在普通Ｘ线平片的摄影中 ,影像的形成是依靠被照体和周围物质的密度差来显示的 ,没有密度差也就不成其为影像。当高密度的物体的边缘有低密度物体的衬托而被Ｘ线束切过时就形成了影像锐利的边缘。但是当物体的密度随厚度缓慢下降时其Ｘ线影像就逐渐失去锐利的边缘或者根本就没 In ordinary X-ray film photography, the formation of the image is dependent on the density of the object and the surrounding material to show the difference, there is no density difference is not as an image. When the edge of high-density objects with low-density objects set off by the X-ray beam when the formation of the sharp edges of the image. However, when the density of an object decreases slowly with its thickness, its X-ray image gradually loses sharp edges or does not