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Surface-Coil Magnetic Resonance Imaging of the Internal Auditory Canal

David L. Daniels' Computed tomography is effective for detecting acoustic neuromas, but not for John F. Schenck2 resolving individual nerves in the internal auditory canal. Surface-coil magnetic reso­ Thomas Foster2 nance (MR) images of the internal auditory canal were obtained using a 1.5 T supercon­ Howard Hart, Jr.2 ducting magnet, a 13.5-cm-diameter surface coil, 3- and 5-mm-thick slices, and partial­ Steven J. Millen3 saturation pulse sequences. Cranial nerves VII and VIII (three branches) were identified on MR images in volunteers and on corresponding cryomicrotomic sections. The nerves Glenn A. Meyer" were obscured in one patient with an acoustic neuroma. Because high-resolution Peter Pech' surface-coil images can demonstrate specific nerves in the internal auditory canal, MR Katherine A. Shaffer' should be a sensitive study to evaluate cranial nerves VII and VIII in patients with facial Victor M. Haughton' paralysis and neurosensory hearing loss that is congenital or caused by small acoustic neuromas.

The effectiveness of magnetic resonance (MR) images in demonstrating the normal and abnormal contents of the internal auditory canal and cranial nerve VII in the petrous bone has been described, but the resolution of specific nerves in the internal auditory canal has not [1-4]. Surface-coil images have increased the signal­ to-noise ratio and the potential to identify individual nerves in the internal auditory canal in contrast to cortical bone and cerebrospinal fluid (CSF), which have negligible signals. Our purpose is to describe the MR appearance of these nerves.

Materials and Methods

Three fresh frozen cadaver heads were sectioned using a cryomicrotome and a previously This article appears in the July/August 1985 described technique [5]. The surfaces of the specimens were serially photographed. In these issue of AJNR and in the September 1985 issue anatomic sections, the branches of cranial nerves VII and VIII and structures in the petrous ofAJR. bone, including the geniculate ganglion, cochlea, vestibule, and greater superficial petrosal Received November 28, 1984; accepted after nerve, were identified by reviewing anatomic, computed tomographic (CT), and MR literature revision January 1 6. 1 985. [1-4, 6-8]. This work was supported by a grant from Gen­ Surface-coil images of five normal volunteers and one patient with a surgically proven eral Electric Medical Systems. intracanalicular acoustic neuroma were obtained. The subjects were studied in General 1 Department of Radiology, Medical College of Electric research MR scanners with superconducting 1.5 T magnets. A 54-cm-diam cylindrical Wisconsin, Froedtert Memorial Lutheran Hospital, coil routinely used for body imaging was used for radiofrequency transmission. A surface 9200 W. Wisconsin Ave. , Milwaukee, WI 53226. Address reprint requests to D. L. Daniels. receiver coil having a diameter of 13.5 cm and tuned to the' H resonance frequency of 63.9 MHz was fixed near the petrous bones of the volunteers. Images were obtained in the axial 2 General Electric Corporate Research and De­ velopment Center, Schenectady, NY 12301 . (parallel to the canthomeatal line) or parasagittal plane. Technical factors included partial­ saturation (PS) pulse sequences, a 350-400 msec repetition time (TR) , 256 x 256 matrix, a 3 Department of Otolaryngology, Medical Col­ lege of Wisconsin , Milwaukee, WI 53226. 19 msec echo time (TE), 1-4 averages, 12.8 cm field of view corresponding to a pixel size of 0.5 x 0.5 mm, and 3- and 5-mm-thick sections. 4 Department of Neurosurgery, Medical College of Wisconsin , Milwaukee. WI 53226. Surface-coil images of the subjects and anatomic sections were correlated to identify specific cranial nerves in the internal auditory canals . The MR and CT images of the patient AJNR 6:487-490, July/August 1985 0195-6108/85/0604-0487 with an intracanalicular acoustic neuroma were correlated . The CT fT 8800 study was © American Roentgen Ray Society performed at Columbia Hospital in Milwaukee. 488 DANIELS ET AL. AJNR :6, July/August 1985

Fig . 1.-A, Axial anatomic section through inferior part of internal auditory canal (arrow). B, Higher magnification. C, Corresponding T1-weighted PS scan, 350 msec TR , 256 x 256 matrix, 4 averages, 3 mm thick. Cisternal and intracanalicular segments of inferior vestibular (IV) and cochlear (C) nerves, vestibule (V), cochlea (Co), horizontal segment (F ') of facial nerve, and greater superficial petrosal nerve (GSP) contrast to lOW-intensity signal from CSF and petrous bone. Cochlear nerve is anterior to inferior vestibular nerve. A = internal carotid artery; P = pons, 4 = fourth ventricle. (Reprinted from [9].)

c

Results and Discussion vidual nerves in the internal auditory canal and have a signal intensity intermediate bet wen fat (high-intensity signal) and The anatomic appearance of the internal auditory canal has cortical bone and CSF (negligible signal) (figs. 1 and 2). In a been described in detail [6, 7J . The internal auditory canal section through the inferior part of the internal auditory canal , contains the facial (VII) and vestibulocochlear (VIII) nerves, the inferior vestibular and cochlear nerves extend to the the latter having three branches. In axial cryomicrotomic anterior margin of the vestibule and the posterior margin of sections parallel to the canthomeatal line and through the the cochlea, respectively. In a section through the superior inferior part of the internal auditory canal , the inferior vestib­ part of the internal auditory canal, the superior vestibular and ular nerve and, anterior to it, the cochlear nerve have almost facial nerves extend to the anterior margin of the vestibule straight lateral courses extending from the brainstem to the and the sharply turning geniculate ganglion, respectively. vestibulocochlear apparatus (fig . 1). In an axial section Sagittal PS surface-coil images commonly demonstrate through the superior part of the canal , the superior vestibular structures that have the configuration of specific cranial nerve and , more anterior to it, the facial nerve have similar nerves in the internal auditory canal (fig. 3). Each nerve has courses (fig. 2) . The facial nerve extends to the geniculate two signal intensities, possibly representing the nerve (central ganglion and turns posteriorly by the medial wall of the area of low signal intensity) and its sheath . epitympanum. In sagittal cryomicrotomic sections through the PS surface-coil images of the intracanalicular acoustic neu­ internal auditory canal, these nerves are seen in cross section roma demonstrate a small mass that obscures the nerves in anterosuperiorly (facial nerve), posterosuperiorly (superior the right internal auditory canal and has uniform signal inten­ vestibular nerve), anteroinferiorly (cochlear nerve), and pos­ sity greater than that of petrous bone (fig. 4). The CT study teroinferiorly (inferior vestibular nerve) (fig. 3). in the same case shows possible abnormal enhancing tissue In axial PS surface-coil images of normal volunteers, struc­ in the right internal auditory canal without widening of the tures are demonstrated consistently that correspond to indi- canal. AJNR:6, July/August 1985 SURFACE-COIL IMAGING OF INTERNAL AUDITORY CANAL 489

Fig . 2.- A, Axial anatomic section through superior part of internal auditory canal (arrow). B, Higher magnification. C, Corresponding PS scan, 350 msec TR , 256 x 256 matrix, 2 averages, 5 mm thick. Characteristic appearance of facial nerve (F) and its geniculate ganglion (G) distinguishes it from superior vestibular nerve (SV) coursing more posteriorly. A = internal carotid artery; P = pons; MA = mastoid antrum; I = incus; M = malleus; Co = cochlea; V = vestibule; 4 = fourth ventricle. (A and B reprinted from [9].)

Fig. 3.-A, Parasagittal anatomic cross section of cranial nerves in internal auditory canal. B, PS image, 400 msec TR , 256 x 256 matrix, 1 averag e, 3 mm thick. Each nerve has two signal intensities, possibly representing nerve (central area of low signal intensity) and sheath. A = internal carotid artery; C = cochlear nerve; F = facial nerve; SV = superior vestibular nerve; IV = inferior vestibular nerve; B = jugular bulb. 490 DANIELS ET AL. AJNR :6, July/August 1985

Fig . 4.- Right intracanalicular acoustic neuroma. A, Axial PS scan, 300 msec TR , 256 x 256 matrix, 4 averages, 3 mm thick. Tumor (arrow) obscures nerves in internal auditory canal. B, Contrast-en­ hanced CT scan. Possible abnormal en­ hancing tissue (arrow) in right canal but not in left. C, Contrast-enhanced CT scan at bone window setting. Internal auditory canals are normal. (A reprinted from [9].)

A c

High-resolution surface-coil images consistently demon­ apparatus and facial nerve. Presented at the annual meeting of strates individual nerves in the internal auditory canal. Early the American Society of Neuroradiology, Boston, June 1984 experience indicates that one sign of intracanalicular tumor is 3. Bydder GM , Steiner RE , Young IR , et al. Clinical NMR imaging obscuring of these nerves. Further work is needed to deter­ of the brain: 140 cases. AJNR 1982;3:459-480, AJR mine the sensitivity of surface-coil imaging for detecting small 1982;139:215-236 acoustic neuromas and congenital anomalies causing neuro­ 4. Young IR , Bydder GM , Hall AS , et al. The role of NMR imaging in the diagnosis and management of acoustic neuroma. AJNR sensory hearing loss. 1983;4: 223-224 5. Rauschning W, Bergstrom K, Pech P. Correlative craniospinal ACKNOWLEDGMENTS anatomy studies by computed tomography and cryomicrotomy. J Comput Assist Tomogr 1983;7:9-13 We thank William A. Edelstein, Cecil Hayes, Ann Shimakawa, 6. Matsushima T, Rhoton AL, Lenkey JC. Microsurgery of the fourth Thomas Bern s, and Laurie Dunk for help in this study. ventricle: part I. Neurosurgery 1982;11 :631-667 7. Valvassori GF, Potter GD , Hanafee WN, Carter BL, Buckingham RA. Radiology of the ear, nose and throat. Philadelphia: Saun­ REFEREN CES ders, 1982 1. Daniels DL, Herfkin s R, Koehler PR , et al. Magnetic resonance 8. Taylor S. The petrous temporal bone (including the cerebellopon­ im ag ing of the internal auditory canal. Radiology 1984;151: 1 05- tine angle). Radial Clin North Am 1982;20:67-86 108 9. Daniels DL, Pech P, Haughton VM . Magnetic resonance imaging 2. Reese DF , Harner SG, Kipert DB , Baker HL Jr. Magnetic reso­ of the temporal bone. Milwaukee: General Electric Medical Sys­ nance display of the intern al auditory canal , oochlea, vestibular tems Group, 1984