Quality control for on-line portal imaging systems

A Quality Control (QC) test suitable for routine daily use has been developed for video based on-line portal imaging systems. It is designed to test for acceptable performance in high contrast spatial resolution. A phantom consisting of five sets of high-contrast rectangular bars with spatial frequencies of 0.1, 0.2, 0.25, 0.4, and 0.75 lp/mm is used for the test. A critical frequency fc for which the square wave MTF is 50% is used as a 'go/no go' decision level for passing the test. Data obtained during a one month period were used to determine fc. The test is shown to provide an automatic and objective measure of the system's imaging performance and is a useful QC tool for routine clinical work.