Answer to Question #7265 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
When calculating effective dose, how is the area that is scanned, such as by a CBCT (cone beam computed tomography) for dental purposes, compared to the whole-body weighting factor? I am confused as to whether the CTDI and effective dose are communicating the same safety limits. Is effective dose calculated from a phantom measurement that is then multiplied by tissue-weighting factors? Is that dose for say, a dental scan, then divided by the whole mass of the body?
When calculating an effective dose, one needs to know the absorbed dose to the organs or tissues being irradiated and the specific tissue-weighting factors for these organs or tissues in order to calculate the dose. The mass of the whole body is not used in calculating the effective dose.
The Computed Tomography Dose Index (CTDI) is based on measuring the absorbed dose in a cylindrical acrylic phantom (either 11 cm in diameter for a head phantom or 21 cm in diameter for a body phantom) with a 10 cm pencil ion chamber in the phantom's center hole and again in one of the phantom's peripheral holes. Depending on which version of the CTDI is being used, a calculation must be performed to determine the average absorbed dose to the phantom. This is useful when comparing several different CT (computed tomography) units, but is limited when trying to calculate an accurate dose. Patient anatomy does not match a solid phantom. When calculating a patient dose, the CT image should be reviewed with a radiologist who can identify the irradiated organs. An anthropomorphic phantom with TLDs (thermoluminescent dosimeters) would provide the most accurate measurement of the absorbed dose to different organs. The new CT scanners will further complicate things as they have a program that actively changes the scan parameters as the patient is imaged to reduce dose.
See Radiation Quantities and Units from the Food and Drug Administration (FDA) website.
Other FDA CT information resources can be found at http://www.fda.gov/cdrh/ct/resources.html.
Steven H. King, CHP