Analog Image Receptors
Film Characteristics
There are four components of radiographic film: a base, the adhesive layer, the emulsion, and the supercoat. The base is made of polyester that is firm but flexible that supports the emulsion and transmits light well. It is tinted blue to control glare from the viewbox. The adhesive layer is the glue that adheres the emulsion to the base. It prevents the emulsion from "bubbling." The emulsion contains a gelatin that holds silver halide crystals (SHCs) in place evenly throughout the emulsion. The gelatin is porous, it swells, and it contracts without dissolving during processing.
The SHCs are the photosensitive agents in the emulsion. They create the density on the film; 95-98% of SHC's are silver bromide (AgBr) with the remaining being silver iodide (AgI). Inside each SHC is a sensitivity speck, usually silver sulfide (AgS). The sensitivity speck is what makes the SHC photosensitive. During latent image formation sensitivity specks attract the silver ions inside the SHC. The supercoat is a hard protective gelatin layer applied on top of the emulsion. It is designed to decrease surface abrasion so the image doesn't get damaged from scratches.
Latent image formation happens as a result of the interaction between x-ray or light photons and the sensitivity speck of the SHC. One SHC has many sensitivity specks. At the time of the exposure, an incident photon (x-ray or light) interacts with one of the halides (bromine or iodine) on the surface of the matrix. As a result, the halides are energized, and electrons are ejected (bromine and iodine become neutral). The ejected electrons are absorbed by the sensitivity speck causing it to become negatively charged. The inner positive silver ions migrate to the negative charge. The positive silver neutralizes the sensitivity speck and then another electron is absorbed, and so on and the latent image is formed.
Three factors make a film faster or more sensitive: the thickness of the emulsion, the size of the SHCs in the emulsion, and the number of sensitivity specks inside the SHC. As emulsion thickness increases, speed increases; a thicker emulsion can accommodate more SHCs. Large crystals are more sensitive (thus faster) than small crystals. Larger SHCs will have more halide ions on the surface to serve as an electron source. The more sensitivity specks, the faster the film. Fast film is great for decreasing patient dose; unfortunately, as film speed increases, sharpness of detail decreases.
Film contrast
Contrast and density of an image can be measured by plotting the information using sensitometry on a graph called a characteristic curve, a H & D curve, or a D log E curve. The five portions of the curve include: (1) base + fog, (2) toe, (3) straight-line, (4) shoulder, and (5) D max. Useful densities lying between .25 and 2.5 (toe to shoulder regions). The steepness of the curve indicates the contrast of the film and the latitude. High contrast film displays more black and white tones and has a steep line (curve). The emulsion layer is thin and the silver halide crystals are small. Low contrast film has more gray tones and a flatter line (curve). The emulsion layer is thick and the silver halide crystals are large.
Film latitude
Film latitude is the "forgiving" characteristic in the emulsion that gives you leeway in your technical factors. Low contrast film has wide latitude whereas high contrast film has narrow latitude. Thick emulsion has wide latitude whereas thin emulsion has narrow latitude.
Exposure Latitude
Exposure latitude is the "forgiving" characteristic that gives you leeway (margin of error) with a given set of exposure factors. Exposure latitude is largely determined by kilovoltage. A variation of a few kilovoltage values at the higher kilovoltage settings have little or no effect on the radiographic image whereas a variation of a few kilovoltage values at the lower kilovoltage settings can have a significant effect on the radiographic image.
System Speed
The speed of an imaging system is designated by a relative speed (RS) number 100, 200, 400, and 800. The higher the number the faster the screen. A slow or detail screen is about 100 and a fast screen is 200 and higher. The RS numbers differ between manufacturers. They are established at 70-80 kVp (with 80 kVp preferred) and are accurate enough to be used to convert exposure techniques from one situation to another. To change your mAs when going from one speed screen to another speed screen, double or cut the mAs in half or use the Conversion Factor formula:
or 
mAs1 = old mAs
mAs2 = new mAs
RS1 = old relative speed
RS2 = new relative speed
Example
What is the proper mAs for use with a 400 RS system when technical factors of 80 kVp and 50 mAs produce an acceptable image with a 200 RS system?
Intensifying Screen Characteristics
Intensifying screens produce a large amount of light when struck by x-rays; 99% of the latent image is formed by the light photons emitted by them. Each screen consists of a base, reflective layer, phosphor layer, and a protective coat.
The base is made of polyester just like the base of the film. It supports the phosphor layer; does not interfere with photons; and does not discolor with age. A discolored base absorbs light instead of transmitting it from the phosphor to the film. The reflective layer reflects light from the phosphor toward film to increase the speed of the screen. It contains a dye to absorb the long wavelengths of light that decrease image detail, however it will slightly decrease the speed of the screen. The phosphor layer contains phosphor crystals that converts x-ray photons to light photons. The phosphor crystals are uniform in size and evenly dispersed throughout the layer. The protective coat protects the phosphors from damage caused by handling.
Phosphors
Phosphors must have a high atomic number to make them efficient at converting x-ray photons into light photons. Conversion efficiency is the measurement of the phosphor's ability to convert x-ray photons to light photons. A screen with a high conversion efficiency is a fast screen and it requires less radiation to produce an adequate density on the film.
Luminescence is the ability of a material to emit light in response to excitation. It occurs when an x-ray photon causes an outer-shell electron of the phosphor to be excited and temporarily leave its orbit; when it returns to its normal state, a light photon is emitted. There are two kinds of luminescence: Phosphorescence and Fluorescence. Phosphorescence is a delayed emission (lag, afterglow) of light from a phosphor and is not acceptable for use with intensifying screens. Fluorescence is an immediate emission of light when excited and is desirable for use in intensifying screens. As phosphors age, they become exhausted and worn out. Eventually all screens will have delayed emissions of light and the intensifying screens must be replaced; usually after five to seven years of use.
The light emitted from the phosphor and the light that the film is sensitive to must match. This concept is called spectral matching. If the film and screens are mismatched, it creates a slower system. A slower system means you have to increase the technical factors (kVp and mAs) to achieve the necessary density. The increase in technical factors results in an increased patient dose.
Types of Intensifying Screens
The original intensifying screen invented by Thomas Edison was made with calcium tungstate (CaWO4) phosphors. The Calcium Tungstate phosphor emits blue violet light and its conversion efficiency is 3 to 5%. It was the phosphor of choice until the rare earth phosphor was introduced in the 1970s. The most common type of intensifying screen in use today are made of rare earth phosphors. The three common types of rare earth phosphors are Gadolinium, Yttrium, and Lanthanum. Rare earth phosphors emit green and blue light and are about twice as fast as a the Calcium Tungstate phosphor. They have a higher absorption rate and better conversion efficiency of 15 to 20%. They also increase x-ray tube life because the filament and target last longer.
Resolution
Resolution is the ability to accurately image an object. A thinner phosphor layer; a smaller phosphor layer; and high concentration of phosphor crystals all result in better resolution. Resolution is measured in line pairs per millimeter (lp/mm). Intensifying screens and film must be in close contact to achieve optimum resolution. Quantum Mottle (noise) is a grainy appearing radiographic image. Noise is caused by an insufficient quantity of photons striking the intensifying screen and is common in very fast rare earth phosphors with screen speeds above 500.
Speed (Sensitivity)
A thicker phosphor layer; a larger phosphor size; and a greater concentration of phosphor crystals all result in faster intensifying screens. The type of phosphor also makes a difference. Rare earth phosphors are faster than Calcium Tungstate phosphors. The Intensification Factor (IF) is an indication of screen conversion efficiency.
Dn = exposure in mR without a screen
Ds = exposure in mR with a screen
Example
A non-screen radiograph of a knee examination results in a skin entrance exposure 200 mR; a screen radiograph of a knee achieving a similar density results in a skin entrance exposure of 10 mR. What is the intensification factor of the film/screen combination?
High efficiency screens result in more light being emitted by the top screen, causing an over exposure on the top of film and underexposure on the bottom of the film. To compensate for this, the top screen is manufactured thinner than the bottom screen.
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