Calculation of the Modulation Transfer Function from the Square Wave Response Function data with an interactive curve fitting software
W.M.S.W. Hassan*
Department of Biomedical Physics and Bioengineering, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD (UK)
Manuscript received: May 19, 1999; revised September 27, 2001
Accepted for publication: December 31, 2001
Abstract
Two ways to calculate the modulation transfer function (MTF) of radiographic screen-film systems from the measured square wave response function (SWRF) data were investigated with an interactive curve fitting software. The measured SWRF data obtained by digitising a radiographic image of a bar pattern test object were fitted to a curve, and the fitted curve was used to calculate the MTF. Satisfactory MTF was obtained by using 12 terms in the calculation. A second version of the calculation included a correction for the normalization at 0.25 cycles/mm of the SWRF data. Measurements from a screen-film combination showed that the MTF of the first version was higher than the second by an average amount of 0.02 units for spatial range 0-3.5 cycles/mm, and on average the MTF of the first version was higher than the second by 10%. Both the SWRF data fitting and the MTF calculation were done within an interactive curve fitting software which made the calculation relatively easy to perform.
KEYWORDS: Medical radiography, modulation transfer function, screen-film system, square wave response function.