Performance Evaluation of Digital X-ray Detectors

Abstract

The performance of X-ray detectors depends on the properties of the X-ray spectrum and the underlying technology of the detector, which together determine image quality. This thesis examines two widely used detector types, indirect and direct detectors, whose design and properties impose physical limitations on image quality. Measuring detector performance is essential for patient safety, which is why the use and sale of X-ray imaging systems are highly regulated. Patient safety is improved by minimizing the radiation dose needed to obtain a diagnosable image. Performance is evaluated to support market access and to verify constancy over time and safe clinical use of X-ray sys-tems. The thesis was conducted as a literature review of textbooks, international standards and measurement guidelines in the EU and the USA. The review focused on the fundamen-tals of medical physics, medical imaging, radiation detectors and performance evaluation methods. Detector performance is defined by three characteristics: contrast, noise and sharpness, which are affected by multiple environmental variables in the X-ray system. Understanding these variables is essential for selecting suitable performance metrics used to quantify de-tector characteristics. The review shows that performance can be evaluated using simple ra-tio-based metrics and more detailed frequency-dependent metrics. The metrics are calculat-ed from test images acquired without a patient in the field, typically using test objects. Typi-cal performance measurements include contrast-to-noise ratio for contrast, signal-to-noise ratio for noise and line pair measurement for sharpness. Computationally simple ratio meas-urements are often more relevant to imaging capability than absolute values. Noise Power Spectrum and Modulation Transfer Function provide a more detailed analysis across the de-tector’s spatial frequency range and are combined in the metric Detective Quantum Efficien-cy, which gives a comprehensive description of the detector’s performance.