Experimental quantification of frame-level active window duration and duty cycle in high frame rate x-ray detectors

Detector frame rate is commonly used as the primary metric to characterize the temporal behavior of x-ray detectors in medical imaging systems. However, frame rate alone specifies only the sampling rate of image frames and does not determine how x-ray interaction signals are temporally integrated to form the measured signal within each frame. In medical imaging applications, an important frame-level component of detector temporal performance is characterized by the actual x-ray active window and by the fraction of the frame time during which the detector is actively exposed to x-rays. These frame-level timing characteristics are crucial for assessing detector radiation dose efficiency, yet they are rarely measured experimentally.

To develop and experimentally validate a method for quantifying the frame-level temporal behavior of high frame rate x-ray detectors beyond frame rate alone, by directly measuring the actual x-ray active window duration and the detector temporal duty cycle during active x-ray exposure.

A rotating test object with sharp lead-foil edges was imaged using a high frame rate photon-counting detector over a wide range of nominal frame-rate settings up to 4500 fps. An independent laser tachometer was used to measure the angular velocity of the rotating object. Motion blur was analyzed using simple kinematic models that establish a direct relationship between the detector active window duration and the angular width of the blurred edge-spread function (ESF) in projection images. The active window duration was estimated from the measured angular blur width. The experimentally measured frame rate was determined independently from the angular displacement of recurring features between frames. The detector duty cycle was computed as the ratio of the measured active window duration to the measured frame time.

The experimentally measured frame rate closely matched the nominal setting over the full tested range, with a linear relationship characterized by a slope of 1.002 and $R^2=0.9999$ . In contrast, the actual x-ray active window duration was consistently shorter than the frame time and decreased with increasing frame rate. A linear dependence of active window duration on frame time indicated a frame-rate-independent inactive window of approximately 0.15 ms per frame. As a result, the detector temporal duty cycle decreased systematically from above 90% at low frame rates to below 40% at the highest tested settings.

Nominal frame rate alone is insufficient to characterize detector temporal performance, because reduced active window duration within each frame substantially lowers the detector temporal duty cycle at high frame rates, with direct implications for radiation dose utilization and efficiency.