Generalizing the MTF and DQE to include x-ray scatter and focal spot unsharpness: application to a new microangiographic system.

Detector characterization with modulation transfer function (MTF) and detective quantum efficiency (DQE) inadequately predicts image quality when the imaging system includes focal spot unsharpness and patient scatter. The concepts of MTF, noise power spectrum, noise equivalent quanta and DQE were referenced to the object plane and generalized to include the effect of geometric unsharpness due to the finite size of the focal spot and the effect of the spatial distribution and magnitude of x-ray scatter due to the patient. The generalized quantities provide performance characteristics that consider the complete imaging system, but reduce to a description of the detector properties without magnification or scatter. We have evaluated a new neurovascular angiography imaging system based on a region of interest (ROI) microangiographic detector using these generalized quantities. A uniform head-equivalent phantom was used as a filter and x-ray scatter source. This allowed the study of all properties of the detector under clinically relevant x-ray spectra and x-ray scatter conditions. Realistic focal spots (0.8 mm nominal), beam energies (60-100 kVp), and detector exposures (0.8-2.3 mR) were used, and the effects of different scatter fractions (0-0.62) resulting from changing the beam size (0-100 cm2) were investigated. The generalized MTF and DQE were found to have very little dependence on the tube voltage and the detector entrance exposure. Magnification, with the focal spot used, results in a large decrease of the generalized DQE at higher frequencies (about 100-fold at 10 cycles/mm), but a significantly smaller decrease at lower frequencies. Scatter on the other hand, causes a constant drop in the generalized DQE (factor of 3 for scatter fraction 0.3) for all frequencies. Our results show that there are tradeoffs in the choice of the different system parameters; therefore this methodology of studying the imaging system as a whole could provide guidance in system design.