Medical Imaging - Shielding - Minimum thickness of lead chart

Reference 1: Minimum thickness of lead.

Reference 2: 75 kV, 100 kV, 125 kV, 150 kV, 175 kV, 200 kV, 225 kV, 300 kV, 400 kV, 500 kV, 600 kV and 900 kV.

Source: http://en.wikipedia.org/wiki/Medical_imaging