Outline briefly the principles of CT scanning.
In a model for CT scanning, a section is divided into four voxels. The pixel numbers P, Q, R, and S of the voxels are shown in figure I below.
I
The section is viewed from the four directions 
, 
, 
and 
. The detector readings for each direction are noted. The detector readings are summed as shown in figure II below.
II
The background reading is 34. Determine the pixel numbers P, Q, R, and S as shown in figure III below.
III
, 
, 
, 
