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DOI:10.2214/AJR.04.1353
AJR 2006; 186:416-423
© American Roentgen Ray Society

Original Research

Digital Volume Tomography: Radiologic Examinations of the Temporal Bone

Carsten V. Dalchow1, Alfred L. Weber2, Naoaki Yanagihara3, Siegfried Bien4 and Jochen A. Werner1

1 Department of Otolaryngology, Head and Neck Surgery, Philipps University Marburg, Deutschhausstr. 3, 35037 Marburg, Germany.
2 Department of Radiology, Massachusetts Eye and Ear Infirmary, Boston, MA.
3 Department of Otolaryngology, Takanoko Hospital, Matsuyama Ehime, Japan.
4 Department of Neuroradiology, Philipps University Marburg, Marburg, Germany.

Received August 27, 2004; accepted after revision November 29, 2004.

 
Address correspondence to C. V. Dalchow (dalchow{at}med.uni-marburg.de).


Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. We evaluated the clinical applicability and the value of digital volume tomography for visualization of the lateral skull base using temporal bone specimens.

MATERIALS AND METHODS. Twelve temporal bone specimens were used to evaluate digital volume tomography on the lateral skull base. Aside from the initial examination of the temporal bones, radiologic control examinations were performed after insertion of titanium, gold, and platinum middle-ear implants and a cochlear implant.

RESULTS. With high-resolution and almost artifact-free visualization of alloplastic middle-ear implants of titanium, gold, or platinum, it was possible to define the smallest bone structures or position of the prosthesis with high precision. Furthermore, the examination proved that digital volume tomography is useful in assessing the normal position of a cochlear implant.

CONCLUSION. Digital volume tomography expands the application of diagnostic possibilities in the lateral skull base. Therefore, we believe improved preoperative diagnosis can be achieved along with more accurate planning of the surgical procedure. Digital volume tomography delivers a small radiation dose and a high resolution coupled with a low purchase price for the equipment.

Keywords: cochlear implant • CT • digital volume tomography • middle ear • middle-ear implant • stapesplasty • temporal bone


Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
In the diagnosis of diseases in the lateral skull base, detailed images with high resolution are mandatory for visualization of small pathologic processes. This requirement explains why CT, introduced in the 1970s, has completely replaced conventional radiodiagnostic techniques [1-3]. With the development of high-resolution CT in the 1980s, it became possible to recognize pathologic processes of the temporal bone in early stages. High-resolution CT has been the method of choice for a long time to assess conductive hearing loss, ear malformations, and destructive processes in the middle and inner ears [3]. It is possible to visualize soft tissues and bone structures because of the detailed display.

Further improvement and optimization of temporal bone imaging with higher resolution is desirable, especially if done without an increase in the radiation dose or additional time for data calculation. An imaging technique with these improvements would allow integration of this new technique into routine daily practice.

Digital volume tomography has been increasingly used in dental surgery [4, 5]. Early in the development of this technique, its value as a high-resolution technique was recognized in the precise planning of dental implantation [6]. It became evident that precise preoperative visualization with the help of 3D reconstruction of the operating field could be accomplished and complications avoided in cases with insufficient bone of the alveolar part of the maxilla or mandible or in the presence of ectopic teeth [4, 7].

Further advancement of panoramic tomography in dental surgery in 2000 led to the introduction of the digital volume tomograph 3DX multiimage micro CT (J. Morita Manufacturing Corp.). Using digital volume tomography, its high resolution and minimum section distance of 0.125 mm allow small pathologic changes to be visualized. With this method, axial, coronal, and sagittal images can be obtained at one examination as opposed to only axial sections with conventional CT. Additional sections in different planes can be reconstructed from the original data. These first dental examinations suggested that digital volume tomography in diseases occurring in the lateral skull base may prove helpful. With limited space requirements, low equipment cost, and a short data calculation, digital volume tomography appears helpful with its increased spatial resolution [8]. Because no experience with digital volume tomography has been reported in the lateral skull base, we evaluated this new technique on temporal bone specimens.

Previous diagnostic techniques such as high-resolution CT or pluridirectional tomography have been evaluated on anatomic specimens [9-11]. To answer these questions regarding clinical applicability and the value of digital volume tomography, we also began our investigation with temporal bone specimens. This allowed us to correlate the radiologic with microscopic findings during preparation of the specimens and to assess the results.


Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
The Accuitomo (J. Morita Manufacturing Corp.) (Fig. 1) was used to perform our examination. From 512 single images with reconstruction increments of 0.125 mm and a cylindric format, 3-cm-high and 4-cm-wide (transverse diameter) images were reconstructed. Routine otoscopic surgical procedures with the operating microscope were performed on temporal bone specimens after removal of excess soft tissues (Carl Zeiss) (Table 1). Subsequently, radiologic control examinations were performed with digital volume tomography.


Figure 1
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  		Fig. 1 —General view of digital volume tomograph (Accuitomo, J. Morita Manufacturing Corp.) with patient's examination chair, radiograph device, 4-inch (10-cm) image intensifier, and control panel integrated in right column.

 

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  		TABLE 1: Methods and Materials Used in Temporal Bone Surgery

 

After targeting the region of interest (ROI) with the help of target light beams, the examination was performed with 60 kV (X-ray tube voltage) and 2 mA (X-ray tube electric current). The temporal bone specimen was attached to a holding device (Temporal Bone Holder, Model Wuerzberg, Storz Medical) and placed on the examination chair in a way that the metallic parts of the device were outside the radiation beam (Fig. 2). As a consequence, the specimens were not examined in typical conventional planes. During the exposure, the X-ray tube rotated along with the opposite sensor in 18 sec and 360° around the center of a conical-shaped radiation beam that hits a 4-inch (10-cm) magnifying screen. With a focal spot of 0.5 mm, the distance from the source to rotational center was 33.5 cm and from the source to image intensifier, 63.5 cm. After acquisition of 512 single images (frames) with a resolution of 240 x 320 pixels with a PC (Pentium 4, Intel; 2.3 GHz) and a volume composed of single units (voxels) 0.125 mm3 in diameter, calculations of the ROI were performed (Table 2). The images of the temporal bone specimens were analyzed with special software (3DX Integrated Information System version 1.52, J. Morita Manufacturing Corp.).


Figure 2
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  		Fig. 2 —Holding device (Temporal Bone Holder, Model Wuerzburg, Storz Medical) with temporal bone specimen indicating positioning for digital volume tomography.

 

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  		TABLE 2: Specifications of the Accuitomo (J. Morita Manufacturing Corp.)

 

The data were displayed after reconstruction on a PC monitor in three planes with vertical orientation to each other and a minimal intersection distance and a minimal section thickness of 0.125 mm. The section angles, section thickness, and the intersection distance were changed at will. The individual structures could be selectively depicted; however, in addition, the sections in the axial, coronal, and sagittal planes were separately displayed and the results shown on a monitor. As an alternative procedure, the collected data were transmitted via the DICOM port and stored in variable picture formats or printed.

We initially examined 12 temporal bone specimens with digital volume tomography. We investigated the middle ear space, auditory canals, and the mastoid air cells in three planes vertical to each other. Image sections were selected to illustrate the oval window niche, stapes footplate, and cochlea with the labyrinth (Fig. 3A). By adapting the angle of the sections, we were able to show the ossicular chain in two different planes. We selected a plane that would illustrate the incus and stapes, the incudostapedial joint, and the stapes footplate together with the oval window niche (Fig. 3B). In a second plane, we showed the head and the handle of the malleus. In addition to the epitympanum, we delineated the facial canal (Fig. 3C). We showed the facial nerve canal in its entire course from the internal auditory canal to the stylomastoid foramen (Fig. 3D).


Figure 3
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  		Fig. 3A —Digital volume tomography images show general view of middle and inner ear of temporal bone specimen. Co = cochlea, ICA = internal carotid artery, JB = jugular bulb, MEC = middle ear cavity. Image shows MEC and inner ear with labyrinth and pneumatized temporal bone with incudostapedial joint. Vestibule and semicircular canals (yellow arrows) and facial nerve canal (black arrow) are seen. Co = cochlea, ICA = internal carotid artery.

 

Figure 4
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  		Fig. 3B —Digital volume tomography images show general view of middle and inner ear of temporal bone specimen. Co = cochlea, ICA = internal carotid artery, JB = jugular bulb, MEC = middle ear cavity. Image shows vestibule (yellow arrow) and semicircular canals, facial nerve canal (black arrow), stapes arch (red arrow), jugular bulb (JB), and long process of incus (white arrow).

 

Figure 5
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  		Fig. 3C —Digital volume tomography images show general view of middle and inner ear of temporal bone specimen. Co = cochlea, ICA = internal carotid artery, JB = jugular bulb, MEC = middle ear cavity. Image shows malleus head (green arrow), malleus handle (gray arrow), vestibule and semicircular canals (yellow arrow), and facial nerve canal (black arrow).

 

Figure 6
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  		Fig. 3D —Digital volume tomography images show general view of middle and inner ear of temporal bone specimen. Co = cochlea, ICA = internal carotid artery, JB = jugular bulb, MEC = middle ear cavity. Image shows entire facial nerve canal (black arrows) between ganglion geniculi (gg) and foramen stylomastoideum (fs) and vestibule and semicircular canals (yellow arrow).

 
For radiologic illustration of middle ear implants on digital volume tomography, an incus stapedotomy was performed with a titanium piston (K-Piston, 5.25 x 0.4 mm, Heinz Kurz GmbH), gold piston (Gold Piston with band hook, 5.5 x 0.6 mm, Heinz Kurz GmbH), and a platinum-band polytetrafluoroethylene (Teflon, DuPont) piston (Platinum-Teflon Prosthesis Type Schuhknecht, 5.5 x 0.4 mm, Audio Technologies). For an incus stapedotomy, the prosthesis was affixed to the loop of the long process of the incus and placed across a created perforation in the stapes footplate to the adjacent vestibule.

Three ossiculoplasties were performed with different implants. One tympanoplasty type 3 was done with an autologous incus (Fig. 4A) and with a partial implant made of gold (Bell Prosthesis, 2.0 mm, Heinz Kurz GmbH) (Fig. 4E). In another ossiculoplasty, a total implant made from pure titanium (titanium Total Implant, 7.5 mm, Spiggle & Theis) was shortened to 4.5 mm and used (Fig. 4D). For incus interposition, the patient's incus was removed, shaped with a diamond drill, and interposed between the handle of the malleus and head of the stapes. The partial prothesis was placed below the handle of the malleus and placed onto the head of the stapes. For the preparation, the Total Implant was placed onto the middle of the stapes footplate below the handle of the malleus. After a macroscopic and radiologic evaluation, a prosthesis dislocation was simulated. For this purpose, the total prosthesis made of pure titanium was repositioned from its optimal position in the middle of the stapes footplate onto the margin of the footplate, adjacent to bone structures next to the facial canal. Subsequently, a radiologic control examination with digital volume tomography was performed.


Figure 7
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  		Fig. 4A —Comparison of otoscopic views of ossiculoplasty with operation microscope (A, C, E) and radiologic control digital volume tomography images (B, D, F) of temporal bone specimen. Co = cochlea, EAC = external auditory canal, ET = eustachian tube, IAC = internal auditory canal, ICA = internal carotid artery, TM = tympanic membrane. Images show autologous incus interposition (green arrows), oval window niche (white arrow, A), round window niche (black arrow, A), and vestibule (yellow arrow, B). White arrow in B indicates long process of incus, gray arrow indicates malleus handle atop incus interposition.

 

Figure 11
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  		Fig. 4E —Comparison of otoscopic views of ossiculoplasty with operation microscope (A, C, E) and radiologic control digital volume tomography images (B, D, F) of temporal bone specimen. Co = cochlea, EAC = external auditory canal, ET = eustachian tube, IAC = internal auditory canal, ICA = internal carotid artery, TM = tympanic membrane. Images show titanium implant (blue arrows) (titanium Total Implant [7.5 mm], Spiggle & Theis), malleus handle (gray arrows), round window niche (black arrow, E) and vestibule (yellow arrow, F).

 

Figure 10
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  		Fig. 4D —Comparison of otoscopic views of ossiculoplasty with operation microscope (A, C, E) and radiologic control digital volume tomography images (B, D, F) of temporal bone specimen. Co = cochlea, EAC = external auditory canal, ET = eustachian tube, IAC = internal auditory canal, ICA = internal carotid artery, TM = tympanic membrane. Images show partial gold implant (red arrows), malleus handle (gray arrows), round window niche (black arrow, C), and vestibule (yellow arrow, D).

 

To evaluate the position of a cochlear implant, implantation was done on a temporal bone specimen. First, a superior mastoidectomy with preservation of the posterior wall was done. After a posterior tympanotomy with skeletonizing, the mastoid segment of the facial canal, round window niche, the incudostapedial joint, and the promontory were identified. A cochleostomy with opening of the scala tympani was performed. Subsequently, an electrode was inserted through the opening to a predetermined demarcated point in the cochlea. After evaluation of the radiologic findings, a comparison examination was made on the temporal bone specimen with the operating microscope.


Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
In the initial radiologic examination of the 12 temporal bones and subsequent preparation of the specimens, the external auditory canal, middle ear cavity with ossicular chain, cochlea, and the labyrinth and the mastoid air cells could be identified on high-resolution images. An analysis of the images showed no pathologic changes in the mentioned structures. Based on the radiologic findings, the preparation of the temporal bone specimens was made, comparing the microscopic findings with the initial digital volume tomography images. The anatomic relationships encountered during the preparation correlated with those obtained from the radiologic findings.

After the initial evaluation by digital volume tomography, the surgical dissection was performed and repeat digital volume tomography was performed. By inspection of the temporal bones with a stapes prosthesis inserted, it was possible to identify the loop of all three prostheses that were affixed to the long process of the incus (Figs. 5A, 5B, and 5C). The inserted titanium and gold prostheses could be shown to the level of the vestibule. The platinum-band Teflon piston was visualized to a level close to the stapes footplate. At the level of the stapes footplate and vestibule, no prosthesis was recognized (Fig. 5C). Blurring from artifacts stemming from implanted metal did not occur. Subsequently, the findings on digital volume tomography were compared with the findings of the operating microscope. All three prostheses were normally positioned in situ. The platinum wire of the platinum-wire Teflon piston terminated above the level of the footplate while the Teflon mantle extended further above the opening of the footplate into the vestibule. Consequently, the macroscopic results were in agreement with the findings seen with digital volume tomography.


Figure 13
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  		Fig. 5A —Artifact-free digital tomography images of temporal bone specimen. Co = cochlea, EAC = external auditory canal, IAC = internal auditory canal. Images of stapes piston show long process of incus (white arrow, A), stapes footplate, and tympanic segment of facial canal (black arrows). Titanium implant (green arrow, A) (K-Piston [5.25 x 0.4 mm], Heinz Kurz GmbH) and gold implant (red arrow, B) (Gold Piston [5.5 x 0.4 mm], Heinz Kurz GmbH) are recognizable reaching from incus into vestibule (yellow arrows).

 

Figure 14
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  		Fig. 5B —Artifact-free digital tomography images of temporal bone specimen. Co = cochlea, EAC = external auditory canal, IAC = internal auditory canal. Images of stapes piston show long process of incus (white arrow, A), stapes footplate, and tympanic segment of facial canal (black arrows). Titanium implant (green arrow, A) (K-Piston [5.25 x 0.4 mm], Heinz Kurz GmbH) and gold implant (red arrow, B) (Gold Piston [5.5 x 0.4 mm], Heinz Kurz GmbH) are recognizable reaching from incus into vestibule (yellow arrows).

 

Figure 15
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  		Fig. 5C —Artifact-free digital tomography images of temporal bone specimen. Co = cochlea, EAC = external auditory canal, IAC = internal auditory canal. Image shows platinum-band polytetrafluoroethylene (Teflon, DuPont) piston (blue arrow) (Platinum-Teflon Prosthesis Type Schuhknecht [5.5 x 0.6 mm], Heinz Kurz GmbH) at level close to stapes footplate. Vestibule (yellow arrow) and tympanic segment of facial canal (black arrow) are also visible.

 
Control examinations of the ossicular prosthesis on digital volume tomography likewise revealed no artifacts. The interpolated autologous incus was visible at the lower part of the tympanic membrane, adjacent to the handle of the malleus, and likewise at the recognizable footplate (Fig. 4B). The position of the gold implant could be identified on digital volume tomography. This implant likewise was positioned below the handle of the malleus on the stapes footplate (Fig. 4D). The proper position of the total implant made of pure titanium could be shown on digital volume tomography. The whole implant was recognized and the shaft of the prosthesis could be differentiated from the prosthesis head plate and foot (Fig. 4F).


Figure 8
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  		Fig. 4B —Comparison of otoscopic views of ossiculoplasty with operation microscope (A, C, E) and radiologic control digital volume tomography images (B, D, F) of temporal bone specimen. Co = cochlea, EAC = external auditory canal, ET = eustachian tube, IAC = internal auditory canal, ICA = internal carotid artery, TM = tympanic membrane. Images show autologous incus interposition (green arrows), oval window niche (white arrow, A), round window niche (black arrow, A), and vestibule (yellow arrow, B). White arrow in B indicates long process of incus, gray arrow indicates malleus handle atop incus interposition.

 

Figure 12
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  		Fig. 4F —Comparison of otoscopic views of ossiculoplasty with operation microscope (A, C, E) and radiologic control digital volume tomography images (B, D, F) of temporal bone specimen. Co = cochlea, EAC = external auditory canal, ET = eustachian tube, IAC = internal auditory canal, ICA = internal carotid artery, TM = tympanic membrane. Images show titanium implant (blue arrows) (titanium Total Implant [7.5 mm], Spiggle & Theis), malleus handle (gray arrows), round window niche (black arrow, E) and vestibule (yellow arrow, F).

 

Simulation of a dislocation of the titanium Total Implant was diagnosed on digital volume tomography. In this case, the titanium Total Implant was replaced during preparation out of its ideal position in the middle of the stapes footplate. As shifting of the implant foot onto the edge of the stapes footplate next to the tympanic segment of the facial nerve canal would lead to a conductive hearing loss and make an ossiculoplasty revision necessary, this preparation was made.

Digital volume tomography visualized the minimal change in implant positioning confirming the dislocation of the prosthesis. The electrode position of the cochlear implant (Nucleus 24 Contour, Cochlear Ltd.) was checked in the temporal bone specimen after insertion with the operating microscope through the cochleostomy into the basal turn of the cochlea. The implant was positioned with its third ring in the cochleostomy opening. The position of the intracochlear electrode was shown on digital volume tomography at different layers. The electrode in the cochleostomy could be seen with the major portion of the individual electrodes within the cochlea adjacent to the modiolus (Fig. 6). By analysis of different imaging planes, it was possible to identify all 22 intracochlear electrodes.


Figure 16
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  		Fig. 6 —Digital tomography image of temporal bone specimen shows intracochlear position of cochlear implant (Nucleus 24 Contour, Cochlear Ltd.) in basal turn (red arrow) of cochlea and cochleostomy (green arrow). Eight single basal electrodes are identified with this image in this section. MEC = middle ear cavity, EAC = external auditory canal.

 


Figure 9
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  		Fig. 4C —Comparison of otoscopic views of ossiculoplasty with operation microscope (A, C, E) and radiologic control digital volume tomography images (B, D, F) of temporal bone specimen. Co = cochlea, EAC = external auditory canal, ET = eustachian tube, IAC = internal auditory canal, ICA = internal carotid artery, TM = tympanic membrane. Images show partial gold implant (red arrows), malleus handle (gray arrows), round window niche (black arrow, C), and vestibule (yellow arrow, D).

 


Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The introduction of high-resolution CT has expanded diagnostic possibilities, especially for temporal bone surgery [1, 3]. With this diagnostic tool, pathologic changes can be recognized at an earlier stage compared with conventional radiodiagnostic methods. With high-resolution CT, the surgical approach can be planned and performed with precision [9]. The 3D visualization allows a detailed evaluation of important structures and simultaneously facilitates orientation for preoperative planning. On the basis of this examination technique, routinely performed with 1-mm sections, not all structures are displayed in detail. When 3D reconstruction of a larger region (obtained from data in the database) is performed, high-resolution display with only slight loss of detail occurs [2, 3, 9, 10]. To provide even more sufficient information, multisectional helical CT with reconstruction increments of 0.3 mm can be done. For highest resolution, a proportional increase in the radiation dose must occur. A distinctly higher resolution obtained with multisectional helical CT is possible with phase contrast-fluorescence tomography [12]. With this examination technique, small changes in the inner ear and labyrinth are shown. However, the radiation dose with this method, which is applied to modify and develop cochlear implant electrodes, is very high. Consequently, this technique is only used in the temporal bone laboratory.

The preoperative diagnosis before insertion of a cochlear implant electrode is of equal value for the postoperative assessment of the proper position of the electrode in the cochlea [13]. An exact diagnosis can prevent possible intraoperative complications. Furthermore, precise postoperative localization of individual electrodes in the cochlea with respect to their position to the modiolus contributes to the assessment of potential hearing improvement. Our examination of the temporal bone specimens shows that digital volume tomography will visualize even the small bone structures of the lateral skull base. Aside from visualization of the tympanic cavity and mastoid air cells, the cochlea and labyrinth are visualized. One advantage is the ability to change the section angle, section thickness, and intersectional distance after the examination to evaluate targeted individual structures. The detailed local resolution, confirmed already in dental surgery [8, 13-16], helps define the ossicles precisely. On the basis of a high resolution with a reconstruction increment of 0.125 mm, small structures such as the ossicles can be demonstrated in a way not always possible with other alternative methods.

Ongoing examinations have shown that the effective radiation dose of examinations with digital volume tomography, with its limited cone beam, has a very small value. A Rando woman phantom (UD-170A and UD-110S, Panasonic) has been used as a subject measuring the effective dose of various organs at the time of projection with digital volume tomography. Focusing the projection, for example, on the upper left molar teeth, the effective dose was calculated as follows: first, doses for the thyroid gland, lungs, esophagus, stomach, colon, liver, urinary bladder, breasts, ovary, testis, red bone marrow, bone surface, thymus, kidneys, small intestines, upper large intestines, skin, brain, eyes, and salivary gland were determined. Each value was corrected with its tissue factor and equivalent dose, and added together to obtain the effective dose. For examinations of the upper molar teeth, temporal mandibular joint, and the middle ear, the resultant effective doses were 6.3 µSv, 9.3 µSv, and 14.2 µSv, respectively. The obtained figures were approximately 1/300-1/100 of that of helical CT [8, 17].

Because of the flexibility of the examination, accompanied by the same quality, single and connected structures can be imaged in detail. It is possible to recognize an interruption of the ossicular chain preoperatively. On the basis of the anatomic location, the entire ossicular chain can be visualized in two different sections on digital volume tomography. Important details are depicted, allowing evaluation of the continuity of the ossicular chain to include the ability to exclude ossicular discontinuity. From the available images, section planes can be selected to illustrate the incus and stapes, including the incudostapedial joint and the oval window niche with the stapes footplate and malleus head and handle. In addition to the epitympanic recess, the bone portion of the facial canal is visible. A soft-tissue mass or erosion of the scutum can be excluded.

Visualization of the location and course of the bony canal of the facial nerve is accomplished by identifying the location of the nerve between the internal auditory canal and the stylomastoid foramen. This is of significance in questionable erosion of the facial canal with exposure of the nerve, diminishing the danger of injury to the nerve during surgery [18].

A further indication of temporal bone digital volume tomography is the analysis of middle ear implants. On digital volume tomography, the loops of all three prostheses, attached to the long process of the incus, could be recognized. The only unidentified bone part, obscured by the surrounding metal, is located within the prosthetic loop. The titanium and gold pistons are demonstrated up to the level of the vestibule. Artifacts impacting the in situ prosthesis were not encountered. Even with digital volume tomography, the proper evaluation of the position of the platinum wire Teflon piston is difficult. The prosthesis consists of a platinum wire that is shown by radiologic means. The Teflon mantle, enveloping the inferior aspect of the prosthesis, is not demonstrated radiologically. This part is 0.5 mm longer than the platinum wire, which means that with a 5.5-mm prosthesis, only 5.0 mm of the wire is shown radiologically. As a consequence, the in situ prosthesis in the temporal bone specimen is not completely visualized on digital volume tomography. The recognizable platinum wire, of which the loop is fixed to the long process of the incus, can be traced to the level of the footplate. The wire terminates about 0.2 mm above the footplate. Realistically, the prosthesis is 0.5 mm longer and the distance is added to the recognizable part. Therefore, the prosthesis can be judged, with a certain degree of certainty, as in situ positioned.

These observations, like the examinations of the gold and titanium pistons, are important in the differential diagnosis of a conductive hearing loss after stapes surgery. It is important to differentiate the possible dislocation of a stapes prosthesis from fixation of the incus and malleus or an incus necrosis. These are important findings because the subsequent operative strategy of revision surgery, associated with possible complications, depends on it. Such visualizations are significant not only for stapes surgery but also for tympanoplasty.

We used three different implants for an ossiculoplasty to be evaluated and visualized on digital volume tomography. In addition to an autologous incus prosthesis, we applied a gold partial prosthesis with a length of 2.0 mm and a 4.5-mm titanium Total Implant. The autologous incus implant was demonstrated to be in direct contact with the malleus handle and stapes head, without adherence to surrounding structures, on digital volume tomography. The radiologic results were confirmed by microscopic examination of the temporal bone specimens. The partial gold implant, recognized between the malleus handle and stapes head, was well illustrated. The structure of the partial implant was visualized artifact-free on digital volume tomography as was the titanium Total Implant, which was demonstrated between malleus handle and stapes footplate.

With digital volume tomography, autologous and alloplastic middle ear implants are well visualized with respect to their position and direction. By positioning a total implant at the edge of the footplate, a slight dislocation of a middle ear implant was simulated. Subsequently, digital volume tomography showed the dislocation of the prosthesis with a difference of 1 mm in location on the stapes footplate compared with its original normal position. In such a situation, a dislocated implant with adherence to the facial nerve canal would result in conductive hearing loss. These findings illustrate the value of digital volume tomography for the diagnosis of a persistent conductive hearing loss after ossiculoplasty.

Similar to the visualization of the middle ear implant position, cochlear implants can be evaluated with digital volume tomography. By using different planes, the position of the complete electrode within the cochlea can be visualized despite its snail-shaped configuration. In addition to evaluation of the intracochlear position of electrodes [13, 19], individual electrodes and their spatial positions in relation to the modiolus are illustrated. A more perimodulary electrode location extends the dynamic range and reduces the stimulation threshold of the implant as the spiral ganglion cells are located in the canal of Rosenthal [20].

Digital volume tomography in dental surgery has value in the diagnosis of small bone lesions. Experience with digital volume tomography in our temporal bone study has shown the value of visualizing and analyzing small pathologic changes in the lateral skull base. Digital volume tomography after ossiculoplasty with autologous and alloplastic middle ear implants showed the position of the prosthesis. The normal position of implants was differentiated from prosthesis dislocation, allowing the establishment of the cause of conductive hearing loss. Furthermore, the normal position of a cochlear implant was actually analyzed, allowing visualization of the position of individual electrodes in relation to the modiolus.

We believe our experience will extend the previous radiologic diagnosis of temporal bone abnormalities by digital volume tomography. The high precision of digital volume tomography will contribute valuable information in the preoperative planning for and potentially the prevention of intraoperative complications. Digital volume tomography combines the advantages of a small radiation dosage and high resolution with significant cost savings.


References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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O. Majdani, K. Thews, S. Bartling, M. Leinung, C. Dalchow, R. Labadie, T. Lenarz, and G. Heidrich
Temporal Bone Imaging: Comparison of Flat Panel Volume CT and Multisection CT
AJNR Am. J. Neuroradiol., August 1, 2009; 30(7): 1419 - 1424.
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