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Optimization of variable temporal averaging in digital fluoroscopy

¹ Regional Medical Physics Department, Newcastle General Hospital, Newcastle-upon-Tyne NE4 6BE, UK and ² Department of Radiology (Medical Physics), School of Medicine, Complutense University of Madrid, 28040 Madrid, Spain

In modern X-ray fluoroscopy systems, the amount of temporal averaging (i.e. persistence) applied to the image is often user selectable. The objective of this work is to quantify the effect of variable temporal averaging on the detection of low contrast test objects moving at a range of known speeds within the digital fluoroscopic image. An image intensifier system with a short-persistence television camera was used to record image sequences of a moving threshold contrast-detail diameter test object onto broadcast-standard U-matic videotape. The image sequences were replayed through an image processing system allowing different amounts of temporal averaging to be applied. The test images were scored by an experienced observer. The temporal averaging time constants produced by the added image processing were measured using a method based on noise correlation. Results are presented showing the trends of threshold contrast with test detail diameter and movement speed. The optimum value of temporal averaging time constant is presented as a function of detail diameter for a range of speeds. By comparison with the limited information available in the literature on organ movement, it is tentatively concluded that for the organ movement speeds expected in the abdomen, the optimum imaging system persistence time constant should be approximately 0.15 s. For the much greater speeds associated with cardiac motion no additional frame averaging, i.e. just the persistence provided by the observer's visual system, appears to be optimal.