EFFECTS OF CONTRAST EQUALIZATION ON ENERGY IMPARTED TO THE PATIENT - A COMPARISON OF 2 DENTAL GENERATORS AND 2 TYPES OF INTRAORAL FILM

Technical evolution in maxillofacial radiology has in the last decade provided faster films and the constant potential generator. The conseq uences of these innovations for radiographic contrast and energy impar ted to the patient are analysed. On the basis of physical measurements a test model has been developed for correcting exposure parameters in order to maintain or restore image contrast. These measurements are e xpressed in and developed from basic radiological concepts and physica l formulas presented in an earlier paper (Helmrot E. et al., Dentomaxi llofac. Radiol. 1991; 20: 135-46). The test model can also be used to demonstrate the balance between contrast and energy imparted to the pa tient in the radiographic process. Changing to constant potential gene rators and faster film may each result in a degradation in contrast, w hich is possible to restore by a controlled adjustment of the kV-setti ng. Maintenance of constant image quality results in a slight reductio n in the net gain in energy imparted, due to the generator and/or film shift. When, for example, a conventional single-pulse generator opera ted at 65 kVp tube potential was replaced by a modern constant potenti al unit, the kV-setting had to be decreased by 5 to 8 kV to maintain t he same radiographic contrast. This correction could be done without i ncreasing energy imparted to the patient, taking into account the fact that the spectral characters of the photon energy are not identical. If, in addition, faster intraoral film with lower film contrast was in troduced, together with the constant potential unit, the kV-setting ha d to be further decreased to maintain the radiographic contrast. Imagi ng the test object of this study, an ivory wedge within a PMMA phantom , with Ektaspeed at the same contrast as Ultraspeed, resulted in a 7-9 kV decrease in kV-setting. With these corrections, the decrease in th e energy imparted was 35-40% which can be compared with the 45-55% obt ained without compensation for loss of contrast.