We investigate the nonlinearity in digital X-ray images to determine the feasibility of a noise reduction process using a
mathematical model, which realizes an accurate digital X-ray imaging system. To develop this mathematical model, it is
important to confirm whether the system is linear or nonlinear. We have verified the nonlinearity of the imaging system
through an analysis of computed radiography (CR) images by using the method of surrogate, a statistical test of
nonlinearity, and the Wayland test. In the method of surrogate, we use the Fourier transform surrogate method. The
Wayland test can be used for evaluating the complexity of the orbit of a signal aggregate called the attractor
reconstructed in a high-dimensional phase space using a nonlinear statistical parameter called the translation error.
Nonlinearity is determined by statistically comparing the translation error of the original data with that of the surrogate
data. X-ray images are obtained under different conditions to investigate the effects of various tube voltages--50 and 80
kV--and dose settings--2 and 10 mAs. We extract 30 profiles from both directions, the directions vertical (V-direction)
and horizontal (H-direction) to the X-ray tube. In the H-direction, nonlinearity is found at all voltage and dose settings.
On the other hand, nonlinearity is found only at 10 mAs and 80 kV in the V-direction. Hence, it can be concluded that
nonlinearity is indicated by a decrease in the quantum mottle, and the factors of nonlinearity exhibit the comprehensive
variation produced by the digital X-ray imaging system.
In the soft copy diagnosis, each pixel of the detector is displayed to the correspondent pixel of liquid crystal display
(LCD). But when the image is displayed at the first time, the entire image may be reduced. We examined the influence
that the difference of image reduction rate on LCD exerts on detection performance by using observer performance
experiment. Moreover, to find the best interpolation method, we investigated the several interpolation methods. We
made a simulation image which is similar to Burger phantom. This image consists of 288 signals, each of a different size
and contrast. The matrix size is the same as Phase Contrast Mammography (PCM). We gradated the simulation image by
using an MTF of a geometric blur, and the image was added to the noise image which is uniformly exposed with PCM.
Then the image was reduced by using the nearest-neighbor, the bilinear, and the bicubic methods. The reduction rates
were calculated as the ratios of the number of pixels of LCDs to those of PCM. We displayed the reduced images on
LCD and examined the detection performance. Results of physical evaluation examined before showed that sharpness
and granularity have worsened both in proportion to the reduction rate. The detection performance deteriorated as the
reduction rate becomes high. In the comparison of the interpolation methods, the detection performance of the nearestneighbor
method was worse than those of other interpolation methods. The bilinear method is the most suitable for the
reduction of the image.
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.
A novel noise power spectrum (NPS) measurement method for medical liquid crystal display (LCD) was developed. A uniform image displayed on an LCD was imaged with a high-resolution single-lens reflex type digital camera (D70, Nikon) equipped with a close-up lens. In order to avoid significant errors (frequency leakages) caused by strong periodic components of the pixel structures, noise profile data was processed by periodic components subtraction, and NPS was calculated from the processed profile with fast Fourie transformation (FFT). Horizontal NPS at the center of display area was measured up to the 10 times of Nyquist frequency. Actual measurements were performed with various models of monochrome two million and three million pixel LCDs to investigate difference of conventional method and our proposed method. Resultant NPS obtained from the conventional method with simple FFT included significant errors over the whole frequency ranges. In contrast, our proposed method could compensate most of those errors. Resultant NPS of our method indicated clearly the difference of noise property between three displays we measured, which corresponded to the visual evaluation for them. In conclusion, our method is very useful for evaluation of noise properties of medical LCD due to the good accuracy and reproducibility.
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.