Truncation errors in calculating the MTF of radiographic screen-film systems from the line spread function

The computed modulation transfer function (MTF) of a radiographic screen-film system may exhibit erroneous oscillations resulting from the necessary truncation of the measured line spread function (LSF). The slit method was extended to permit evaluation of the LSF to approximately 0.001 of its maximum value. This reduces the MTF oscillations to a negligible level. The maximum MTF deviation was determined as a function of the truncation. Results show that for LSFs measured in the usual manner (0.01 truncation) the maximum MTF deviation is from 0.04 to 0.07, depending on the screen-film system. A theoretical analysis to estimate the error due to truncation is described, and a correction procedure is developed.