We developed a new, interspersed, adaptive forced-choice method of general applicability, and used it to study perception in x-ray fluoroscopy. We measured detectability of low-contrast objects in noisy image sequences and determined x-ray dose levels for equivalent detectability of test (typically pulsed fluoroscopy at 15 acq/sec, hereafter called pulsed-15) as compared to reference (conventional fluoroscopy at 30 acq/sec, hereafter called pulsed-30). We interspersed reference and test in order to reduce effects of subject effort and attention. After 200 total presentations, we obtained absolute detectability of reference and test and an equivalent perception dose ratio (EPDR) for test as compared to reference. For this technically demanding application, implementation features such as real-time creation of noisy image frames and fast maximum-likelihood estimation of detectability were critical. We derived parameter uncertainties and proved applicability with Monte Carlo simulations and experiments. The interspersed, reference/test method lowered experimental standard deviations due to the removal of day-to-day variations in absolute detectability. Reliability of comparisons of subject response times was also improved. A variety of results in x-ray fluoroscopy has been obtained with this new method. Examples are a dose savings of pulsed- 15 as compared to pulsed-30, a saturation of the detectability response as one increases the number of frames in the display loop, effects of temporal filtering, and effects of motion.