Answer to Question #9544 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

The following question was answered by an expert in the appropriate field:

When it comes to radiation from x rays, is it true that scatter radiation in the patient is prevented from escaping the body if a lead shield is present and therefore it scatters back into the patient? And when a shadow shield is used, does its thickness vary from the normal lead shield since it is nearer to the x-ray tube? If yes, by how much?

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
Answer posted on 5 April 2011. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.