Eric Laurence Gingold PhD, Presenter: Consultant, Agfa-Gevaert Group

A practical method for calculating the 2-dimensional MTF of digital imaging systems has been developed, and initial results presented.

Traditionally, MTF evaluation required sophisticated precision-machined tools with careful alignment required under laboratory conditions. The slit and edge MTF methods perform 1-dimensional assessment of imaging systems, while the 2-dimensional pinhole method is useful for focal spot evaluation but cannot characterize digital area detectors. The goal of this project was to utilize the ubiquitous star resolution test pattern, available to most medical physicists, to estimate the 2-dimensional MTF of digital imaging systems. Star pattern images are easy to obtain using a wide range of x-ray imaging technologies. A circular profile around the center of the star generates a square wave of a spatial frequency that depends on the profile's radius. A series of profiles at varying radius are obtained, and each square wave is analzyed to determine its fundamental spatial frequency and its modulation contrast amplitude. By repeating this process over a range of radii (spatial frequencies) up to the imaging system's Nyquist frequency, the square wave response function (SWRF) is obtained. The MTF is calculated from the SWRF by computing its Fourier Transform and taking the first harmonic at each spatial frequency.

The proposed method was tested on a variety of digital imaging technologies including storage phosphor, direct flat panel, indirect flat panel, image-intensifier based, and CT. By varying the source-object distance and object-image distance, the relative effects of geometric and detector unsharpness on system resolution were observed. The spatial resolution performance of various acquisition modes, imaging system designs and imaging technologies are compared.    

The proposed method has the potential to put the power of MTF system analysis into the hands of clinical medical physicists. Its primary attributes are simplicity, low cost, and convenience. The analysis software can be deployed on the Internet, in the form of a web-based service that requires only the uploading of star pattern images in standard formats.

Gingold, E, A Practical Method for Estimating the 2-Dimensional Modulation Transfer Function (2D-MTF) of Digital Imaging Systems Using a Star Resolution Test Pattern.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6011467.html