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- Sandborg, Michael, 1961-
(författare)
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Erbium filter in diagnostic radiology: calculations of contrast and patient mean absorbed dose
- 1989
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Ingår i: Optimisation of image quality and patient exposure. - London : British Institute of Radiology. - 0905749219 ; , s. 169-170
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Konferensbidrag (refereegranskat)abstract
- Rare earth materials instead of aluminium as added filter have been reported to reduce the mean absorbed dose in the patient. With these filters, the energy spectrum can be shaped to yield high energy absorption in the detector (film or screen). Exchange of filter results in a change in contrast. The image contrast and the mean absorbed dose in the patient have been investigated in the case of detection of a small contrast detail in a homogeneous phantom. Image contrast is defined as the difference in optical densities on the film, behind and beside a small contrasting detail. The Monte Carlo photon transport technique has been used to calculate the mean absorbed dose in the phantom and the energy imparted to various intensifying screens. Different Xray spectra were obtained by varying tube potential and filter thickness as well as filter material. Using the image contrast as the image quality parameter, the optimization condition is to find that combination of tube potential, filter material and filter thickness that gives the lowest mean absorbed dose in the patient for a given contrast. The mean absorbed dose in a 2(H) mm thick water phantom is reduced by 1025% using an erbium filter at a tube potential which will give the same primary contrast as obtained with a 2 mm aluminium filter. Contrast equivalent tube potential, kVcq, is achieved at higher values than with a non ACcdgc filter like copper. With the same increase in tube load, the dose reduction using an erbium filter (130 mg/cm") is approximately as high as with a copper filter (200 mg/cm2). Dose reduction at equal contrast and filmblackening as with 2 mm aluminium added filter is higher at lower tube potentials. When the high atomic number of the detector material is at least ten below that of the filter, it is slightly favourable to dose reduction. The change in image contrast when exchanging the aluminium filter for one of erbium requires a change in tube potential to restore the contrast. This can be achieved with a decrease in patient mean absorbed dose at the expense of an approximate increase of two in tube load using 130 mg/cm" erbium as the added filter.
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