What is the difference between photon-counting CT and normal CT?

Differences Between Photon-Counting CT and Conventional CT

The evolution of computed tomography (CT) technology has led to significant advancements in diagnostic imaging. One of the latest breakthroughs is the introduction of photon-counting CT, which differs from the traditional, or conventional, CT in various ways. Here, we discuss the main differences between these two types of CT technologies.

Principle of Operation

  • Conventional CT: Uses energy-integrating detectors that measure the total energy of all the x-ray photons absorbed. The signal produced is proportional to the total energy deposited by the x-ray photons.
  • Photon-Counting CT: Directly counts each incoming x-ray photon and can measure the energy of each photon. This allows for better differentiation between different types of tissue and materials.

Image Quality and Resolution

  • Conventional CT: Generally produces good-quality images, but can be limited in resolving structures that have similar densities or in differentiating between types of tissues with slight differences in atomic numbers.
  • Photon-Counting CT: Offers improved image resolution and contrast by accurately detecting and measuring the energy of individual photons. This results in higher quality images with better tissue differentiation.

Radiation Dose

  • Conventional CT: The energy-integrating nature of traditional CT can sometimes require higher doses of radiation to achieve sufficient image quality for diagnosis.
  • Photon-Counting CT: Typically requires a lower radiation dose compared to conventional CT, as it is more efficient in its use of the x-ray photons that contribute to image formation.

Spectral Imaging Capabilities

  • Conventional CT: Limited in spectral imaging capabilities, primarily providing images based on the density and shape of objects.
  • Photon-Counting CT: Enables spectral imaging by distinguishing between photons of different energies, allowing for material decomposition and improved diagnostic information on tissue composition and function.


  • Conventional CT: Widely used in a variety of diagnostic applications across medical fields, including detecting fractures, tumors, and infections.
  • Photon-Counting CT: While also applicable across a variety of diagnostic scenarios, it shows particular promise in fields requiring high resolution and contrast, such as cardiac, breast, and neuro imaging, as well as in areas benefiting from spectral imaging, like material differentiation in kidney stones or plaque characterization in coronary arteries.
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