The effects of BID length and shape on the surface area and volume of tissue exposed during dental radiography.

Using projection geometry, algebraic formulas, and different beam indicating devices (BIDs), the surface area and volume of tissue exposed for a hypothetical patient was determined. Combinations of 4 BID lengths and shapes were used: 4-inch (10.2 cm.) pointed plastic, 8-inch (20.3 cm.) round plastic, 14-inch (35.6 cm.) round aluminum, and 14-inch (35.6 cm.) rectangular aluminum. As each subject BID was progressively lengthened and collimated, the area and volume of tissue exposed was reduced. The calculated area and volume of exposed tissue for the 14-inch rectangular aluminum open-ended lead-lined BID was 16.6 cm.2 and 327.7 cm.3, respectively. This represented 9% of the area and 8% of the volume of tissue exposed using the 4-inch round pointed plastic BID or a 91% (area) and 92% (volume) reduction in patient exposure.