COMPUTED TOMOGRAPHIC ANATOMY OF THE CANINE INNER AND MIDDLE EAR

MARCO Russo, DVM, PHD, EUCENIO M. COVELLI, MD, LEONARDO MEOMARTINO, DVM, PHD, CHRISTOPHER R. LAMB, MA, VETMB, ARTURO BRUNETTI, MD

A series of high-resolution computed x-ray tomography (CT) images of the normal canine middle and inner ear are presented to serve as a reference for optimal interpretation of clinical CT images of animals with diseases affecting this region. Vetrrinarv Rccdiology & Ultrasound, Vol. 43, No. I , 2002, pp. 22-26.

Key words: dog, ear, otitis, computed x-ray tomography (CT).

Introduction

HE CANINE EAR is an anatomically and physiologically T complex structure that is difficult to evaluate using con- ventional radiography or planar tomography. Radiography is widely used in the diagnostic work-up of dogs with signs of chronic ear disease;'-7 however, there is evidence that radiography is insensitive for identification of lesions af- fecting the inner or middle ear.s37*8 Use of computed x-ray tomography (CT) should enable more accurate diagnosis of otitis media in the dog"','-'" and more accurate depiction of lesion margins in animals that are candidates for radiation therapy of neoplasms affecting this region." Clearly, knowledge of the normal anatomy of this region as depicted on CT images is a necessary precursor to optimal clinical use of this modality. There have been several studies of the CT anatomy of other regions of the canine head,24 including the brain,' 'L" orbit,'' and nasal cavity,I7 but no detailed studies of the ear anatomy were found, although this region is covered superficially in certain textbooks of tomographic anatomy."

The purpose of the current study was to describe the normal anatomy of the inner and middle ear of the dog as depicted using CT.

From the Dipartimento di Scienze Cliniche Veterinarie, Centro di Ra- diologia Veterinaria (Russo, Meomartino, Brunetti), and Dipartimento di Diagnostica per Immagini e Radioterapia (Covelli), Polo delle Scienre e delle Tecnologie per la Vita, Universith degli Studi di Napoli Federico 11, 1-80137 Naples, Italy, and the Department of Small Animal Medicine and Surgery (Lamb), The Royal Veterinary College, Hawkshead Lane. North Mymins, Hertfordshire AL9 7TA, U.K.

Address correspondence and reprint requests to Dr. M. Russo, Diparti- mento di Scienre Cliniche Veterinarie, Facoltii di Medicina Veterinaria, Polo delle Scienze e delie Tecnologie per la Vita, Unjversiti degli Stiidi di Napoli Federico 11, Via Delpino I , 1-80137 Naples, Italy.

Received May 10, 2001; accepted for publication July 28. 2001.

Material and Methods

Five healthy adult dogs (3 medium size and 2 small size) were fasted for 24 h prior to scanning, then prernedicated with acepromazine; general anesthesia was then induced with intravenous phenobarbital and maintained with oxy- genhalothane. A saline solution was given by slow intra- venous infusion during the anesthetic period. For transverse scans, the dogs were placed in sternal recumbency using a trough-shaped sponge to support the head and cervical spine; the head was extended with the hard palate parallel to the scanning table, and was secured in this position with tape. For dorsal scans, the dogs were placed in right lateral recumbency with the head flexed. A lateral scout image* was made to confirm correct positioning, and for selection of the transverse and dorsal images. Contiguous transverse 1-mm images of the skull were obtained, beginning imme- diately rostra1 to the tympanic bullae and continuing to a level just caudal to the petrous temporal bones. Contiguous dorsal 1-mm images were obtained, beginning at the level of tympanic bulla and continuing to a level corresponding to the dorsal margin of the petrous bone. The CT examination required approximately 20-30 min for image acquisition and reconstruction. Images were reconstructed with a high- resolution algorithm and displayed using bone window (center 200-300 HU; width 2000-3000 HU). Images were treated using proprietary illustrating softwarei to obtain line diagrams of aural structures. Diagrams were labeled with reference to standard anatomy

Results

The normal anatomy of the inner and middle ear is pre- sented in a series of labeled. transverse and dorsal CT im-

"Somatom HiQ, Siemens, Erlangen, Germany. +Photoshop, Adobe Systems Italia SRL, Milan, Italy.

22

VOL. 43, No. I CT OF C A N ~ N E EAR 23

ages sequentially displayed from rostra1 to caudal (Figs 1-8).

Discussion

CT imaging enables identification of all the major struc- tures of the middle and internal ear of the dog, such as the tympanic membrane, the auditory ossicles, the tympanic bulla (middle ear), the cochlea, the internal acoustic meatus, and the semicircular canals (inner ear). Images were ac- quired for the current study using thin slices, a wide window setting, and reconstructed using a bone algorithm to opti- mize resolution of the fine osseous structures of the ear. These parameters are also recommended for clinical use because without them there may be difficulty judging tym- panic wall thickness in the presence of fluid or soft tissue within the bulla.8,'y Dorsal and transverse images provide complementary views of the major anatomic structures. In

B

FIG. 2. Transverse CT image at the level of the cochlea (av. aqueductus vestibuli; ch, cochlea; eam, external auditory meatus; in, incus; ma. malleus; tb, tympanic bulla; tc, tympanic cavity; vw, vestibular window).

particular, the tympanic membrane was best visualized in the dorsal images. However, the visualization depends strongly on appropriate window selection. Therefore, good visualization of the tympanic membrane could be possible also with transverse images.

It is hoped that these anatomic reference images will provide the basis for a more precise definition of pathologi- cal conditions of the middle ear and, possibly, abnormalities affecting the inner ear. Recent reports have emphasized use of magnetic resonance (MR) imaging for diagnosis of otitis in dogs?op22 It seems likely on the basis of these reports that MR imaging will be the imaging modality of choice for animals with suspected otitis or vestibular syndrome be- cause of the potential to identify small lesions affecting aural and neural soft tissue structures; however, CT may be

ment is suspected.

mL * '*'

FIG. I. Transverse CT image at the level of the middle ear ductus vestibuli; ch, cochlea; eam, external auditory meatus: e

, aqUe-

panic recess; in, incus: ma, malleus; t, tentorium cerebelli (), a useful alternative to MR imaging when osseous involve-

tympanic bulla; vw. vestibular window).

24 RUSSO ET AL. 2002

Flci. 3. Dorsal CT image at the level of the tympanic bullae (bo, basioccipital hone; bs, basisphenoid bone; hm, head of mandible; tb, tympanic bulla; tnij, temporomandibular joint).

t

FIG. 4. Dorsal CT image at the level of the middle ear ( e m , external auditory meatus: ma, malleus; mt, tympanic membrane).

vw

FIG. S . Dorsal CT image at the level of the cochlea (aim, articulatio incudomallearis: ch, cochlea; vw, vestibular window; ma, malleus; mo. modiolus).

VOL. 43, NO. I CT OF C A N I N E EAR 25

FIG. 6. Dorsal CT image at the level of the internal acoustic canal (csc, caiialis spiralis cochleae; ct. cribta transversa; iac, internal acouhtic canal).

sa

FIG. 7. Dorsal CT image at the level of the inner ear (csa, anterior semicircular canal)

v

FIG. 8. Transverse CT image at the level of the middle ear (tb, tympanic bulla; sb, septum bullae).

26 RUSSO ET AL. 2002

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