Use of Wiener filtering in the measurement of the two-dimensional modulation transfer function

This paper presents a new method for the measurement of modulation transfer function (MTF) using Wiener filtering. The method, unlike conventional methods using slit or edge devices, allows the direct determination of the MTF in all directions at one step. An image containing a precise circular region is acquired and its Fourier transform is calculated. In the absence of any blurring, the Fourier transform yields a simple Bessel function. Because of the symmetry in the convolution theorem, the roles of the blurring function and object can be interchanged, allowing the blurring function to be recovered using a Wiener filter. We simulated this process to understand the effects of attenuation, signal-to-noise ratio, and circle size. Images were simulated containing a circular region and degraded by spatial domain blurring with a Gaussian convolution kernel and by additive Poisson noise. The determined MTF matches the expected MTF except for a slight high frequency overestimate due to noise aliasing, which can be compensated. This method allows one to easily measure the two-dimensional MTF, particularly in systems which have an asymmetrical point spread function such as computed radiography. The method can be used as a tool for quality assurance and for comparing the resolution characteristics of various digital radiography systems.