The HVL in soft tissue and the AAPM and IEC exposure indices

Full text: Manual exposure settings for radiographic projections were once based on a points system which assumed that the HVL in soft tissue is 3.0 cm and that each of 1.0 em of soft tissue corresponded to a change of 25% image receptor dose. A set of mAs steps and equivalent kVp steps was estimated that would give appropriate technique factors for changes in patient thickness. With the advent of rare-earth screen-film systems and AEC systems the points system fell into disuse. Screen-film systems have almost entirely been replaced by CR or DR stems and recently, standardised exposure indices indices have been recommended by the AAPM and IEC to provide exposure guidance for these systems. If the fundamental assumptions on which the points system was based are still valid for modern high-frequency generators and digital imaging systems, then there would be an elegant correspondence between the predictions of the points system and the requirements for correction of exposure errors indicated by the AAPM and IEC indices. This study estimated the HVL and attenuation per cm in soft tissue using computer simulation, finding that practically, the HVL is between 2.0 and 5.0 cm and attenuation per cm ranges frm 15 to 25%. The study concluded that agreement between the points system predictions and the true effects of technique factors changes on dose to the image receptor was moderately good, that use of the points system and technique charts based on this system should be encouraged and that use of the IEC or AAPM digital exposure indices should be standardised. (author)