Imaging characteristics of the new screens HG-M(AD system) and XG-S(EX system) were examined in terms of both image quality and x-ray utilization efficiency. For the evaluation of these screens, NEQ(u), which represents an overall evaluation of the imaging properties of the screen-film system, was measured to determine image quality, and DQE(u), which represents detective quantum efficiency, was measured to determine x-ray utilization efficiency. The conventional screen HR-8, which has similar sensitivity, was examined in the same way, and the results were compared with those of the new screens. The new screens were found to have inferior graininess but superior sharpness, NEQ(u), and DQE(u) in comparison with the HR-8 screen. These results indicate that the new screens improve both image quality and detective quantum efficiency. Therefore, the HG-M and XG-S screens, which include the new phosphor technology, can be considered new-generation screens with improved image quality performance for sensitivity. In use, these screens provided superior image quality to the conventional screen with identical exposure.
The keyhole data acquisition technique, which is designed to shorten scanning time, is generally used for dynamic studies. We used a computer simulation technique to perform a quantitative examination of the effect of keyhole rate on image quality. We found that, when the keyhole rate is less than 30%, the error of pixel intensity increases. Further, the better a tissue is enhanced, the larger the error of pixel intensity. The error is most severe in geometrical images with additional defect, reaching 25% at maximum. Error arises from the discontinuity in k-space data in the phase-encoding direction. Examiners need to be careful not to set the KHR too low, especially when dynamic study is required for the evaluation of a small structure that is also expected to be well enhanced, such as the pituitary gland. In summary, we recommended a KHR of at least 40%.
Observer performance tests were conducted to establish the effects of the noise and film contrast of screen-film systems on the detection of low-contrast signals. Three screen-film systems with different noise levels and film contrast were used in the experiments. The observer performance tests were carried out by the contrast-detail(C-D) diagram method and receiver operating characteristic(ROC) analysis. The results of observer performance tests with the C-D diagram method and ROC analysis indicated that the screen-film system with low noise level and high film contrast provided higher detectability than the system with low noise and low contrast. The screen-film system with high contrast showed greater improvement in detectability than the system with low contrast and low noise at low optical densities, despite the increase in noise.
A basic investigation of fluid attenuated inversion recovery imaging(FLAIR) was carried out using inversion recovery(IR) echo planar imaging(EPI). The null point changed with the number of shots, TR, and dummy pulses in the case of multiple shots. When a dummy pulse is not used, the null point becomes shorter with the number of shots. On the other hand, when the dummy pulse is used and the change is from a single shot to two shots, the null point is significantly reduced. However, the null point did not change even though the number of shots was subsequently increased.
The influences on blood vessel images of different imaging delays after medium infusion and different k-space scanning was investigated in simulation and phantom experiments. For 3D SPGR(sequential phase order), it was found that too long a delay in the timing of blood vessel imaging resulted in a loss of data in the low frequency range of the k-space and only the edge of the blood vessel image was detected. Therefore, it seemed difficult to obtain good contrast under such conditions. Form the present results, it was concluded that the most favorable blood vessel images would be obtained under MRA conditions that provide high homogeneity and good contrast during scanning in k-space.
We present our system for polygraph quality control, i.e., gain adjustment and zero correction of the amplifiers and recorders of two polygraphs that are used for monitoring blood pressure and ECG in cardiac catheterization. We examined the zero drift and frequency characteristics of the equipment, for which the gain and zero level of the amplifiers and recorders showed unstable drifting for several hours after the power supply was turned on. We devised plugs for adjusting the amplifiers and recorders. These simple devices made it easy to adjust gain and zero level, and therefore improved the quality control of the equipment in everyday practice.