Validity of the line-pair bar-pattern method in the measurement of the modulation transfer function (MTF) in megavoltage imaging



The measurement of the modulation transfer function (MTF) of an imaging device is a common requirement in evaluating radiographic detector performance. Such measurements are considered mandatory in detector development research, and may also be carried out as part of routine quality assurance (QA) checks of image quality. Traditionally, MTF measurement has been performed by imaging either a narrow slit or a sharp edge in order to generate a line spread function, whose Fourier transform provides the MTF on a near-continuous frequency domain. Much less commonly employed is the method of square-wave line-pair modulations, in which the modulation response to bar resolution targets contained in a bar pattern is used to estimate the MTF at discrete spatial frequencies. While the slit and edge methods offer advantages of accuracy and a well-known standardized protocol for measurement based on several decades of development, their major limitation is the difficult and time-consuming experimental setup that is necessary to ensure accurate measurements. On the other hand, the bar pattern offers the advantage of a quick, simple, and easy measurement without the need for a complex experimental setup, with the main disadvantages of the technique being a pseudo-normalization that may lead to an overestimated MTF, and corrections for removing higher-order frequency harmonics that require interpolating between discrete spatial frequencies. Therefore, bar patterns are traditionally used for qualitative imaging applications like detector QA in terms of relative and arbitrarily defined spatial resolution metrics, while slit and edge methods are preferred for quantitative MTF measurements. Compared to diagnostic x rays, MTF measurements using megavoltage x rays are further complicated by low x-ray attenuation and excessive Compton scattering. In this work, a method to measure the MTF of megavoltage x-ray detectors based on imaging square-wave line pairs with improved near-zero-frequency normalization was developed as an adaptation to previously reported methods. Monte Carlo simulations were used to identify an improved normalization condition with which the accuracy of the MTF determined from line-pair modulations could be enhanced considerably compared to previously used techniques. Slit, edge, and bar-pattern measurements were performed to obtain the MTF of commercial megavoltage imaging devices including portal film and electronic portal imaging devices. A comparison of the MTF measurements from the three techniques was used to ascertain the validity of the proposed bar-pattern method for accurate and reliable measurement of MTF for megavoltage imagers. Statistical analyses revealed no significant differences between the bar-pattern method and the standard slit and edge techniques, indicating very good agreement (mean difference within ±3%). These results indicated the potential for line-pair bar patterns to be used more effectively than in the past for traditional QA imaging as well as for quantitative MTF measurement in detector development research.