There are several different ways for generating digital X-rays available:
Storage phosphor system
Those systems are widely used today, due to the attractive price and reasonable image quality. These systems however require a 2 step process, where you first use a "standard" cassette as it was used in the film era and perform a normal X-ray exposure. In a second step the storage phosphor plate is unloaded to a laser scanner where the latent image is being read out and digitized. Image quality in terms of resolution is comparable to film/screen combinations whereas dynamic range is very much extended and superior to film.
Intensifying screen / CCD system
A cost-effective approach to an online X-ray detector is a combination of a intensifying screen (e.g GdOS or even CsI), some kind of optics (fiber or lenses) and one or multiple CCD cameras. This solution however suffers from serveral drawbacks: the minification (e.g. 43*43 cm -> 1,5*1,5 cm) needs space, so these devices are bulky and due to physics the loss of light is propotional to the minification factor. But even when you cool down the CCD to grab those very small signals you generate an additional source of noise which results in increased dose requirement.
GdOS flatpanel detector
GdOS is a very well known material for intensifying screen in film/screen combination. This material has a high light output, a very good X-ray absoption and a light spectra which can be easily detected by TFTs. The disadvantage of this material is the unavoidable light scattering within the scintilator layer. GdOS is availlable only in form or a power which has to be embedded into a binder. The scatter within this emulsion actually determines the spatial resolution of the complete detector. Because you can only use one intensifying screen with a TFT, whereas you will normally use two with film, the resoltion will always less than a comparable film/screen combination.
Only a few materials like Se are offering the possibilty for a true direct digitalradiography (DDR). In this case X-ray photons are directly converted into electrical charge, which is detected by a large TFT matrix. To move the generated charge to the TFTs a high voltage (approx. 10V/µm) has to be applied. Because amorphous Se has no grain structure there is practicable no bluring of the latent charge image. For this reason Se detectors offer the highest spatial resolution possible - superior to any other detector type. The disadvantage is the handling of the high voltage and the rather low absorption of Se. If your application is mammography you will be using low kV (e.g. 35 kV). In this case the stopping power of a 200µm Se layer will be totally sufficient - Se will be an ideal choice. If you are interested in general radiography and you want to use 70 kV and above, then Se is only a valid choice if your detector has a thickness of the Se layer of 1000µm or more. Otherwise you will miss a considerable number of X-ray quanta, resulting in an increased dose requirement.
CsI flatpanel detector
CsI is a scintilator material used in all X-ray image intensifiers. The huge advantage over GdOS is the possibility to grow needle like cristalls to form a conversion layer for X-rays. This particular feature of CsI allows to use a rather thick layer without compromising resolution. CsI flatpanel detectors are the ideal choice for applications, which do not require the highest spatial resolution (only Se can achieve this) but need a trade-off between low dose, low noise and resonable resolution. Overall image quality of images from flatpanel detectors are considered to be superior to those of film/screen combinations or storage phosphor systems.
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