[NIOBI-X filter: reduction of patient exposure and effect on image quality].

Patient protection against ionizing radiation has become more and more important and the subject has been included in all the latest ICRP publications. Recently a 50 microns Niobium filter (NIOBI-X) has come out, which, installed on tungsten anode X-ray apparatuses, allows patient dose reduction by absorbing the radiation which does not contribute to the image. In order to verify the NIOBI-X actual effect on image quality and on patient dose reduction, image resolution, image contrast and patient exposure (air-Kerma) were measured using a bone-equivalent stepped wedge. The NIOBI-X, added to the total filtration of the X-ray apparatus, allows an air-Kerma reduction of about 50% from 60 to 130 kV. In the case of bone-equivalent material exposure the filter causes a slight contrast loss and does not affect, or even improves, image resolution. Before choosing the NIOBI-X filter for all the X-ray apparatuses a cost-benefit analysis should of course be made by evaluating results and cost related to other kinds of filter. With particular regard to Aluminium filters, whose prices are far lower than Niobium ones, it has to be pointed out that: 1) in order to have the same air-Kerma reduction the Aluminium filter thickness must be quite heavy (2.8 mm) which causes practical problems of use; 2) the Aluminium filter causes image quality loss especially at lower kVs.