X-ray intensifying screen

opened x-ray film cassette with the white intensifying screens and an x-ray film

X-ray intensifying screens support X-ray recordings with X-ray films by converting the X-ray radiation into visible or ultraviolet light in an intermediate step , which then exposes the X-ray film.

Basics

X-rays penetrate X-ray films largely unchanged without exposing the films. Only a very small part (approx. 3 - 5%) of the X-ray radiation hitting X-ray films is absorbed and leads to direct exposure of the film. Film emulsions are much more sensitive to light than to X-rays. The dose can be reduced by a factor of 2 to 40, depending on the type of amplifier, by using intensifying screens that absorb X-ray radiation much more efficiently and indirectly expose the X-ray films by generating luminescent light.

X-ray intensifying foils are pressed onto both sides of the X-ray film in a special cassette. The phosphors in the intensifying screens absorb a considerable part of the X-ray photons that act and emit the absorbed energy again via luminescence in the form of visible or ultraviolet light. This light - each X-ray quantum generates whole groups of light photons - exposes the directly adjacent X-ray film much more efficiently than X-ray photons acting directly on the X-ray film can.

The intermediate step via the conversion of X-ray photons into light photons leads - in the literal sense - to the exposure of the X-ray film via detours. Because of these detours, the X-ray film is not exposed with the same punctual accuracy by the individual X-ray photons, but by groups of slightly scattered light photons that reach the adjacent X-ray film, which leads to a certain unavoidable blurring of the X-ray image, the so-called foil blurring. In order to keep the foil blurring as low as possible, intensifying screens must lie close to the film. This is ensured by the contact pressure in the cassette.

In medical diagnostics intensifying screens with fluorescent salts from rare earths are used. The higher the desired gain (sensitivity), the thicker the film must be and the greater the film blurring.

Because of the considerable dose reduction, the use of intensifying screens is mandatory for almost all medical X-rays with X-ray films. There is only one exception in dentistry , where X-rays are taken with the X-ray film positioned in the mouth (intra-oral recording using dental film ). No intensifying screens are used here because the foil blurring would impair the image quality (spatial resolution) too much for dental purposes.

construction

X-ray intensifying screens have a multilayer structure. An intermediate layer, a luminous layer containing the phosphor and a protective layer are located on a carrier layer.

The backing layer of polyester provides the mechanical integrity of the intensifying screen.

The intermediate layer can have reflective properties for particularly highly reinforcing films. For intensifying screens with particularly fine definition, the intermediate layer can have absorbing properties or can be omitted.

If the intermediate layer is designed to be absorbent ( absorption layer ), it should absorb those light photons which were inexpediently emitted by the phosphor to the back of the intensifying screen and, after scattering there, could reach the film and additionally expose it diffusely, which would impair the image quality (fogging). If the intermediate layer is designed to be reflective (e.g. by adding titanium oxide ) ( reflective layer), it directs the light photons sent to the rear to the front of the intensifying screen in order to additionally expose the X-ray film. This increases the gain factor of the intensifying screen. However, because of the detours that reflected light photons take on their way to the X-ray film, the film blurring becomes greater.

As phosphors , the blue glowing today are mainly terbium -activated lanthanum or green glowing terbium-activated gadolinium used.

The protective layer consists of a hard, transparent clear lacquer. It should reduce the mechanical wear of the intensifying screen, improve the sliding properties when changing the film and avoid electrostatic charges on the intensifying screen.

Sensitivity classes

The coordination between the emission spectrum of the intensifying screen and the spectral sensitivity of an X-ray film is crucial but very complex. It would be very disadvantageous to combine a blue intensifying screen with a predominantly green-sensitive X-ray film. In order to ensure an optimal compromise between image quality and dose requirements, the correct combination between X-ray film and intensifying screen, as developed and offered by the manufacturers, is necessary.

In order to be able to compare and assess the sensitivity of the entire system of X-ray film and intensifying screen, a standard was created. The film-foil combination of a certain X-ray film and a certain intensifying screen based on calcium tungstate was set as a benchmark and defines the dimensionless speed class SC 100. Another film-foil combination that has the same film blackness with only the half the dose, therefore belongs to sensitivity class SC 200. If only a quarter of the dose is required, the corresponding film-foil combination belongs to sensitivity class SC 400. There were or are film-foil combinations of sensitivity classes 50, 100, 200 , 250, 400 and 800. For the various indication areas in medicine, there are legal regulations as to which sensitivity classes may be used.

history

In 1896, in the first year after Wilhelm Conrad Röntgen discovered X- rays , Mihajlo Idvorski Pupin invented the method of placing a sheet of paper coated with fluorescent substances next to the photographic plate, which drastically reduced the exposure time. Thomas Alva Edison identified the blue calcium tungstate (CaWO ₄ ) as a suitable phosphor, which quickly became the standard for intensifying screens . It was not until the 1970s that calcium tungstate was replaced by intensifying screens with luminescent substances (lanthanum oxybromide, gadolinium oxysulphide) based on rare earths that were even better for amplifying and more finely detailed . The use of intensifying screens in the production of dental films was investigated by Voss and Hickel, among others, but has not caught on because of the loss of image quality.

literature

D. Gregersen: The use of film-foil combinations of different system sensitivities in panoramic layer recordings , dental dissertation, Hamburg, 2001. Retrieved on December 11, 2016.

Individual evidence

↑ Olaf Dössel: Imaging methods in medicine. From technology to medical application. 2nd Edition. Springer Vieweg Verlag, Berlin / Heidelberg 2016. ISBN 978-3-642-54406-4 , doi : 10.1007 / 978-3-642-54407-1 .
↑ A. Voss, R. Hickel: Dental film with intensifying screen. Dtsch Zahnärztl Z 42, 798-802 (1987).

[1] Olaf Dössel: Imaging methods in medicine. From technology to medical application. 2nd Edition. Springer Vieweg Verlag, Berlin / Heidelberg 2016. ISBN 978-3-642-54406-4 , doi : 10.1007 / 978-3-642-54407-1 .

[2] A. Voss, R. Hickel: Dental film with intensifying screen. Dtsch Zahnärztl Z 42, 798-802 (1987).