Imaging performance of a flat panel-based chest radiography system was evaluated using a recently introduced parameter: system detective quantum efficiency (DQE), i.e. DQE<sub>SYS</sub>. The DQE<sub>SYS</sub>calculation includes the signal to noise (SNR) transfer efficiency of the x-ray detector (detector DQE) and of the antiscatter device (DQE<sub>ASD</sub>). Posterior Anterior (PA) and bedside imaging techniques were evaluated using Poly(methyl methacrylate) (PMMA) thicknesses of 90, 130, 160 and 190 mm, equivalent to the lung and mediastinum regions covering a range of three patient sizes. Detector DQE was measured for beams without scatter using aluminum filters with similar half-value-layer (HVL) as the PMMA blocks. The grid efficiency (DQE<sub>ASD</sub>) was calculated from the primary and scatter grid transmissions for the four PMMA thicknesses. Acquisition settings were 120 kV (grid in) for the bucky PA technique and 105 kV (grid out) for bedside imaging. Results showed an increase in the DQE<sub>SYS</sub> for PA examinations with increasing PMMA thickness, opposite to the detector DQE. This can be attributed to the increasing efficiency of the antiscatter grid (i.e. DQE<sub>ASD</sub>) as PMMA thickness is increased, consistent with the expected result that grid use is important for the thicker patients. DQE<sub>SYS</sub> for bedside was lower than for PA, this is because no grid is used for bed examinations and DQE<sub>SYS</sub> reverts to detector DQE. DQE<sub>SYS</sub> was successfully used to evaluate the performance of the system in the presence of scatter radiation with the antiscatter device in place, results showed the importance of this type of devices for chest radiographies.