Perception of fluoroscopy last-image hold

Last-image hold (LIH) is used in x-ray fluoroscopy systems as a convenience and dose savings feature. In the case of an image sequence, temporal filtering in the human visual system (HVS) reduces perceived noise. In the case of a constant, single image frame, this phenomenon is not present: the image looks noisier, and low-contrast objects disappear. Using low-contrast, stationary cylinder and disk phantoms in noise, perception of single frames are compared with that of conventional 30 acq/s continuous fluoroscopy (continuous). The dose of continuous is fixed at Q/acq, and the dose of single-frame presentations is varied in order to determine an "equivalent-perception dose" for a paired-comparison task. The equivalent-perception dose depends upon the shape and size of an object. As cylinder diameter increases from 1 to 21 pixels, the equivalent-perception dose decreases from 4.6 to 2.8 Q/acq. At equal equivalent-perception dose values, the relationship between cylinder and disk diameters are determined; a cylinder diameter of 10 pixels is roughly equivalent to a disk diameter of 20 pixels. For interventional angiography, an average equivalent-perception dose of approximately 3.5 Q/acq for a single-frame presentation is predicted. Thus processing by the HVS effectively reduces noise variance by a factor of 3.5, corresponding to an effective averaging time of 3.5 frames or 120 ms. Several variance reduction techniques are suggested to create an LIH frame having perception equal to the fluoroscopy sequence.