Answer to Question #10576 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:

If the effective dose for a lumbar spine series is 180 mSv* do the organs such as the kidney, bladder, liver receive the same effective dose or does most of the radiation go to the skeletal bones?
Effective dose is a tricky beast. It is actually not a dose to any part of the body. Let me explain. 

In order to account for differences in the health consequences of radiation doses to different organs or tissues, a tissue weighting factor is assigned to the major organs and tissues of the body. The absorbed dose to the organ is multiplied by the tissue weighting factor. This is done for all exposed organs/tissues and these are added together to get the effective dose. Effective dose allows the risks from nonuniform irradiation of the body to be compared.

If, instead of effective dose, you meant absorbed dose, then the answer is that there is significant attenuation by the body so the entrance dose is much higher than the exit dose. Organs/tissues closest to where the x rays enter the body receive higher doses than those deeper in the body or where the x rays exit. For this reason, chest x rays are typically taken with the patient’s back towards the x-ray tube to reduce the breast dose. 

Kent Lambert, CHP

*The typical effective dose for a lumbar spine series is 1.8 mSv.
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