Automatic exposure for xeromammography

During mammography without intensifying screens, exposure measurements are carried out behind the film. It is, however, difficult to construct an absolutely shadow-free ionization chamber of adequate sensitivity working in the necessary range of 25 to 50 kV. Repeated attempts have been made to utilize the advantages of automatic exposure for xero-mammography. In this case also the ionization chamber was placed behind the Xerox plate. Depending on tube filtration, object thickness and tube voltage, more than 80%, sometimes even 90%, of the radiation is absorbed by the Xerox plate. Particularly the characteristic Mo radiation of 17.4 keV and 19.6 keV is almost totally absorbed by the plate and cannot therefore be registered by the ionization chamber. This results in a considerable dependence of the exposure on kV and object thickness. Dependence on tube voltage and object thickness have been examined dosimetrically and spectroscopically with a Ge(Li)-spectrometer. Finally, the successful use of a shadow-free chamber is described; this has been particularly adapted for xero-mammography and is placed in front of the plate. (orig)