Measurement and correction of the effects of lag on contrast–detail test results in fluoroscopy

Abstract

Persistence of the video signal between TV frames, an effect also known as image lag, can lead to anomalously good contrast–detail test results for fluoroscopy systems. In this practical paper, a simple method is described which quantifies lag in fluoroscopy systems and corrects for its effect on threshold contrast. A digital framestore was used to acquire temporally contiguous fluoroscopy images. Correlation of the variance between an initial base TV frame and successive later frames was then measured via the correlation coefficient. Plotted against time, this function defines a time constant which characterizes the rate at which the initial variance pattern is replaced by incoming quantum noise. A survey of seven fluoroscopy units incorporating vacuum TV camera tubes found a mean time constant of 0.06 s.The relative change in contrast–detail performance was then measured as a function of applied digital frame averaging for two separate fluoroscopy units. A time constant was found for each frame averaging mode using the correlation of variance between frames. These measurements were used to derive a function which corrects contrast–detail results obtained for a unit with a measured nominal time constant to the typical vacuum camera tube time constant of 0.06 s. The correction is shown to significantly reduce the spread of contrast–detail results obtained over a range of temporal filtration settings.