Vestibular Aqueduct Measurements in the 45° Oblique (Pöschl) Plane

Abstract

Background and purpose: The 45° oblique (Pöschl) plane allows reliable depiction of the vestibular aqueduct, with virtually its entire length often visible on 1 CT image. We measured its midpoint width in this plane, aiming to determine normal measurement values based on this plane.

Materials and methods: We retrospectively evaluated temporal bone CT studies of 96 pediatric patients without sensorineural hearing loss. Midvestibular aqueduct widths were measured in the 45° oblique plane by 2 independent readers by visual assessment (subjective technique). The vestibular aqueducts in 4 human cadaver specimens were also measured in this plane. In addition, there was a specimen that had undergone CT scanning before sectioning, and measurements made on that CT scan and on the histologic section were compared. Measurements from the 96 patients' CT images were then repeated by using findings derived from the radiologic-histologic comparison (objective technique).

Results: All vestibular aqueducts were clearly identifiable on 45° oblique-plane CT images. The mean for subjective measurement was 0.526 ± 0.08 mm (range, 0.337-0.947 mm). The 97.5th percentile value was 0.702 mm. The mean for objective measurement was 0.537 ± 0.077 mm (range, 0.331-0.922 mm). The 97.5th percentile value was 0.717 mm.

Conclusions: Measurements of the vestibular aqueduct can be performed reliably and accurately in the 45° oblique plane. The mean midpoint width was 0.5 mm, with a range of 0.3-0.9 mm. These may be considered normal measurement values for the vestibular aqueduct midpoint width when measured in the 45° oblique plane.

Fig 1.

The vestibular aqueduct as seen on axial (A), coronal (B), sagittal (C), and the 45° oblique (Pöschl) (D) planes (arrows). It can be seen along its entire longitudinal length on the 45° oblique plane, but only partially on the other planes. It also appears wider on the axial, coronal, and sagittal planes, due to the oblique orientation of its cross-section relative to these planes, which may lead to overestimation of its width when measurement is made in these planes.

Fig 2.

CT image of the vestibular aqueduct in the 45° oblique plane. The midpoint of the vestibular aqueduct is identified, and a line (shown in black) is drawn perpendicular to its wall. The width is measured along this line.

Fig 3.

The vestibular aqueduct of a cadaveric temporal bone displayed in the 45° oblique plane (arrow), in a histologically processed microtome section (A) and in a CT image (B).

Fig 4.

A graph of the distance along a line drawn through the midpoint of the vestibular aqueduct (x-axis) plotted against CT attenuation in Hounsfield units at each point along this line (y-axis). The optimal percentage attenuation is denoted on the graph. Through radiologic-histologic correlation by using the cadaveric temporal bone specimen, the OPA was found to be 30%.

Fig 5.

Scatterplot showing comparison between the subjective and objective techniques. Each point denotes the midpoint vestibular aqueduct measurement made by using the subjective (visual assessment) technique (x-axis) plotted against that made by using the objective (modified full width at half maximum/OPA) technique (y-axis). The Pearson correlation coefficient is r = 0.566.