[Analysis of Scattered Radiation in an Irradiated Body by Means of the Monte Carlo Simulation: Variation of the Subjective Contrast Due to Difference in the Location of Inhomogeneous Region].

When an inhomogeneous medium such as bone, whose composition or density are clearly different from that of soft tissue of human body, exist in irradiated body, a subjective contrast of X-ray image changes by the location of these inhomogeneous medium. This cause due to the change of behavior of scattered photons in the body depends on the location of inhomogeneous medium besides due to the influence of a penumbra. But this mechanism is not explained clearly yet. In this paper, it was analyzed by means of the Monte Carlo simulation that what kind of difference occurs to a subjective contrast by the difference in location of inhomogeneous medium in water phantom and that a change in behavior of scattered photons in the phantom influences a subjective contrast by what kind of mechanism. In this case the inhomogeneous medium is bone, whose effective atomic number and density are higher than that of water, the subjective contrast of X-ray image degrades when bone is located near the entrance surface (upper position) than located near the exit surface (lower position). This is caused by the number of scattered photons, originated in primary photons incident upon the zone besides the region from entrance surface to exit surface including inhomogeneous medium and incident on the area of shadow of inhomogeneous medium on the image detector, is greater in case of the upper position than in case of the lower position. In the lower position, many of these scattered photons are interacted in bone located near the exit surface by the photo-electric absorption and only a small amount is incident on the image detector.

Investigation of measurement accuracy of factors used for detective quantum efficiency measurement in digital radiography.

In the detective quantum efficiency (DQE) evaluation of detectors for digital radiography (DR) systems, physical image quality indices such as modulation transfer function (MTF) and normalized noise power spectrum (NNPS) need to be accurately measured to obtain highly accurate DQE evaluations. However, there is a risk of errors in these measurements. In this study, we focused on error factors that should be considered in measurements using clinical DR systems. We compared the incident photon numbers indicated in IEC 62220-1 with those estimated using a Monte Carlo simulation based on X-ray energy spectra measured employing four DR systems. For NNPS, influences of X-ray intensity non-uniformity, tube voltage and aluminum purity were investigated. The effects of geometric magnifications on MTF accuracy were also examined using a tungsten edge plate at distances of 50, 100 and 150 mm from the detector surface at a source-image receptor distance of 2000 mm. The photon numbers in IEC 62220-1 coincided with our estimates of values, with error rates below 2.5%. Tube voltage errors of approximately ±5 kV caused NNPS errors of within 1.0%. The X-ray intensity non-uniformity caused NNPS errors of up to 2.0% at the anode side. Aluminum purity did not affect the measurement accuracy. The maximum MTF reductions caused by geometric magnifications were 3.67% for 1.0-mm X-ray focus and 1.83% for 0.6-mm X-ray focus.

[Study of appropriate dosing in consideration of image quality and patient dose on the digital radiography].

Recently about 90% of radiographs have been taken by the digital radiographic system in Japan, but the exposure dose of the patients are about ten-times different among the systems. We understood it by a surveytaken in 2007. We studied the visual evaluation with varying exposure doses using the image phantom of the lumber AP, lumber lateral and hip AP. Additionally we measured quantum efficiency (DQE) of the digital systems. We also studied the exposure index (EI) of IEC standard to see whether it is able to be the sensitivity index among the digital systems. DQE in 1.0 cycle/mm of CR, FPD (GOS), FPD (CsI, a-Se) became 0.2-0.25, 0.3, 0.5, respectively. Our results display that the dose reduction is relative to DQE. The visual evaluation results also show that dose reduction is possible among the systems. From these results, we are able to reduce the exposure dose of the patients at the clinical site. We also suggest that we manage the exposure dose using the E.I of the IEC standard.

[Investigation of error factors in analysis of digital noise power spectrum].

The noise power spectrum (NPS) measurement is important for assessing noise properties of digital radiography systems, and its measurement method was standardized in International Electrotechnical Commission 62220-1 (IEC). However, improvement of its accuracy is not easy due to random data analysis. In this study, regarding error factors in the NPS measurement using 2-dimensional (2D) Fast Fourier transform, we investigated effects of overlap of region of interests (ROIs), number of average lines in 2D frequency space, directional dependence of frequency property, and detrending techniques. If the number of average lines was set so as to obtain a similar frequency range to IEC, total matrix size was the most important factor and error rate was decreased with increasing of the size. For images, including many trends, detrending using 256 x 256-pixels ROI and second-order polynomial fitting was the most effective. Consistent with the previous report, the overlap of ROIs was not effective for improving accuracy. Contrary to the previous report that indicated effectiveness of 128 x 128-pixels ROI for detrending, we demonstrated less affectivity of the ROI size, other than 256 x 256-pixels.

[Influence on the physical characteristics of medical color liquid crystal displays with increase in aperture ratio].

We measured the physical characteristics of 2-million(2M)and 3-million(3M)color liquid crystal displays(LCD)whose aperture ratio was increased and compared them with conventional models. The results showed the influence of the increased aperture ratio on the physical characteristics of the LCDs. We evaluated resolution by means of modulation transfer function(MTF)and evaluated granularity by means of noise power spectrum(NPS). Each of the measurements was done with a high-resolution single-lens reflex-type digital camera. A decrease of MTF depending on sub-pixel structures was recognized. A decrease in the cross sub-pixel direction was recognized in the 2M model, and a decrease in the sub-pixel direction was recognized in the 3M model. As for NPS, a reduction was recognized in the sub-pixel and the cross sub-pixel direction in both models. As a result, an improvement in granularity was recognized. The improvement in granularity was large with the color LCDs whose aperture ratio was increased. The increase of an aperture ratio influenced both MTF and NPS, and the results depended on the shape and size of the sub-pixel cells.

[Proposal and verification of presampled MTF measurement by simple analysis using the edge method].

The edge method adopted by the International Electrotechnical Commission (IEC) for presampled modulation transfer function (presampled MTF) measurement has the advantage that the complexity and limitations of image acquisition are less than those of the slit method. On the other hand, it has the disadvantage that the maintenance of accuracy is difficult because of the noise amplification caused by the differentiation. In order to reduce this disadvantage, we proposed an effective method in which many edge profiles are synthesized and then the synthesized edge profiles are bound into regular interval bins. Furthermore, the influences of some factors in edge profile analysis on measurement accuracy were examined. The appropriate width of one bin was 10-20% of the sampling interval. Measurement error of the inclination angle of the edge should be made up to 0.05 degrees . The range of the angle was allowed to be from 1 to 5 degrees. If these conditions were fulfilled, accurate measurement up to the Nyquist frequency (fn) was possible. Using an actual computed radiography (CR) system, the presampled MTF was obtained by the slit method and our edge method. The two results showed good agreement.