Scanning-slit photon counting x-ray imaging system using a microchannel plate detector.

An experimental prototype of a novel photon counting x-ray imaging system was evaluated. This system is based on an "edge-on" microchannel plate (MCP) detector and utilizes scanning slit imaging configuration. The detector is capable of photon counting, direct conversion, high spatial resolution, controllable physical charge amplification, quantum limited and scatter free operation. The detector provides a 60 mm wide field of view (FOV) and its count rate is 200 kHz for the entire FOV. The count rate of the current system is limited by the position encoding electronics, which has a single input for all events from the entire detector, and incorporates a single channel ADC with 1 micros conversion time. It is shown that the count rate can potentially be improved to clinically acceptable levels using multichannel application specific integrated circuit (ASIC) electronics and multi-slit image acquisition geometry. For a typical acquisition time used in this study, the image noise was measured to be less than the typically acceptable noise level for medical x-ray imaging. It is anticipated that the noise level will be also low after the implementation of the ASIC electronics. The quantum efficiency of the detector was measured to be 40%-56% for an energy range of 50-90 kVp for MCPs used in this study and can be improved to > 80% using MCPs with the optimized parameters. Images of resolution and anthropomorphic phantoms were acquired at an x-ray tube voltage of 50 kVp. The value of contrast transfer function for the detector was measured to be 0.5 at a spatial frequency of 5 lp/mm. The intrinsic spatial resolution of the system is 28 microm FWHM and was limited by the accuracy of the time-to-digital conversion of the position encoding electronics. Given the advantages of the edge-on MCP detector such as direct conversion and physical charge amplification, it can potentially be applied to mammography and chest radiography.