#319 INNER EAR ANOMALIES: AN EVALUATION OF 3D RECONSTRUCTIONS Nedim Durakovic, Mary Beth Cunnane, and Daniel J. Lee

Introduction

Subjects: This retrospective study examined the temporal bone CT scans of 20 pa-tients (20 ears) who had undergone cochlear implant evaluation. Nine of the scans did not have abnormalities of the inner ear identified on the initial scan, while the other 11 scans had a variety of abnormalities that included cochlear hypoplasia, cochlear nerve aperture enlargement/hypoplasia, and semicircular canal hypopla-sia.Scanning Procedure: CT scanning was performed on one of two CT scanners. Some of the scans were obtained on a 4 channel multidetector CT (SOMATOM Plus 4 Volume Zoom; Siemens, Erlangen, Germany). Scans performed on this scanner were obtained using 0.5mm collimation at 320 mAs and 120 kVp. Recon-structions were performed at 0.5mm intervals with a FOV of 100. Pediatric scans performed on this scanner were altered to lower the radiation dose by lowering the tube current to 120 mAs. The remainder of the scans were obtained on a 40 chan-nel multidetector CT scanner (Volume Zoom; Siemens, Erlangen, Germany). These scans were acquired using 0.6mm collimation at 320 mAs and 120 kVp. Pediatric scans were dose adjusted using 120 kVp and 175 mAs. These images were recon-structed at 0.2mm intervals using a FOV of 100. In both cases the matrix size of the final images was 512 by 512. Reconstruction: Raw data from the CT scanner was processed with commercially available 3D reformatting software (Voxar 3D; Barco, Edinburgh, Scotland). Volume rendered views were created after limiting the field of view to the bony labyrinth. Sculpting tools were then used to isolate the internal auditory canal, cochlea and semicircular canals. Scan Evaluation: Reconstructions were reviewed by four different readers (radiology resident, first year neuroradiology fellow, and two Otolaryngology resi-dents). The four readers were blinded to the clinical data. The four readers inter-preted the 2D scans first with axial and coronal views. They evaluated three distinct structures: the cochlea, cochlear nerve aperture, and semicircular canals. Evalua-tions were scored by each reader on a five point scale (1. definitely normal, 2. likely normal, 3. unsure, 4. likely abnormal, and 5. definitely abnormal). Afterwards, 3D re-constructions were made available along with the 2D views and the same scoring system was used for the three structures.Statistical Analysis: Results from the 20 cases were recorded in an excel spread-sheet and averages were obtained for each reader and each structure to summa-rize the findings. These scores were then compared to those of a neuroradiologist who reexamined all of the scans and scored them as the expert reader. Further sta-tistical analysis was not done because of the small sample size.

Methods

Summary and Discussion

Results Results

We greatly appreciate the help of Leena Hamberg for her assistance with 3D reconstructions and the four readers for their time.

Acknowledgements

1 2

3

4

Reader 2D 2D/3D

1 67% 77%

2 62% 63%

3 67% 75%

4 72% 82%

TABLE 1: Percent of Abnormalities Correctly Identified

Structure 2D 2D/3D

Cochlea 67% 77%

Aperture 62% 63%

SCC 67% 75%

TABLE 2: Average Percent of Structures Correctly Identified

Objectives

Above: For each structure examined (cochlea, cochlear nerve aperture, and semicircular canals), 3D reconstructions improved the detection of anomalies. The greatest improvement was seen with anomalies of the cochlea and semicircular canals, while minimal improvement was seen at the cochlear nerve aperture.

Above: All four readers improved in agreement with the neuroradi-ologist after viewing the 3D reconstructions. Reader 1 and 2 were radiology resident and neuroradiology fellow while readers 3 and 4 were otolaryngology residents. For the otolaryngology residents the improvement was considerable often differing from the expert read by one point or less on the ranking scale.

With the addition of 3D recon-structions, all four readers had improved agreement with the expert reader

With the addition of 3D recon-structions, detection of mal-formations at the three struc-tures examined improved

In two cases of a severally malformed inner ear, 3D reconstruc-tions did not improve agreement with the expert reader

Agreement on minor malforamations improved with the addition of 3D reconstructions

- To develop a fast and reliable method of generating 3D reconstructions of the inner ear

- To quantify the value of 3D reconstructions in diagnosing inner ear anomalies

- To identify specific structures where 3D reconstructions aided in identifying malformations - To identify cases where 3D reconstructions improve reader agreement

- 3D CT reconstruction from 2D scans can be done rapidly and reliably:

- The addition of 3D reconstructions to 2D scans improved agreement between trainees and the expert reader:

- 3D reconstructions aided in reader agreement for malformations of the cochlea and semicircular canals:

- The addition of 3D reconstructions is most useful in improving certainty with minor malformations

Departments of Otolaryngology and Radiology, Massachusetts Eye and Ear Infirmary, Boston, USA Departments of Otology, Laryngology, and Radiology, Harvard Medical School, Boston, USA

References

Conventional Temporal Bone CT scan is a standard diagnostic tool for evaluating the inner ear anatomy of cochlear implant candidates. A variety of malformations can occur in the inner ear as outlined by Jackler and Sennaroglu (See Figure 1). CT scans are effective in identifying these malformations, including cochlear hypo-plasia, cochlear nerve aperture (fossette) hypoplasia, and semicircular canal hypo-plasia.

Several studies have pointed out the potential role of three-dimensional (3D) re-constructions of CT scans as a way of improving our understanding of inner ear anatomy. Potential uses for the reconstruction include the preoperative evaluation of individual patient anatomy as well as an educational tool for understanding com-plex anatomical structures.

Our aim is to contribute to this literature by quantifying the value of 3D reconstruc-tions. In this study, trainees examined 20 ears and ranked how certain they were about the degree of dysplasia. This was done on 2D scans and afterwards with the addition of 3D scans. Rankings were then compared to an expert reader’s.

Above: The four scans above illustrate a normal case. A) The cochlear aperture is seen posterior to the cochlea (C) in the axial view. B) The cochlea is also seen in the coro-nal view. C) The semicircular canal (SCC) and internal auditory canal (IAC) are seen in the axial view. D) The 3D reconstruction illus-trates all three structures.

Above: The 2D axial scan was suspicious for an underdeveloped semicircular canal. With the addition of the 3D reconstruction, two readers agreed with the expert reader in scoring the malformation as “definitely abnormal.”

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IAC SCC

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Above: An underseg-mented cochlea is seen in axial scan (A) and 3D reconstruction (B). A resident reader initially ranked the scan as “unsure,” but after the addition of the 3D view changed the ranking to “definitely abnormal.”

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Above: The 2D scan was suspicious for hy-poplasia of the cochlear nerve aperture. After ex-amining the 3D scans, the reader was able to rank the aperture as “definitely abnormal.”

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Right: A) The axial scan illustrates a cystic cochlea with poor visualization of the co-chlear nerve aperture. B) There is a cystic vestibule and a hypoplastic semicircular canal as well. C) On the 3D reconstruction the relationship between all three structures is seen with the hypoplastic aperture, cystic cochlea and vestibule, and only two devel-oped semicircular canals.

All four readers were able to identify the ab-normal structures on 2D scans and ranked them as “definitely abnormal.” The 3D recon-structions provided extra details about the anatomy, but did not lead a change in the agreement with the expert reader.

Figure 2: Normal Anatomy

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Figure 3: Case #1 Figure 4: Cystic Cochlea

Figure 5: Cochlea

Figure 7: Semicircular Canal

Figure 6: Cochlea Nerve Aperture

1. Jackler RK, Luxford WM, House WF. Congenital malformations of the inner ear: a classification based on embryogenesis. Laryngoscope 1987;97: 2–14.

2. Sennaroglu L, Saatci I. A New Classification for Cochleovestibular Malformations. Laryngoscope 2002; 112: 2230–2241.

3. Fatterpekar GM, Doshi AH, Dugar M, Delman BN, Naidich TP, Som PM. Role of 3D CT in the evaluation of the temporal bone. Radiographics 2006; 26 Suppl 1: S117-32. 4. Rodt T, Ratiu P, Becker H, Bartling S, Kacher DF, Anderson M, et al. 3D visualisation of the middle ear and adjacent structures using reconstructed multi-slice CT datasets, correlating 3D images and virtual endoscopy to the 2D cross-sectional images. Neuroradiol-ogy 2002; 44: 783–790.

5. Jun BC, Song SW, Cho JE, Park CS, Lee DH, Chang KH, Yeo SW. Three-dimensional reconstruction based on images from spiral high-resolution computed tomography of the temporal bone: anatomy and clinical application. The Journal of Laryngology & Otology 2005; 119: 693–698.

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BRight: The three scans illustrate a severe malformation. A) The coronal scan illustrates an underdeveloped semicircular canal. B) The cochlea is identified as cystic in the axial scan. C) On the 3D reconstruction the rela-tionship between all three structures is seen with a cystic cochlea and and only one fully developed semicircular canal.

As with the prior case all four readers were able to identify the abnormal structures with definite certainty on the 2D scans. With the addition of the 3D reconstructions, the initial ranking of “definitely abnormal” was not changed.

Figure 1: Sennaroglu Classification

Above: Illustration of the classification scheme developed by Sennaroglu for some of the inner ear malformations.2

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Our data thus far has demonstrated an association between the addition of 3D reconstructions and improved detection of inner ear anomalies. This is likely to have a greater impact for otolaryngologists than radiologists since radiology train-ing relies on generating 3D mental images from 2D scans. Further observations need to be made in order to evaluate a statistical significance of this relationship.

Our methods describe a rapid and reliable method of performing 3D reconstructions of the inner ear. The potential clini-cal or educational application of 3D recontructions requires that they are economical as well as reliable. Further valida-tion should be performed with a comparison to histopological sections.

Our study design with rankings of “likely abnormal” and “definitely abnormal” did not allow for a sensitive test of the added value of 3D reconstructions in cases of severe malformations. These malformations were obvious on the 2D scans and there was limited value of the 3D scan in these cases. In contrast this ranking system was helpful in the cases of minor malformations.

The 3D structures of the cochlea and semicircular canals were more obvious than those of the cochlear nerve aperture. Future reconstructions could examine the cochlear nerve canal seperately from the other inner ear structures.