A new paradigm in portal imaging QA: fast measurements of modulation transfer function (MTF) and detective quantum efficiency (DQE) using line-pair bar-patterns

Abstract

In portal imaging, the role of electronic portal imaging devices (EPIDs) to implement complex radiation therapy protocols is crucial, and regular quality assurance (QA) of EPID image quality is necessary to ensure treatment efficacy. The modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) are universal metrics that can completely describe radiographic image quality. These metrics are independent of measurement geometry and the user, and allow intra and inter detector performance evaluation. Though NPS and DQE calculations are straightforward, conventional MTF measurements using slit/edge phantoms are difficult and time consuming. Therefore, such measurements are not feasible within routine clinical QA. Currently, EPID performance is monitored using image quality indices obtained from commercial QA phantoms. Such methods may be qualitative or sensitive to imaging conditions, and hence not appropriate for rigorous QA. In this work, a simple and quick method for EPID QA is presented based on a line-pair bar-pattern for fast MTF measurement, coupled with standard NPS and DQE measurements, all of which can be carried out within two minutes. The method to determine MTF from line-pair modulations was developed based on theoretical analyses and Monte Carlo simulations to identify accurate and reliable measurement conditions. The bar-pattern based QA method was tested with two clinical EPIDs and found to be in excellent agreement with slit/edge measurements. It was also implemented for weekly QA checks, and compared with established commercial QA methods. This bar-pattern based QA was more sensitive to potential decrease of EPID image quality.