Soft tissue, on the other hand, isn't as dense, and contains mostly lower atomic number elements, like carbon, hydrogen, and oxygen. So more of the X-rays penetrate tissues like lungs and muscles, darkening the film. 相反的,軟組織不是那麼緻密,大部分含有較低原子序數的元素,如碳、氫及氧。所以比較多的X-射線能穿透如肺及肌肉組織,使底片變得較黑。