A method for total x-ray imaging system evaluation: Application to a microangiographic detector for neurovascular procedures.

摘要

Detector characterization with the 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 the Modulation Transfer Function (MTF), Noise Power Spectrum (NPS), Noise Equivalent Quanta (NEQ) and Detective Quantum Efficiency (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, the geometric configuration, 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 for no magnification or scatter. We evaluate 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 system under clinically relevant x-ray spectra and x-ray scatter conditions. Realistic focal spots, beam energies, and detector exposures were used, and the effects of different scatter fractions resulting from changing the beam size or the detector-to-patient airgap were investigated. The ideal detectability and the detection probability for a 2 Alternative Forced Choice Experiment (2-AFC) were calculated under the same conditions, for clinically relevant objects, such as small blood vessels, and stent struts. The objects were simulated inside the uniform human head equivalent phantom. The patient (or phantom) entrance Dose Area Product (DAP) required for a 75% object detection probability was calculated, taking into account the system parameters and limitations.