A physics primer on photon-counting detectors in CT: Physics, signal formation, and performance.

Photon-counting detector CT (PCD-CT) is moving from laboratory development to clinical deployment. However, its threshold-bin data arise from a tightly coupled detector-readout chain that can be difficult to understand as a whole. These data are not a direct readout of photon energy, but the outcome of a sequence of physical and electronic processes that begins with x-ray interaction and charge creation and ends with threshold decisions applied to processed electrical pulses. Understanding this chain is essential for interpreting detector behavior, evaluating performance, and identifying the origins of spectral distortion, count loss, and instability under clinical operating conditions. This primer presents a physics-grounded framework for semiconductor photon-counting detectors in CT by organizing the discussion around a single causal chain: energy deposition, charge creation, charge transport, signal induction, pulse formation, and final event counting and multi-threshold energy binning. Within this framework, we show how charge sharing, detector pixel geometry, dead time, pileup, dark current, contact-controlled leakage, and operating conditions shape spectral response, count-rate performance, threshold stability, and reproducibility at clinical flux. By linking detector physics, waveform formation, threshold logic, contact physics, operating conditions, and practical performance characterization within one coherent framework, this primer aims to give medical physicists and imaging researchers a scientifically rigorous and operationally useful understanding of how photon-counting CT detectors work and what governs their performance.