Ear study with 64 slices Multidetector CT

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1 Cabeza y cuello Puesta al día Ear study with 64 slices Multidetector CT María Lourdes Mallo, Cecilia C. Giordanengo, Carlos A. Bertona, Juan José Bertona, Cecilia Gigena, María Paula Florez Abstract The ear is a complex anatomic structure. Its study and understanding represent a constant challenge for the radiologist. As a consequence, the computed tomography becomes an essential tool for its examination. The conventional tomographic examination with both axial and coronal reconstruction of the image allows a satisfactory visualization of different structures. However, the study requires long periods of time for its acquisition and uncomfortable or intolerable positions for some patients. Besides, the characterization of some ear s structures becomes limited. The 64 slices multidetector computed tomography enables an optimal study of the ear s small and complex anatomy, performing volumetric acquisitions with 0.5 mm slices, which leads to accurate multiplanar reconstructions in the three basic planes, in curved planes and three dimensional reconstructions. These make a detailed visualization of its structures in few seconds, avoiding discomfort in the patients. Our purpose in this article is to demonstrate the utility of reconstructions particularly in oblique planes, reaching an optimal visualization of the temporal bone. Key words: Inner ear. Multidetector computed tomography. Temporal bone. INTRODUCTION From the advent of Computed Tomography (CT), the study of temporal bone structures is limited to axial and coronal (1) orientations, with slices of the lowest thickness (usually of slices between 1 and 3 mm) (2,3). In that way, the identification of smallest and most complex anatomic ear structures is limited. Furthermore, it is necessary to consider time exploration in order to avoid movements and uncomfortable position in coronal plane orientations. The incorporation of 64 slices Multidetector Computed Tomography (64MDCT), by reconstructing the image in any space plane, with acquisition in few seconds and with a tolerable position for all patients, enables a detailed observation of the ear s anatomic repairs and progress in addressing different diseases (3). 64 MDCT PROTOCOL Data Acquisition: The temporal bone CT SCAN was performed in our institution using a 64 channels multidetector scanner Aquilion Toshiba. The exploration was conducted in the standard axial plane with helicoidal technique and volumetric acquisition (120 kv, 200 ma, rotation time of 0,5 seconds, thickness of the section of 0,5mm, matrix 512 l x 512). The head of the patient was located in a neutral position and the time of study was of 4 to 6 seconds. Image reconstruction and post-processing: The data analysis was performed in a Vítrea work station. The temporal bones were visualized in three basic planes (axial, sagital and coronal), reconstructed in isometric way (with the same resolution as the original acquisition) every 0,5, 1 or 2 mm. In addition, there were performed 3D reconstructions, multiplanar and oblique curves, and Maximum Intensity Projections (MIP), according to the structure of clinical interest or as the findings: ossicular chain, round and oval windows,the cochlea, vestibular aqueduct, semicircular canals and facial nerve canal. EXTERNAL EAR It comprises the pinna and external auditory canal (EAC). The EAC has an italic S shape on its way to the tympanic membrane. Its two internal thirds are osseous and slightly narrower than the lateral third, which is cartilaginous. The tympanic membrane shows the intermediate limit of EAC, separating the external ear from the tympanic cavity (2, 5, 6, 7). The exploration of these structures with 64 MDCT, with volumetric and 3D reconstructions allowed a very detailed view of them (Fig. 1), resulting a valuable help in the detection of pathological features (Fig. 2). Servicio de Diagnótico por Imágenes - Clínica Privada, Velez Sarfield, Córdoba - Argentina Correspondencia: María Lourdes Mallo marialma02@hotmail.com Recibido: agosto 2010; aceptado: septiembre 2010 Received: august 2010; accepted: september 2010 SAR-FAARDIT 2010 RAR - Volumen 74 - Número Página 1

2 Ear study with 64 slices Multidetector CT MIDDLE EAR Ossicular chain Hammer and incus The handle of the hammer and the incus are not clearly defined by conventional scan of the temporal bone, which consists on axial and coronal acquisitions (1). Oblique coronal planes (Figs. 3-4), oriented along long axis of these two ossicular structures provide a better identification of them. The oblique sagital plane (Fig. 5) also enables its visualization and provides an alternative to the axial plane for the examination of incude-maleolar joint (8). The reconstruction of this plane of hammer and incus, resemble a molar tooth (9-12) (Fig. 6). These reconstruction planes can be helpful for studying the erosions of the ossicular chain, colesteatoma, traumatic dislocations, and congenital anomalies of the ossicular chain and oval window (10-11). Stapes- oval window The oval window is located laterally to the vestibule, above and behind the promontory. It connects the tympanic cavity with the vestibule and it is occupied by the stapes footplate (2-3). Its longitudinal diameter is of 3-4 mm and its vertical is of 1,5-2 mm. Its conventional topographical visualization is optimum on the coronal plane, and it is not satisfactory visualized on the axial plane (1). A double-oblique orientation of the reconstruction s axial plane enables an optimum exhibition of the stapes and oval window (Fig. 7). An oblique sagital reconstruction is useful to confirm the presence of both branches of the stapes, next to the oval window. The combination of sagital, axial and oblique Fig. 1. 3D reconstruction of external ear with 64 MDCT. (A) skin, (B & C) bone. Fig. 2. 3D reconstruction of external ear with 64 MDCT. (A & B) Atresia of the external bone of EAC, occupied by compact bone (Arrows). (C) Skin fold. Página 2 RAR - Volumen 74 - Número

3 María Lourdes Mallo et al. Fig. 3. CT with Multiplanar Reconstruction (MPR) double oblique coronal of the hammer, shows hammer s head (C), handle (M) and neck (CU). Also, tensor timpani (TT). Fig. 4. CT with double oblique coronal MPR of incus, is visualized the body of the incus (C), short process (SP) and long process (LP). Fig. 6. Incude-malleolar joint, 3D bone reconstruction. Fig. 5. CT with oblique double sagital MPR of incude-malleolar joint (AIM). The incus body (CY), long process (LP); handle of hammer (MM). reconstructions can assist in the evaluation of congenital anomalies, in traumatic dislocations and in fixation of the stapes in patients with hearing loss (10). Round Window The round window is located behind and below the oval window, in a shallow pit. It corresponds to the tympanic orifice of the cochlea s tympanic scale and its diameter is 2 mm (2). It can be clearly represented on axial images (Fig. 8), while it is not possible to do it on the coronal plane, due to its orientation (1). It is more convenient to use oblique sagital reconstructions (Fig. 9). The radiological study of both windows is important in several pathologies, such as otosclerosis and malformations (13). INTERNAL EAR The internal ear is the bone labyrinth which contains the membranous labyrinth (2). Cochlea It is a spiral-shape structure with two and a half turns. It is located anterolateral to the IAC and contains the organ of Corti. Its long axis can be represented in axial or coronal standard images. The 64 MDCT, by oblique reconstructions (Fig. 10), enables the evaluation of superior, middle and inferior turns, as well as the evaluation of the opening of the cochlear nerve. Curve reconstructions enable to unroll the cochlea, getting insight into its morphology and dimensions. Finally, volumetric imaging provides morphological and anatomical detail, which allows to visualize the total of its turns (3-4). RAR - Volumen 74 - Número Página 3

4 Ear study with 64 slices Multidetector CT Semicircular Canals They are constituted by three conducts (superior, posterior and lateral), situated behind and above the vestibule. The superior semicircular canal is orientated in an approximately 90 angle to the long axis of the temporal bone. Coronary images are enough to determine its integrity (1-2-14). Vestibule and vestibular aqueduct The vestibule is part of the internal ear s labyrinth, situated between the semicircular conducts and the cochlea. It contains the utriculus and saculus, which are part of the membranous labyrinth. The aqueduct or vestibular conduct contains the endolymphatic conduct and communicates the vestbiule s saculus with the endolymphatic saculus (2). It is not usually visualized in coronary images and it is hard to see it in axial images (1) (Fig. 11). The vestibular conduct s course is better identified in the sagital plane (Fig. 12), which is useful in the diagnosis of dilated vestibular aqueduct syndrome, an anomaly more common of the osseous labyrinth observed in patients with a deep sensorineural hearing loss (15). Facial nerve canal It is a bony channel where the facial nerve is located in its intratemporal course. It starts at the back of the internal auditory canal. Its first portion, of 4 mm in length, is between the cochlea (front) and the semicircular canals (back). It is often injured in longitudinal bone fractures. The second portion or tympanic portion starts when it bends back, almost at right angle, to follow a parallel path to the lateral semicircular canal. It measures 10 mm long and may be dehiscent. The third portion or descending portion, is 15 mm long and continues to the stylomastoid foramen (2-16). With the image reconstruction in an oblique sagi- Fig. 7. CT with oblique double axial MPR of stapes (E) in the oval window (VO). Fig. 8. axial MPR of round window. Fig. 9. CT with sagital oblique MPR of round window (VR), carotid canal (CC), labyrinthic segment of facial nerve canal (NFL). Fig. 10. cochlea s short axis (Arrow), CT with oblique MPR, where the inferior turn can be observed. Página 4 RAR - Volumen 74 - Número

5 María Lourdes Mallo et al. tal plane (Figs ) and curve reconstructions, the whole longitude between the tympanic and mastoid segments of the facial nerve can be represented in just one image (8-9). The use of this reconstruction plane can help in the diagnosis of the nerve anomalies and fractures in the path of the facial nerve canal. Carotid conduct The intrapetrosal path of the internal carotid artery (ICA) is perfectly visible with a tomography regarding its bone repairs. With 64 MDCT it is also possible to perform angiographic reconstructions for suspected arterial injuries. This requires iodinated contrast injection (Figs. 16, 17 and 18). Cochlear implant 64 MDCT in presurgical study for cochlear implant is an excellent method for anatomical and pathological detail of the temporal bone region (Fig. 18). Volumetric images provide information about the anatomy of the ossicular chain, the inner ear and its relationship to vascular structures, the carotid artery and the internal jugular vein. Curve reconstructions enable to unroll the cochlea, getting insight into its morphology and dimensions. This information is very useful for the i surgeon. In the postsurgical study, 64 MDCT is an excellent method to control the correct positioning of the electrodes of the implant, since it avoids image artifices Fig. 11. CT, standard axial image. Vestibule (V) Posterior semicircular canal (CSP) and lateral (CSL). Fig. 12. CT with MPR, oblique sagital image, where the vestibular conduct is visualized (CV) in its origin, common area of CSS and CSP (ZC). Fig.13. 3D reconstruction: semicircular canals CCS: superior CCL: lateral CCP: posterior. Fig. 14. CT with MRP, oblique sagital image where superior semicircular conducts (CSS) and lateral (CSL) and facial nerve canal (NF) are visualized. RAR - Volumen 74 - Número Página 5

6 Ear study with 64 slices Multidetector CT with an acquisition of submillimetrical slices and optimum resolution (17) in just a few seconds. CONCLUSION 64 Channels MDCT is an excellent imaging method for the study of the ear s anatomy and pathology. It enables through volumetric acquisition with submillimetrical slices in a few seconds and with a comfortable position for the patient, a display on different planes of the space, with three dimensional reconstructions of the ear s anatomy. Therefore, they allow a more comprehensive approach to different pathologies and optimized the planning for possible surgery. Bibliography 1. Venema HW, Phoa SS, Mirck PG, Hulsmans FJ, Majoie CB, Verbeeten B Jr. Petrosal bone: coronal reconstructions from axial spiral CT data obtained with 0.5-mm collimation can replace direct coronal sequential CT scans. Radiology 1999;213(2): Williams PL, Warwick R, Dyson M, Bannister LH, eds. Gray s anatomy. New York: Churchill Livingstone Editors; 1989: Reisser C, Schubert O, Forsting M, Sartor K. Anatomy of the temporal bone: detailed three-dimensional display based on image data from high-resolution helical CT a preliminary report. Am J Otol 1996; 17(3): Calhoun PS, Kuszyk BS, Heath DG, Carley JC, Fishman EK. Three-dimensional volume rendering of spiral CT data: the- Fig. 15. CT with MPR, oblique sagital image where the facial nerve canal, tympanic (NFT) and mastoid (NFM) portions are visualized. Lateral semicircular conduct (CSL). Tensor timpani muscle (MTT). Fig. 16. CT, 3D reconstruction of aneurysm in intrapetrosal portion of ICA. Fig. 17. CT, 3D reconstruction of aneurysm in intrapetrosal portion of ICA. Bone lisis. Fig. 18. CT, MIP reconstruction of aneurysm in intrapetrosal portion of ICA. Página 6 RAR - Volumen 74 - Número

7 María Lourdes Mallo et al. ory and method. RadioGraphics 1999; 19(3): Fatterpekar GM, Doshi AH, Dugar M, Delman BN, Naidich TP, Peter M. Som PM. Continuing Medical Education: Role of 3D CT Evaluation of the Temporal in the Bone. RadioGraphics 2006; 26:S117-S Donaldson JA, Duckert LG, Lambert PM, Rubel EW. Surgical anatomy of the temporal bone. New York: Raven Edit.; 1992: S Curtin HD, Sanelli PC, Som PM. Temporal bone: embryology and anatomy. In: Som PM, Curtin HD, eds. Head and neck imaging. St. Louis: Mosby Edit; 2003: Chuang MT, Chiang IC, Liu GC, Lin WC. Multi-detector row CT demonstration of inner and middle ear structures. Clin Anat 2006; 19(4): Lane JI, Witte RJ, Driscoll CL, Camp JJ, Robb RA. Imaging microscopy of the middle and inner ear. I. CT microscopy. Clin Anat 2004; 17(8): Park K, Moon SK, Cho MJ, Won YY, Baek MG. 3D micro-ct images of ossicles destroyed by middle ear cholesteatoma. Acta Otolaryngol 2004; 124(4): Meriot P, Veillon F, Garcia JF, et al. CT appearance of ossicular injuries. Radiographics 1997; 17(6): Yamada M, Tsunoda A, Muraoka H, Komatsuzaki A. Threedimensional reconstruction of the incudostapedial joint with helical computed tomography. J Laryngol Otol 1999; 113(8): Zeifer B, Sabini P, Sonne J. Congenital absence of the oval window: radiologic diagnosis and associated anomalies. Am J Neuroradiol 2000; 21(2): Lemmerling M, Vanzieleghem B, Dhooge I, Van Cauwenberge P, Kunnen M. CT and MRI of the semicircular canals in the normal and diseased temporal bone. Eur Radiol 2001; 11(7): Berrettini S, Forli F, Bogazzi F, et al. Large vestibular aqueduct syndrome: audiological, radiological, clinical, and genetic features. Am J Otolaryngol 2005; 26(6): Tuccar E, Tekdemir I, Aslan A, Elhan A, Deda H. Radiological anatomy of the intratemporal course of facial nerve. Clin Anat 2000; 13(2): Manual de implante coclear. Medel Medical Electronics, Insbruck (Austria), Accesible en: Fig. 18. CT, 3D images and MPR. Cochlear implant. EXTERNAL: microphone, speech processor, transmitter. INTERNAL: receiver- stimulator: sends signals to the electrodes. Electrodes: stimulate nerve cells inside the cochlea. RAR - Volumen 74 - Número Página 7