The application of dual energy subtraction technique to sialography

(1)图像处理系统的升级：在图像处理计算机上增加了随机存储器和磁光盘单元。提高了图像文件存储的处理速度和容量。双能量减影抑制骨骼图像的软件系统。通过计算机模拟对X射线滤光片的滤材、管电位和增强屏的最佳选择进行了计算机模拟，发现用Gd_2O_2S和CaWO_4作滤色器，用Gd_2O_2S和CaWO_4作滤色器，用铜或锡作盒式滤光器，用10OkVcp作管电位的组合具有较高的性能。(3)体模实验用不同直径的聚晶硅管填充造影剂、骨模楔子和水浴。在用这种方法产生的这些模体的图像中，可以识别出厚度大于0.5 mm的造影剂代码。但造影剂的对比度明显降低，噪声增加。(4)讨论从理论上证明，碘对比度的降低是由于骨与碘衰减系数的能量依赖关系相似。噪声的增加是由胶片-屏幕系统颗粒的统计相加性质引起的。尤其是高速滤网在铜/锡滤光片后面产生的高能量图像中的噪声被放大4倍或5倍。因此，目前的双能量减影技术不适合提高腮腺造影术的图像质量。

(1) Grade up of the image processing systemRandom access memories and a magnetic optical disk unit were added to the image processing computer. The processing speed and capacity of image file storage were advanced. The soft ware system for the dual energy subtraction to suppress the bone image. It took within 10 minutes to process all the procedure for subtraction including input scan of x-ray film.(2) Simulation for optimizationThe optimum selection of x-ray filter materials, tube potential and intensifying screen was searched for by means of computer simulation, and found that the combination of gadrinium or tungsten for dichromizing x-ray filter, Gd_2O_2S and CaWO_4 for the screen and copper or tin for in-cassette-filter and 1OOkVcp for tube potential were expected to have high performance.(3) Phantom experimentPhantoms were made of polyethyrene tube of various diameter filled with contrast agents and bone phantom wedge and a water bath. In the images of these phantoms produced by this method, codes of contrast media thicker than 0.5mm were recognized. The remarkable reduction of contrast of the contrast media and increment of the noise were, however, observed.(4) DiscussionIt was theoretically proved that reduction of the iodine contrast was due to the similarity of the energy dependency of attenuation coefficient among the bone and the iodine. The increment of the noise was caused by the statistically additive nature of the granuality of the film-screen systems. Especially, the noise in the high edergy image, which was produced by high speed screen place behind the copper/tin filter, magnified by factor 4 or 5. It was concluded, therefore, that the present dual energy subtraction technique was not suitable for improvement of image quality of the sialography.