracranial segment of the third nerve—coronal high-resolution T1W images through the superior orbital fissure ( A ) and orbital apex ( B ). ( A ) The oculomotor nerve ( black arrow ) and optic nerve ( white arrow ) enter the superior orbital fissure and orbit. ( B ) Once in the orbital apex the oculomotor nerve divides in a superior ( black arrow ) and inferior ( gray arrow ) branch. Also visible is the optic nerve ( white arrow ).


The Trochlear Nerve or Cranial Nerve IV


The fourth cranial nerve provides motor innervation to only a single muscle, the superior oblique muscle.


Trochlear nucleus


The trochlear nucleus is situated at the level of the inferior colliculus, just ventral to the aqueduct, posterior to the MLF and just inferior to the oculomotor nerve complex. The localization of the nucleus very close to the MLF explains why lesions that involve the trochlear nucleus most often cause both a fourth nerve palsy and an internuclear ophthalmoplegia (INO). Axons leaving the nucleus course dorsally around the aqueduct and the left and right fascicular segments cross each other at the level of the superior medullary velum before exiting the midbrain at its posterior surface, just caudal to the inferior colliculus ( Fig. 17 ). These are two unique features of the fourth cranial nerve: (1) it is the only cranial nerve crossing the midline, hence a superior oblique muscle is always innervated by the contralateral trochlear nucleus and (2) it is the only cranial nerve exiting the brainstem at its dorsal surface.




Fig. 17


Trochlear nucleus—axial m-FFE image at level of inferior colliculus. The trochlear nucleus ( dot ) is located posterolateral to the MLF and anterolateral to the aqueduct. The fascicular segment ( line ) then courses posteriorly to exit the midbrain just below the inferior colliculus. The contralateral nerve is crossed at this point and the cisternal segment of the nerve ( line outside brainstem ) then follows the lateral border of the brainstem until it reaches the cavernous sinus.


Cisternal segment


Once outside the brainstem, the nerve is best visualized on heavily T2W images in the axial plane. The nerve is extremely small and is therefore sometimes difficult to find. The trick is to look for the superior medullary velum, a small flat band at the back of the superior part of the fourth ventricle. At that level, the nerves exit the brainstem in a nearly horizontal mediolateral direction until they reach the free edge of the tentorium ( Fig. 18 ). They then course anteriorly around the brainstem and pass through the gap between the superior cerebral artery and superior cerebellar artery, lateral to the oculomotor nerve. Because the nerves have to turn around the complete brainstem, they are the cranial nerves with the longest intracranial segment.




Fig. 18


Cisternal segment of the trochlear nerve—axial DRIVE image at level of medullary velum. The left ( gray arrows ) and right ( white arrow ) trochlear nerves exit the brainstem at the level where the medullary velum ( black arrow ) is seen. Note the displacement of the nerve on the left caused by a vessel.


Cavernous segment


The trochlear nerve then enters the lateral wall of the cavernous sinus below the oculomotor nerve ( Fig. 19 ). As already mentioned, the cavernous segment of the trochlear nerve initially is situated below the third nerve but then gradually ascends along the lateral border of the third nerve as the nerves approach the SOF and once in the SOF the trochlear nerve can be found at the superolateral border of the third nerve.




Fig. 19


Cavernous segment of the trochlear nerve—coronal contrast-enhanced T1W image. Both trochlear nerves ( white arrows ) are entering the wall of the cavernous sinus. At that level the oculomotor nerves are still seen in the prepontine cistern ( black arrows ).


Extracranial segment


The trochlear nerve then crosses over the superior border of the third nerve to run medially toward the superior oblique muscle. This nerve courses above the annulus of Zinn and does not pass through the annulus like the oculomotor and abducens nerve. The cavernous and extracranial segments are again best seen on coronal high resolution T1W images.


The Trigeminal Nerve or Cranial Nerve V


The trigeminal nerve is both motor and sensory to the muscles of mastication and of course also has a large sensory territory.


Trigeminal nuclei—intra-axial segment


Four nuclei, 1 motor and 3 sensory, are located in the brainstem ( Fig. 20 ). The “motor nucleus” is located in the lateral pontine tegmentum and supplies the muscles of mastication and the tensor veli palatini and tensor tympani muscle. The “pontine” or “principal” sensory nucleus (PSN), which can be found lateral to the motor nucleus and anterolateral to the fourth ventricle at the level of the root entry zone (REZ), processes discriminative tactile sensation from the skin of the face. The “mesencephalic nucleus” is a superior extension of the PSN in the midbrain up to the level of the inferior colliculus. It is the only nucleus in the CNS composed of unipolar neurons. It receives afferent fibers for facial proprioception (teeth, hard palate, temporomandibular joint [TMJ]). Their peripheral processes supply stretch receptors in the muscles of mastication and periodontal ligaments of the teeth. The spinal nucleus is a caudal extension of the PSN and descends from the level of the lower pons down to spinal cord level C3. The nucleus merges with dorsal gray matter of the cord. This nucleus mainly receives tactile, nociceptive, and thermal information from the entire V1, V2, and V3 trigeminal areas.


Mar 28, 2017 | Posted by in NEUROLOGICAL IMAGING | Comments Off on MRI of the Cranial Nerves—More than Meets the Eye: Technical Considerations and Advanced Anatomy
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