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Computed Tomographic Anatomy of the Temporal Bone

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Computed Tomographic Anatomy of the Temporal Bone 379 Computed Tomographic Anatomy of the Temporal Bone Chat Virapongse 1 With the recent development of high-resolution computed tomography (CT), there is Stephen L. G. Rothman a growing need to explore the full potential of this new method in demonstrating the E. Leon Kier detailed anatomy of the temporal bone. For this purpose, dry skulls with intact ossicles Mahammad Sarwar were scanned in axial and coronal projections. The detailed CT anatomy of t he temporal bone was documented, complemented by images from live patients. Because of its superior contrast resolution, CT was able to demonstrate numerous structures, such as the tympanic membrane, ossicies, and supporting structures, hitherto never or poorly visualized by any other method. In addition, the ease by which axial sections of the temporal bone could be obtained is of great benefit in displaying several structures previously difficult to evaluate. Computed tomographic (CT) scanning has proven to be indispensable in the evaluation of intracrani al pathology, but its role in the evalu ation of the temporal bone anatomy and pathology has not been fully explored [1]. Recent improve­ ments in CT scanners have made avail able detailed information of the temporal bone [2], and certain structures that were previously poorly visible by other methods are now clearly seen [1 -6]. The wealth of anatomic data displayed in vari ous projections on CT poses a diagnosti c chall enge to neuroradiologists and clinicians. Furthermore, the understanding of the CT anatomy of the temporal bone is difficult due to complex stru ctural relations that cannot be visuali zed on a single plane [7]. Our systematic CT analysis of the temporal bone was under­ taken to demonstrate and document thi s detailed anatomy. Materials and Methods All scans were obtained with a Pfizer 0200FS scanner in a " neuropack " configuration. The scanner contains a detector array of 30 calcium fluoride c rystals, each 2.5 x 3. 5 mm . The detectors are collimated so that only the central 1.5 x 1.5 mm are open to th e x-ray beam. The x-ray beam width is narrowed to 2 mm by a manual slide, and the sli ce thickn ess This article appears in the July / August 1982 is collimated to 2 mm by a removable stainless steel tube-side collimator. issue of AJNR and the October 1982 issue of The scanning algorithm is modified by increasing the sampling rate by a factor of two AJR. and by decreasing the translation arm speed to about 40 sec. The combinati on of these Received March 9, 1981; accepted after revi­ two software modifications and decreasing the detector size improves the geometric sion January 6, 1982. resolution all owing visualization of 0.75 mm pi ns in the American Association of Physicists Presented at the annual meeting of the Ame ri­ in Medicine phantom. Th e image is then back-projected onto 0.3 mm pixels and recorded can Society of Neuroradiology, Chicago, April in the usual manner. 1981 . Hounsfield [8] suggested that scans of the bones of the middle ear would not be 'All authors: Department of Diag nos ti c Radiol­ degraded by graininess at pixel sizes greater than 0 .25 mm. We have successfully back­ ogy, Section of Neuroradiology, Yale Universi ty School of Medicine, 333 Cedar St., New Haven , projected the epithympanum into 0 .15 mm pi xels, but suggest that, unless the sampling CT 06510. Address reprint req uests to C. Vira­ rate would again be halved, 0 .3 mm pixels seem a better compromise. Because of the 2 pongse. small pixel size, the zone of reconstruction is constric ted to only 200 c m . It is possible to reconstruct only one temporal bone at a time, although both are scanned AJNR 3:379-389, July / August 1982 0195-6108/ 82 / 0304- 0379 $00.00 simultaneously. This disadvantage is circumvented by storing th e raw data on disk and © American Roentgen Ray Soc iety recomputing the opposite temporal bone from th is data at th e completi on of the study. If it 380 VIRAPONGSE ET AL. AJNR:3, July / August 1982 is cru cial to see both temporal bones at the time the scans are Ex ternal Auditory Canal obtained, it is possible to back-project the scan onto 0.5 mm pixels and display a rectangular area of reconstruction aligned to encom­ Anatomy. In the adult, the external auditory canal is about pass both temporal bones. The resolution in this scanning mode is 2-3 cm in length, oriented directly along the coronal plane. not as good as in the 0.3-mm-pixel scan, but if the data are stored Except for the most superior part, it is completely sur­ on disk or magnetic tape, th e two temporal bones can be recom­ rounded by the tympanic bone, which forms an incomplete puted with 0 .3 mm pixels at the termination of th e examination. ring over the meatus. It is covered superiorly by the squa­ More than 80 unprepared dry skull s were examined in an attempt mous temporal bone. The mandibular fossa, containin g the to find skulls with intact ossicular chains. Ossicles are absent in mandibular condyle, constitutes its anterior relationship, commercially avail able skulls as th e result of destruction of the while the mastoid process and air cells are situated poste­ li gaments, tendons, and the tympanic membrane during the prep­ riorly. Most medially, at the attachment of the tympanic arati on process. In vivo, these soft-tissue structures form the natural support of the ossicles, tethering them to each wall of the middle membrane, the most posterosuperior edge of the tympanic ear cavity. It is not uncommon to find an intact ossicular c hain either rim protrudes slightly into the canal, forming the posterior in one or both ears in a newborn prepared skull, since often the (greater) tympanic spine (fig. 1 F) . Superiorly, the squamous tympanic membrane is left intact, providing th e ossicles with their temporal bone provides the most superior attachment of the lateral support. In the adult skull , as a rule, the ossicles are absent. pars flaccida of the tympanic membrane. Most of the dry skulls examined had lost their tympanic membranes Observations. The anterior and posterior walls are best and ossicles. visualized in the axial projection (figs. 18 and 1 C), while the Dry skulls were scanned in the axial and coronal planes. Two coronal plane is well suited for visualization of the roof and techniques were considered and compared. A " low" kilovoltage the inferior wall (figs. 28-20). The posterior tympanic spine technique using 80 kV and 50 mA and a " high" kilovoltage tech­ can be visualized on the axial view as a sharp projection nique using 140 kV and 35 mA were performed on each skull. The latter technique offered the best detail, and all the CT scans in this extending anteriorly at the junction of the middle ear and study, including scans of clinical subjects, were performed in this external canal (fig . 1 F). The axial projection also provides manner. All scans were obtained at 40 sec. excell ent visualization of the relations between the mandib­ Forty-five patients were scanned. In most, only th e axial projec­ ular fossa and the external auditory canal. tion was used, primaril y due to the ease of patient positioning and patient comfort. Th e coronal projection was attempted in some, but occasionall y resulted in a poor im age due to patient motion. At the Middle Ear termin ation of the stud y, reconstruction of the opposite ear was Anatomy. The middle ear is a narrow cavity separating the performed . inner and external ear. This flattened rectangular chamber The illustrations in our anatomi c study are a combination of those provided from dry skulls and those from our normal c linical subjects. is oriented along the same oblique plane as the temporal The individual images were chosen to display discrete anatomic bone. The many structures that traverse this space plus the structures, some of wh ich are best displayed in the dry skull, while irregularity of its inner wall add to its overall complexity. oth ers require delineation of soft tissue best demonstrated in live The middle ear is divided into the mesotympanum or patients. tympanum proper (the region of the middle ear cavity di­ rectly contiguous with the tympanic membrane), the epitym­ panic recess, and the hypotympanum. The ossicles for the Observations and Discussion most part reside within the epitympanic cavity, with the Our observations are divided into sections based on each major part of the ear and also on arbitrary grouping of a set of structures of special interest. Each section contains an anatomic description foll owed by observations and com­ Fig . 1.-Abbreviations. ments, so that each set of anatomic structures is dealt with AA = ad itus ad anlrum MF = mandibular fossa sequenti ally in its entirety. C = cochlea OW = oval window CA = cochlear aqueduct PE = pyramidal eminence CC = carotid canal PL = posterior incudalligament CCO = crus communis PSC = posterior semicircular CN = cochl ear division of eighth canal nerve PTS = posterior tympanic spine CP = cochleariform process RW = round window niche EAC = external auditory canal SC = semicanal ER = fossa incudis and epitym- SEP = septum separating th e H--~4-~~~~~~--7T~~~~~~---­ panic recess semicanal from the eustachian G--~------------~~ ET = eustachi an tube tube FC = facial canal SML = superior mall eal ligament FI = foveate impression SPS = sphenopetrosal synchon- i~~~~~~~~~~!1~~~:i~~~~~~~ FIN = fossa incudis drosis FR = facial recess SSC = superi or semi circular canal ~====~~==~==~~~~~======= G = geniculum ST = sinus tympani I = incus SU = subiculum lAC = intern al auditory canal TM = tympanic membrane IMJ = incudomall eal joint TS = tympanic spine LSC = lateral semicircular canal V = vestibule M = malleus VA = vestibular aqueduct A MA = mastoid antrum VN = vestibular nerve AJNR:3, July/ August 1982 CT OF TEMPORAL BONE 381 ElJSTACH IIW 1UFE B c E STPffS& [Nfl!.
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