Flat panel and photon-counting detectors may seem similar at first sight, but they work in very different ways. They both consist of a matrix of sensitive pixels which are made of layers of a sensitive material and electronics, but they collect information in very different ways.
A flat panel detector is used for example in hospitals. Its surface is covered in scintillating material. When it comes in contact with x-ray radiation, it converts it into visible light in an analog process. The pixel starts to glow. The next layer contains a photo diode, which converts the light into electrical signal.
The current created by the diode is accumulated in the capacitor, and the charge is measured by a digital voltmeter, which gives out digital information about the pixel. These values together form an image.
The first layer of a photon counting detector is a semiconductor sensor. When a particle hits its surface, it creates an electrical charge, which is then transferred to the electronics in lower layers.
It passes through an amplifier to a counter, which stores a complete amount of hits during exposure and ignores noise. To make sure the counter only receives relevant data, we can set a threshold for a certain signal strength. This allows us to select only the photons which generate the clearest image and uncover hidden details.
The main difference between a flat panel and photon-counting detector is the precision of output information. In a flat panel, the useful data and noise are collected. This results in a noisy image. In comparison, with the photon-counting method, we receive clean data and avoid noise completely.
Another disadvantage of flat panel detectors is their limited dynamic range. Just like a digital camera, flat panel image can be underexposed when the detector receives too little information. On the other hand, too strong of a signal will result in an overexposed image, just like when you attempt to take a photo against the sunrays. This is avoided in photon-counting detectors, because of their fully digital principle.
