Answer to Question #10922 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:


I had a CT (computerized tomography) head angiogram a few months ago. The total DLP (dose length product) was 3,032 mGy cm and the cumulative CTDIvol was 249.18 mGy. From my understanding, taking the DLP × the conversion factor k gives you the effective dose. In this case: 3,032 × .0023 = 6.97 mSv. The .0023 is the conversion factor for a head CT. However, the CT angiogram was done with and without contrast and on the patient protocol there was a third part of the scan named HEAD ANGIO. It seems the scan I had involved three parts: (1) head scan with contrast, (2) head scan without contrast, (3) head angiogram scan. This seems excessive and even though the effective dose seems quite low, the CTDIvol seems rather high. It is my impression that the CTDIvol is the absorbed dose to the organ, in this case the brain. I'm curious, is the CTDIvol or the absorbed dose I received high and something to be concerned about? Should I even be concerned about the CTDIvol since the effective dose provides a better estimate of exposure to radiation? If it helps, the total mAs was 8,566.


Yes, you are correct that you had three scans. Based on your information, your scan data can be summarized as in the following table.

Scan # CTDIvol DLP Scan Length (cm)
1 Noncontrast 66.05 1,129 17
2 Contrast 34.56 695 20
3 Delay Scan 66.05 1,129 17

There is nothing excessive about your scans. Your hospital seems to have used a routine scan protocol for your diagnosis.

There are many different CT head scan protocols and each protocol may vary depending on what the doctors are looking for. This includes rotation time, mAs, slice thickness, and pitch. The mAs is only one of the many parameters that affect dose and as such, you cannot single it out from the rest. These scan parameters are set to achieve the best diagnostic results.

CTDIvol is not an absorbed dose to the brain. It is a physics parameter measured with a Plexiglas cylinder test object. As such, there is no need for additional concerns. The dose and CTDIvol is proportional to the mAs (tube current-time product), i.e., the higher the mAs, the higher the dose. The brain is one of the most radioresistant organs in the human body. No damage to the brain is expected from regular CT scans. As a patient it is important to remember that CT scans have a medical benefit that when properly performed and clinically indicated, outweigh the potential risks (benefit vs. risk).

Terry Yoshizumi, PhD CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 19 May 2014. 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.