Production and use of X-rays — practice question

Outline briefly the basic principles behind CT scanning.

In a CT scanning model, a section is split into four voxels. The pixel numbers $P$, $Q$, $R$ and $S$ for the voxels are shown in Fig. 12.1. The section is examined from the four directions $D_1$, $D_2$, $D_3$ and $D_4$. The detector readings for each direction are recorded. The detector readings are added as shown in Fig. 12.2. The background reading is $34$. Find the pixel numbers $P$, $Q$, $R$ and $S$ as shown in Fig. 12.3.