Ari M. Blitz, MDa∗, Asim F. Choudhri, MDb, Zachary D. Chonka, MDa, Ahmet T. Ilica, MDa, Leonardo L. Macedo, MDc, Avneesh Chhabra, MDd, Gary L. Gallia, MD, PhDe and Nafi Aygun, MDa,     aDivision of Neuroradiology, The Russell H. Morgan Department of Radiology and Radiologic Science, The Johns Hopkins Hospital, Phipps B-100, 600 North Wolfe Street, Baltimore, MD 21287, USA; bDepartment of Radiology, University of Tennessee Health Science Center, Le Bonheur Neuroscience Institute, Le Bonheur Children's Hospital, 848 Adams Avenue-G216, Memphis, TN 38103, USA; cCedimagem/Alliar, Diagnostic Center, 150 Centro, Juiz de Fora, Minas Gerais 36010-600, Brazil; dThe University of Texas Southwestern, 5323 Harry Hines Blvd, Dallas, TX 75390-9178, USA; eDepartment of Neurosurgery, Neurosurgery Skull Base Surgery Center, The Johns Hopkins Hospital, Phipps 101, 600 North Wolfe Street, Baltimore, MD 21287, USA. E-mail address: Ablitz1@jhmi.edu

∗Corresponding author.

Various methods of cross-sectional imaging are used for visualization of the cranial nerves, relying heavily on MR imaging. The success of the MR imaging sequences for visualization of cranial nerves depends on their anatomic context at the point of evaluation. The heterogeneity of opinion regarding optimal evaluation of the cranial nerves is partly a function of the complexity of cranial nerve anatomy. A variety of approaches are advocated and variations in equipment and terminology cloud the field. This article proposes a segmental classification and corresponding nomenclature for imaging evaluation of the cranial nerves and reviews technical considerations and applicable literature.

Keywords

Cranial nerve segments

Cross-sectional imaging

MR imaging

Key points

Introduction

The 12 pairs of cranial nerves (CNs) arise directly from the brain within the cranial vault (with the exception of spinal rootlets of CN XI, which arises from the rostral cervical spine). The CNs serve a variety of highly specialized functions, including those necessary for vision, movement of the eyes and face, and identification and consumption of food. The branching patterns and/or proximity of CNs to each other at points along their course may allow localization of pathology on clinical grounds.1 MR imaging plays an important role in the localization and identification of pathology as well as presurgical planning. A variety of modalities have been used in the imaging evaluation of CNs. Clinically, the first cross-sectional imaging study to directly demonstrate the CNs was pneumoencephalography. During pneumoencephalography, the introduction of subarachnoid air surrounding the CNs allowed for visualization of the optic, oculomotor, trigeminal, and hypoglossal nerves within the basal cisterns.2 The advent of CT enabled visualization of the region of the CNs with a greater degree of detail and with injection of intrathecal contrast; the CNs were visualized as linear filling defects within the subarachnoid space.3 In both pneumoencephalography and CT cisternography, visualization was principally limited to the cisternal/subarachnoid course of the CNs and pathology was implied by alterations in the adjacent osseous structures. With the advent of MR imaging, cross-sectional examination of the structures of the head and neck without ionizing radiation became possible, with the ability to acquire images in any arbitrary plane allowing for the examination to be tailored to the CN in question. MR imaging is now the standard mode of imaging of the CNs and is the focus of this article.

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 surprising because the tissue discrimination generally improved with higher field strengths.6 Although 3T evaluation is generally preferred over 1.5T evaluation, diagnostic images may be obtained at either field strength, in particular when 3T MR imaging is not available, of questionable safety, or otherwise deemed inappropriate.

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) imaging for detection of the cisternal trochlear nerve and found that the rate at which the nerve could probably or definitely be identified rose significantly from approximately 23% to 100%. Fig. 1 demonstrates visualization of the trochlear nerve on 0.4-mm, 0.5-mm, and 0.6-mm isotropic constructive interference in the steady-state (CISS) images. Although increasing spatial resolution may improve visualization of small structures, the trade-off with respect to length of acquisition, reduced coverage, and/or decreased signal-to-noise ratio renders optimal coverage in all cases difficult. In the authors' practice, multiple 3-D sequences are typically used and include CISS imaging for the highest spatial resolution acquisition. The typical CISS acquisition includes 0.6-mm isotropic voxels with coverage of the entirety of the posterior fossa, skull base, and upper face. In select cases where CN IV palsy has been clinically diagnosed, a higher spatial resolution is often used.

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 artifacts and thinner sections and suggested that MR imaging evaluation of the cisterns be performed with 3-D technique.13 When isotropic 3-D images are acquired, post hoc reconstructions can be made in any arbitrary plane, which is often useful in evaluating the complex anatomy of the CNs and surrounding structures.

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 blood-nerve barrier or blood-brain barrier, it is detected by the presence of an increase in intensity on postcontrast MR images with T1 weighting. Perhaps owing to lack of a similar barrier mechanism or increased blood flow, enhancement of the ganglia of the CNs may be detected as a physiologic finding.14...