Answer to Question #11177 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:

Q

How can the absorbed dose be calculated using the parameters that are on the console of a computed tomography (CT) unit, e.g., computed tomography dose index volumetric (CTDIvol), pitch, precontrast series, contrast series, and delay series?

A

Normally, it is difficult to convert a CTDIvol to an absorbed dose or effective dose for a number of reasons. First is the definition of absorbed dose versus effective dose. Absorbed dose is the mean energy imparted to a mass of tissue. Effective dose uses the specific absorbed doses to various organs multiplied by tissue-weighting factors for each organ to estimate the health risk from an exposure compared to a uniform absorbed dose to the body as a whole.

First, let's take a look at the absorbed dose. In a CT scan, generally the whole body is not directly irradiated by the scan. Usually, only a limited portion of the body is irradiated. For example, in an abdominal CT scan, the irradiated volume extends from the thoracic diaphragm to the pubis. The head, chest, and legs are not directly irradiated and are only exposed to minor scatter levels of radiation. 

However, even within the abdomen itself, the dose is not uniform. Due to the absorption of the x rays by the abdominal tissues, the peripheral tissues will have a higher absorbed dose, while inner core tissues will have a lower absorbed dose. The core-tissue absorbed dose in the abdomen is about one-half the peripheral-tissue absorbed dose.

Generally, the CTDIvol number that appears on the CT scan is a weighted-average absorbed dose to a cylindrical phantom used to perform the calibration measurements. Using a 32-centimeter (cm)-diameter cylindrical phantom, measurements at a depth of 1 cm and at the center of the phantom are made. The CTDIvol estimates the mean tissue dose by adding one-third of the central measured dose to two-thirds of the peripheral measured dose.

The problem, however, is that generally the body cross sections of most people are closer to an oval than a circle and not everyone has a diameter of 32 cm. So to come up with an absorbed dose estimate for an individual, you would need to convert the CTDIvol dose from the cylindrical phantom to an appropriately sized anthropomorphic model. In addition, to generate an effective dose you would need to estimate the mean absorbed dose to each organ within the irradiated volume and then use the International Commission on Radiological Protection (ICRP) organ tissue-weighting factors to convert the absorbed doses into an effective dose estimate.

Consequently, the CTDIvol value listed on the scan is a good "ballpark" absorbed dose estimate for tissues within the x-ray field, but to convert it to an individualized absorbed or effective dose is a lot more complicated. Fortunately, there are many groups that have studied this problem in detail, and they provide various methods to convert the CTDIvol and/or dose-length product (DLP) information into an individualized dose estimate.

One method is available from the ImPACT website, impactscan.org/ctdosimetry.htm. ImPACT has developed an Excel spreadsheet system that uses Monte Carlo phantom data and machine characteristics to determine dose estimates for the various organs from the scan parameters and derives an effective dose estimate.

Also available are online CT dose calculators that use DLP or CTDI data; anthropomorphic phantoms for newborns, 1-, 5-, and 10-year-olds, and adults; CT study type; and diameter data to derive a size-corrected effective dose estimate. You can find one at radiation-dose.com.

The above cited sites are only examples and are not necessarily endorsed by the Health Physics Society.

Mike Bohan
Radiation Safety Officer

References

  • Huda W, Ogden KM, Khorasani MR. Converting dose-length product to effective dose at CT. Radiology 248(3): 995–1003; 2008.
  • McNitt-Gray MF. AAPM/RSNA physics tutorial for residents: Topics in CT–radiation dose in CT. RadioGraphics 22: 1541–1553; 2002.
  • International Commission on Radiological Protection. Recommendations of the International Commission on Radiological Protection. New York: Pergamon Press; ICRP Publication 26, Ann. ICRP 1(3); 1977.
  • International Commission on Radiological Protection. 1990 Recommendations of the International Commission on Radiological Protection. New York: Pergamon Press; ICRP Publication 60, Ann. ICRP 21(1–3); 1991.
  • International Commission on Radiological Protection. 2007 Recommendations of the International Commission on Radiological Protection. New York: Pergamon Press; ICRP Publication 103, Ann. ICRP 37 (2–4); 2007.
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 23 February 2015. 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.