We measured modulation transfer functions (MTFs) of liquid crystal displays (LCDs) by rectangular waveform analysis. This method is taking a picture of the bar pattern on the monitor surface with a digital camera, and analyzing the picture with a personal computer. The monitors used are the monochrome LCDs of 1M(about 1 million of the number of pixels), 2M, 3M, and 5M, and the color LCDs of 1M, 2M, 3M. The display of 2M used IPS system and VA system. 3M and 5M of the monochrome LCDs were examined when there was a protective filter or not. Two or three displays are used for each system. In both the monochrome and the color LCDs, MTFs became high as the matrix size increased. In the monochrome LCDs, MTF in horizontal direction was higher than MTF in vertical direction. And there was no difference when a protective filter was used or not. MTFs of the color LCDs had little difference in horizontal direction and the vertical direction. MTFs of the LCDs are influenced on the form and the fill factor of a pixel, and composition of sub-pixels.
The international standard IEC 62220-1 about DQE measurement of digital X-ray equipment was published in 2003, but mammography systems aren’t applied to this IEC standard because the factor affect measurement is complicated. Especially, the influence to the pre-sampling MTF by edge method when X-ray beam is oblique to detector. The influence of nonuniformity of x-ray intensity by the heel effect on digital Wiener spectrum (WS) doesn’t become clear. A 0.1mm-thick tungsten edge was imaged in the position where X-ray beam was perpendicular to detector plane and in 6cm from chest wall, respectively. And the pre-sampling MTFs were obtained from these edge images. The calculation area of the digital WS within irradiation area was moved in parallel direction to X-ray tube axis, and the digital WS were calculated. The pre-sampling MTFs and the digital WS are calculated by the method based on the IEC proposal. We used MAMMOMAT3000(SIEMENS), MGU-100B(TOSHIBA), M-IV(LORAD) and Senographe DMR+(GE) as X-ray generator. Images were obtained by FCR PROFECT CS (Fujifilm medical). In all equipments and both arrangement directions of the edge test device, pre-sampling MTFs are almost the same, even if the arrangement places of the edge test device varied. In all equipments, when the calculation area was moved about 10cm, the digital WS of the anode side was higher 7.2-17.9% than those of the cathode side. It was found that the dose of anode side was lower about 20% than cathode side from the profile of an exposure image. We think that digital WS modified the nonuniformity of the dose by the heel effect is obtained by multiplying the digital WS by the compensation coefficient obtained by the dose profile, in low spatial frequency.
The method of calculating DQE of a general digital imaging system is proposed by IEC and it is coming to the stage of final draft. However, about digital mammography, nothing is decided yet. This research examines the evaluation method for image quality of a digital mammography with clinical equipment through physical evaluation of the mammographic computed radiography (CR) systems under clinical conditions. We used two CR systems. One consisted of a single plate image reader (FCR PROFECT CS, Fuji), which includes dual-side reading and 50-micron pixels. Other consisted of a single plate image reader (FCR 5000H, Fuji), which includes single-side reading and 100-micron pixels. Digital characteristic curves, presampling MTFs and digital Wiener spectra were measured as indices of image quality. Presampling MTFs were measured from slit and edge images at 28kV. Digital Wiener spectra were measured at 28kV with breast equivalent filter. Presampling MTFs with both readings were almost the same. Digital Wiener spectra with dual side reading were superior to those with single side reading. NEQ of CR system with dual side reading was superior to that with single side reading because of the good efficiency of light condensing. New mammographic CR systems with dual side readings should be a further powerful tool for detecting low-contrast lesions in breast. Wiener spectra need to determine exposure conditions, in order to perform comparison between institutions, since it is strongly influenced of beam quality and a dose. We also compared overall characteristic curves, overall MTFs and overall Wiener spectra of a new CR system with them of a screen-film system. Although MTF was calculated by the slit method, it is necessary to examine another method in quest of MTF including the effect of image processing of CR system.