絞り装置による散乱線除去効果のMTF

In the radiographic system, scattered radiations from object make the image quality poor. As clean-up equipment, X-ray grids and collimator are generally used. The efficiency of collimator had been geometrically evaluated concerning extra focal radiation and penumbra. As to the influence of scattered radiations on the image quality some more quantitativeevaluation is necessary.<BR>In this paper, we tried to show the clean-up effect of scattered radiations of collimator by means of the technique of Modulation Transfer Function (MTF).<BR>Let MTF of systems with collimator and without collimator be X(ω) and Y(ω)respectively. The MTF C(ω) of clean-up effect of scattered radiations by collimator is to be obtained by the Vol.4 No.2 1974 69following Equation. C(ω)=X(ω)/Y(ω)<BR>Where C(ω) is changed by radiation fields.<BR>From the theory above mentioned, we studied about the MTFs of clean-up effect of scattered radiations on each radiation field of 10×10cm.,20×20cm. and 30×30cm.. As a phantom, we used acrilic board of 9cm. thick, and the conditions of exposure are 80kVp. and 100mA..<BR>The experimental results are shown in Figure 3. It is proper that the improvement of image in narrow radiation field is better than in wide one. If the clean-up effect of scattered radiations is evaluated by MTF, then the difference of result by mesurement method is solved, we can evaluate the clean-up effect quantitatively.<BR>This method is useful to the selection of the optimum combination of X-ray grid and radiation field, and these results are useful to the selection of equipments according to radiographic portions. The effective selection of equipments is useful to the reduction of patient exposures and the improvement of capability of diagnosis.