Abstract
Abstract: :
Purpose: Optic nerve diameter (OND) measurements are classically performed using A-scan mode of ultrasonography. The B-scan mode can also be used for this purpose, but is not believed to be as reliable as the A-scan. With the advent of 3-dimensional B-scan ultrasonography (3DUS) however, interactive sectioning of the scanned 3D-volume image block has made examination of coronal views possible. The purpose of this study is to determine if coronal scans can be used to measure the OND in normal adult human eyes. Methods: Adult human eyes were randomly selected to undergo 3DUS using the 3D i-scan Ultrasound Tomography System (Ophthalmic Technologies Inc. [OTI], Toronto, Ontario, Canada). Following instillation of a topical anesthetic (0.5% Proparacaine HCl) and with the eye looking straight at an overhead fixation target, the ultrasound probe was applied directly on the temporal sclera just outside the cornea (cursor up). Scanning was performed once a crisp image of the optic nerve shadow was obtained. Using the saved 3D-volume, the pathway of the orbital portion of the optic nerve from the disc and backward was closely observed. Coronal sections of the optic nerve yielding a configuration closest to a circle were chosen for measurement. We assumed that a perpendicular cut of the optic nerve would bear a circular shape. The longest diameter measured was presumed to be the OND. Distance from the point of measurement to the disc surface (DS) was likewise recorded. Coronal sectioning, as well as OND and DS measurements was repeated three times to assess intra-observer variability. Results: Twenty-three normal eyes of 13 male and 6 female subjects were randomly selected for inclusion in the study. Age ranged from 16 - 82 years. Mean OND was found to be 4.40 mm (range: 3.64 - 5.26), with a standard deviation of 0.54 mm. Mean OND was 4.53 mm (range: 3.76 - 5.26) in males, and 4.11 mm (range: 3.64 - 4.90) in females. The OND was not dependent on age. D varied from 1.44 - 6.83 mm, but was not correlated with OND. Conclusions: Coronal scans may be a tool for measuring the OND in human eyes. A prospective randomized study will be required to assess the accuracy of 3DUS-OND over the conventional A or B-scan methods of ultrasonography.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, S • imaging/image analysis: clinical • neuro-ophthalmology: optic nerve