Computed Tomographic Anatomy of the Temporal Bone

379 Computed Tomographic Anatomy of the Temporal Bone Chat Virapongse 1 Stephen L. G. Rothman E. Leon Kier Mahammad Sarwar With the recent developm...
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379

Computed Tomographic Anatomy of the Temporal Bone

Chat Virapongse 1 Stephen L. G. Rothman E. Leon Kier Mahammad Sarwar

With the recent development of high-resolution computed tomography (CT), there is a growing need to explore the full potential of this new method in demonstrating the detailed anatomy of the temporal bone. For this purpose, dry skulls with intact ossicles 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 o f 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 intrac rani al path o logy, but its role in the evalu ation of the temporal bone anatomy and pathology has not been fully exp lored [1]. Recent improvements in CT scanners have made avail ab le detailed information of the temporal bone [2], and ce rt ain structures that were previou sly poorly visib le by other method s are now c learly seen [1 -6]. The wealth of anatom ic data disp layed 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 du e to complex stru ctural re lations that can not be visualized on a single plane [7]. Our systematic CT analysis of the temporal bone was undertaken to demonstrate and document thi s detailed anatomy.

Materials and Methods

This article appears in the July / August 1982 issue of AJNR and the October 1982 issue of AJR.

Received March 9, 1981; accepted after revision January 6, 1982. Presented at the annual meeting of the Ame rican Society of Neuroradiology, Chicago, April 1981 . ' All authors: Department of Diagnosti c Radiology, Section of Neuroradiology, Yale Universi ty School of Medicine, 333 Cedar St., New Haven , CT 06510 . Address reprint requests to C. Virapongse. AJNR 3:379-389, July / August 1982 0195-6108 / 82 / 0304 - 0379 $00.00 © American Roentgen Ray Society

All scans we re obtained w ith a Pfizer 0200FS scanner in a " neuropack " co nfiguration. The scan ner contains a detector array of 30 calc ium fluorid e c rystals, each 2.5 x 3. 5 mm . The detectors are co llim ated so that on ly the c entral 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 manu al slide, and th e slice thi c kn ess is co llim ated to 2 mm by a removable stainless steel tube-side co llim ator . Th e scannin g algorithm is modifie d by inc reasin g the samplin g rate by a factor of two and by decreasing the translation arm speed to about 40 sec. The combin ati on of th ese two software mod ifications and decreasing the detector size improves th e geom etri c resolution all owing visualization of 0.75 mm pi ns in the Americ an Associ ation of Phy sici sts in Medicine phantom . Th e im age is then back-projected onto 0. 3 mm pi xe ls and recorded in the usual manner. Houn sfield [8 ] suggested that scan s of the bon es of the middl e ear wou ld not be degraded by graininess at p ixel sizes greater th an 0 .25 mm . We have successfull y bac kprojected the epithympanum in to 0 .15 mm pi xels, but suggest th at, unless th e sa mplin g rate wo uld again be halved , 0 .3 mm pixels seem a better compromi se. Because of th e small pixel size, the zone of reconstruction is constri c ted to only 200 c m 2 . It is possible to reconstruct only one temporal bon e at a tim e, alth ough bo th are scanned simu ltaneously. This disadvantage is c irc umvented by storing th e raw data on d isk and recomputing the opposite temporal bone from th is data at th e co mpl eti on o f th e stu dy. If it

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is c ru c ial to see both temporal bon es at the tim e th e scans are obtained, it is possible to back-project the scan onto 0.5 mm pi xe ls and display a rectang ular area of reconstruction align ed to encompass both temporal bon es. Th e reso lution in thi s scanning mode is not as good as in the 0 .3-mm-p ixel scan, but if the data are stored on disk or mag neti c tape, th e two temporal bones ca n be recomputed with 0 .3 mm pixe ls at th e termination of th e examination. More th an 80 unprepared dry sk ull s were examined in an attempt to find skull s with intact ossic ular c hain s. Ossic les are absent in co mmerc ially available sk ulls as th e result of destruction of th e ligaments, tendons, and the tympanic membrane during the preparati on process. In vivo, these soft-ti ssue stru ct ures form the natural support of th e ossic les, teth erin g them to each wall of the middle ear cavity. It is not unco mmon to find an intact ossicu lar c hain either in one or both ears in a newborn prepared sk ull , since often th e tympani c membrane is left intac t, provid ing th e ossic les with th eir lateral support. In the ad ult sk ull , as a rul e, th e ossic les are absent. Most of th e dry sk ulls examined had lost their tympanic membranes and ossicles. Dry sk ulls were scanned in th e ax ial and co ron al planes. Two techniques were co nsi dered and co mpared . A " low" kilovoltage techniqu e using 80 kV and 50 mA and a " high " ki lovoltage technique using 140 kV and 35 mA were performed on each sk ull. Th e latter techniqu e offered th e best detail, and all th e CT scans in thi s stud y, inc luding scans of c linica l subjects, were performed in thi s manner. All scans were obtained at 40 sec. Forty-five pati ents were scanned . In most, only th e axial projection was used, primaril y due to the ease of patient positioning and pati ent co mfort. Th e co ronal projection was attempted in some, but occasion all y resulted in a poor image d ue to patient motion . At th e termin ation of th e stud y, reconstru ction of the opposite ear was perform ed . Th e illustrations in our anatomic study are a co mbination of those provided from dry skulls and those from our normal c linical subjects. Th e individual images were c hosen to display d iscrete anatomic stru ct ures, some of wh ic h are best displayed in th e dry skull , w hile oth ers require delineation of soft tissue best demonstrated in live patients.

Observations and Discussion Our obse rvation s are divided into sections based on each major part of th e ear and also on arbitrary grouping of a set of structures of special interest. Each section contains an anatom ic description fo ll owed by observations and comments, so that each set of anatomic structures is dealt with sequenti ally in its entirety .

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AJNR :3 , July / August 1982

Ex ternal Auditory Canal Anatomy. In the adult, the external auditory canal is about 2-3 cm in length, oriented directly along the coronal plane. Exce pt for th e most superior part, it is completely surrounded by the tympanic bone, which forms an incomplete ring over the meatus. It is co vered superiorly by the squamous temporal bone. The mandibular fossa , contain in g the mandibular condyle, constitutes its anterior relationship , while the mastoid process and air ce lls are situated posteriorly. Most medially, at the attachment of the tympanic membrane , the most posterosuperior edge of the tympan ic rim protrudes slightly into the canal, forming the posterior (greater) tympani c spine (fig. 1 F) . Superiorly, the squamous temporal bone provides the most superior attachment of the pars flaccida of the tympanic membrane. Observation s. The anterior and posterior walls are best visualized in th e axial projection (figs. 18 and 1 C), while the corona l plane is well suited for visualization of the roof and the inferior wall (figs . 28-20). The posterior tympanic spine can be visualized on the axial view as a sharp projection extending anteriorly at the junction of th e middl e ear and external canal (fig . 1 F). The axial projection also provides excell ent visu alization of the relations between the mandibular fossa and th e external aud itory canal.

Middle Ear Anatomy. The middle ear is a narrow cavity separating the inner and extern al ear. This flattened rectangular chamber is oriented along the same oblique plane as the temporal bone. The many structures that traverse this space plus the irregularity of its inner wall add to its overall complexity. The middle ear is divided into the mesotympanum or tympanum proper (the region of the middle ear cavity directly contig uous with the tympanic membrane) , the epitympanic recess , and the hypotympanum . The ossicles for the most part reside within the epitympanic cavity, with the

Fig . 1.-Abbreviations. AA = ad itu s ad anlrum C = cochlea CA = coc hlear aq uedu ct CC = carot id ca nal CCO = cru s communis CN = coc hl ear division of eighth nerve CP = cochleariform process EAC = ex tern al auditory canal ER = fossa inc udis and epitympani c recess ET = eustac hi an tube FC = fac ial ca nal FI = fovea te impression FIN = fossa inc udis FR = facial recess G = gen iculum I = inc us lAC = intern al aud itory canal IMJ = incudomall eal joint LSC = lateral semici rcular canal M = malleus MA = mastoid antrum

MF = mandibular fossa OW = oval w indow PE = pyramidal em inence PL = posterior incudalligament PSC = posteri or sem icircu lar canal PTS = posterior tympanic spi ne RW = round w indow nic he SC = sem icanal SEP = septum separating th e semicanal from the eustachian tube SM L = superi or mall eal ligament SPS = sphenopetrosal sync hondrosis SSC = superi or semi circ ular canal ST = sinus tympani SU = subicu lum TM = tympanic membrane TS = tympanic spine V = vestibule VA = vestibular aqueduct VN = ves tibu lar nerve

AJNR: 3, July / August 1982

CT OF TEMPORAL BONE

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F Fig. 1 .-A-G . A, Coronal section of right temporal bone shows levels of ax ial views in B-O . Level A is most inferior. (Fig . 4 provides a more complete anatomiC representation .) B and C, Leve l A, live patient, passing through basal turn of cochlea. Trefoil appearance formed by round window niche , sinus tympani , and facia l recess. Air-filled bony part o f eustach ian tube is directed anterom edially parall el to carotid ca nal. 0 and E, Level B, live

G patient, also passes through basal turn of cochl ea. Depressions along posteri or aspect of tympanic cavity not as rounded as in B , but appear more fl attened and shallow. F and G, Level C, live patient. passes through ossicular processes and ova l window. V-shaped lucency formed by int ernal audi tory canal and ca nals for two divisions of eighth nerve . Oval window appea rs as breach in continuity of otic capsule.

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AJNR:3, July / Au gusl 1 982

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fac ial ca na l. Circ ular lucency in petrous bone form ed by lateral semic irc ular canal posterolatera!ly. Vestibule and c ru s communis anterom edi ally. Facial nerve passes anterol aterally dorsal to basal turn o f coc hlea and turns sharpl y posteri orl y at genu .

AJNR :3, July / August 1982

CT OF TEMPORAL BONE

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