547IMAGING & THERAPEUTIC TECHNOLOGY

Virtual Labyrinthosco-py: Visualization of theInner Ear with Interac-tive Direct VolumeRendering1

Bernd F. Tomandl, MD • Peter Hastreiter, Dipl.-Ing. • Knut E. W.Eberhardt, MD • Christof Rezk-Salama, Dipl.-Inf. • Ramin Naraghi, MDHolger Greess, MD • Urs Nissen, MD • Walter J. Huk, MD

Computed tomography (CT) is the modality of choice for detailed im-aging of the bony labyrinth. Usually, information about the complexthree-dimensional anatomic structures of the inner ear is presented astwo-dimensional section images. Interactive direct volume rendering isa powerful method for visualization of the labyrinth. Unlike other visu-alization methods, direct volume rendering enables direct visualizationof the bony labyrinth without explicit segmentation prior to the visual-ization process. Direct volume rendering was applied to visualizationof the structures of the temporal bone in five patients without patho-logic conditions and four patients with pathologic conditions. In allcases, clear representations of the bony labyrinth and the facial canalwere provided. Because standard CT examinations combined with in-teractive visualization based on direct volume rendering are used, themethod is fast and flexible. Therefore, this approach is applicable inroutine clinical work. Problems occur in patients with effusion in thetemporal bone because adjustment of imaging parameters for properdelineation of the target structures is difficult in this situation. How-ever, direct volume rendering can produce meaningful images of highquality even in these problematic cases. The term virtual labyrinthosco-py is suggested for visualization of the labyrinth by using direct volumerendering.

Abbreviations: FOV = field of view, MIP = maximum intensity projection, SSD = shaded surface display, 3D = three-dimensional

Index terms: Computed tomography (CT), image processing • Computed tomography (CT), three-dimensional • Computed tomography (CT),volume rendering • Data fusion • Ear, labyrinth, 213.12117

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1From the Division of Neuroradiology (B.F.T., K.E.W.E., W.J.H.), Computer Graphics Group (P.H., C.R.S.), Department of Neurosurgery(R.N., U.N.), and Institute of Diagnostic Radiology (H.G.), University of Erlangen-Nuremberg, Schwabachanlage 6, D-91054 Erlangen, Ger-many. Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received March 8, 1999; revision requested April 28 and receivedJune 2; accepted June 11. Address reprint requests to B.F.T. (e-mail: bernd.tomandl@stud.uni-erlangen.de).

©RSNA, 2000

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IntroductionFor detailed imaging of the temporal bone in pa-tients with fractures, congenital malformations,or osteodystrophies involving the inner ear, com-puted tomography (CT) is still the modality ofchoice (1,2). Typically, information about thecomplex three-dimensional (3D) anatomic struc-tures is presented as two-dimensional section im-ages. Multiplanar reconstruction is helpful forfurther evaluation but still provides only two-di-mensional images. In the past, it was difficult andtime-consuming to perform 3D visualization ofthe tiny structure of the inner ear on the basis ofhigh-resolution CT data (3,4).

Recently, direct volume rendering has receivedincreasing attention within the medical commu-nity (5–7) because it includes all of the image datain the visualization process. Thereby, it providessemitransparent views, thus giving a good impres-sion of the spatial relationships of all structures.So far, direct volume rendering has been appliedto visualization of the trachea, the colon, andvarious blood vessels. Depending on the targetstructures, names like virtual bronchoscopy (7) andvirtual colonoscopy (8) have been suggested. Ac-cording to this nomenclature, we suggest theterm virtual labyrinthoscopy for visualization of thelabyrinth by using direct volume rendering. Ow-ing to the tiny and complex structures of the in-ner ear, a fast and convenient analysis requireshigh-quality 3D representations and real-time ma-nipulation of the viewing direction and the colorand opacity values at any stage of the visualiza-tion process. These features are provided by in-teractive direct volume rendering.

Materials and Methods

PatientsImage data from five patients (three men, twowomen; mean age, 55 years) examined with spiralCT angiography for evaluation of an incidentallydiscovered aneurysm were used to reconstructhigh-resolution images of the temporal bone (seethe “CT Protocol” section). These patients hadno known affliction of the temporal bone and nohistory of hearing problems, dizziness, or othersymptoms related to the vestibulocochlear sys-

tem. Two female patients with tumors in the re-gion of the temporal bone (one chondrosarcoma,one schwannoma of the glossopharyngeal nerve)were also included in the study. For the investi-gation of congenital malformations, we used im-age data from a female patient with an enlargedvestibular aqueduct and a male patient with acomplex, Mondini dysplasia–like malformationof the inner ear.

CT ProtocolAll examinations were performed with a spiralCT scanner (Somatom Plus 4; Siemens MedicalSystems, Erlangen, Germany). In the five patientswithout pathologic conditions of the temporalbone, contrast medium was administered for CTangiography. The section thickness was 1 mm andthe table speed was 1 mm/sec, resulting in a pitchof 1. The axial images were reconstructed with ahigh-resolution algorithm in steps of 0.5 mm anda field of view (FOV) of 60 mm2. By using a 5122

matrix, the resulting voxel size was 0.5 ´ 0.12 ´0.12 mm. We used the same protocol to evaluatethe patient with chondrosarcoma.

The image data of the patient with schwannomaof the glossopharyngeal nerve were used for pre-operative therapy planning. Because fusion withmagnetic resonance (MR) images was envisaged,reconstruction of the section images was performedwith an FOV of 180 mm2. For image registration,a two-step procedure based on anatomic land-marks and consecutive automatic adjustment wasapplied.

The two patients with congenital anomalieswere scanned with a section thickness of 1 mmand a table speed of 1.5 mm/sec (pitch = 1.5).Reconstruction was performed with steps of 1mm and an FOV of 150 mm2 to allow compari-son of both sides.

PostprocessingAfter transferring the image data to a high-endgraphics workstation (Onyx Reality Engine II;Silicon Graphics, Mountain View, Calif), 3D vi-sualization based on interactive direct volumerendering was performed with a tool developedby the Computer Graphics Group of the Univer-sity of Erlangen-Nuremberg (9–11). By using thehardware capabilities of high-end graphics work-stations, semitransparent representations of the

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a. b.

c. d.Figure 1. Visualization process based on high-resolution spiral CT data on the structures of thetemporal bone. (a) Section image shows a volume of interest selected for 3D visualization (yellowsquare). (b) Anterolateral opaque 3D representation based on direct volume rendering shows theselected volume of interest. (c) Anterolateral view of the right labyrinth shows an intermediatestep, during which color and opacity values are adjusted interactively. (d) Anterolateral semitrans-parent view shows the bony labyrinth.

structures of the inner ear were calculated. Be-cause direct volume rendering considers all of theimage data, no explicit segmentation prior to thevisualization process was required. As a result ofthe hardware assistance, both color and opacityvalues were adjusted interactively to delineate allstructures related to the inner ear in real time.

After an appropriate setting for optimal delinea-tion of the target structures is defined, the colorand opacity table can be stored and used for fur-ther studies. Owing to the tremendous accelera-tion of the visualization process (Fig 1), the whole

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procedure, including the transfer of the data, wasperformed in less than 20 minutes. The standard-ized user interface ensured intuitive manipulationof any object in real time. This manipulation in-cludes so-called clip planes, which can be appliedto cut off disturbing structures virtually withinthe 3D representation. The software also allowsdistance measurements directly within the 3Dscene.

For documentation of the visualization results,high-resolution screen images of selected direc-tions of view were stored. The main target struc-tures were (a) the semicircular canals, (b) the am-pullae and vestibule, (c) the cochlea, (d) the inner

a. b.Figure 2. Direct volume rendering of a normal left labyrinth and facial canal. (a) Anterolateralview shows the relationship of the mastoid and labyrinthine segments of the facial nerve (arrows)to the bony labyrinth. (b) Anterolaterocaudal view shows the relationship of the bony labyrinth tothe facial canal.

auditory canal, and (e) the mastoid and labyrin-thine segments of the facial canal. In the cases ofpathologic conditions, additional screen imagesthat showed the best possible representation ofthe pathologic condition were stored.

ResultsIn the five patients without pathologic conditionsof the temporal bone, high-quality 3D visualiza-tion of the main target structures was possible(Fig 2). The selected volume of interest was largeenough to show the entire facial canal as it runsthrough the temporal bone. For optimal delinea-tion of the bony labyrinth, structures surroundingthe otic capsule were suppressed from visualiza-tion with clip planes (Fig 3).

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a. b.Figure 3. Direct volume rendering of a normal left labyrinthincluding clip planes, which suppress disturbing structures withinthe 3D representation. (a) Superior view clearly shows the semi-circular canals, ampullae, vestibule, cochlea, and inner auditorycanal (arrow). (b) Anterolateral view shows the turns of the co-chlea and the orthogonal semilunar canals. Arrow = inner audi-tory canal. (c) Anterolateral view shows the cochlea in detailand allows measurement of its diameter in a given clip plane.

c.

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In the patient with chondrosarcoma, there wasan effusion of the temporal bone, making identi-fication of structures difficult (Fig 4). In this case,a large volume of interest was required for in-

a. b.

c. d.Figure 4. Chondrosarcoma of the temporal bone. (a) Gadolinium-enhanced axial T1-weightedMR image shows a partly cystic, enhancing tumor (short arrows). There is effusion in the tempo-ral bone (long arrow). (b) Shaded surface display (SSD) image shows bone destruction in the apexof the pyramid (arrow). (c) Anterolateral view obtained with direct volume rendering shows thedestruction caused by the tumor (arrow). (d) Anterosuperior view obtained with direct volumerendering does not show the inner ear clearly due to effusion within the temporal bone. Still, it ispossible to see the mastoid segment of the facial canal (arrows).

spection of the tumor and its relationship to sur-rounding structures. To overcome this problem,clip planes are a helpful tool for suppression ofdisturbing structures.

In the patient with schwannoma of the glos-sopharyngeal nerve, data from MR imaging, which

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a. b.

c. d.Figure 5. Schwannoma of the glossopharyngeal nerve inside the jugular foramen. (a) Fusion of MR and CT im-ages shows a contrast-enhanced tumor (arrow) in relation to the skull base. (b) Fusion image after application ofclip planes shows that the tumor has been eliminated to provide a view of the enlarged jugular foramen (arrow). (c,d) Superoposterior (c) and posteromedial (d) semitransparent views show the spatial relationship of the inner earand the tumor. Interactive measurement of distances assists in therapy planning.

The images of the two patients with congenitalmalformations, which were reconstructed with awide FOV (150 mm2), resulted in 3D visualiza-tions of worse quality compared with those of theother patients. However, the reconstructed im-ages were still of diagnostic value (Figs 6, 7).

shows the contrast material–enhanced tumor, werefused with data from spiral CT, which shows theosseous structures. Thereby, the spatial relation-ship of the tumor and the temporal bone includ-ing the inner ear was clearly demonstrated (Fig 5).Despite use of a wide FOV (180 mm2), the 3Dvisualization based on direct volume renderingwas of good diagnostic quality.

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DiscussionIn 1993, Casselman et al (12) described use of ahigh-resolution MR imaging sequence for imag-ing of the inner ear. By using a 3D constructiveinterference in the steady state sequence andmaximum intensity projection (MIP), 3D visual-ization of the membranous labyrinth in livingsubjects was easily performed.

Before this development, it was very difficultto produce 3D images of the inner ear from im-aging studies in living subjects. For spatial under-standing of the complex anatomy of the temporalbone, including the inner ear, drawings in anatomybooks were the only source of information (13).

However, 3D visualization of the bony laby-rinth from CT data is difficult due to the tiny sizeof the structures and their location within thetemporal bone, which consists of many air-filledchambers. Therefore, it is almost impossible tovisualize the structures of the inner ear with anycommonly used technique without time-consum-ing explicit segmentation.

Volume Renderingversus Other CommonlyUsed Visualization TechniquesVarious visualization techniques have been appliedto produce 3D representations of the inner earbased on CT data. Reisser et al (4) and Frank-enthaler et al (14) used automatic segmentationtools to extract the target structures from the im-

a.

b. c.Figure 6. Enlarged vestibular aqueduct. (a) Section images (presented from superior [left] toinferior [right]) show an enlarged vestibular aqueduct (arrows). (b) Superior view obtained withdirect volume rendering shows the spatial relationship of the funnel-like enlarged vestibular aque-duct (arrow) to the other structures of the inner ear. (c) Posterior view obtained with direct vol-ume rendering shows the inside of the “funnel” (arrow).

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age data set. The method used by Frankenthaleret al (14) produced beautiful 3D images of thelabyrinth, the ossicles, and even vessels. However,a time-consuming process of several hours is re-quired for segmentation of the image data and re-construction of the geometric model. Therefore,it is difficult to apply this approach in routineclinical work. Conversely, direct volume render-

ing takes into account all of the information in-herent in the CT data and provides a semitrans-parent representation of all structures. Becausetime-consuming preprocessing is not required,the analysis focuses on the visualization processexclusively.

c. d.Figure 7. Complex deformity of inner ear structures. (a) Section images (presented from infe-rior [top left] to superior [bottom right]) show a small, irregular cochlea (short arrow); a short in-ner auditory canal (long arrow); a long labyrinthine segment of the facial nerve (arrowhead); anddeformity of the temporal bone. (b) SSD image (posterosuperior view) of the skull base showswinglike deformity of the petrous parts of the temporal bones (arrows). (c) Superior view obtainedwith direct volume rendering shows the deformities. (d) Anterior view obtained with direct volumerendering shows only one turn of the cochlea (arrow).

a. b.

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For a better understanding of our approach,direct volume rendering is compared with SSDand MIP, which are commonly used for 3D visu-alization.

Shaded Surface Display.—In SSD, user-selectedupper and lower thresholds are used to define aspecific range of Hounsfield units to be displayed.Owing to the nature of the CT data, the result isopaque surface representations. To enhance theimpression of depth, light effects producing shad-ows are included in the visualization. The draw-back is that only the first voxel within the selectedrange of Hounsfield units along every ray of sightis displayed. Therefore, all structures behind thatvoxel are invisible (15). In theory, it should bepossible to depict the structures of the inner earjust by selecting thresholds between the low valueof air and the high value of bone. However, ow-ing to partial volume effects, it is impossible tosuppress the bone structures surrounding the in-ner ear. With a lot of patience, it is possible tocreate 3D representations (Fig 8a) that requiretime-consuming work but are of little diagnosticvalue.

Maximum Intensity Projection.—The othercommon approach, which is based on MIP, doesnot require selection of any thresholds. MIP dis-plays only the brightest voxel along every ray ofsight. As a result, all darker voxels in front of orbehind a brighter voxel are not displayed. Thus,the depth information is lost, but some of thedensity information is preserved (15). Ambigu-ities easily occur because the displayed voxels arenot necessarily part of the target structure, andimportant parts of a structure may disappear withonly slight changes in viewing direction.

Obviously, the method is of no value for display-ing hypoattenuating structures like the labyrinthdue to the distribution of the data values withinthe structures of the temporal bone. However,the method is applicable for 3D display of the in-ner ear on the basis of high-resolution MR im-ages (Fig 8b) (12).

Interactive Direct Volume Rendering.—Incontrast to SSD and MIP, direct volume render-ing uses all of the information contained inside avolume, thus allowing production of more mean-ingful images. By assigning a specific color andopacity value to every attenuation value of the

a. b.Figure 8. Commonly used visualization techniques. (a) SSD image (anteromedial view) shows the inner ear. Onlyparts of the structures are seen. (b) MIP image (anterolateral view) from high-resolution MR imaging data showsthe inner ear.

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CT data, groups of voxels are selected for display.Depending on the selected opacity (high opacityproduces low transparency, low opacity produceshigh transparency), nontransparent images simi-lar to SSD images or semitransparent 3D repre-sentations providing views of all interior struc-tures of the temporal bone are possible (5–7).

The software developed by the ComputerGraphics Group of the University of Erlangen-Nuremberg is accelerated by hardware, thus tak-ing advantage of the blending and interpolationcapabilities of high-end graphics workstations.This feature allows real-time manipulation of anyvisualization parameter and guarantees high-quality 3D visualization of the inner ear (9–11).By interactively adjusting color and opacity val-ues, the transition between soft tissue and bonestructures is clearly displayed within the semitrans-parent representation, thus taking advantage ofthe partial volume effect. The use of clip planesallows one to cut off parts of the volume virtually,making visualization of the target structures moreconvenient. In addition, interactive measurementof point distances directly within the 3D sceneassists in therapy planning (Figs 3c, 5c, 5d).

For high-resolution visualization, as shown inour examples, sophisticated software in combina-tion with high-end graphics workstations is man-datory. Most of the volume rendering packagescommercially available today do not provide therequired high image quality.

Problems and LimitationsA major problem in 3D visualization is effusionwithin the temporal bone. To delineate the struc-tures of the inner ear and the facial canal, it is im-portant to have no objects of similar attenuationin the vicinity. Otherwise, it is difficult to suffi-ciently differentiate the target structures (Fig 4).Choosing a smaller volume of interest and apply-ing clip planes improve the results achieved withdirect volume rendering. In comparison, methodsbased on automatic segmentation perform poorlyin this situation.

Virtual labyrinthoscopy can be performed withdata from routine spiral CT examinations. Afterselection of an appropriate volume of interestwithin the section images (Fig 1), the whole visu-alization process is performed in less than 20 min-utes without any explicit segmentation. To pro-duce high-quality images, it is mandatory to usehigh-resolution CT images with the smallest pos-sible section thickness, reconstruction of overlap-ping sections, and a narrow FOV mainly contain-ing the target structures.

The relationship between the FOV and imagequality is demonstrated in two cases of congenitalmalformations (Figs 6, 7). The section images inthese cases were reconstructed with an incrementof 1 mm and an FOV of 150 mm2. The result wasdecreased quality of the 3D visualization.

If comparison of both sides is necessary, two ormore sets of section images with a different FOVhave to be reconstructed from the raw data. As arule of thumb, the quality of the 3D images in-creases with decreasing size of the voxels of CTdata.

Clinical Use ofVirtual LabyrinthoscopyIn our preliminary study, we demonstrated theusefulness of virtual labyrinthoscopy in cases ofcongenital malformations. By using interactivedirect volume rendering, the relationship betweenthe structures of the inner ear is understood moreeasily. The method is also useful for preoperativetherapy planning in patients with tumors of thetemporal bone. In these cases, the location of thetumor relative to other structures inside the tem-poral bone is conveniently analyzed within thesemitransparent representation. In the future, itshould be possible to integrate this visualizationapproach into surgical navigation systems, thusproviding the surgeon with additional orientationduring surgery in the area of the inner ear.

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ConclusionsVirtual labyrinthoscopy is a new approach to in-teractive 3D analysis of the inner ear. Major ad-vantages of this technique are that it is reliablyperformed with standard spiral CT examinationsand does not require explicit segmentation of theimage data. Because the whole procedure is per-formed within a very short time, it is suitable forapplication in routine clinical work. Problems oc-cur in patients with effusion in the temporal bonebecause adjustment of imaging parameters forproper delineation of the target structures is diffi-cult in this situation. However, direct volumerendering is the only technique to produce mean-ingful images of high quality even in these prob-lematic cases. The fast availability of a 3D repre-sentation as additional information after routineexaminations with spiral CT makes this methoda useful tool for further analysis of anomalies andtumors within the temporal bone.

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