Anatomic Considerations, Nomenclature, and Advanced Cross-sectional Imaging Techniques for Visualization of the Cranial Nerve Segments by MR Imaging

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Key points

  • The cranial nerves (CNs) pursue a complex course through tissues with widely varying MR imaging signal characteristics as they extend from brainstem nuclei into the fluid-filled subarachnoid spaces and ultimately pass through the skull base to exit the cranium.

  • In turn, the reported success of the variety of available MR imaging sequences for visualization of the CNs depends largely on their anatomic context at the point of evaluation.

  • Consideration of the general segmental architecture of the

Technical considerations for MR imaging acquisition field strength

Fischbach and colleagues4 studied T2-weighted spin-echo imaging of the CNs at 1.5T and 3T, the two most commonly available field strengths of clinical MR imaging units, and found that images acquired at higher spatial resolution on the 3T scanner nonetheless also had higher clarity and signal-to-noise ratio. The detection of perineural spread of neoplastic disease in the face initially not detected on 1.5T evaluation was possible on repeat examination at 3T.5 Such results are not generally

Coil choice

Various approaches to coil choice and combination have been advocated7 although phased-array head coils are typically used in the clinical setting and are adequate for most applications.

Voxel size and coverage

Thin-section imaging significantly improves detection of the CNs8 although visualization of the cisternal trochlear (CN IV), abducens (CN VI), and accessory (CN XI) nerves may remain challenging. Due to its small caliber and proximity to multiple vascular structures, visualization of CN IV is particularly dependent on the spatial resolution of the sequences acquired. Choi and colleagues9 compared conventional resolution (0.67 mm × 0.45 mm × 1.4 mm) to high-resolution (0.3 mm × 0.3 mm × 0.25 mm)

2-D versus 3-D imaging

Initially, MR imaging tailored to the CNs required careful attention to 2-D slice angulation to best demonstrate the CN in question.10 Modern MR imaging equipment allows 3-D acquisition from which post hoc reconstruction in multiple planes can be created, often better demonstrating the CNs.8, 11, 12 One study of the cisternal components of the CNs in the cerebellopontine angle cistern with fast spin-echo technique found that 3-D imaging was superior to 2-D imaging due to suppression of flow

Injection of intravenous contrast agents

The central nervous system (CNS) components of the CNs (including the entirety of the ophthalmic (CN I) and optic (CN II) nerves, which are properly tracts of the CNS rather than nerves per se) are at least partly isolated from the contents of the bloodstream by the blood-brain barrier and do not normally demonstrate visible contrast enhancement. The components of the CNs in the peripheral nervous system (PNS) are likewise separated by the blood-nerve barrier. When there is disruption of the

Nomenclature

International consensus on anatomic nomenclature has been established in the Terminologia Anatomica.17 The accepted terms for the CNs are olfactory nerve (CN I), optic nerve (CN II), oculomotor nerve (CN III), trochlear nerve (CN IV), trigeminal nerve (CN V), abducens or abducent nerve (CN VI), facial nerve (CN VII), vestibulocochlear nerve (CN VIII), glossopharyngeal nerve (CN IX), vagus nerve (CN X), accessory nerve (CN XI), and hypoglossal nerve (CN XII). The system defines multiple named

Anatomic segments

After emerging from the brain, each of the CNs courses through the cerebral spinal fluid (CSF) before it traverses the meninges, extending through an associated skull base foramen to emerge into the head and neck. Along this path, the nerves are surrounded by CSF in the subarachnoid space, venous blood in the interdural compartment, bone within the skull base foramina, and various soft tissues after exiting from the skull. Although the anatomic course of each of the CNs is different, some

Summary

Imaging of the CNs presents a challenge due to their small size and course. This article proposes a segmental classification system for radiologic evaluation of the CNs, which the authors hope proves useful in clinical practice while providing a framework for future high-resolution CN imaging research. MR imaging is currently the gold standard technique and a variety of pulse sequences are available to demonstrate the CNs. The optimal imaging approach depends largely on which sequence is best

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