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Home , Bony labyrinth, Cochlear aqueduct, Epitympanic recess, Eustachian tube, Facial canal, Pyramidal eminence

379

Computed Tomographic of the Temporal

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 . For this purpose, dry with intact 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 of the middle 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 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 . It is covered superiorly by the squa­ More than 80 unprepared dry s were examined in an attempt mous temporal bone. The , 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 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 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 = OW = CA = PE = CC = PL = posterior incudalligament CCO = crus communis PSC = posterior semicircular CN = cochl ear division of eighth canal PTS = posterior tympanic spine CP = cochleariform process RW = 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 = 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 = SU = subiculum lAC = intern al auditory canal TM = tympanic membrane IMJ = incudomall eal joint TS = tympanic spine LSC = lateral semicircular canal V = vestibule M = VA = A MA = VN = AJNR:3, July/ August 1982 CT OF TEMPORAL BONE 381

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Fig. 1 .-A-G. A, Coronal section of right temporal bone shows levels of patient, also passes through basal turn of cochlea. Depressions along pos­ axial views in B-O. Level A is most inferior. (Fig. 4 provides a more complete terior aspect of not as rounded as in B, but appear more anatomiC representation.) B and C, Level A, live patient, passing through fl attened and shallow. F and G, Level C, live patient. passes through ossicular basal turn of cochlea. Trefoil appearance formed by round window niche, processes and oval window. V-shaped lucency formed by internal auditory sinus tympani , and facial recess. Air-filled bony part of is canal and canals for two divisions of eighth nerve. Oval window appears as directed anteromedially parallel to carotid canal. 0 and E, Level B, live breach in continuity of otic capsule. 382 VIRAPONGSE ET AL. AJNR:3, July / Au gusl 1 982

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Fig. 1.-H-M. H and I, Through level D, juvenile temporal bone in live facial canal. Circular lucency in petrous bone formed by lateral semicircular palient. Tensor tympanic and stapedial tendons arise from cochleariform and canal posterolatera!ly. Vestibule and cru s communis anteromedially. Facial pyram idal processes, respectively. J and K, Through level E, dry skull, nerve passes anterolaterally dorsal to basal turn of cochlea and turns sharply slightly above oval window , passing through ossicul ar processes and vesti­ posteriorl y at genu . bule. L and M, Level F, live pati ent, passes through semi circular canal and AJNR :3, July/ August 1982 CT OF TEMPORAL BONE 383

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houses the tendon of the tensor tympani (fig. 1 H). Th e tendon crosses the middle ear at a 90° angle to th e mu scle, where it attaches to the malleus at the neck of the malleus. Therefore, the cochleariform process acts as a fulcrum for this pulleylike action. Observations. Axial scans provide the best visualization of the eustachian tube, semicanal, and carotid canal. On ly the air-containing bony part of the eustachian tube is visu­ alized on CT (figs. 1 B and 1 D) . It and the semicanal run parallel to the carotid canal, directed toward the nasophar­ yn x. The eustachian tube and semicanal, separated by a bony septum that partially surrounds the tensor tympani p muscle, are visualized in the axial projection (fig. 1 D) . These structures are difficult to identify on the coronal projection , handle of the mall eus and the long process of the in cus particularly when the petrous bone is well aerated, si nce extending downward into the mesotympanum. The hypotym­ sections are obtained perpendicular to their orientation. panum is empty and essentially featureless. It is the small est OccaSionally, however, they can be visualized (fig. 2A). of the three divisions of the middle ear cavity (fig. 2B). Th e communicates with the mastoid antrum Medial Wall via the aditus (figs. 1 Nand 10). Korner septum, the remnant of the petrosquamosal suture [9], is a thin osseous partition Anatomy. The promontory, the part of the labyrinthine vertically oriented along th e sagittal plane eccentrically pro­ capsule covering the basal turn of the cochlea, occupies the truding downward into th e mastoid antrum (fig. 2C). major part of the anteromedial wall of the tympanic cavity. It extends posteriorly up to the oval and round windows and then diverges into two ridges of bone, call ed superiorly the Anterior Wall ponticulus and inferiorly the subiculum, both forming the Anatomy. The anterior wall of th e middle ear cavity has anterior margin of the sinus tympani. The subiculum also two openings: the eustachian tube and the semicanal for th e forms the posterior aspect of the round window niche. The tensor tympani. Both structures run parallel and almost sinus tympani is a concavity in the posteromedial aspect of contiguous to the carotid canal , which is more medially th e middle ear. It is surrounded laterally by the pyramidal situated (figs. 1 B-1 E) . The bony eustachian tube is usually eminence, superiorly by the pyramidal turn of the facial air-filled for about 2- 3 cm. The cartilaginous extension is nerve and lateral semicircular canal, and anteriorly by the collapsible and usually not air-filled. The semicanal extends promontory, oval, and round windows. further posterior than the eustachian tube and passes below The second part of the facial canal traverses the middle the geniculum (genu of the seventh nerve), ending slightly ear anteroposteriorly along the superomedial aspect of the anterior to the oval window. At its point of termination , th ere tympanic cavity. This part is usually covered by a thin rim of is a small process called the cochl eariform process th at bone. At times, the coverin g can be partiall y deficient, 384 VIRAPONGSE ET AL. AJNR:3, July / August 1982

particularly near the oval window and cochleariform process which may not assume a pointed triangular configuration [10, 11]. The oval and round windows are located posteriorly but may be blunt and broad. along th e medial wall. The oval window is superior and sli ghtly ventral to the round window, where a deep depres­ sion can be seen representing the cochlear fossula (niche). Lateral Wall The oval window communicates with the scala vestibuli, Anatomy. Th e lateral wall is covered to a large extent by while th e round window with the scala tympani; both are the tympanic membrane (fig. 4). The scutum (drum spur), compartments of the perilymphatic system surrounding the which forms the lateral wall of the epitympanic recess, . projects downward for the attachment to the tympanic mem­ Observations. The axial plane provides good visualization brane at the tympanic incisura. of the promontory, fossular cochlea (round window), and Observations. The coronal and axial views are suited for th e subic ulum (figs. 1 B and 10). The round window niche visualizing the lateral wall. The scutum is best seen in can be recognized as a sharp, deep, bony depression dorsal coronal section (fig. 2B). to the basal turn of the cochlea. The ridge of bone separating the round window from the sinus tympani is the subiculum (figs. 1 B and 10). The structures of the posteromedial wall .rioof and Floor of th e tympanic cavity are best seen in this projection. The Anatomy. The tegmen tympani forms a thin bony roof oval window is the only exception. This ovoid dehiscence, separating the epitympanic recess from the temporal lobe. longer in its anteroposterior dimension than superoinferior It extends posteriorly to cover the aditus and the mastoid dimension (2.99 mm versus 1.41 [12]), is difficult to visualize antrum , where it is called the tegmen mastoideum. The floor on the axial projection owing to partial volume effect, the of the middle ear also consists of a thin bony shell, which sli ce thickness being 2 mm. When seen, it appears as a separates the major vessels from the middle ear cavity. The breach in continuity of the otic capsule lateral to the vesti­ carotid and the jugular vein are separated most bule, th e stapedial superstructure within the middle ear inferiorly by the caroticojugular spine, which has the ap­ overlying this dehiscence more laterally (figs. 1 F and 1 G). pearance of an inverted triangular structure wedged be­ It is easily seen, however, on direct coronal projection (fig. tween these two major vessels [9]. 20). Observations. The coronal projection is best for demon­ strating Korner septum and the tegmen tympani (fig. 2C). Posterior Wall The thickness of the osseous floor and the hypotympanum and its relationship to the major blood vessels are best Anatomy. The posterior wall of the middle ear cavity is a assessed on this view (fig. 20). complicated structure consisting of several ridges and de­ The caroticojugular spine can be demonstrated on axial pressions. Inferiorly, the middle ear cavity appears to extend CT as an area of bone separating the carotid canal from the into the mastoid and labyrinthine capsule in a trefoil pattern jugular bulb on a low section through the temporal bone. of bony depression. The most medial one is the round Korner septum, the tegmen tympani, and mastoideum are window nic he, th e middle is the sinus tympani , and the most difficult to identify on this projection. lateral is the facial recess (figs. 1 Band 1 0). The round window ni che is separated from the sinus tympani by the subiculum, and th e sinus tympani is separated from the Ossicles, Tendons, and Ligaments facial recess by the pyramidal eminence. The pyramidal eminence contains the and projects into Anatomy. The auditory ossicles provide a lever mecha­ the middle ear cavity in an anterosuperior direction toward nism for the transfer of sound energy. The malleus, incus, the stapedial head in a beaklike manner. The pyramidal and form a chain from the tympanic membrane to eminence is a ridge of bone, extending in an anterosuperior direction along the posterior wall , which partially hides the tympanic sinus (fig . 3). The penetrates the Fig. 2.-Abbreviations.

posterior wall of the middle ear above and slightly lateral to C = coc hl ea MH = malleus handle the pyramidal eminence and turns 90° (pyramidal turn) to CC = carot id cana l MP = masto id process descend verti call y, toward the . CCO = cru s comm uni s OC = occ ipital cond yle CF = cri sta fal ciformi s OW = ova l window Observations. The axial projection is ideal for the visual­ EAC = extern al auditory canal P = promontory ization of the sinus tympani and the facial recess. The ease ET = eustac hi an lube PE = pyramidal eminence with which the sinus tympani can be demonstrated on the FC = facial cana l PSC = posteri or semi circular HC = hypog lossal canal canal axial projection has important clinical relevance, since this HT = hypotympanum SC = semicanal structure is difficult to visuali ze on c linical examination, I = incus SCU = scutum lAC = internal auditory canal SM F = stylomastoid foramen being partially hidden from view by the more lateral pyram­ JF = iugular fossa SPS = sphenopetrosal synchon- idal em inence (fig. 1 H). The size and configuration of these JT = drosis recesses, however, are subject to individual variation from KS = Korner Septum SSC = superior semicircul ar canal LSC = lateral semic ircular canal TT = teg men tympani rounded deep recesses to more flattened shallow ones (figs. M = malleus V = vestibule 1 B and 10). This is also true for the pyramidal eminence, MA = mastoid antrum AJNR:3, July/ August 1982 CT OF TEMPORAL BONE 385

A B

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Fig. 2.-Coronal CT sections of dry skull, rig ht temporal bone. Successive Through basal run of cochlea of intern al auditory canal. D, Through internal slices 2 mm apart from anterior to posterior. A is most anterior. A, Through auditory canal and vestibule. E, Through vestibule and . F, carotid canal and eustachian tube. B, Through cochlea and mall eus. C, Through jugular fossa and vertical part of facial canal. 386 VIRAPONGSE ET AL. AJNR:3, July/ August 1982

Faciat conal

Fig. 3. -Posteromedial aspect of left middle ea r viewed from external ea r. Fig . 4 .- Coronal vi ew of right ear shows relati on betwee n structu res Promontory extends into two ridges, ponti culus superiorl y and subiculum comprisin g inner, middle, and extern al ear. Ossicul ar chain can be seen from inferiorl y, forming ventral aspect of sinus tympani. Pyramidal eminence it s attachment to tympanic membrane, extending across middle ear to laby­ contains stapedius muscle, partiall y obscuring sinus tympani. (Reprinted from rinthine capsul e. Eustachi an tu be occupying anterior wall of tympanic cavity [ 12].) passes anteromedially toward .

the (fig. 4). The mall eus, lying most ventrall y, membrane. The anterior malleal li gament passes from th e consists of a head , a manubrium (handle), and an anteri or petrotympani c fissure (glaseri an fissure) to insert into the process (fig. 5A). The head articul ates with th e body of th e anterior mall eal process. Mediall y, the tensor tympani ten­ inc us, while the manubrium and th e lateral process are don attaches itself to th e neck of the mall eus. It functions attached to the tympan ic membrane more laterall y. The primarily to dampen the mall eus when the mall eus is sub­ head is round with a diameter of about 2- 3 mm. The anteri or jected to vigorous sound vibrations (figs. 1 H and 11). Su­ process provides an attachment for th e anteri or mall eal periorly, the head of the malleus is attached to the roof of li gament. the epitympanic recess by the superi or mallealligament (fig. The inc us, th e largest of th e three ossicles, has a body 1 N). The in cus is supported by th e posterior inc udal liga­ and a short and long process. The long process extends ment (fig. 1 L). The tendon of th e stapedius muscle, which downward into the mesotympanum parall el and postero­ inserts into th e head of th e stapes, serves a functi on similar medial to the mall eal handle. Its most inferi or extension (the to that of the tensor tympani tendon . The stapedial footplate lenticular process) arti culates with the head of th e stapes is fastened to the rim of th e oval window by the annular (fig. 1 H) [3, 13]. The short process projects dorsall y from li gament. the body, where it is attached to th e posterior wall of the From th e foregoing description, it is quite apparent that epitympanic recess by the posterior in cudal li gament (figs. th e incus has the least support of all three ossicles, thus 1 L and 1 M). The body of the incus, including its short expl ai ning its in creased susceptibility to traumatic disloca­ process, is about 4-5 mm along its anteroposterior dimen­ tion. sion. The epitympanic recess, where the inc udal body and Observations. The molar tooth configuration formed by mall eal head are located, narrows posteriorly and may be the mall eus and incus is a familiar lateral tomographic [9, separated from the short process by 1-2 mm . This part of 14, 15] projection of th e auditory ossicles. On axial CT, the epitympanic recess is call ed the fossa inc udis (fi g. 1 L). secti ons are obtai ned perpend icular to the superorinferior The stapes consists of a head, which articulates with the orientation of the ossic les. Essentiall y, sections pass incus, anterior and posterior crura, formi ng a crural arch through either head-body or the long processes of the (superstructure), and the stapedial foot plate. The foot plate ossicles. Higher cuts corresponding to the level of the configuration is complementary to the oval window. internal auditory canal will usuall y also pass through the Th e ossicles are held suspended in th e middle ear by a epitympanic recess and the incudomalleal articulati on (fig. seri es of struts consisti ng of li gaments and tendons (fig. 1 L), while lower cuts corresponding to th e external canal, 58). Some of th ese ligaments are merely folds of th e mucous drum, and mesotympanum will pass through the handle of membrane that cover the entire middle ear cavity and the the malleus and long process of the incus, both appearing ossicul ar chain . These mucosal folds are considered li ga­ as two small densities (fig. 1 J). The stapedial superstructure, ments, because they playa supportive role in maintaining forming an arch over the oval window, li es on an axial pl ane, the integrity of the ossicular chain. and, therefore, can be vi sualized in its entirety using thi s Laterall y, the malleal handle is attached to the tympanic projection. The cochleari form process, tensor tympani ten- AJNR:3, July I August 1982 CT OF TEMPORAL BONE 387

Fig. 5.-A, Normal anatomy of audi­ tory ossicles. B, Attachments of various MALLEUS: SUPERIOR MALLEAL L1G. to head of molieul ligaments and tendons to auditory ossi­ SUPERIOR INCUDAL LIG. head to body of Incul cles. (Reprinted from [1 2].)

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Attachment of tympanic membrane to manubrium t base (foot plate) ANNULAR LIGAMENT to moroln of vestibular fenestra A B don, pyramidal eminence, and stapedial tendon are also at about the same level. Frequently, a section passing through the stapedial crura will reveal all these structures (figs. 1 F and 1 H). The axial scan is also ideal for visualizing the superior malleal ligament and the posterior incudal ligament (figs. 1 Land 1 M). The coronal projection should provide superior visualization of the incudostapedial joint [13]. This was not achieved in our study.

Inner Ear

Anatomy. The consists of osseous and membra­ nous labyrinths surrounded by dense compact bone call ed the otic capsule [16]. The bony labyrinth is directed along the same oblique plane as the temporal bone (fig. 6). It consists of the cochl ea, vestibule, and (fig. 4). The cochlea consists of 2'12 turns: the api cal, middle, and basal turns, the last bei ng th e larg est and most distinc­ tive . The rounded basal turn of the cochlea is separated from the middle ear by the promontory. Posteriorly, it is Fig . 6.-Schema of left temporal bone viewed from above shows relati ons of external , middle, and internal ear. Note relati on of auditory tube to internal connected to the vestibule that su rrounds the deli cate mem­ carotid artery; of genu of the seventh nerve with basal turn of cochlea and, branous and . The vestibule is perforated more posteriorl y, lateral semic irc ular canal; of vestibular aqueduc t passing directly posterior to its intracranial opening along posterior aspect of temporal posterolaterally by the oval and round windows. The semi­ bone. circular canals are attached to the vestibule in a series of arches oriented along all three planes, so that head move­ ment in any direction can be detected. The superior semi­ posterior semicircular canal appearing as a dot postero­ circul ar canal (fig. 1 P) is anteriorly situated close to the medially (fig. 1 L). The bony covering over the lateral sem i­ internal auditory canal and the cochlea. The posterior and circular canal is best evaluated in this projection. An accu­ lateral (horizontal) semicircular canals are more dorsally rate assessment of its integrity is helpful in ruling out ero­ positioned deep within the mastoid bone (fig 2E) . The medial sions and fistulae formation before surgical exploration of arms of the superior and posterior sem icircular canals com­ the middle ear in destructive lesions such as cholesteato­ bine to form a common crus that is oriented superomedially mas. (fig. 2E) . The coronal projection is probably the best for vi sualizing Observations. The axial projection is excellent for visual­ the cochlea [1 7]. Virtually th e entire extent of the basal turn izing the entire lateral semicircular canal, since it lies on a can be visualized on the one section (fig. 2B). The middle horizontal plane. This structure and the vestibule combine and apical turns (columella) are difficult to separate since to produce a lucent ring, with the posterior arm of th e they seem to blend into one another. 388 VIRAPONGSE ET AL. AJNR:3, July/ August 1982

Internal Auditory Canal pared with the ventral arm . On axial CT, th is canal widens posteriorly and merges with the foveate impression along Anatomy. The course of the internal auditory canal is the posterior aspect of the petrous pyramid. directed along the coronal plane similar to, but slightly more cephalad than, th e external auditory canal [18). The falci­ form crest (crista falciformis), which consists of a thin pi ece Facial Canal of horizontal bone, is thickest at th e fundus of the internal Anatomy. The facial canal can be divided into three sec­ auditory canal. It divides the internal auditory canal into a tions: th e first two parts are called the horizontal segments; superi or and inferior compartment [19-21] (fig. 2C). The the last the vertical segment [26- 28). facial nerve and the superior division of the vestibular nerve The facial canal begins at the fundus of the internal li e in the superior compartm ent, wh il e the cochlear and the auditory canal. It courses anteriorly ending at the geniculate inferior division of the vestibular nerve lie in the inferior ganglion . At this point, the canal makes a hairpin turn and compartment. At the fundus of the canal, each nerve exits extends posteriorly along the medial wall of the tympanic through its own foramen. cavity to the pyramidal eminence. This part of the canal lies Observations. The axial projection is best suited for visu­ superior to the semicanal for the tensor tympani, cochlear­ ali zing the exit foramina at the fundus. At this point, the iform process, oval window, lateral semic ircular canal, sinus internal auditory canal forks in a Y configuration. The ventral tympani, and pyramidal eminence. arms represent the canals for the cochlear and facial ; Behind the pyramidal eminence, the canal turns sharply the dorsal for the vestibular nerves (fig. 1 F). The facial canal downward toward the stylomastoid foramen and is related lies directly above the cochlear canal (figs. 1 L and 1 N). The anteriorly to the external auditory canal and middle ear and coronal projection is ideal for visualizing the integrity of the posteriorly to the mastoid air cells [29, 3D). falciform crest, which can be seen as a linear density Observations. The axial projection usuall y provides ex­ medially dividing th e intern al auditory canal into two com­ cellent visualization of the horizontal parts of the facial partments (fig. 20). canal. It is particularly good for demonstrating the first segment of the facial nerve as it leaves the internal auditory Aqueducts canal (figs. 1 L and 1 N). The second part of the facial canal traversing the middle Anatomy. The vestibu lar and cochlear aqueducts drain ear is difficult to visualize, owing to its thin bony covering, the and the , respectively. The vestib­ which is microscopically perforated by blood vessels [10, ul ar aqueduct consists of ventral and dorsal arms, which are 11). Occasionall y, when this bony covering is thick, the directed posteriorly in a widened inverted V configuration entire horizontal course of the facial nerve can be demon­ [22-24). The termination of th e aqueduct widens to accom­ strated on one section . modate the [25). This widening along the In the normal adult, the canal of the descending segment posterior aspect of the petrous bone forms a small plateau of the facial nerve is difficult to identify in the axial plane call ed th e foveate impression . due to surrounding mastoid air cells. It can be readily The cochl ear aqueduct provides a potential communica­ identified in young children and in patients with mastoid tion with the subarachnoid space. It arises from the scala disease who have obliterated mastoid air cell s. The coronal tympani near the round window and extends mediall y to projection is helpful for visuali zing the genu of the facial open along the medial aspect of the petrous bone below the nerve. It is also useful in assessing the vertical part of the internal auditory canal and is separated from the jugular facial nerve (fig. 2F). fossa by a th in ridge of bone. While the cochlear aqueduct often assumes a funnel shape at its intracranial opening, the size and meatal configuration of both aqueducts are subject Reconstruction to considerable vari ation. Observations. The axial projection is ideally suited for In a cooperative patient, it is possible to obtain a series of demonstrating the cochlear aq ueduct. This structure is usu­ axial scans and reformat them in any number of planes. This ally seen at the level of the carotid canal and eustachi an technique has the advantage of being able to display the tube (fig. 10) and may appear as a th in, slitlike structure complex anatomy in all of the conventional projections, muc h like the vestibular aqueduct or as a widened canal. It although only exposing the patient to radiation one time. should not be mistaken for the intern al auditory canal, whic h There are two technical drawbacks to this tec hnique. The li es just above it and is considerably larger. These two resolution of reformatted images along the z axis is limited structures are separated by about 3 -4 mm of bone on the to 1.5-2.0 mm. As a result, some slight degradation of the medial aspect of the petrous pyramid; sequenti al scans image is expected when compared with direct coronal or should be carefully scrutinized in order to differentiate these sagittal scans. Patient motion is also very critical when structures. The vestibular aqueduct can also be visualized evaluating small bony structures. The minute proximity of in this projection (fig. 1 L) lying medial to the posterior th e many important intratemporal structures make even a semicircular canal. small amount of patient motion intolerable. Therefore, it is In our study, only the dorsal arm of the vestibular aqueduct desirable to perform the scanning sequence as quickly as can be visualized, probably owing to its larger size com- possible to limit patient motion. AJNR:3, July / August 1 982 CT OF TEMPORAL BONE 389

Comment 8. Hounsfi eld GN. Pi cture quali ty of computed tomography. AJR 1976;127 :3-9 A large number of structures previously not vi sualized or 9 . Gui nto TC, Hi madi GM. Tomographic anatomy of the ear. poorly visualized by oth er means can now be seen by CT. Radiol Clin North Am 1974 ;3: 405-417 These inc lude: the tympani c membrane, the ossi cles and 10. Wil brand HF. Multidirectional tomography of the facial canal. their assoc iated tendons and ligaments, th e eustachi an Acta Radiol [Oiagn] (Stockh) 1975; 16: 6 54-672 tube, and th e semi canal for the tensor tympani, the coc h­ 11 . Wilbrand HF, Bergstron B. Multidirectional tomography of de­ leariform process. The round window nic he, subiculum, fects of the facial canal. Acta Radiol [Oiagnj (Stockh) sinus tympani, th e first portion of the facial canal, th e canals 1975; 16 : 223 -240 for the vesti bular and cochl ear nerves, and the cochl ear 12. Anson BJ, Donaldson JA. Surgical anatomy of the temporal bone and ear, 2d ed. Phil adelphi a: Saunders, 1973 : 239 aqueduct can also be seen. 13. Matsubara R, Konrad H, Hanafee WN . Incudostapedial joint in For maximum resolution, scanning patients directly in the health and disease. AJR 1978;13 1 :307-3 10 two projections is advantageous. When this is not possible, 14. Schatz CJ, Vi gnaud J. The inclined lateral projection : a new reformatted images can be obtained, although there is some view in temporal bone tomography. Radiology 1976;11 8: loss in resolution. 335- 361 There are many c linical implicati ons of this new meth od­ 15. Potter GO. The lateral prOj ection in tomography of the petrous ology. CT provides a precise technique for the evaluation of pyram id. AJR 1968;104 :194-200 soft-tissue masses within th e ai r-filled middle ear and mas­ 16. Donaldson JS. Normal anatomy of the inner ear. Otol Clin North toid. Defining th e extent of erosion or destructi on of the Am 1975;2 :267- 269 walls of th e middle ear, th e capsule of the semi circ ul ar 17. Vi gnaud J, Juster M, Lerich H, Li chtenberg R, Korach G. Radioanatomi e de la cochlea. Acta Radio/ 1969;9: 11 7 -1 22 canals, and the ossicular chain is crucial in the evaluation 18. Valvassori GE, Pi erce RH . The normal auditory canal. AJR and treatment of and other middle ear le­ 1964;92 : 1 232- 1 240 sions. It is also very helpful in evaluating postoperative 19 . Pait TG, Zeal A, Harris FS, Paullus WS, Rh oton A. Microsur­ complications from a mastoidectomy. CT is especially useful gical anatomy and dissection of the temporal bone. Surg Neural in assessing the tympanic membrane and in visuali zing 1977;8: 363 - 392 partly hidden structures su ch as the sinus tympani. Detaj led 20. Rh oton AL. Mic rosurgery of the intern al acoustic meatus. Surg anatomi c knowledge is essential for th e in creasingly impor­ Neural 1974;2: 3 11-3 18 tant role that high-resolution CT can be expected to have in 2 1. Vi gnaud J, Grall Y, Elbaz P, Paleirac R. 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