Clinical Study
Eye (2009) 23, 1436–1441; doi:10.1038/eye.2008.258; published online 19 September 2008
Effects of scan circle displacement in optical coherence tomography retinal nerve fibre layer thickness measurement: a RNFL modelling study
C Y L Cheung1, C K F Yiu2, R N Weinreb3, D Lin1, H Li1, A Y Yung1, C P Pang1, D S C Lam1 and C K S Leung1,3
- 1Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, Hong Kong, PRC
- 2Department of Applied Mathematics, Hong Kong Polytechnic University, Hong Kong, PRC
- 3Hamilton Glaucoma Center, Department of Ophthalmology, University of California, San Diego, CA, USA
Correspondence: CKS Leung, Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, 147K Argyle Street, Kowloon, Hong Kong. Tel: +852 27623080; Fax: +852 27159490; E-mail: tlims00@ hotmail.com
Received 7 November 2007; Revised 17 July 2008; Accepted 17 July 2008; Published online 19 September 2008.
Abstract
Objective
To study the effect of optical coherence tomography (OCT) scan circle displacement on retinal nerve fibre layer (RNFL) measurement errors using cubic spline models.
Methods
Forty-nine normal subjects were included in the analysis. In one randomly selected eye in each subject, RNFL thickness around the optic disc was measured by taking 16 circular scans of different sizes (scan radius ranged from 1 to 2.5 mm). The RNFL profile in each eye was constructed with a mathematical model using a smoothing spline approximation. Scan circle (diameter 3.4 mm) RNFL measurements (total average, superior, nasal, inferior, and temporal RNFL thicknesses) obtained from eight directions (superior, superonasal, nasal, inferonasal, inferior, inferotemporal, temporal, and superotemporal) displaced at different distances (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 mm) from the disc centre were then computed by a computer program and compared to the 'reference standard' where the scan circle is centred at the optic disc. RNFL measurement error was calculated as the absolute of (RNFL thickness (displaced) – RNFL thickness (reference standard)).
Results
The respective mean average, superior, nasal, inferior, and temporal RNFL measurement errors were 2.3
2.0, 4.94.5, 4.13.8, 6.27.6, and 3.83.5 
m upon 0.1 mm scan circle displacement, and 12.111.4, 27.818.4, 21.718.6, 34.822.9, and 15.210.7 m upon 0.7 mm scan circle displacement. Significant differences of average and quadrant RNFL thicknesses were evident between centred and displaced scan circle measurements (all with P<0.001). RNFL measurement error increased in a monotonic fashion with increasing distance away from the disc and the change was direction-dependent.
Conclusions
RNFL measurement error varies with the direction and distance of scan displacement. The superior and the inferior RNFL measurements are most vulnerable to scan displacement errors, whereas the average RNFL thickness is the least susceptible. Obtaining a well-centred scan is essential for reliable RNFL measurement in OCT.
Keywords:
RNFL, OCT, scan circle displacement, cubic spline model, 3D RNFL profile, RNFL measurement error
