The detective quantum efficiency of fluoroscopic systems: The case for a spatial-temporal approach (or, does the ideal observer have infinite patience?)

The detective quantum efficiency (DQE) of an imaging system describes how well the ideal observer performs for a specific imaging detection task. While it can be calculated from measured image data and used to quantify system performance, it is rarely used for assessing fluoroscopic systems. This is primarily due to the effects of system tag. Lag results in a temporal averaging of image signals that reduces noise. As a consequence, the measured DQE of fluoroscopic systems having lag will be erroneously high relative to systems having less lag. This effect can be substantial, resulting in measures of the DQE that can be 15-40% greater than the "lag-free", DQE. The description of a spatial-temporal NPS and DQE is presented as a means of accommodating system tag. The spatial-temporal NPS has units mm(2)s and the spatial-temporal DQE is unitless. Using this generalized interpretation, the (conventional) spatial NPS and DQE are described as sections of the spatial-temporal NPS and DQE along the zero temporal-frequency axis. Calculation of spatial-temporal metrics requires determining an effective temporal aperture related to the temporal MTF. It is shown, both theoretically and experimentally, that the spatial component of the spatial-temporal DQE of a system operating in a fluoroscopic mode is the same as the conventional DQE of the same system operating in a radiographic mode under quantum-noise limited conditions.