High-Resolution CISS MR Imaging With and Without Contrast for Evaluation of the Upper Cranial Nerves Segmental Anatomy and Selected Pathologic Conditions of the Cisternal Through Extraforaminal Segments Ari M. Blitz, MDa,*, Leonardo L. Macedo, MDb, Zachary D. Chonka, MDa, Ahmet T. Ilica, MDa, Asim F. Choudhri, MDc, Gary L. Gallia, MD, PhDd, Nafi Aygun, MDa KEYWORDS High-resolution MR imaging Cranial nerves Segmental classification CISS Upper cranial nerves KEY POINTS High-resolution isotropic three-dimensional (3D) magnetic resonance imaging acquisition relying heavily on constructive interference in the steady state (CISS) with and without contrast has largely replaced 2D techniques previously used for the evaluation of the cranial nerves (CNs) at the authors’ institution and allows for high-contrast evaluation of the CNs along a greater portion of their extent than was previously possible. The cisternal and dural cave segments of the upper CNs, with the possible exception of the fourth cranial nerve, are well evaluated with noncontrast CISS. The interdural and foraminal segments of the CNs are revealed after the administration of intrave- nous contrast. The proximal extraforaminal segments of the CNs are often well visualized on 3D high-resolution imaging with CISS. Evaluation for pathologic contrast enhancement in segments of the upper CNs which were previously not well seen, and on a scale previously not well depicted, is now possible with high- resolution 3D technique. a Division 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; b Cedimagem/Alliar Diagnostic Center, 150- Centro, Juiz de Fora, Minas Gerais 36010-600, Brazil; c Department of Radiology, Uni- versity of Tennessee Health Science Center, Le Bonheur Neuroscience Institute, Le Bonheur Children’s Hospital, 848 Adams Avenue-G216, Memphis, TN 38103, USA; d Department of Neurosurgery, Neurosurgery Skull Base Surgery Center, The Johns Hopkins Hospital, Phipps 101, 600 North Wolfe Street, Baltimore, MD 21287, USA * Corresponding author. E-mail address: [email protected] Neuroimag Clin N Am 24 (2014) 17–34 http://dx.doi.org/10.1016/j.nic.2013.03.021 1052-5149/14/$ – see front matter Ó 2014 Elsevier Inc. All rights reserved. neuroimaging.theclinics.com 18 Blitz et al INTRODUCTION Siemens Verio (Ehrlangen, Germany) with 0.6- mm isotropic resolution. Although long used for the evaluation of the course Although for the purposes of this discussion the of the cranial nerves (CNs) within the subarachnoid 1 authors use the nomenclature established in the space, the recognition of the utility of enhance- article by Blitz, Choudhri, Chonka and colleagues ment on postcontrast constructive interference in within this issue for CN I to VI, CN I and CN II are the steady state (CISS) imaging allows for the eval- properly considered tracts of the central nervous uation of the remainder of the extended course of 2 system (CNS) rather than true cranial nerves pass- the CNs once they exit the subarachnoid space. ing into the peripheral nervous system (PNS) and In this article, the authors review the recent litera- do not entirely conform to the same anatomic ture on the evaluation of the CNs with CISS and outline as CN III to VI. Although classically anato- analogous sequences and describe the use of mists describe nerves from their point of origination such techniques for the evaluation through the ex- to their destination, the mixed afferent and efferent traforaminal segments. In many instances, such components of several cranial nerves makes this techniques enable visualization of nearly the entire challenging and potentially confusing. For the course of many of the upper CNs, and the relevant sake of simplicity, descriptions are given here anatomy is described here following the following the nerve from their point of apparent segmental classification suggested in the article origin at the brainstem peripherally into the extrafor- by Blitz, Choudhri, Chonka and colleagues within aminal components. this issue (Box 1). CN dysfunction may arise from pathologic con- CISS imaging combines true and reversed fast ditions interrupting fiber bundles at any point imaging in steady state precession (FISP) imaging from the CN nuclei to the end organ innervated to allow for high-signal, high-spatial resolution by the nerve or often even from lesions within path- imaging with suppression of banding artifacts ways that themselves innervate the CN nuclei. Le- caused by unbalanced imaging gradients and field sions within the brain, cranial nerve nuclei, or fiber inhomogeneities. CISS is a gradient echo tech- tracts, as they course to exit the brain and give rise nique, and images acquired have the appearance to the CNs, are often associated with other signs of of strong T2 weighting but, in fact, demonstrate damage to the CNS4; such lesions are discover- some degree of mixed weighting, which allows 3 able on imaging of the brain. Imaging of the cranial for the visualization of enhancement. nerve nuclei and course of fibers within the sub- The application of precontrast and postcontrast stance of the brainstem itself is beyond the scope CISS imaging has several significant advantages of this discussion, which focuses on imaging the over other techniques used for cranial nerve eval- CNs from their apparent origin at the surface of uation. CISS imaging allows for the acquisition of the brainstem through their exit from the skull high spatial resolution isotropic 3-dimensional base foramina. The interested reader will find (3D) imaging in a clinically acceptable time period, excellent information on the location and course allowing for the evaluation of the entirety of the of the nuclear and fascicular segments, for skull base and posterior fossa at 0.6-mm isotropic instance, in the recent text by Naidich and col- resolution in less than 6 minutes. The addition of leagues.5 Additionally, it is important to note that contrast allows for the depiction of the CNs in the entirety of the extraforaminal course of the up- most segments with a single sequence, and it is per CNs is complex and limited space precludes simultaneously possible to assess for pathologic treatment of the entirety of the topic beyond contrast enhancement. Unless otherwise noted, some introductory remarks within this article. the images in this article were acquired on a 3T CN I: OLFACTORY NERVE Box 1 Anatomic segments of the cranial nerves The olfactory nerve provides the sense of smell. The portions of the olfactory nerve visible on mag- a. Nuclear netic resonance (MR) imaging, namely, the olfac- b. Parenchymal fascicular tory bulb (OB) and olfactory tract (OT), are not c. Cisternal components of a cranial nerve in the formal sense d. Dural cave but rather a forward extension of the telenceph- alon,6 more properly a tract of the CNS. As such, e. Interdural CN I does not follow the typical segmental pattern f. Foraminal of CNs III to XII. For the purposes of this discus- g. Extraforaminal sion, the authors define cisternal, foraminal, and extraforaminal segments. The olfactory tracts Evaluation of the Upper Cranial Nerves 19 carry information on smell from the cisternal syndrome.12,13 Idiopathic olfactory loss is com- segment into the olfactory stria to the primary mon, and patients with idiopathic olfactory loss olfactory cortex located in the inferomedial aspect demonstrate decreased OB volumes compared of the temporal lobe. with normal controls.14 Idiopathic anosmia can be a harbinger of Parkinson disease or Alzheimer CN I.c (Cisternal): Anatomy disease.15–17 Intra-axial and extra-axial tumors, CN I.c consists of the OB and the OT (Fig. 1). The most frequently meningioma, arising from the OB sits above the cribriform plate and receives anterior skull base can result in anosmia. The input from primary sensory neurons. The CNS- vulnerability of the CN I.c to head trauma and post- PNS transitional zone (TZ), located in the OB, is traumatic olfactory dysfunction is well docu- 18 histologically distinct from that of the other CNs mented and also correlates with OB/OT size. with a cell type, the ensheathing cell, not found in other CNs.7 The OB is uniformly well visualized CN I.f (Foraminal): Anatomy 8 with CISS. The neurons of the OB project poste- Up to 100 unmyelinated axons of the olfactory riorly into the OT, which in turn extends posteriorly neuroepithelium form bundles (olfactory fila, the toward the brain. The OT is distinguished from the olfactory nerves proper) (see Fig. 1B white arrows) OB by a change in caliber, although the lack of a that extend intracranially through the openings of constant landmark has led to variability in identifi- the cribriform plate before penetrating the dura. cation between subjects and readers, hampering The numerous fila and the foramina in the cribri- clinical evaluation and research. Some investiga- form plate are not reliably visualized with the cur- tors advocate spin echo imaging rather than rent imaging resolution. CISS for the evaluation of CN I.c to avoid artifacts in this region caused by air tissue inferface.9 In the CN I.f: Selected Pathologic Conditions authors’ experience, diagnostic images of the anterior cranial fossa and paranasal sinuses can Traumatic injury to the cribriform plate resulting in be obtained with CISS, with the significant advan- loss of olfaction or cerebrospinal fluid (CSF) leak tage of also allowing for the assessment of (CSF rhinorrhea) may occur in this region. contrast enhancement at a high spatial resolution. CN I.g (Extraforaminal): Anatomy The quantitative size evaluation of OB and OT has demonstrated a correlation between OB/OT vol- The primary olfactory neurons are located in the ume and olfactory function and an age-related neuroepithelium lining the superior nasal cavity. decrease in OT volume.10,11 CN I.g: Selected Pathologic Conditions CN I.c: Selected Pathologic Conditions The most common cause of anosmia involving the The congenital absence of CN I.c may be detected CN I.f is rhinosinusitis.