Physics, Techniques and ProceduresIntensifying screen
a sheet of crystals of inorganic salts (called phosphors) which emit fluorescent light when excited by
X-ray radiation (see
fluorescence). The sheets are used to intensify the effect of X-rays during exposure of
X-ray film.
The intensifying screen was discovered by Wilhelm Conrad Roentgen himself on November 8, 1895, simultaneously with the discovery of X-rays. He was experimenting with a high energy
cathode ray tube (CRT) and had enclosed the tube in black cardboard. When passing a high-voltage discharge through the tube, he noticed a faint light from a piece of paper left on a work bench. The paper was covered with a thin layer of
barium platinocyanide, and this first "intensifying screen" sent out fluorescent light caused by the mysterious X-rays. The intensifying screen as part of a
film-screen system, has been one of the most important components in all modern radiology. Today, the
film-screen cassette is gradually being replaced by alternative
X-ray detectors such as the
photostimulable phosphor plate and
selenium plate (see also
X-ray image transducer).
The intensifying screen or a pair of screens is nearly always used with
X-ray film in radiography (see
film-screen radiography). Film may be used as the only radiation detector, but because it has a relatively low atomic number (that of
silver halide), film is relatively
radiolucent. At direct exposure of film, only about 5% of the
X-ray photons will be absorbed by the film and react with the
emulsion. For comparison, a high-speed
calcium tungstate screen will absorb approximately 40% of the
X-ray photons. Furthermore, each absorbed
X-ray photon will be converted into many light photons. The efficacy of the screen in converting X-rays into light photons is called the intrinsic conversion efficiency. The efficiency of
calcium tungstate is about 5%. A 50 keV
X-ray photon when absorbed by calcium tungstate (by
photoelectric absorption), would be converted into about 17 000 light photons of 3 eV energy at 100% efficiency. Since the efficiency is only 5%, the actual number of light photons emitted is 850. Approximately half of these will escape from the screen to expose the emulsion. About 100 light photons may be sufficient to form one
latent image centre.
There are two major types of phosphors and therefore intensifying screens; the calcium tungstate screen (CaWO
4) and
rare earth screens (such as La
2O
2S:Tb, Gd
2O
2S:Tb, Y
2O
2S:Tb), where terbium (Tb) is often used as an impurity or activating substance. But even a metal sheet can be used as an intensifying screen, see
metal screen. The fluorescent input and output screens of the
image intensifier are very similar to intensifying screens.
The spectral output of the phosphor must be matched to the response of the film (
Fig. 1). Calcium tungstate screens emit blue light of continuous spectrum with a peak wavelength at about 430 nm. The term "blue screen" refers both to the screen itself and to the blue sensitive film used together with the CaWO
4 screen. Rare earth screens emit light in narrow lines with strong peak(s) in the green part of the spectrum but smaller ones also in the blue, blue-green and yellow regions. The term "green screen" may be used. It is absolutely necessary to use green sensitive film with these screens to make sure that useful transmitted radiation is not lost.
Many factors affect the speed of a screen. The phosphor type determines 1) the
X-ray radiation absorption efficiency, 2) the radiation to light conversion efficiency, and 3) the thickness of the phosphor (varies between 100-200 μm). The fraction of X-rays absorbed by a pair of calcium tungstate screens is about 20-40% depending on the speed (determined mainly by screen thickness), while rare earth screens absorb about 60%. The radiation to light conversion efficiency of calcium tungstate is about 1/3 or 1/4 of that of rare earth screens (about 5% vs. 12 - 18%). Observe that most often two screens are used in the film-screen cassette. The relative speed of film-screen combinations is normalized to 100 which corresponds the basic calcium tungstate screen with the basic film. Speed values vary from 20 and 50 (slow screens) through 100, 200, 400, and 800 to 1 600.
An essential basic feature of the two screen types is related to the position of the
K edge on the energy axis. Tungsten (W) being a heavy element has its K edge at 69.5 keV, while that for rare earth elements is in the vicinity of 50 keV. Most
X-ray spectra used in conventional radiography have their mean energy between 40-50 keV, signifying that also for this reason rare earth screens are more effective than CaWO
4 in absorbing
X-ray quanta.
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