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21 March 2010

Answer to Question #6846 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

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

Is there a higher risk to the patient from the secondary (scatter) radiation resulting from the ionizing radiation produced during a CT (computerized tomography) scan when shielding is used? In other words, does the scatter radiation produced within the body become "trapped" and thereby pose a greater risk due to the 360-degree rotation of the source, etc.?

In recent years bismuth shielding has been evaluated for dose reductions for the eye and breasts in pediatric CT. Results demonstrate up to 42 percent in the eye dose and 29 percent in the breast dose in a five-year-old pediatric phantom. Based on the results one can conclude that the radiation dose and risk are reduced in CT with the use of the bismuth shield.

It is not clear what the "trapped" radiation means in your description. I assume that you meant normal attenuation and interaction processes through photoelectric and Compton effects in the body. Based on published articles (see below), there is no data that show that scattered radiation in the body contributes to the organ dose increase.

Three main factors may explain this result as follows:

A direct CT beam above the organ of interest, e.g., breast or eye, is attenuated with a bismuth shield, which reduces the organ dose significantly.
The beam from the 180-degree opposite entry into the organ will be attenuated in energy by the body and reduce the number of photons that reach to the organ from behind. This may not affect the dose at any rate from this exposure.
The small fraction of radiation that reaches to the shield after it passed through the body will be absorbed by the shield and, thus, will not have any effects on the organ dose.

Terry T. Yoshizumi, PhD

References:
Mukundan S, Wang PI, Frush DP, Yoshizumi TT, Marcus J, Kloeblen E, Moore M. MOSFET dosimetry for radiation dose assessment of bismuth shielding of the eye in children. American Journal of Roentgenology 188:1,648-1,650; June 2007.

Fricke BL, Donnelly LF, Frush DP, Yoshizumi TT, Varchena V, Poe SA, Lucaya J. In-plane bismuth breast shield for pediatric CT: effects of radiation dose and image quality using experimental and clinical data. American Journal of Roentgenology 180:407-411; February 2003.

Answer posted on 30 October 2007. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of 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 specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.