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Experimental measurement of the modulation transfer function of differential phase contrast CT systems

This paper concerns an experimental method to quantify the spatial resolution of an experimental DPC-CT system via modulation transfer function (MTF) measurement. Note that conventional metal wire-based MTF measurement methods are no long applicable to DPC-CT, because: (i) The refractive signal generated by the high density metal is out of the dynamic range of DPC-CT, and (ii) A DPC-CT system is not sensitive to input pulse finer than a detector pixel. These technical challenges were overcome by a new experimental design, in which a graphite rod was used as the probe to simultaneously measure the MTFs of the DPC-CT and the associated absorption CT. An appreciable difference in MTFs between DPC-CT and absorption CT was observed in our experimental benchtop system: At the 10% MTF level, the MTFs of the DPC-CT and the absorption CT are 4.7 cycles/mm and 5.2 cycles/mm, respectively. Such a difference in MTF leads to the conclusion that the grating interferometer used by the DPC-CT system has a frequency-dependent response to input DPC-CT signals.