A method for the determination of the two-dimensional MTF of digital radiography systems using only the noise response

We present a new method that enables the determination of the two-dimensional MTF of digital radiography systems using the noise response measured from flat-field images. Unlike commonly-used methods that measure the one-dimensional MTF, this new method does not require precision-made test-objects (slits/edges) or precise tool alignment. Although standard methods are dependent upon data processing that can result in inaccuracies and inconsistencies, this method based on the intrinsic noise response of the imager is highly accurate and less susceptible to such problems. A cascaded-linear-systems analysis was used to derive an exact relationship between the noise power spectrum (NPS) and the presampled MTF of a generalized detector system. The NPS was then used to determine the two-dimensional MTF for three systems: a simulated detector in which the "true" MTF was known exactly, a commercial indirect flat-panel detector (FPD), and a new solid-state x-ray image intensifier (SSXII). For the simulated detector, excellent agreement was observed between the "true" MTF and that determined using the noise response method, with an averaged deviation of 0.3%. The FPD MTF was shown to increase on the diagonals and was measured at 2.5 cycles/mm to be 0.086±0.007, 0.12±0.01, and 0.087±0.007 at 0, 45, and 90°, respectively. No statistically significant variation was observed for the SSXII as a function of angle. Measuring the two-dimensional MTF should lead to more accurate characterization of the detector resolution response, incorporating any potential non-isotropy which may result from the physical characteristics of the sensor, including the active-area shape of the pixel array.