Abstract
Purpose: :
To convert paper tonography tracings to digital files allowing curve fitting and other forms of analysis as well as stable longterm storage and convenient transfer of data.
Methods: :
Paper chart recordings are used to record the output of an electronic Schiotz tonometer during tonography. Data extracted from these records can be used to determine C, the facility of aqueous outflow. In this study, paper records were converted to digital color bitmap files using a flat bed scanner (Canon, LiDE 35) attached to a personal computer. A digital conversion of the tracing was produced using WinDIG 2.5 software and was stored as a computer file of approximately 1000 points of X,Y (Time,Schiotz Scale Reading) data. Data were analyzed using curve–fitting statistical analysis software (GraphPad Prism 4). As tonometry is based on charting the decay of pressure induced by application of the Schiotz tonometer, an attempt was made to fit the data to an exponential decay curve.
Results: :
17 tonograms previously evaluated by standard manual methods were selected for study, all were successfully digitized. Curve fitting as judged by the R2 value was better using an exponential decay model than with linear regression (mean R2=.8973 vs .8744, p <.05). C values calculated from the curve fitted using an exponential decay model were within .01 of the C value derived by traditional manual methods in 10 of the 17 cases, and within .03 in 15.
Conclusions: :
Converting tonographic tracings to a string of data points has several advantages and requires minimal equipment. Data stored in this fashion are not only more stable and more easily shared, they are also in a form ready for the powerful analytic software now available on a personal computer. This allows more objective and precise characterization of the data than by traditional visual assessment, as well as testing and comparison of various mathematical models.
Keywords: outflow: trabecular meshwork • image processing • clinical research methodology