Answer to Question #10922 Submitted to "Ask the Experts"
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)|
|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