Activity

 If the number in the ASCII file, for a given voxel, is for instance between 221 and 230, then the activity for that voxel is set to 5. Bq.

利用linear或range把voxelized phantom的灰度值转换成activity的时候，比如linearTranslator/setScale 1. Bq ，指的是每个voxel的activity.