Design principles for advanced new low-dose photon-counting x-ray detectors for medical radiography

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X-ray imaging methods have been under development for over 100 years since the discovery of x rays by Röntgen in 1895, and the complete dependency of modern medicine on x-ray-based diagnostic radiography continues to motivate research to improve costs, efficacy, safety and the potential for screening and early diagnoses. However, there are known risks associated with exposure to radiation and it has been estimated that 1 in 200 of all cancer deaths can be attributed to diagnostic x-ray imaging. This critical need to produce better images with less radiation is the driving motivation for this proposal. The ability of a detector to produce high-quality images with low patient exposures is described by the detective quantum efficiency (DQE) of the detector. The DQE is a number between 0 and 1, expressed as a function of spatial frequency, giving performance relative to an ideal detector. Low-frequency DQE values describe how clearly large lesions can be visualized for a given exposure. High-frequency values describe visualization of small structures and fine detail. An improvement in DQE can be used either to increase image quality or reduce patient exposure, depending on the clinical preference. A new generation of detectors is currently under development that can measure the energy of x-ray photons for use in advanced imaging procedures including CT. While it is essential these photon-counting systems be designed to ensure low dose to patients, the tools required to develop optimal designs are primitive and inadequate for the task. As a consequence, we believe some detectors currently under have little chance of success, while others are not being developed to exploit the full potential of advanced applications. The full potential of these new detectors can only be achieved by developing the mathematical tools that relate physical design parameters and materials to the resulting image quality. We will build on previous contributions and the successes of Fourier-based linear systems to develop a cascaded-systems description of signal and noise as it pertains to photon-counting energy-discriminating detectors. We will introduce x-ray photon energy as an independent parameter in a multi-dimensional approach that will, for the first time, accommodate spatial correlations in image signals that result from energy-dependent x-ray interactions. It is these correlations that degrade the DQE of imaging systems, and are ultimately responsible for poor image quality requiring the use of higher radiation exposures to patients. Results from this research will provide the first means to develop optimal detectors for these advanced applications, and to assess and verify their potential to produce high-quality images while maintaining low levels of x-ray exposure to patients.