Hi does the CTDl volume on the dose page for an abdominal pelvic CT include the scatter from having the arms down , extended scan to the knee and a brace on the femur ? I am not sure if it includes the scatter for the in field organs . I know it doesn’t include for the out of field organs . Can you tell me and also do you know anyone to hire to do Monte Carlo calculations for CT scans ? Thank you

CTDIvol is a measure of the radiation produced by a Computed Tomography (CT) scanner. The CT Dose Index (CTDI) was defined by the US FDA shortly after the birth of CT, in 1981. It is a common misconception that CTDIvol (the “vol” is for volume) indicates the radiation dose received by a patient from a CT scan. Rather, CTDIvol is an “index” that reflects average dose to a plastic cylinder (called a “phantom”) that mimics the radiation absorption of an average-sized adult. Therefore, CTDIvol is a standard for assessing the radiation output from the scanner during a CT scan. It represents the absorbed dose that would be received by a patient whose radiation absorption properties are the same as that standard plastic cylinder that was used by the FDA to define the measure. CTDIvol is not an accurate indication of the radiation dose received by an individual patient whose body size, shape, and composition are much different from the standard plastic cylinder. That is why the word “index” was included in the name. Although CTDIvol is not any individual patient’s dose, it is sometimes used as a rough approximation. When used as an approximation of patient dose, it should be understood that there is a large uncertainty (potential error) in the value.

In CT scanning, there is a section of anatomy that is directly exposed by an x-ray beam. This is called “primary” radiation. A portion of the primary radiation is deflected or "scattered" outside of the imaged volume and delivers radiation dose to adjacent anatomy. So, the radiation dose to any point inside the imaged volume is a sum of the primary radiation received at that point and the scattered radiation from scans of adjacent anatomy. The CTDIvol index always includes both primary and scattered (also known as secondary) radiation, regardless of the type of examination (head, chest, abdomen/pelvis, etc). Anatomy and objects that are outside of the primary beam do not generate any scatter and have no impact on CTDIvol.

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Hello, If I understand the American Association of Physicists in Medicine (AAPM) report 293 correctly, the SSDE and CTDIvol values for children are approximately the same. This is because, as I understand it, the fH16 factor (conversion factor) is quite small even for newborn infants (e.g. between 0.9 and 1.05 for a one-year-old child). In fact, for children older than 1 year, the fH16 value becomes smaller and therefore the SSDE value also becomes smaller. This means, for example, that the CDTIvol value shown in the dose report of a 3-year-old child after a CT scan should actually be smaller. Please allow me to ask 4 questions about all this: 1- Is my summary above correct? 3- Is this formula correct to convert CDTIvol to SSDE? SSDE= fH16*CTDVol 3- How can I convert SSDE value to effective dose? 4- When we calculate with the conversion factors in the AAPM 204 report, sometimes there is a big difference between SSDE and CDTIvol, why do the conversion factors in the AAPM 293 report do not cause a big difference between SSDE and CTDIvol? Yours sincerely.