SHAPING X-RAY-SPECTRA WITH FILTERS IN X-RAY-DIAGNOSTICS

The influence on image contrast, tube load and patient mean absorbed d ose of different ways of shaping diagnostic X-ray spectra by placing f ilters in the beam is derived for two radiographic models (abdominal s creen-film radiography and intra-oral dental radiography) using a comp utational model. The filters are compared at either equal tube load (k eeping tube potential constant) or equal contrast (adjusting the tube potential with the different filters), but always at equal energy impa rted per unit area to the image receptor. Compared at equal tube load and relative to standard aluminium filtration, reductions in the mean absorbed dose in the patient of 15-25% can be achieved using filters o f Cu, Ti, W and Au (increasing the tube load by 30-40% compared with s tandard aluminium filtrtion). However, contrast is also reduced by 7%. Compared at equal contrast, the dose reductions are smaller, about 10 %. Filters of copper are generally recommended, as are filters of alum inium. The use of bandpass filters (K-edge filters) should be restrict ed to examinations where the need for substantial variation in tube po tential from patient to patient is small. The benefit of using thicker filters than those commonly used today (increasing tube load by facto rs of 1.4-2.0 compared with no added filter) is small as the dose redu ction is most rapid for small initial values of added filters, and the increase in tube load increases steadily with increasing filter thick ness.