Technique of Multi-Planar CT Image Reconstruction for the Evaluation of Superior Semicircular Canal Dehiscence Syndrome

Abstract

Superior semicircular canal dehiscence (SSCD) syndrome is an unusual cause of vertigo that was first identified by Minor in 1998. The patients initially described by Minor presented with vertigo, oscillopsia, and/or dysequilibrium related to sound, changes in middle ear pressure, and/or changes in intracranial pressure due to an absence of the bony layer that normally covers the superior semicircular canal.1 Subsequent clinical studies have shown that the condition may lead to a variety of vestibular and/or auditory symptoms that mimic other otologic disorders. These symptoms include autophony, ear blockage or fullness, conductive hearing loss, pulsatile tinnitus, dizziness or vertigo with head movements, and general disequilibrium.2
 
 In a patient with the appropriate symptoms, the diagnosis of superior semicircular canal dehiscence syndrome rests on the identification of a dehiscence in the bone overlying the semicircular canal on coronal high-resolution temporal bone computed tomographic scans (white arrowhead, Figure 1).
 It must be emphasized that due to the low specificity of images taken at 1.0- and 1.5-mm collimation, current radiologic literature advocates the use of coronal reformatted images based on submillimeter (0.4 – 0.6-mm) collimated axial scans. It must also be stressed that although reformatted coronal images are sufficient for the radiologic evaluation of SSCD in most cases, oblique reformatted images in the Stenver and Pöschl planes are necessary in equivocal cases.3 When operative management becomes necessary, these views are indispensable for proper surgical planning.
 
 The images in oblique planes of reconstruction can be created on radiologic imaging software that allows multiplanar reconstruction (MPR) of the raw axial CT image data set. This software allows the original data set to be simultaneously viewed in the standard sagittal, coronal and axial orthogonal planes of orientation (Figure 2) and manipulated into non-orthogonal or arbitary planes of orientation (oblique and double-oblique).
 The images in this article were made using syngo CT (version 2010B) software (Siemens AG, Berlin and München). This software has a set of toggle buttons in the 3D task window that allow manipulation of the primary image data set (Figure 3).
 The orthogonal axial image at the level of the superior semicircular canal is used as the primary reference image (Figure 4). The image has been zoomed in and panned using the Zoom/Pan function to center the image on the superior semicircular canal to be viewed. With the Free View Mode activated, aligning the first reference line in the primary reference image to the long axis of the petrous bone and centered between the arms of the superior semicircular canal (white bar, Figure 4) creates a secondary reference image in the Stenver plane (Figure 5). Aligning the second reference line in the primary reference image to the short axis of the petrous bone and running through both arms of the superior semicircular canal (black arrow, Figure 4) creates the tertiary image in the Pöschl plane that allows visualization of the superior semicircular canal as a complete ring (Figure 6). As the superior semicircular canal may not be perfectly vertically oriented in the orthogonal axial reference image, adjustments in the vertical reference line of the secondary reference image in the Stenver plane to run along the long vertical axis of the superior semicircular canal are made (white bar, Figure 5). This will allow perfect visualization of the ring of the superior semicircular canal and any dehiscences of the overlying bone in the Pöschl plane (white arrowhead, Figure 6).
 Imaging of the inner ear structures in non-orthogonal planes of orientation are not usually provided to the clinician by radiology centers in the Philippines. With a small investment in time and effort at the radiology workstation, it is possible for the clinician to view the inner ear structures, especially the semicircular canals, in their actual anatomic planes. This will allow for the accurate diagnosis and managment of less common, but treatable causes of otologic symptoms.