Abstract Photon-counting computed tomography (PCCT) is an important development in diagnostic imaging. It uses detectors that register individual X-ray photons and classify them according to their energy. This technology improves the visualization of fine details and provides spectral information that supports material differentiation. This article reviews its principles, advantages, applications, and challenges, emphasizing that image improvement and dose reduction depend on the examination protocol and diagnostic task. Keywords Photon counting; computed tomography; semiconductor detectors; spectral imaging; spatial resolution; radiation dose. Introduction Computed tomography measures X-ray attenuation from multiple angles to reconstruct cross-sectional images of the body. Photon-counting technology introduces a more detailed method of recording transmitted radiation, supporting improved images and diagnostic information. The first clinical system using this technology received clearance from the U.S. Food and Drug Administration in 2021. Physical Principles Conventional detectors use scintillators to convert X-rays into light and then into electrical signals, integrating the deposited energy over a specified period. Photon-counting detectors use semiconductor materials to convert photons directly into electrical pulses, allowing individual photons to be counted and classified into energy ranges. Image Quality Improvement and Spectral Imaging The detector design allows small detection elements to improve spatial resolution. Energy thresholds also help exclude most electronic noise. Spectral information enables the separation of contributions from specific materials, such as iodine, and the generation of virtual monoenergetic images, adding information beyond conventional anatomical imaging. Radiation Dose and Contrast Material Improved detection efficiency may allow reductions in radiation dose or contrast material in selected examinations while maintaining appropriate diagnostic image quality. However, the achievable reduction varies according to the patient and protocol; therefore, no fixed percentage applies to all cases. Clinical Applications Applications include coronary artery and stent imaging, as well as detailed assessment of the lungs, bones, and inner ear. Higher resolution may reduce blooming artifacts from calcifications and stents, while spectral images support assessment of iodine distribution and material differentiation. The benefits remain dependent on the examination type and clinical question. Technical Challenges Challenges include pulse pileup when photons arrive close together in time and charge sharing between detector elements, which may affect photon counts and energy estimates. High-resolution images also require greater processing and storage resources, and image noise may increase when very thin sections are reconstructed. Conclusion Photon-counting CT advances the integration of anatomical detail and spectral information. Effective use requires appropriate protocols and joint evaluation of image quality and radiation dose, ensuring that the technology’s capabilities meet the actual diagnostic need. Almustaqbal University – The First University in Iraq