Diagnostic performance of reformatted isotropic thin-section helical CT images in the detection of superior semicircular canal dehiscence

Introduction

Superior semicircular canal (SSC) dehiscence syndrome, first described by Minor et al., ¹ is a pathology that implies vestibular and cochlear symptoms resulting from the communication of the SSC and the adjacent middle cranial fossa.

The symptomatology can be characterized by the presence of dizziness, autophony and hearing loss; vertigo may be induced and aggravated by loud noises (Tullio phenomenon) ² and/or pressure variations secondary to intense efforts or in the setting of the Valsalva maneuver (Hennebert’s sign). ³

The dehiscence of the SSC leads to the “third mobile window” (in addition to the oval and round windows), in which the mechanical stimulations (sound and/or pressure) that activate the cochlea may induce endolymphatic flow within the superior semicircular canal and the vestibular labyrinth, leading to dizziness.^4,5

Potential etiologies for SSC dehiscence include abnormal development of the ducts, congenital defects, chronic otitis, cholesteatoma, and fibrous dysplasia.^1,4,5

Physical examination signs to diagnose SSC dehiscence syndrome include vertical-torsional eye movements that align with the plane of the SSC evoked by sound and/or pressure stimuli.^6–8

Computed tomography (CT) imaging has an important role in the demonstration of the dehiscence of the SSC confirming the diagnosis of SSC dehiscence syndrome.^9,10 However, temporal bone CT accuracy depends on the availability of multidetector row CT capable of thin-section collimation with isotropic voxel allowing multiplanar reformation of the images.^11–14

Previous studies reported the sensitivity and specificity of CT imaging using 1-mm collimation or 0.5-mm collimation in the detection of SSC dehiscence.^9,12–14

However, significant differences have been found between autopsy-proven versus CT-detected SSC dehiscence.^15–17 It has been proposed that reformatted CT images oriented parallel (plane of Pöschl) and orthogonal (plane of Stenver) to the plane of the SSC may improve the sensitivity and specificity in detecting SSC dehiscence.^10–13

The aim of our study was to establish the diagnostic performance of 0.5-mm collimation temporal bone helical CT in the diagnosis of SSC dehiscence syndrome comparing the accuracy of reformatted images oriented in the standard axial and coronal planes, as compared with the accuracy of reformatted images oriented parallel and perpendicular to the SSC.

This information is fundamental for the radiologic diagnosis of this entity, particularly in cases where surgical correction of the abnormality is considered.

Material and methods

CT scans

All patients and controls underwent helical 0.5-mm-collimated temporal bone CT. Thin-section CT scans were obtained in the axial plane with a 64-row CT scanner (Brilliance CT, Philips, Best, The Netherlands) with the following parameters: tube voltage = 120 kV, effective mAs = 330, slice thickness 0.55 mm, spacing 0.3 mm, pitch = 0.7, field of view (FOV) = 180 mm, image matrix 768 × 768.

A volume between 3.5 and 4.5 cm was acquired through the temporal bone.

The acquisition time was 30 seconds. The raw data were reconstructed by using the bone algorithm.

Reformatted images of the left and right temporal bones of patients and controls were obtained from this volumetric isotropic dataset for the purpose of this study by using a dedicated workstation (Philips, Best, The Netherlands).

Standard axial and coronal images through the temporal bone were reconstructed at 0.1-mm increments.

Fifteen oblique reformatted images, perpendicular to the SSC (plane of Stenver), were obtained with a space gap of 0.1 mm (Figure 1). OPEN IN VIEWER

Figure 1.

Plane of Stenver. Orientation of the plane of reformation perpendicular to the superior semicircular canal demonstrated on the axial scout image.

Fifteen oblique reformatted images, parallel to the SSC (plane of Pöschl) (Figure 2), were obtained with a space gap of 0.1 mm to allow visualization of the SSC in its sagittal plane (Figure 3). OPEN IN VIEWER

Figure 2.

Plane of Pöschl. Orientation of the plane of reformation parallel to the superior semicircular canal demonstrated on the axial scout image.

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Figure 3.

Thin-section reformatted computed tomography image oriented in the plane of Pöschl demonstrating an intact superior semicircular canal (arrowhead) visualized entirely in its sagittal plane.

Results

The diagnosis of SSC dehiscence syndrome was established clinically on the basis of the characteristic symptoms and evidence of vertical-torsional eye movements in the plane of the SSC evoked by sound and/or pressure stimuli. There were 25 left ears solely affected and 17 right ears solely affected in the 42 patients. There were no cases of bilateral involvement of SSC dehiscence syndrome.

On CT, SSC dehiscence was defined by the absence of high-attenuation bone coverage on the reformatted images.

Diagnostic performance results for group A and group B images evaluation are presented in Table 1.

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Table 1.

Diagnostic performance results for group A images and group B images in the detection of superior semicircular canal (SSC) dehiscence in 42 patients clinically diagnosed with SSC dehiscence syndrome and in 42 control participants.

	Sensitivity	Specificity	AUC	Accuracy	TP	TN	FP	FN
Group A images	85.7%	100.0%	0.929	92.9%	36	42	0	6
Group B images	97.6%	100.0%	0.988	98.8%	41	42	0	1

Group A images evaluation demonstrated an AUC of 0.929 (sensitivity 85.7%, specificity 100%, TP 36, TN 42, FP 0, FN 6) with an overall accuracy of 92.9%.

Figure 4 shows a representative case of SSC dehiscence, demonstrated on the reformatted standard coronal image. OPEN IN VIEWER

Group B images evaluation demonstrated an AUC of 0.988 (sensitivity 97.6%, specificity 100%, TP 41, TN 42, FP 0, FN 1) with an overall accuracy of 98.8.

Representative cases of SSC dehiscence demonstrated on the reformatted images oriented in the plane of Stenver and in the plane of Pöschl are presented in Figure 4 and 5, respectively. OPEN IN VIEWER

Although the differences between the two AUCs were not statistically significant, evaluation of group B images alone showed an improved diagnostic performance over group A images alone.

In particular, for group A images, SSC dehiscence was correctly demonstrated in 36 out of 42 temporal bones, with six FN cases, whereas for group B images, SSC dehiscence was correctly demonstrated in 41 out of 42 temporal bones, with only one FN case.

Discussion

In our study, the accuracy of thin-section (0.5 mm) helical CT for the detection of dehiscence of the SSC was improved by using images reformatted in planes parallel (plane of Pöschl) and perpendicular (plane of Stenver) to the SSC (Figures 5 and 6). In a previous study, Branstetter et al. ¹³ suggested that these additional reformatted images are not routinely needed to diagnose SSC dehiscence, indicating that they should be performed only when the diagnosis remains uncertain after review of the standard coronal reformatted images. OPEN IN VIEWER

However, Branstetter et al. ¹³ acknowledged as a limitation of their study their failure to include an “oblique reformation only” group, an aspect that is indeed addressed by our study.

Moreover, our results demonstrate that these additional reformatted images alone, oriented in the parallel and perpendicular plane to the SSC, provide an accuracy of 98.8% compared to the accuracy of 92.9% of the standard axial and coronal reformatted images alone. Thus, these additional reformatted images alone provide an improvement of the accuracy in the diagnosis of SSC dehiscence syndrome, which results in six missed cases by evaluating the group A images compared to the only one missed case by evaluating the group B images.

The explanation of the reduced diagnostic performance of group A images relies on the well-known phenomenon of partial volume averaging, which affects mainly those images in which the roof of the SSC is oblique to the coronal plane of the section and therefore fills only partially the voxels that compose the image. ¹² Conversely, the effect of partial volume averaging can be reduced by using a parallel plane to the SSC for the visualization of the roof of the SSC, thus allowing virtually complete filling of the voxels that comprise the image.

Although these additional reformatted images require more time for the post-processing, no additional radiation or scanning time are necessary and the benefit in terms of improvement in the accuracy for the diagnosis is clearly demonstrated.

While SSC dehiscence syndrome can be diagnosed on clinical grounds, CT imaging is required to confirm the diagnosis and to localize the portion of the SSC that is dehiscent; this aspect of the diagnosis is particularly critical to confirm whenever surgical treatment is contemplated.^12,19,20

The surgical treatment consists in surgically obliterating the defect in the superior canal by using either a trans-mastoid or middle fossa approach.^17,21

Previous studies suggest that radiologic evidence of SSC dehiscence may be overestimated, indicating that a dehiscence at the arcuate eminence may be observed in asymptomatic patients.^14,16,17

It should be pointed out, however, that these studies are based on older scanning protocols with 1.0-mm-thick direct transverse and coronal images, or with 1.0-mm-thick reformatted images in the plane of Pöschl and Stenver.

Other studies, based on 0.5-mm collimation CT scan with orthogonal reformatted images, reported a higher prevalence of SSC dehiscence in comparison to the prevalence reported in a post-mortem study of 1000 temporal bones, confirming that advanced, thin-section, multirow CT scans improve the diagnostic accuracy of the technique in the detection of the SSC dehiscence. ¹⁹

Our results confirm that multislice temporal bone CT with 0.5-mm collimation and reformatted images to the planes of Pöschl and Stenver improves the specificity and the positive predictive value of the technique as reported in the literature.^12,13 Moreover, this study provides evidence that temporal bone CT with 0.5-mm collimation and reformatted images in the plane of Pöschl and Stenver alone provides the best overall diagnostic performance (accuracy 98.8%).

This study, as previous studies in the literature,^9,12–15,19 is limited by the lack of surgical confirmation of the CT diagnosis but, clearly, patients with negative radiologic findings are not likely to be considered for surgical exploration and, similarly, not all patients with a radiologic diagnosis and clinical evidence of SSC dehiscence choose to undergo surgery.

It was proposed that multidetector row CT imaging of SSC dehiscence could be superseded by digital volume tomography (DVT), also known cone beam computed tomography (CBCT), which provides fast image acquisition with relatively high three-dimensional resolution, and lower radiation exposure compared with multidetector row CT.^22–25

However, it should be mentioned that not all image data sets for DVT from different companies are equally accurate, and that the majority of the studies in the literature are based on in vitro examinations,^24,25 thus, further studies are needed to validate DVT in the detection of SSC dehiscence in a large sample population.

In conclusion, our results provide evidence that proper CT examination with 0.5-mm collimation and reformatted images into the Pöschl and Stenver planes improves detection of potentially treatable SSC dehiscence in patients with vestibular symptoms induced by sound and/or pressure stimuli.

DVT has the potential to be an excellent alternative to multidetector row CT for inner ear imaging with lower radiation to patients and equal or higher resolution depending on the unit used.