The attenuation of X-ray through the object is non-linear and the gray level of image is non-linear response of object thickness.
射线穿过物体的衰减是非线性的，经过影像系统得到的图像灰度与物体厚度是非线性的关系；
2
The image reconstruction problem of linear attenuation coefficients is very difficult and central in transmission TGS.
在TGS透射测量中，线性衰减系数值的图像重建问题是TGS的难点和核心问题。
3
Many factors work on the target volume calculation, such as PET image acquisition mode, scatter correction, attenuation correction, reconstruction method, image display mode, positron pharmacy.