X-ray fluorescent screens and film contrast

[ChumpusRex]

It's common practice in radiography, especially medical radiography where radiation doses are relevant, to use a fluorescent screen to expose photographic film, rather than rely on the effect of x-rays on the film itself.

I understand the concept of fluorescence and the overall photon gain/quantum efficiency gain of using a heavy metal fluorescent material (e.g. Calcium Tungstate or Gadolinium oxysulphide) to absorb the x-rays photons and emit numerous visible light photons. This subsequently increases the overall sensitivity (speed) of the system.

However, a fluorescent screen is also stated to increase the film contrast (gamma) significantly.

As gamma is a log-log measurement this would suggest that there is a non-linear process in the screen. As surely, if the conversion was linear (e.g. 100 light photons for 1 x-ray photon) then the system gamma would be unaffected.

Does anyone know what this non-linearity is, and what factors affect it?

I've already been through about half-a-dozen radiography texts, all of which mention the effect on gamma, but none of which explain it.

 

[imabug]

I have to give just a partial answer here, because all my texts on the matter are at the office (which is where I'm not at the moment). Most of what I put here is recollection from a fuzzy memory. It's been a while since I last worked with film/screen...most of what I do now is on CR.

There are lots of non-linearities in film or film/screen systems. They come from things like the emulsion composition, emulsion particle shapes and sizes, the screen composition and grain to name a few off the top of my head.

Film makers tailor the film gamma and shape of the H&D curve by changing the shape and size of the emulsion crystals. They can also do the same with the screen.

Also, the conversion process in the screen is not entirely linear either. Keep in mind that your conventional x-ray beam is polychromatic, and x-ray photons of different energies will produce different numbers of light photons (which is a statistical process).

For more details on film and film/screen in medical imaging, I highly recommend the two following books by Art Haus

Basics of Film Processing in Medical Imaging
Advances in Film Processing Systems Technology and Quality Control in Medical Imaging

BTW, Calcium Tungstate or Gadolinium oxysulphide have been replaced long ago as screen materials by rare Earth compounds.

If you're still interested by tomorrow, I'll look up some more details and post a hopefully better reply (unless someone beats me to it)

 

[imabug]

BTW, Calcium Tungstate or Gadolinium oxysulphide have been replaced long ago as screen materials by rare Earth compounds.

duh, so used to CR phosphors, I'm forgetting screen/film things. Gadolinium oxysulphide is one of the newer rare Earth phosphors.

 

[imabug]

"The Physics of Radiology" by Johns and Cunningham has this to say about differences between H&D curves for film and film/screen:

The "no screen" film shows a linear increase in density with exposure up to a density of about 1.0 and then gradually starts to saturate. In contrast, the response of the film used with a screen is sigmoidal. The difference can be explained as follows. The film without screen is exposed by x-ray photons, each of which deposits enough energy to sensitize a single grain of silver, hence the initial linear relation; as the silver grains are used up the curve starts to show saturation.

When films are exposed with screens the x-ray photons are absorbed in the screens, which emit visible and ultraviolet light. It is these light photons that cause 99% of the exposure to the film. Since it requires the absorption of several of these light photons to sensitize a single grain of silver, we obtain a sigmoidal response at small densities.