How should low-contrast detail detectability be measured in fluoroscopy?

The relationship and precision of four methods for measuring the low-contrast detail detectability in fluoroscopic imaging were studied. These included the physical measurement of the accumulation rate of the square of the signal-to-noise ratio (SNR(rate)2), two-alternative forced-choice (2-AFC) experiments, sixteen-alternative forced-choice (16-AFC) experiments and subjective determination of the threshold contrast. The precision and sensitivity of the threshold contrast measurement were seen to be modest in the constancy testing of fluoroscopic equipment: only large changes in system performance could be reliably detected by that method. The measurement of the SNR(rate)2 is suggested instead. The relationship between the results of the various methods were studied, and it was found that human performance can be related to SNR(rate)2 by introducing the concept of the effective image information integration time (t(eff)). When measured for an unlimited observation time, it depicts the saturation of human performance in detecting a static low-contrast detail in dynamic image noise. Here, t(eff) was found to be about 0.6 s in 2-AFC tests and 0.3 s in 16-AFC tests.