Abstract
The present theory underlying the concept of the modulation transfer function of magnification radiography MTFm, although mathematically consistent, raises fundamental questions in the practical application. It has not been demonstrated that, for objects of finite thickness, the exposing operation is a linear and isoplanatic process which can be described by a convolution integral in the spatial domain and a multiplication in the spatial frequency domain. In other words, it needs to be shown that OTFm satisfies the relation Sout( nu )=OTFm( nu )Sin( nu ) where OTFm( nu ) is the optical transfer function of magnification radiography, and Sin( nu ) and Sout( nu ) are the input and output Fourier spectra of an object, respectively. It is difficult to verify this relationship experimentally by means of ordinary diagnostic equipment, since important physical parameters, such as focal spot size, cannot be measured accurately. Therefore, experiments have been performed using a blood vessel phantom and a focal spot simulated by an 241Am source and a small lead aperture. From a comparison of theoretical and experimental data, it can be concluded that OTFm, and therefore also MTFm, are indeed transfer functions in the sense of the above equation.