The modulation transfer function (MTF) is a fundamental measure of spatial resolution of an imaging system, and can be measured by imaging a slit, edge or a bar-pattern. In portal imaging, the MTF has been measured using the slit and edge techniques, requiring very thick collimation to minimize the effect of megavoltage scatter and laborious alignment procedures. A simpler and quicker method for measuring MTF is presented: the bar-pattern. This method has been successfully used in diagnostic imaging. In portal imaging, this method is sensitive to the measurement of MTF(0) due to lateral scattering of megavoltage x-rays. A lack of a precise measurement of MTF(0) can lead to an over-estimation of MTF. The slit and bar-pattern techniques were studied using Monte Carlo simulations on a kinestatic charge detector (KCD), which uses a slot photon beam and a scanning high-pressure gas multi-ion chamber. The experimental condition for measuring MTF(0) was determined. MTF measurements using the slit and bar techniques, as well as those from Monte Carlo simulations, were subsequently observed to be in good agreement (i.e. one standard deviation of measurement). The bar-pattern method, being easier and simpler than the slit or edge techniques, provides a fast MTF measurement.