Answer to Question #10577 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
In x-ray medical imaging equipment that still uses film, how many different speeds of film are used? Does using a 200-speed film for a chest x ray have twice the radiation dose compared to using 400-speed film to take the chest view?
Unfortunately, there is no simple answer as the speed depends on the speed of the film and the cassettes, plus the type and quality of photographic processing.
Most manufacturers provide three speeds of photographic film—200, 400, and 800. Changing from one speed film to a higher speed film will result in a 50 percent reduction of patient dose, e.g., going from 200 to 400 speed results in a 50 percent reduction of dose, as does going from 400 to 800 speed. This assumes that the technologist uses the same type of cassette and reduces the technique factors appropriately, e.g., reduces the exposure time by 50 percent.
Screen-film combinations can produce speeds from 25 to 1,300 (http://www.crcpd.org/Docs/Screen-filmSpeedCombos_040506.pdf). Combinations used clinically range from 100 to 400. Higher speeds can sometimes be used clinically, but there is some loss in image quality (increased noise) with speeds in excess of 400. Speeds below 100 are typically used for detail imaging, i.e., imaging the extremities.
If the facility processes the radiographic film according to the manufacturer's instructions, then all films would provide the speed intended. However, the developer solutions in the processors at facilities are often not maintained at the appropriate chemical activity level. This can result in slower film (requiring more radiation dose to the patient) and poor-quality images (often reduced contrast, making small, important details more difficult to visualize).
These variables make it essential for the facility to have a quality-control program under the direction of a medical physicist. This assures optimized image quality and radiation dose to the patient.
Joel E. Gray, PhD