The modulation transfer function (MTF) of radiographic systems is frequently evaluated by the system's line spread function (LSF) using narrow slits. The conventional slit method requires LSF tail approximation, which is achieved by exponentially extrapolating the LSF tails beyond 1% of peak value. However, the estimated MTF at low frequencies from extrapolation may not reflect the true performance of the system. In this study, a monotone spline regression technique for LSF tail approximation is developed to improve the accuracy of MTF estimation at low frequencies. This technique is based on the underlying physical principles of the system response. The advantages of this technique are demonstrated with simulated examples of which the true MTFs are known. The application of this measurement technique is also demonstrated.