Fundamental imaging performance in terms of Modulation Transfer Function (MTF) was investigated for a high resolution CMOS based imaging sensor and a commercial Computed Radiography (CR) unit. The CMOS device consists of a 33.91 mg/cm2 Gd2O2S:Tb scintillating screen, placed in direct contact with a CMOS photodiode array. The CMOS photodiode array, featuring 1200×1600 pixels with a pixel pitch of 22.5 μm , was used as an optical photon detector. The MTF was measured using the slanted-edge method (measuring the Edge Spread Function–ESF), as well as by using pin-hole phantoms (measuring the Point Spread Function–PSF). The experimental procedure was performed under the representative radiation quality (RQA) settings, RQA 5 (70 kVp digital radiography) and RQA M2 (28 kVp digital mammography) recommended by the International Electrotechnical Commission Reports 62220-1 and 62220-1-2 respectively. It was found that the detector response function was linear for the exposure ranges under investigation. MTF for the pinhole method was found to increase by diminishing pin-hole size up to the detector pitch (22.5 μm ). Additionally, our results showed that for the same RQA quality, MTF was comparable in the whole spatial frequency range by both ESF and PSF (50 μm ) methods. MTF of the CMOS imaging sensor was found better compared with the CR unit as well as compared with previously published data for other CCD and CMOS sensor.