INTENSIFYING SCREENS

INTENSIFYING SCREENS

Intensifying screens are used because they decrease the x-ray dose to the patient, yet still afford a properly exposed x-ray film. Also, the reduction in exposure allows use of short exposure times, which becomes important when it is necessary to minimize patient motion. The x-ray film used with intensifying screens has photosensitive emulsion on both sides. The film is sandwiched between two intensifying screens in a cassette, so that the emulsion on each side is exposed to the light from its contiguous screen. Remember, the screen functions to absorb the energy (and information) in the x-ray beam that has penetrated the patient, and to convert this energy into a light pattern that has (as nearly as possible) the same information as the original x-ray beam. The light, then, forms a latent image on x-ray film. The transfer of information from x-ray beam to screen light to film results in some loss

of information.

Construction

An intensifying screen has four layers:

1. a base, or support, made of plastic or cardboard

2. a reflecting layer (Ti02)

3. a phosphor layer

4. a plastic protective coat

The total thickness of a typical intensifying screen is about 15 or 16 mils (1 mil= 0.001in. = 0.0254 mm).Base. The screen support, or base, may

be made of high-grade cardboard or of a polyester plastic. The base of Du Pont intensifyingscreens is a polyester plastic (Mylar*)that is 1 0 mils thick ( 10 mils = 2 54f.Lm). Kodak X-Omatic and Lanex screens have a similar (Estart) base 7 mils thick.

Reflecting Coat.

The light produced by the interaction of x-ray photons and phosphor crystals is emitted in all directions. Much of the light is emitted from the screen in the direction of the film. Many light photons, however, are also directed toward the back of the screen (i.e., toward the base layer) and would be lost as far as photographic activity is concerned. The reflecting layer acts to reflect light back toward the front of the screen. The reflecting coat is made of a white substance, such as titanium dioxide (Ti02), and is spread over the base in a thin layer, about 1 mil thick. Some screens do not have a reflecting layer (e.g., Kodak X-Omatic fine and regular).

Phosphor Layer.

The phosphor layer, containing phosphor crystals, is applied over the reflecting coat or base. The crystals are suspended in a plastic (polymer)containing a substance to keep the plastic flexible. We will discuss the phosphor in more detail later. The thickness of the Protective Layer. The protective layer applied over the phosphor is made of a plastic, largely composed of a cellulose compound that is mixed with other polymers. It forms a layer about 0.7 to 0.8 mils thick. This layer serves three functions: it helps to prevent static electricity; it gives physical protection to the delicate phosphor layer; and it provides a surface that can be cleaned without damaging the phosphor layer.

Phosphor

The original phosphor used in x-ray intensifying screens was crystalline calcium tungstate (CaW04). New (since about1973) screen phosphor technology is being developed to increase screen speed over that available with calcium tungstate, and has resulted in the introduction of a bewildering variety of screens and corresponding films. Let us first consider calcium tungstate and then review some of the new phosphors. Natural calcium tungstate (scheelite) is no longer used because synthetic calcium tungstate of better quality is produced by

fusing sodium tungstate and calcium chloride under carefully controlled conditions. The first commercial calcium tungstate screens were made in England and Germany in 1896; they were first made in the United States in 1912. The calcium tungstate crystal must be absolutely free of any contaminant if it is to fluoresce properly.

An intensifying Action of Screens.

Intensifyingscreen is used because it can converta few absorbed x-ray photons into many light photons. The efficiency with which the phosphor converts x rays to light is termed the intrinsic conversion efficiency of the phosphor; this is more accurately defined as the ratio of the light energy liberated by the crystal to the x-ray energy absorbed. The intrinsic conversion efficiency of calcium tungstate is about 5%. The ability of light emitted by the phosphor to escape from the screen and expose the film is termed the "screen efficiency."The intensification factor of a screen is the ratio of the x-ray exposure needed to produce the same density on a film with and without the screen (intensification factor is commonly determined at a film density of 1.0).

Speed of Calcium Tungstate Intensifying Screens. Several factors determine how "fast" or "slow" a calcium tungstate screen will be. These include thickness of the phosphor layer, size of the phosphor crystals, presence or absence of light-absorbing dye in the phosphor layer, and phosphor conversion efficiency. Of course, the faster screen will allow a lower x-ray exposure to the patient, but a price for this speed must be paid. The speed of a calciumtungstate screen and its ability to record detail are in reciprocal relationship; that is, high speed means less detail. This statement is also true for new screen phosphors, but the subject is more complex because higher speed does not always require a thicker screen. These screens are classified as fast, medium (par speed), and slow (detail), with intensification factors in the range of 100, 50, and 25, respectively. A thicker phosphor layer will result in a faster screen because the thick layer will absorb more x-ray photons than a thin layer. Thick screens will be faster but will cause a decrease in the clarity of the image recorded on the film. This decrease in image clarity is primarily caused by diffusionof light in the phosphor layer

Screen-Film Contact

The cassette in which the intensifying screens are mounted provides a light-tight container for the film. It also serves to hold the film in tight contact with the screens over its entire surface. With good film screen contact a dot of light produced in the screen will be recorded as a comparable dot on the film. If contact is poor, this dot of light will diffuse before it reaches the film, so that its radiographic image is unsharp. There is a simple method for testing film-screen contact. A piece of wire screen is placed on the cassette, and a radiograph of the wire screen is made. The sharpness of the image of the wire in all regions of the film is compared, and any areas of poor film-screen contact become obvious because the image of the wire appears fuzzy.

Cleaning

Intensifying screens must be kept clean. Any foreign material on the screen, such as paper or blood, will block light photons and produce an area of underexposure on the film corresponding to the size and shape of the soiled area. Cleaning will eliminate the "high spots" on a screen; these high points are the major source of excessive wear. The cause of screen failure is mechanical attrition. Under normal conditions of use, x-ray photons will not damage screens. Screens are best cleaned with

a solution containing an antistatic compound and a detergent; the solution should be applied gently (never rub vigorously) with a soft lint-free cloth. The cassette should never be closed after cleaning until it is absolutely dry.

RARE EARTH MATERIAL

Chemists divide the periodic table of the elements into four basic groups: alkaline earths, rare earths, transition elements, and nonmetals. The term "rare earth" developed because these elements are difficult and expensive to separate from the earth and each other, not because the elements are scarce. The rare earth group consists of elements of atomic numbers 57 (lanthanum) through 71 (lutetium), and includes thulium (atomic number 69), terbium (atomic number 65), gadolinium (atomic number 64), and europium (atomic number 63). Because lanthanum is the first element, the rare earth group is also known as the lanthanide series . Lanthanum (La) and gadolinium (Gd) are used in the rare earth phosphors. A related phosphor, yttrium

(Y), with atomic number 39, is not a rare earth but has some properties similar to those of the rare earths.