Photon-counting CT (PCCT) and energy-integrating CT (EICT) system offer differing image quality attributes, but it is uncertain if such attributes manifest themselves differently across phantom vs patients. This study investigated if image quality assessment with a multi-sized phantom can predict patient population trends in PCCT and EICT. Under IRB approval, 56 patients underwent clinical CT on a PCCT and four EICT scanners from two vendors (A, B). Additionally, the Mercury Phantom 3.0 (GAMMEX Sun Nuclear), representing different adult population sizes, was imaged with the same conditions and scanners. Modulation Transfer Function 10% frequency (MTF-f10), Global Noise Index (GNI), and Noise Power Spectrum average frequency (NPS-fav), peak frequency (NPS-fpeak), and 10% frequency (NPS-f10) were evaluated in both patients and Mercury Phantom (MP). The related percentage differences between PCCT and EICT images were assessed. Median differences between patients imaged with PCCT and EICT-