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High-resolution isotropic 3-dimensional (D) MR imaging with and without contrast is now routinely used for imaging evaluation of cranial nerve anatomy and pathologic abnormality.
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Previous work has highlighted the utility of sequences, including constructive interference in steady-state without and with intravenous contrast, in such cases.
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The extraforaminal segments are well-visualized on these techniques, especially in the setting of contrast against varying tissue types.
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The extraforaminal
High-Resolution Isotropic Three-Dimensional MR Imaging of the Extraforaminal Segments of the Cranial Nerves
Section snippets
Key points
Imaging approaches
At the authors’ institution, high-resolution isotropic 3D MR imaging protocol consists of the following sequences: precontrast and postcontrast 3D constructive interference in steady-state (CISS), 3D T2-weighted short-tau inversion recovery (STIR) SPACE (Sampling Perfection with Application optimized Contrasts using different flip angle Evolution), precontrast volumetric interpolated breath-hold examination (VIBE), and postcontrast VIBE with fat saturation (Table 1).4 In addition, standard
Technique (constructive interference in steady-state)
CISS sequence is a high-spatial resolution, refocused gradient echo sequence that can be obtained with isotropic resolution in a clinically acceptable acquisition time. Images can be reconstructed in any plane. CISS images have the appearance of T2-weighted images, although increased signal is easily recognized after administration of gadolinium-based contrast material in tissues that show contrast enhancement on T1-weighted images. Precontrast and postcontrast CISS sequences can be performed
Discussion
CN dysfunction can occur as a result of pathologic abnormality at any point along the nerve fibers, including the extraforaminal segments. The wide range of pathologic entities affecting CN#.a to CN #.f has been discussed elsewhere.1, 2, 3 This article highlights pathologic abnormality localized to the extraforaminal segment, with characteristic examples featured along with the discussion of normal CN anatomy.
Several entities may affect CNs in a similar manner, leading to the corresponding CN
Cranial nerve II: optic nerve, anatomy
The optic nerve is unique in that it is an extension of the central nerve system and not a CN by strict definition. It is surrounded entirely by meninges rather than Schwann cells, therefore all segments are considered intradural (segmentation of CN II has been discussed elsewhere), and the extraforaminal portion is located within the orbit after passage through the optic canal. CSF fluid and the optic sheath surround the optic nerve, separating it from the adjacent orbital fat (Fig. 3).
Cranial nerve III.g: oculomotor nerve, anatomy
The oculomotor nerve enters the orbital apex via the superior orbital fissure. It then branches into superior and inferior divisions. The superior division supplies the superior rectus muscle and levator palpebrae, whereas the inferior division innervates the medial and inferior rectus muscles (Fig. 5), as well as supplying parasympathetic innervation to the ciliary ganglia responsible for the pupillary reflex.2, 12
Cranial nerve IV.g: trochlear nerve, anatomy
CN IV.g exits the skull in the superolateral portion of the superior orbital fissure, external to the annulus of Zinn, along with the frontal and lacrimal branches of the ophthalmic division of CN V.1.g,12 eventually innervating the superior oblique muscle.
Cranial nerve VI.g: abducens nerve, anatomy
CN VI.f travels through the central portion of the superior orbital fissure along with the CN III.f to enter into the orbit and innervates the lateral rectus muscle.12
Cranial nerves III.g, IV.g, and VI.g: selected pathologic abnormalities
A wide range of causes of ocular CN palsies exist, including diabetic neuropathy, trauma, demyelinating disorders, congenital abnormality or absence,13 and idiopathic disease.14, 15, 16 Pathologic abnormalities that involve the ocular motor CNs can present with classic symptoms localizing to a specific CN palsy. However, given the close proximity of these nerves within the orbital apex, pathologic abnormalities involving 1 nerve may also masquerade as palsy of another.2 Symptoms such as
Cranial nerve V: trigeminal nerve
The trigeminal nerve is the largest of the CNs, with 3 main divisions supplying sensation to different regions of the face, as well as motor innervation to the muscles of mastication via the third division (CN V.3). The extraforaminal segment of each division and their associated pathologic abnormalities are discussed individually.
Cranial nerve VII.g: facial nerve, anatomy
The facial nerve is composed of a complex combination of motor, sensory, and parasympathetic fibers. The main extraforaminal trunk exits the temporal bone at the stylomastoid foramen along the posterior belly of the digastric muscle, and then immediately into the parotid gland.53 Just proximal to the parotid gland, the facial nerve gives off the posterior auricular nerve and branches to the posterior belly of the digastric and stylohyoid muscles.3 Within the parotid gland, the facial nerve
Cranial nerve VIII
CN VIII alone does not extend beyond the skull and, for this reason, does not formally have an extraforaminal segment, rather it passes through foramina to innervate the inner ear within the temporal bone.
Cranial nerves IX to XI
Due to the close anatomic location of the extraforaminal segments of CNs IX to XI, they are discussed here together, with particular emphasis on the vagus nerve.
Cranial nerve XII:g: selected pathologic abnormality
CN XII.g can be affected anywhere along its course to produce classic symptoms of CN XII palsy, tongue weakness on protrusion, with particular pathologic abnormalities having a predilection for particular segments of the nerve. Given the location of CN XII.g, it has an extensive course in the head and neck, and it can be involved in malignant processes originating from any of these spaces, including SCC; salivary gland tumors; such as adenoid cystic carcinoma63, 65; and lymphoma, which may
Summary
This article describes the normal high-resolution anatomy and appearance of several featured CNs, with specific emphasis on the extraforaminal portions. The superior visibility of many distal extraforaminal branches of these CNs on CISS sequences allows for assessment of involvement of these distal branches by pathologic assessment. The high signal-to-noise ratio, along with high spatial resolution with isotropic 3D images that can be reconstructed in multiple planes, makes this sequence
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2021, Oral Surgery, Oral Medicine, Oral Pathology and Oral RadiologyCitation Excerpt :The neural abnormality produced a fairly bright signal or nerve thickening on 3D-CUBE fs images (Figure 3) compared with 2-dimensional images. The reconstructions of the long axis of the nerve showed signal or caliber alterations on the affected branches of the nerve, which is helpful for clinicians in preoperative treatment planning.20 Compared with the studies of Terumitsu et al.,14,15 we not only used 3.0-T MRI for imaging but also adopted more scanning sequences and applied more comprehensive image reconstruction methods.
Advanced Magnetic Resonance Imaging of the Skull Base
2021, Seminars in Ultrasound, CT and MRICitation Excerpt :MR cisternography sequences provide exquisite delineation of the anterior, central, and posterior skull base; cerebrospinal fluid and meninges; and complex neurovascular foramina for transit of cranial nerves, arteries, and veins. The various segments of the cranial nerves include that can be distinguished include nuclear, parenchymal fascicular, cisternal, dural cave, interdural, foraminal, and extraforaminal.10-15 These high-resolution sequences can also be used to delineate the anatomy of arachnoid cysts and adhesions, aqueductal stenosis and webs, and meningoceles.16-21
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Disclosure: Dr A. Blitz has been on the medical advisory board for Guerbet. He is also partially funded by R21 NS096497 and FAIN U01DC013778 grants.