Flat panel detector

A flat-panel detector for X-rays ( solid state detector ) is generated in digital form, a flat, modern X-ray detector, the X-ray images.

Construction

Indirect solid-state detectors convert the incoming X-ray radiation by means of a scintillator into visible light. Including a semiconductor is usually made ​​of amorphous silicon ( a-Si) from which the integrated circuit is realized. Per pixel, there is a capacitor, a thin film transistor ( TFT) and a photo diode (TFD ). The photodiode converts light into electrons. The capacitor stores charge and this by means of the transistor of each pixel may be read individually. The transformation of the X-rays into visible light results in indirect solid-state detectors to diffusions degrade the sharpness and resolution of the images.

To counteract structured scintillators are used, which consist of many individual scintillators with about 5-10 microns in diameter. Thereby, similar to fiber optic cables, the dispersion decreases. As thicker scintillator may be used so that the efficiency of the detectors is improved.

Direct flat panel detectors use instead of the scintillator and the photodiode only one sensitive to X-ray photoconductor that generates the arrival of photons charges, which are then sucked out with electrodes. The amount of charge of a pixel is proportional to the incident radiation.

The photoconductor consists mostly of amorphous selenium, which has a high sensitivity to X- rays and a very good spatial resolution. On the selenium layer, an electric field is applied. Caused by the irradiation electrons and holes diffuse in the direction of the applied field. In direct conversion, there is practically no dispersion, since the charges move perpendicular to the surface of the selenium layer, and in the direction of the electric field. The readout electronics is very similar to that of the indirect solid-state detector.

Reading the TFT array

In the capacitor, the charges are stored which are generated in proportion to the incident X-rays. If the line on the gate -controlled gate potential is set high, the TFT turns on and the pixels of a row can be read. By amplifiers and multiplexers the image is read out row by row and digitized (ADC). This process is referred to as active matrix readout.

Pixel size and spatial resolution

Since the read-out mechanism is integrated in the detector itself, the spatial resolution is directly on the structure -dependent ( as opposed to X-ray imaging plate, where the selection process is decisive), and thus limits from each other by the size of the individual pixels (TFT, capacitor ) and their distances. Typical values ​​are 2.5 to 3.6 lp / mm (line pairs per millimeter ), at a detector element size of 139-200 microns and Matrizengrößen between 2000x2000 and 3000x3000 pixels. Special applications, however, require higher resolutions and thus smaller image elements that need to be 100 microns and smaller. The to be calculated by the size and spacing of the pixels resolution, there is only the theoretical maximum value. The effective value is lower due to the above component properties ( scattering, etc.). Direct Solid- state detectors are due to the low scattering very close to the maximum value. The resolution is also affected by the geometric fill factor. The percentage of the sensitive area in a pixel element is referred to as space factor.

Efficiency

The efficiency of a detector is the ability of X-rays to capture and is also known as DQE ( Detective Quantum Efficiency) respectively. The higher the DQE, the lower the dose of the radiation must be used. The DQE is dependent on several factors, among other things, by the detector itself, the quality of the radiation, the dose, and the spatial frequency. Solid-state detectors have a higher DQE than based on footage detectors at high doses. However, at lower doses and higher spatial frequencies, the DQE deteriorated. Studies have shown that solid-state detectors in comparison with the same dose, a higher image quality and lower dose to achieve comparable image quality. This allows up to 50% lower doses are used. Furthermore, the investigation time is less in comparison to X-ray film techniques by 50 to 68%.

Formats

Solid-state detectors are available in small sizes for dental radiology and in large format ( 20 × 20 cm to about 43 × 43 cm) for conventional radiology available.

Costs and distribution

Due to the high cost ( at least 300,000 euros, according to Siemens Webzine ) of the equipment, largely image plate systems and existing X-ray film systems are still in use. Over the last 3-4 years, however, the solid-state detector technology has prevailed more and more against the imaging plates at new acquisitions. For example, were in the United States from 1998 to 2003 still about 2500 image plates based devices compared to approximately 750 solids based reinstalled, but from 2003 to early 2007, only 750 to more than 1500 devices.

Special Applications

In Austria, in early April 2006, the first solid-state detector was built into a bus. This bus is used for tuberculosis and After screening checks for the province of Lower Austria. The major innovation is the first use of a solid-state detector, resulting in many advantages over X-ray result on X-ray film or X-ray imaging plates.

Conclusion

Solid-state detectors are very beneficial because of their technology. Despite lower doses of radiation can be obtained similar images. Also, saved by the integration of the readout mechanism, a lot of time and material. The fact that the images are digitized directly, even archiving is simple and space -saving. A disadvantage, however, on the one hand the high cost, on the other hand also involves a disadvantage of integrating, as this X-ray apparatus and patient must be equipped with an expensive detector system. Memory films such as can also be read later. Also, there are no studies about the durability of the systems.