Answer to Question #7237 Submitted to "Ask the Experts"

CT Scanning uses a highly collimated fan beam that takes axial images through the area of interest. Earlier CTs used a sequential slice method and then moved the table to the next adjacent slice. Newer-model CTs use a spiral technique where multiple slices are acquired simultaneously as the table is moved through the gantry. The scanned volume of the body is exposed to the primary x-ray beam and in this area, the dose is the highest.  However, outside of the scanned area, the body is only exposed to scatter and leakage radiation from the shielded x-ray tube in the CT gantry.

When the primary x-ray beam emerges from the collimator and traverses the body, generally three things happen to the x rays. First, an x ray can be absorbed by a structure within the scan plane. Since this x ray doesn't get to the detector array, a dark spot is detected. Second, an x ray traverses the body and is not absorbed; this registers as a bright spot on the detector array. These two processes, absorption and transmission, result in the reconstructed axial image of light and dark areas that you see. The third process is when an x ray is only partially absorbed in the scanned volume and the resultant x ray is scattered off-axis. Scattering of x rays is a very inefficient process and less than one percent of the energy in the primary beam is lost as scatter. Therefore, immediately outside the scanned volume the doses are much lower and they diminish rapidly as you move away from the scan plane, due to the inverse square law. Most of the  scatter that results in dose to the rest of the body is directed internally, so a lead apron or a thyroid shield outside the scan plane will have minimal effect on these internally scattered x rays.

The lead apron and thyroid shield are in the right geometry to absorb some of the leakage radiation that emanates from the x-ray tube as it traverses over the body, but the x-ray tube housing is required by the Food and Drug Administration regulations to be well shielded. The inherent shielding of the tube housing is much greater than an apron provides. So, the additional lead apron and thyroid shield have little real effect on the dose to the body outside the scan plane. However, many patients are so fearful of radiation that for psychological/social reasons, it may be appropriate to use aprons in an effort to reassure them that they will receive the least amount of radiation possible.

There have been some recent developments using bismuth breast shields in the scan plane to reduce the direct exposure of breast tissues in adolescent young girls. This reduces the dose to the breast tissue and doesn't cause too many image artifacts. Possibly, this is what the studies you heard about were really talking about.

Mike Bohan, RSO

References
McNitt-Gray. AAPM/RSNA physics tutorial for residents: Topics in CT, radiation dose in CT. RadioGraphics 22(6):1,541-1,553; 2002.

Fricke BL, Donnelly LF, Frush DP, Yoshizumi T, Varchena V, Poe SA, Lucaya J. In-plane bismuth breast shields for pediatric CT: Effects on radiation dose and image quality using experimental and clinical data. AJR 180:407-411; 2003.