Purpose:
To describe a simple method of measuring anterior segment lesions from digital slit lamp images, and confirm reliability of the technique.
Methods:
All images where taken with a Sony DKC–5000 digital photo camera on a Haag–Streit BQ 900 slit lamp, imported into a Macintosh G3 computer running Adobe Photoshop 5.0. For system calibration, 10 reference photos were taken of a standard PD ruler at 16x magnification, refocusing on the ruler for each photo. Using Adobe Photoshop© the number of pixels per mm2 (area) and one mm (linear) were recorded for each photo. Descriptive statistics in MS Excel were used to calculate mean, standard deviation, and confidence interval from the reference data. An Excel spreadsheet was set up to convert pixels to mm and mm2. Interrater reliability was determined by two observers who independently calculated the area of 69 epithelial defects. A Bland and Altman plot was used to demonstrate the agreement between the two doctors.
Results:
From the reference photos the pixel mean per mm was 138 +/– 0.5. (The 95% confidence interval was 1.002 to 0.998 mm). The pixel mean per mm2 was 19,588 +/– 579 pixels (The 95% confidence interval was 1.02 to 0.98 mm2). Interrater reliability was excellent as measured by an intraclass correlation coefficient ICC(2,1) = 0.99. From the Bland and Altman plot it was determined that the values calculated by Provider 2 will fall between 27% below and 20% above that of Provider 1.
Conclusions:
This simple method, which allows accurate measurements from digital images using common off–the–shelf software, is a valuable tool for clinical documentation as well as research purposes.
Keywords: cornea: epithelium • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)