Sandra Hopkins Final Report.pdf - University of Surrey

digital detectors means that the systems should cope with this reduced contrast. The disadvantage ofadditional copper is a reduction in the tube output and it is necessary to increase the output from thex-ray unit to compensate for the absorption in the copper filter. This increases tube loading andreduces x-ray tube life.Measured fluoroscopy dose using a standard Perspex phantom were broadly similar on the twosystems whereas the patient audit for fluoroscopy dose was higher on the new system. This may bedue to the fact that dose is very dependent on patient thickness when pre-filters are employed.Figure 30 shows the variation of kV with mA for the Siemens Axiom Artis. At small patientthicknesses 0.9mm of copper is placed in the beam. As patient thickness increases so the mAincreases until it comes to a maximum allowable value for the x-ray tube. At this point the amountof copper reduces from 0.9mm to 0.6mm, the kV increases to improve penetration and the mAvalues reduce back to minimum values. As the patient thicknessVoltage v's Current (uA)11500010500095000850007500065000550004500014000 19000 24000 29000 34000 39000 44000 49000 54000Figure 30: kV/mA operating curve for the Siemens Axiom Artiscontinues to increase the mA again increases to maximum allowable values and at this point thecopper reduces to 0.3mm, 0.2mm and then later to 0.1mm copper. When the level of Copper isreduced, the kilovoltage increases, mA reduces and the patient dose is increased. This means thatfor measuring patient dose for the Siemens system the point on the kV/mA curve criticallydetermines the patient dose. A previous study using a standard phantom, determined that copperfiltration of 0.35mm would reduce the entrance dose by 58% with a mean increase in tube loading37