Secondary radiation is produced both by the beam passing through the patient's body and also by its passage through the table. Numerous centers of SR are set up through the irradiated part (figure 1-22). The radiation thus produced tends to fog the film and cover the image. This often makes the use of grids necessary.

Figure 1-21. Two PA radiographs of the hand using 50 kVp. mass factors: A, 100; B, 50.
Figure 1-22. Absorption of secondary radiation by lead strips in the grid.

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