X-Ray Detector Arrays for Dental, Medical and Security Applications
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Background of X-ray imaging
The photographic film is the most widely used detection medium in X-ray imaging applications. It has been used since the discovery of X-rays at the end of the last century. But the principal disadvantage of the X-ray film is its low sensitivity due to the poor absorption. Only about 1 % of the incoming radiation is absorbed in the film. In addition the film needs to be chemical developed before it can be viewed. Major advantage of course is the fact that the film area can be large. This is necessary in medicine, where large organs or objects need to be imaged.
The prospects of digital X-ray imaging systems compared to the photographic film are the higher sensitivity due to an increased absorption and the avoidance of time and material consuming chemical processing. The image is immediately available on the monitor which allows real-time operation. Also the possibility to apply software image processing tools supports the user to analyse the image. For example the image can be coloured to identify interesting areas more easily.
Such systems have already been introduced on the market. The most common sensor concept is the silicon based CCD (charge coupled device). Other detector materials than silicon, such as gallium arsenide, cadmium telluride and cadmium zinc telluride, are now under extensive investigation for array applications. The advantage using these materials is a higher absorption coefficient resulting in a higher sensitivity compared to silicon devices. The higher performance may be used for obtaining images with higher signal to noise ratio. Alternatively, it may be used for reducing the radiation dose.
Detector arrays for digital X-ray imaging systems may be either of the linear or area array type. The former consists of one or a few rows of detector pixels and requires that the object of interest utilised must be scanned. Area arrays are two-dimensional filled with rows and columns of pixels. They require no scanning procedure. For example, for luggage inspection linear detectors are often used since one scanning direction is handled by the moving transportation band.
In contrast to the ultraviolet and the infrared wavelength range, focusing optics is not available for X-rays (or it is very expensive or cumbersome to use). Instead, a shadow image is taken, where the image necessarily is larger than the object to image. When real-time imaging is needed, fluorescent screens, sometimes combined with image intensifiers, can be used. Even in this case the quantum efficiency is low, and large X-ray doses have to be utilised, which may be hazardous to the patient.