インバータ式X線発生装置の諸特性 : マンモグラフィ用X線発生装置

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A dynamic mammographic study was carried out for measuring the precision of tube voltage, tube current, and irradiation time; percentage ripple of X-ray tube voltage; linearity and reproducibility of the X-ray-output(fluorescence quantity); radiation quality and photographic effect. Two inverter-type X-ray generators from different manufactures were used. The values obtained were processed with a personal computer. The same X-ray tube unit with a molybdenum target was used for both X-ray generators. Acrylate was used as the X-ray absorber. For both X-ray generators, the percentage average error(PAE) of tube voltage and tube current was rather minimal, 0.2 to 1.0%; the coefficient of variation(C)in the tube voltage, tube current, and X-ray output was also minimal, 0.001 to 0.03. Under conditions of 25 to 40 kV, 10 to 400 mA, X-ray generator A produced a percentage ripple of X-ray tube voltage of from 1.4 to 5.3% for each X-ray loading, and the percentage ripple was greater at lower tube voltages and larger tube currents. With X-ray generator B, the percentage ripple of X-ray tube voltage was 4.7 to 30.5%, and it varied depending on the amount of X-ray loading. The percentage ripple was greater at lower tube voltages and smaller tube currents, and this tendency was more significant when the tube current was less than 100 mA. As regards the photographic effect, the tube voltage of X-ray generator B was lower than that of X-ray generator A by about 5 kV, because generator B showed a greater percentage ripple of X-ray tube voltage. This became more evident when a molybdenum filter(0.03 mm) was used. An X-ray generator, having the most potential, should have the smallest possible percentage ripple of the X-ray tube voltage and coefficient of variation, with the highest possible reproducibility. However, although a generator may have good reproducibility, its X-ray output could vary widely even at the same tube voltage and current time product, if the tube voltage waveform fluctuates to a significant extant(i.e., the percentage ripple of tube voltage is high)in response to X-ray loading. In mammographic imaging, in which the effective tube voltage is low, a high percentage ripple of tube voltage reduces the ineffective ness of the molybdenum-characteristic radiation, increasing soft X-rays, and has a higher irradiation dose. Based on the above, the best possible X-ray generator for mammographic imaging should have a percentage ripple of tube voltage equivalent to that provided by a constant pottential X-ray generator(3 to 4%).
1995-05-01

インバータ式X線発生装置の諸特性 : マンモグラフィ用X線発生装置

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