Analog X-ray Imaging Recommended Book: Walter Huda, REVIEW OF RADIOLOGIC PHYSICS By: Maisa Alhassoun

Intensifying Screens A. Screens -Intensifying screens contain phosphor crystals that absorb x-ray photons and emit many more visible light photons, which expose the film as shown in Fig The screen therefore converts the x-ray pattern to a light pattern, which is recorded on film.

-Intensifying screens improve the efficiency of radiographic imaging over use of film alone. -The use of intensifying screens decreases the mAs required for a given film density, resulting in a lower patient dose. -Shorter exposure times also decrease x-ray tube loading and image blur caused by patient motion.

-Intensifying screens are typically made of a polyester base for support with a 40 to 200 μm thick phosphor layer. -About 50% of the radiation incident on a cassette containing a pair of screens will be absorbed. -Absorption efficiency refers to the percentage of x-ray photons absorbed in a screen and is typically 25%.

-Conversion efficiency refers to the percentage of the absorbed x-ray energy that is converted into light energy and is typically 10%. -The intensification factor is the ratio of exposures, without and with intensifying screens, required to obtain a given film density. -Intensification factor depends on the absorption and conversion efficiency of the screen. -Typical intensification factors are 30 to 50.

B. Screen materials -Screens contain high atomic number materials to maximize the absorption of x-rays. -Calcium tungstate (CaWO4) was used in intensifying screens until about 1970.

-Tungsten has a high K-shell binding energy (69.5 keV) compared with the mean photon energy levels normally used in diagnostic radiology; therefore, absorption was less than optimal. -For example, an examination performed at 100 kVp corresponds to a mean photon energy level of about 40 keV, which is well below the K-shell binding energy in the screen.

-Rare earth screens are "faster" than calcium tungstate because they have a higher absorption efficiency at the mean x-ray energies normally used in radiology. -Rare earth screens also have a higher conversion efficiency and produce more light for a given amount of deposited x- ray energy. -Table 4.2 summarizes the K-shell binding energy levels of the rare earth elements used in screens.

-Gadolinium oxysulfide (Gd2O2S) emits mainly green light, and lanthanum oxybromide (LaOBr) and CaWO4 emit mainly blue light. -The light output—that is, wavelength (λ) or color—of a screen and the light sensitivity of the film must be matched (spectral matching). -Conventional film is sensitive to ultraviolet and blue light. -Orthochromatic film is also sensitive to green light.

C. Cassettes

-The film and screens are held in a light-tight cassette. -Screens are usually permanently mounted inside the cassette. -A thin layer of resilient foam holds the screen tightly against the film when the cassette is closed.

-The front of the cassette is made of a minimally attenuating material, such as aluminum or carbon fiber. -Dual-screen, dual-emulsion systems are frequently used to improve x-ray absorption. -Intensifying screens can be significant sources of image artifact. -Scratches, stains, hair, dust, cigarette ash, and talcum powder are all potential sources of image artifacts.

-As part of a quality control program, all screens should be regularly cleaned. -Cassettes should also be evaluated for good screen/film contact. -Screen/film contact is evaluated by taking an image of a wire mesh and ensuring that the resultant image permits visualization of the mesh.

D. Screen/film speed -The speed of a screen/film combination is inversely related to the exposure required to produce a given density. -As the speed increases, the exposure required decreases.

-The speed of a screen/film combination used in radiology ranges from approximately 50 to Screen/film speed is normally expressed relative to a CaWO4 standard assigned a speed of Screen speed increases with increasing screen thickness, absorption, and conversion efficiency. -Both screen and film must be specified when assigning speed to any screen/film combination.

-High-speed screens are generally thicker and have decreased spatial resolution. -Thick screens increase image blur because of increased diffusion of light in the screen before striking the film. -Detail screens are thinner and therefore slower, but they have better spatial resolution.

-Fast screen/film combinations are used for abdominal studies: slow films are used for extremity examinations. -Single-emulsion single-screen systems are used for bone detail and mammography.

-To ensure the correct film density, phototiming systems are generally used. -A phototimer measures the actual amount of radiation incident on the screen/film and terminates the exposure when the correct amount has been received. -Table 4.3 lists a range of screen types used in radiology, as well as their clinical applications.