Answer to Question #9544 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
While x rays do backscatter off of a lead apron, this is a very inefficient process and contributes very little additional dose upon escaping the body. At all angles, in the x-ray energy range of 50-300 kilovolt potential (kVp), the backscatter is less than 0.3 percent of the incident exposure. The other 99.7 percent of the exposure is mostly absorbed by the apron, and a very small portion penetrates through the apron as leakage.Shadow shields generally have a uniform thickness and usually are placed as close to the x-ray collimator as possible. This allows them to be small and easily adjusted to the right anatomical position, while using the light field function to aim the x-ray tube. The area under the shadow will be protected by the shield, which is why they are called "shadow shields." Since a shadow shield is trying to minimize exposure to a small portion of the body (usually the gonads) its thickness is usually enough to reduce the exposure to less than 0.1 percent of the unshielded exposure. The distance the shield is placed from the x-ray source does not adjust the shielding value, just the portion of the body protected within the shadow it projects.
Sometimes, x-ray compensators are used to modify the x-ray field intensity when trying to image portions of the body where the thickness changes rapidly. In these cases, the problem is that thick portions of the body will produce an underexposed image and thin areas will produce an overexposed image. The x-ray compensators correct for this problem by varying their thickness in the opposite direction, allowing for a uniformly exposed image where all structures are imaged properly. While these types of filters do vary in thickness, they are usually not made out of lead.
Michael J. Bohan