Persistence of the video signal between TV frames can lead to anomalously good contrast-detail test results. This is practical paper describes a simple method of quantifying and correcting for the effects of persistence in fluoroscopy. A digital framestore was used to acquire temporally contiguous fluoroscopy images. Correlation of the variance between a nominal TV frame and successive frames was then measured via the correlation coefficient. Plotted against time, this function defines a time constant which characterises the propagation of variance from an arbitrary initial frame to later frames. A survey of 7 C-arm fluoroscopy units incorporating standard vacuum TV camera tubes (typically of Vidicon design) 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. From these measurements, a function was derived which corrects contrast-detail results obtained for a unit with a measured nominal time constant to the typical vidicon time constant of 0.06 s. The correction is shown to significantly reduce the spread of c-d results obtained over a range of temporal filtration settings.
Marshall NW, Kotre CJ. Measurement and correction of the effects of lag on contrast-detail test results in fluoroscopy. Physics in Medicine and Biology 2002; 47: 947-960.
Dr. John Kotre, NGH Unit, Newcastle upon Tyne. Tel: +44 (0)191-233-6161.
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