What is Photon Counting in X-ray Imaging?
Inside a photon counting detector, the resulting charge distribution of each single interaction between incident X-rays and the active detector material is read-out separately. This enables photon-counting technology to extract energy resolved information on a per-interaction basis.
How is Photon-Counting different from Charge Integration?
In charge integrating detectors, the cumulative charge distribution per exposure window is read out per pixel. This resulting signal is an intensity signal, comparable to the brightness in a black-and-white photography. In a photon counting detector, the energy of each incident X-ray photon can be compared to one or more energy levels. This results in a spectral image, comparable to a colour photography.
Figure 1 shows a random distribution of X-ray photons around 40 kV.
Figure 1: X-rays with random energy arriving over time
For each photon an integrating detector would see a corresponding charge that scales with the energy of the photon. During the exposure time all these charges are accumulated. The plot in figure 2 shows this accumulation over time. For simplicity, it is assumed that the charge is linearly equal to the energy.
Figure 2: Integrated Signal of the distribution in Figure 1. For simplicity, the signal for each photon is assumed to be its energy.
When using a photon counting detector, it is possible to set one or several thresholds. These thresholds correspond to energy levels of the incident radiation. The resulting charge cloud of each X-ray photon is compared to these thresholds and sorted into energy bins accordingly.
Figure 3: The same X-ray distribution as in figure 1 with a threshold (red line) at 40 kV. All photons below the threshold are depicted in light blue, all photons above the threshold are depicted as dark blue.
Figure 3 shows one exemplary result. Here, one single threshold is used at a corresponding photon energy of 40 kV. Only the photons above this threshold are regarded to be the signal of interest. All other photons are below 40 kV and therefore discarded. For each photon above the threshold, an internal counter is increased, which is depicted in Figure 4.
Figure 4: Counts of the photons in Figure 3 which are above the 40kV threshold. Each step corresponds to an increase of the counter value by one.
It is important to notice, that all this happens before one single frame is being read out.