Focal-film distance (FFD) is the the distance between the center of the anode of the x-ray tube (the focal spot) and the film (top of cassette). Also known as, Source-image distance (SID) which this measurement effects magnification, distortion, and x-ray beam intensity. To assist with reducing these types of negative effects, we only use two FFD types, a long distance (70 cm) and a short distance (40 cm). The short distance is most commonly used for all xray tests, calibration verification, and alignment procedures.
- ↑ Thomas. Patrick. Jr. "Basic Radiograpghic Procedures". p. 3. http://chiro.org/radiology/ABSTRACTS/procedures.pdf