Flat panel detector for X-rays

from Wikipedia, the free encyclopedia
Indirect flat image detector.jpg

A flat panel detector for x-rays ( solid state detector ) is a flat, modern x-ray detector that generates x-ray images in digital form .

construction

The indirect solid-state detectors convert the incoming X-rays into visible light with the help of a scintillator . Underneath is a semiconductor - usually made of amorphous silicon (a-Si) - from which the integrated circuit is made. There is a capacitor, a thin film transistor (TFT) and a photodiode (TFD) per pixel . The photodiode converts the light into electrons. The capacitor stores this charge and each pixel can be read out individually with the help of the transistor. By converting the X-rays into visible light, indirect solid-state detectors cause diffusions, which deteriorate the sharpness and resolution of the images.

To counteract this, structured scintillators are used, which consist of many individual scintillators with a diameter of approx. 5–10 μm. As with fiber optic cables, this reduces the spread. For example, thicker scintillator layers can be used, so that the efficiency of the detectors is improved.

Structured Scintillators.jpg

Instead of the scintillator and the photodiode, direct flat-panel detectors only use a photoconductor which is sensitive to X-rays and which generates charges when photons arrive, which are then extracted with electrodes. The amount of charge on a pixel is proportional to the incident radiation.

DirekterFlachbildDetektor.jpg

The photoconductor usually consists of amorphous selenium , which has a high sensitivity to X-rays and a very good spatial resolution. An electric field is applied to the selenium layer. The irradiation creates electrons and holes that diffuse in the direction of the applied field. With direct conversion, there is practically no scattering, since the charges move perpendicular to the surface of the selenium layer and in the direction of the electric field. The readout electronics are very similar to those of the indirect solid-state detector.

Photoconductor.jpg

Reading out the TFT arrays

The charges that are generated proportionally to the incident X-rays are stored in the capacitor. If the gate potential controlled via the gate line is set high, the TFT switches through and the pixels in a row can be read. The image is read out row by row and digitized (ADC) by amplifiers and multiplexers. This process is known as an active matrix readout.

TFTArrayauslesen.jpg

Pixel size and spatial resolution

Since the read-out mechanism is integrated in the detector itself, the spatial resolution is directly dependent on the structure (in contrast to X-ray imaging plates, where the read-out process is decisive) and is thus limited by the size of the individual picture elements (TFT, capacitor) and their spacing. Typical values ​​are between 2.5 and 3.6 lp / mm (line pairs per millimeter), with a detector element size of 139-200 μm and matrix sizes between 2000 × 2000 and 3000 × 3000 picture elements. Special applications, however, require higher resolutions and thus smaller picture elements that have to be 100 μm and smaller. However, the resolution to be calculated from the size and spacing of the picture elements only indicates the theoretical maximum value. The effective value is lower due to the above-mentioned component properties (scatter, etc.). Direct solid-state detectors come very close to the maximum value due to the low scatter. The resolution is also influenced by the geometric degree of filling. The percentage of the sensitive area in a pixel is called the fill factor.

Efficiency

The efficiency of a detector describes the ability to capture X-rays and is also known as DQE (Detective Quantum Efficiency). The higher the DQE, the lower the radiation dose used must be. The DQE depends on several factors, including the detector itself, the quality of the radiation, the dose and the spatial frequency. Solid-state detectors have a higher DQE at high doses than detectors based on film material. However, the DQE deteriorates at lower doses and higher spatial frequencies. Studies have shown that solid-state detectors achieve a higher image quality with the same dose and approximately the same image quality with a lower dose. This means that doses up to 50% lower can be used. In addition, the examination time is 50 to 68% less compared to X-ray film techniques.

Formats

Solid-state detectors are available in small formats for dental radiology and in large formats (from 20 × 20 cm to approx. 43 × 43 cm) for conventional radiology.

Cost and Distribution

In 2007, due to the high acquisition costs (at least 300,000 euros according to Siemens Webzine), image plate systems and existing X-ray film systems were still largely used. In the last 3-4 years, however, solid-state detector technology had become more and more popular over imaging plates for new acquisitions. z. In the United States, for example, between 1998 and 2003 around 2500 memory plate-based devices were installed compared to around 750 solid-state-based devices, but from 2003 to the beginning of 2007 only 750 compared to 1500 devices.

Special applications

In Austria, the first solid-state detector was installed in a bus in early April 2006. This bus is used for post- tuberculosis and preventive checks for the state of Lower Austria. The big innovation is the first-time use of a solid-state detector , which has many advantages over X-rays on X-ray film or X-ray storage plates.

conclusion

Solid state detectors are very advantageous because of their technology. In spite of lower radiation doses, comparable images can be obtained. In addition, the integration of the readout mechanism saves a lot of time and material. Because the images are digitized directly, archiving is also easier and saves space. On the other hand, the high costs are disadvantageous on the one hand, and the integration also has a disadvantage on the other hand, since it means that outpatient X-ray apparatus must also be equipped with an expensive detector system. Image plates e.g. B. can also be read in later. There are also currently no studies of the durability of the systems.

literature

Web links