Answer to Question #10599 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:

Two years ago I got a CT (computerized tomography) scan of my head with and without contrast. I’m afraid that I got a lot more radiation than allowed in that scan.

Scan parameters were:
Total mAs: 4,159
Total DLP (dose length product): 2,156

I read that to get the total radiation dose in mSv, I need the conversion rate which I found on the Internet. For the head of an adult person it is 0.0021. So, 2,156 × 0.0021 = 4.5 mSv. How do I know they used a conversion rate of 0.0021 and did not use 0.0053 for example? If they used a conversion rate of 0.0053 (the conversion rate for a neck scan or for a baby scan), the total radiation is 11 mSv. Is there any proof that they used a conversion rate of 0.0021? 

Please help me. I’m 26 years old. Did that unnecessary scan I got two years ago destroy my life?  
The conversion factor to estimate effective dose from CT exams does not influence the DLP value displayed on the dose summary report you received. DLP (dose length product) is the product of CTDIvol (a dose from the CT scanner used as a reference) and the CT scan length.
A nationwide survey of CT examinations conducted by the Council of Radiation Program Control Directors indicates the median effective dose for an adult head CT scan is 2.5 mSv. The effective dose estimate for each scan you received is 2.26 mSv (4.5 mSv for both scans). In other words, the estimated effective dose you received is about the same as the typical doses seen nationwide.
The American Association of Physicists in Medicine states that the “risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short time periods are too low to be detectable and may be nonexistent.” The effective dose estimate from your CT scans is well below these levels and involves minimal, if any risk.
Karen Brown, MHP, CHP, DABR                                            
Steve King, MS, CHP, DABMP

Editors’ Note:
Even if CT scans or other diagnostic images do not reveal anything wrong, they should not automatically be considered unnecessary. These exams eliminate possible causes of the medical condition and this is important information to know. 
Answer posted on 25 March 2013. 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.