Photon counting

Photon counting is a technique in which individual photons are counted using some single-photon detector (SPD). The counting efficiency is determined by the quantum efficiency and any electronic losses that are present in the system. Photon counting is a technique in which individual photons are counted using some single-photon detector (SPD). The counting efficiency is determined by the quantum efficiency and any electronic losses that are present in the system. Many photodetectors can be configured to detect individual photons, each with relative advantages and disadvantages,including a photomultiplier, geiger counter, single-photon avalanche diode, superconducting nanowire single-photon detector, transition edge sensor, or scintillation counter. Charge-coupled devices can also sometimes be used. Single-photon detection is useful in many fields includingfiber-optic communication,quantum information science,quantum encryption,medical imaging,light detection and ranging,DNA sequencing,astrophysics, andmaterials science. In radiology, one of the major disadvantages of X-ray imaging modalities is the negative effects of ionising radiation. Although the risk from small exposures (as used in most medical imaging) is thought to be very small, the radiation protection principle of 'as low as reasonably practicable' (ALARP) is always applied. One way of reducing exposures is to make X-ray detectors as efficient as possible, so that lower doses can be used for the same diagnostic image quality. Photon counting detectors could help, due to their ability to reject noise more easily, and other advantages compared to conventional integrating (summing) detectors. Commercial photon counting mammography machines have been produced. Although such systems are not widespread, there is some evidence of their ability to produce comparable images at lower doses than other digital mammography systems with flat panel detectors. Photon-counting computed tomography is another key area of interest, where the ability to discriminate between photon energies could significantly improve the ability to distinguish tissue types when reconstructing an image. The number of photons observed per unit time is the photon flux. The photon flux per unit area is the photon irradiance if the photons are incident on a surface, or photon exitance if the emission of photons from a broad-area source is being considered. The flux per unit solid angle is the photon intensity. The flux per unit source area per unit solid angle is photon radiance. SI units for these quantities are summarized in the table below.