Systems and methods for determining an extended low contrast detectability performance function for an operating range for a core operating mode of a radiographic imaging system using actual reconstructed images characterize the contrast performance of a radiographic imaging system over its operating range and for any patient size based on the off-line calibration, uses ordered pairs of flux index and contrast index for each scanned object to provide a contrast index for each protocol for each contrast set, and uses the ordered pairs of flux index and contrast index to determine an extended low contrast detectability performance function for the operating range of a radiographic imaging system. Extended low contrast detectability performance data compilation and methods of clinical use, and low contrast phantom configurations and methods of calibration are also disclosed.